Shotcrete FAQ's

Why Shotcrete

Technical Questions and Answers Archive

The ASA Technical Questions & Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.

User agreement: The answers provided to submitted questions are intended for guidance in planning and executing shotcrete applications. This information is intended only for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations, and who will accept responsibility for the application of the material it contains. The American Shotcrete Association provides this information based on the best knowledge available to them and disclaims any and all responsibility for the information provided. The American Shotcrete Association will not be liable for any loss or damage arising therefrom.

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Question 1:

What is the difference between shotcrete and Gunite?

Answer:

Shotcrete is an all-inclusive term to describe the spraying of concrete or mortar that may be accomplished through either a dry- or wet-mix process. Gunite refers only to the dry-mix process in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. Because complete mixing of the water and dry ingredients is not possible in the nozzle, mixing is completed as the material impinges on the receiving surface, through manipulation of the nozzle. This requires a very highly skilled nozzleman, especially in the case of thick or heavily reinforced sections. Large aggregate is seldom used with the dry-mix process. Wet-mix shotcrete involves pumping of a previously prepared mixture, typically ready mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The mixture usually contains minus 1/2 in. aggregate, although larger-size aggregate has also been used.

The use of the term “shotcrete” first occurred in Railroad Age magazine more than 50 years ago in place of the then proprietary word “Gunite,” and has been used by the American Concrete Institute since at least 1967 to describe all sprayed concrete or mortar.

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Question 2:

How should I design joints for shotcrete?

Answer:

Shotcrete is concrete forced or impelled through a hose using a pressurized air system. Therefore, the guidelines for jointing concrete are no different than for concrete placed by other methods.

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Question 3:

I want to specify ACI Nozzleman Certification in my next project. A contractor has told me that there are no certified nozzlemen in the project area. How can I verify that information? What should I direct this contractor to do?

Answer:

Go to the ACI website, www.concrete.org, and click on the Certification tab. A button will appear for the Certified Personnel Directory. Click this button. Using Search Option 2, customize the search by type of certification and location. Please note, ACI will identify the individual by name, city, and state only. The individual address, telephone, or employer is not available from ACI or the ASA. Education for ACI Certification is available through the ASA office. Contact ASA for the roster of ASA Educators. Certification exams are conducted by ACI-approved examiners in strict compliance with ACI certification policies.

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Question 4:

We have a project that calls for new 6 in. concrete shearwalls formed and placed against the existing structure from the basement up to the fourth floor to enable an additional seven floors to be added to the structure. Our engineer has suggested that the new shearwalls be constructed using shotcrete. We are not familiar with using this system for structural applications. Most of the information we have gotten relates to using shotcrete for swimming pools and cosmetic applications. What advice can you provide?

Answer:

The use of shotcrete for structural applications has been documented in numerous articles in Shotcrete, Concrete International, and other publications. The key is to find a shotcrete contractor experienced in structural applications. Investigate the contractor’s project history to determine his/her experience. A contractor experienced in this type of structural enhancement will be most helpful in achieving the desired result in an economical and timely manner.

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Question 5:

We are building a new home. Foundation contractors who place traditional basement walls tell us they would never go into a house built with walls constructed using shotcrete. When used for walls, can shotcrete be of equivalent strength as placed concrete?

Answer:

Shotcrete is a method of building a structure using a concrete mixture. A shotcrete mixture likely would exceed the compressive strength of most mixtures used for placed walls because the application of shotcrete requires a much lower water-cementitious material ratio than commonly found in residential wall mixtures. A shotcrete mixture will have a water-cementitious material ratio of approximately 0.50, yielding a compressive strength of about 4000 psi at 28 days. Poured wall mixtures have ratios of approximately 0.70 and compressive strengths of 2500 to 3000 psi. The lower water-cementitious material ratios of shotcrete mixtures produce other benefits such as reduced shrinkage and lower permeability. Additionally, the greater compaction of shotcrete achieved through the velocity of placement improves compressive strength and durability.

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Question 6:

We will be using shotcrete to repair a concrete box culvert that has some minor spalling. Do we need to apply a bonding agent before applying the shotcrete? How should we prepare the surface?

Answer:

No bonding agent is required. A key to a successful repair is proper surface preparation. The surface receiving the shotcrete must have the deteriorated material completely removed, be thoroughly cleaned, and in a saturated surface-dry condition (SSD) at the time of shotcrete appli­cation. Another key item is proper curing and protection following shotcreting. Details can be found in the Task Force 37 Report “Guide Specification for Shotcrete Repair of Highway Bridges.” The document is available from the American Associ­ation of State Highway and Transportation Officials (AASHTO), Washington, DC.

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Question 7:

I am a structural engineer working on a project in Southern California. We are creating specifications for the use of shotcrete for basement walls. However, I cannot find any information on compressive strength requirements for shotcrete in the building code. We are basing our design on compressive strengths ranging from 3500 to 4500 psi. Are there minimum and maximum allowable compressive strengths for shotcrete?

Answer:

To the best of our knowledge, there is no maximum compressive strength limitation. The minimum compressive strength would be dictated by your structural calculations as it would be with any structural concrete design. The most common compressive strength specifically encountered by ASA members in your area is a minimum of 4000 psi at 28 days.

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Question 8:

Our firm has no experience designing for shotcrete applications. We have been investigating the process and would like to know what we should be looking for as the shotcrete is placed. Are there special features or problems in shotcreting?

Answer:

Proper placement is the most important element in achieving good shotcrete results. Most defects that occur in shotcrete are due to poor placement. Shotcrete success depends largely on the skill and actions of the nozzleman. The nozzleman’s goal is to achieve adequate compaction and good encasement of the reinforcement (if present) with no entrapped rebound or hardened overspray. For this reason, it is important to require that the nozzleman be ACI certified for the application. There are specific certifications for both wet and dry processes as well as vertical and overhead applications. If the nozzleman is certified, the probability that you will get the desired results is significantly increased. For more information on certification, visit the ASA website, www.shotcrete.org, and click on Certification.

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Question 9:

I know air entrainment is required in concrete exposed to cycles of freezing and thawing while saturated. However, the shotcrete I am going to be applying on a project in Chicago is on a vertical surface where the water will essentially run off the surface. Do I still need to worry about air content?

Answer:

You are correct in stating that entrained air is necessary in concrete that is exposed to freezing and thawing while critically saturated. Even vertical walls can get critically saturated in places. Because you are working in a part of the country that experiences significant freezing and thawing, it is imperative that you maintain sufficient air content in the shotcrete. Remember, you are going to lose some air content in the placement process so the air content of the shotcrete mixture going into your pump must be higher than the desired in-place air content. It is a wise idea to do some testing in advance of the actual shotcreting to determine how much air content you will lose.

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Question 10:

We are going to be using shotcrete for repairs in a parking structure. We have no experience performing this work and will be subcontracting this portion of the job. What should we be watching for when the shotcrete is being applied?

Answer:

Surface preparation is a critical operation. The substrate must be prepared properly. All deteriorated concrete must be removed. This is generally accomplished with light-duty chipping hammers, scarifiers, or scabblers. The remaining concrete is then sandblasted or waterblasted to remove the concrete “bruised” by the initial removal operation. The objective is to create a clean, sound surface with the proper surface roughness to receive the shotcrete.

After the surface preparation, the substrate must be saturated with clean water and then allowed to dry to a saturated, surface-dry condition immediately prior to shotcreting. Shotcrete should not be applied to a bone-dry surface as the substrate will absorb water in the shotcrete mixture intended for hydration of the cement. Also, a bone-dry surface will tend to allow plastic and drying shrinkage cracks to form. Conversely, a surface that is wet at the time of shotcreting will result in a high water-cement ratio (w/c) at the interface between the substrate and the shotcrete. High w/c at the interface will result in significantly lower bond strengths.

As with all concrete, proper curing and protection is critical. Failure to cure properly will result in lower shotcrete strengths and may cause some delaminations if drying shrinkage causes stresses that exceed early bond strength. Plastic shrinkage cracking and “crazing” may also result from failure to cure and protect properly. Moist curing is the preferred method of curing. If moist curing is not feasible, membrane curing compounds may be used.

Finally, be sure the nozzleman who will be applying shotcrete on your project is certified by the American Concrete Institute (ACI). Certified nozzlemen have been trained and tested on the requirements for proper shotcrete application. Insisting on this certification dramatically increases the probability that you will get the desired results.

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Question 11:

I am currently involved in the design of a large retaining wall for a job in Boston. One option under consid­eration is the use of soil nails with shotcrete lagging. The design anticipates a 100-year service life. What can I tell my client to realistically expect from the shotcrete option? Is shotcrete durable in the freezing-and-thawing conditions in this area? What is the best way to improve the longevity of the product?

Answer:

The simplest way to clarify things is to advise your client that shotcrete is not a product but a process. Shotcreting is a process of installing concrete at a high velocity. Because the concrete is installed at a high velocity, it will have a higher density than conventional concrete in most cases. The increased density will provide reduced permeability and higher durability.

A shotcrete mixture can be designed and proportioned to meet virtually any job requirement. In this case, air entrainment must be specified. Whenever any concrete mixture (shotcrete mixtures included) will be exposed to freezing and thawing while critically saturated, air entrainment must be part of the mixture. The amount of air entrainment required depends on the maximum size of the coarse aggregate used. In general, for a mixture with a maximum-sized coarse aggregate of 3/8 in. (10 mm), the air content should be about 8% as-batched for a severe exposure condition.

Another key to longevity is reduction of permeability. As a mixture becomes denser, the transmission of fluids through the mixture becomes more difficult. This is especially critical when trying to protect reinforcing steel. When chloride ions and oxygen reach reinforcing steel, corrosion is initiated. Increasing the density by using products like silica fume, slag cement, and fly ash dramatically decreases permeability.

Discuss the curing and protection plan with the contractor prior to the start of shotcreting. Failure to cure and protect properly is the most common reason for poor concrete or shotcrete performance.

Another often overlooked element in obtaining an extended type of service life is maintenance of the concrete structure. By periodically cleaning the concrete and applying an appropriate surface sealer, materials that may lead to deterioration are removed from the surface and not allowed to penetrate the pore structure of the concrete.

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Question 12:

I am a civil engineer working on the rehabilitation of a low fixed crest concrete dam of 6 foot height. After stitching of cracks and patch repairs, we want to specify a 2.5" shotcrete facing on the down-stream side to protect from high velocity-induced erosion. The up-stream side will be sealed with a betonite-clay liner to save costs. To get a very dense concrete, we are thinking of 8000 psi airentrained, fiber-reinforced mixture. Should we use a WWF reinforcement? Should this be a wet or dry application?

Answer:

Whether to use the wet or dry process depends primarily on your production schedule. With wet you will get much higher production; it will be easier to entrain air; and rebound and dust will be less. It is suggested that you use a wet-mix, steel fiber reinforced, air entrained, silica fume shotcrete, mechanically connected with L-bar anchors and small diameter bars (not mesh) spanning between the anchors. For precedence with this type of retrofit of the face of a dam, see the publication on "Seismic Retrofit of Littlerock Dam, by Forrest, Morgan in ACI, Concrete International, November, 1995, pp. 30-36, or an abbreviated version of the paper in the ASA Shotcrete Magazine, May,1999, pp. 46-55. If you must specify the shotcrete you can use ASTM C 1436, “Specification for Materials for Shotcrete”, which will cover all the materials mentioned, including fibers. For a general shotcrete specification you should review ACI 506.2. You should not use welded wire fabric and fibers together. Fibers will hang up on the mesh causing voids behind the mesh. I recommend a steel fiber meeting ASTM C 1436, Type I, Deformed at approximately 85 lbs/c.y. (50 kgs/c.m.). The steel fibers will tend to lie in the plain of the shotcrete surface; however, you should be aware that some fibers may protrude from the surface, and over time will corrode. Thirty years of experience shows corrosion is only to carbonation depth (2-3 mm), and corrosion of one fiber does not effect other fibers nor disrupt the shotcrete. Staining of the shotcrete surface is a possibility. Some spray a thin ( ½ in.) layer of non-fibrous shotcrete as a final finish to cover fibers.

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Question 13:

I am an architecture student and would like any information you could provide in regard to the proper and typical mix ratios of cement to sand.

Answer:

The best reference for shotcrete Questions in general is ACI 506 - Specification for Shotcrete. It is available from the American Concrete Institute.

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Question 14:

I have come across the term "spacing factor" and have been unable to find a definition. What is a spacing factor?

Answer:

The term "spacing factor" refers to the distance between air bubbles in hardened concrete. All concrete has some air bubbles, usually in the range of 1 or 2%, referred to as "entrapped air". These bubbles provide no freeze/thaw protection. Where freeze/thaw protection is desired, air bubbles are intentionally introduced, or entrained, into the plastic concrete mixture. These microscopic bubbles protect the mortar portion of the concrete by providing space for water in the concrete to expand during the freezing process. If these bubbles were not available for this purpose, the expansion of the water would damage the mortar. An important characteristic of a good air-void system is the spacing factor. Bubbles need to be in close proximity so the water migrating through the concrete does not have to travel far to find a bubble in which the water can expand. Ideally the spacing factor will be less than 0.008 in. This analysis is performed on hardened concrete by a trained petrographer using test method ASTM C 457. There usually is some slight variance between petrographers evaluating the same concrete sample.

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Question 15:

I am looking for any information regarding the use of construction joints for permanent shotcrete wall facing. I have found information on placing shotcrete over existing construction joints but none regarding the use of construction joints for the shotcrete wall facing itself.

Answer:

In many experiences, the spacing and design of the joints are the same as you would expect for a cast in place wall. Walls have been constructed with no joint, with contraction and expansion joints, with a joint that is caulked, with joints containing waterstop, and just about anything else you might see in a cast in place wall. In short, it is suggested to look to the direction given for cast in place concrete. The construction joint should be designed similar to the needs of any cast in place wall.

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Question 16:

I have a project wherein some 25,000 sq ft of existing shotcrete is to undergo varying degrees replacement, repair and restoration.

  • It is on slopes varying from 1:1 to 1:10 or so.
  • It is approximately 40 years old in most cases.
  • It is in a fairly arid climate (Southern New Mexico) with little rainfall and typically low humidity.
  • The subgrade is non-plastic gravelly sandy material.
  • It was reinforced with wire mesh (looks like 6x6x10x10).

I'm interested in any techniques and/or materials that might be applicable.

Answer:

I recommend reading the following publications in Shotcrete Magazine: "Shotcrete for Ground Support: Current Practices in Western Canada", by C.Chan, R Heere, & D. R. Morgan, Part I printed in Winter 2002, and Part II printed in Spring 2002. "Soil and Rock Slope Stabilization Using Steel Fiber Reinforced Shotcrete in North America", by M.Ballou & M Niermann, Summer 2002.

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Question 17:

Can shotcrete be painted like other concrete? Can an elastomeric paint, 100% acrylic latex house paint or solvent acrylic be used? I have a customer who wants to paint a tank which uses shotcrete. With normal concrete the surface must be 30 days or older, pH is approximately 7-8 and moisture content is low, remove efflorescence or laitance, etc., then it is ready to paint or coat. Do the same restrictions for shotcrete?

Answer:

Shotcrete is pneumatically applied concrete. All surface prep work for concrete will be the same for shotcrete applications. Before a recommendation can be made, is this tank going to be painted on the outside or the inside? Second if this tank is to be painted on the inside, what will be put in it? The environment in which this tank is located also plays a key part in determining what type of paint or coating application. If this a tank that has been in operation, what was stored in it? Testing of the concrete in this case is important, in order to determine what method of surface prep would be needed to achieve a good coating bond.

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Question 18:

I am a general contractor who hired a company to shotcrete a new swimming pool. They began on Friday, a very hot day, and they were placing concrete very slowly (27 yards in 4 hours). Their pump broke down and they were unable to complete the job that day so they returned on Monday. My Question is about the "cold joint" between the work on Friday and the work on Monday. What is your opinion of this situation?

Answer:

On large swimming pools, it is not unusual to have joints that are left over a weekend or longer. The key is the means by which the joint is dealt with. As with any concrete joint, the surface needs to be clean and free of laitance or other contamination. This can be accomplished by cleaning the joint while it is green on the first day or by cleaning with waterblasting, sandblasting, or wire brushing after the surface has gotten hard. As long as the joint is clean, all gloss has been removed, and the joint is dampened the structure should not be impacted by the joint. Also, 27 cubic yards in 4 hours is not necessarily slow production. Depending upon the circumstances, I would think that 27 cy in 4 hours was quite productive.

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Question 19:

My company manufactures a polyester geogrid that is coated with PVC. We sell these grids into underground mines, as well as many aboveground civil engineering products. We have a new grid that may work very well as an auxiliary reinforcement for shotcrete-type products. Can you tell me what the pH is for these products? The type that we would be exposed to is used in underground mines to reinforce the mine roofs.

Answer:

The most commonly used estimates for pH of concrete are 13 for plastic (fresh) concrete and about 10 for hardened concrete with a little age to it.

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Question 20:

Is there any reference that differentiates between temporary shotcrete work and permanent shotcrete work, as far as inspection/testing requirements?

Answer:

Temporary lagging of shotcrete must meet some standard as it is the shoring holding back the earth. If reinforcing is used in the design of the temporary shoring it must be fully encapsulated to provide the design strength of the lagging as specified in the design. A temporary structure may have a low safety factor but the strength of the rebar and shotcrete must meet the design specifications. Many times it is more important to do good shotcrete for the temporary shoring just because it has a lower factor of safety and therefore less allowance for poor construction practices.

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Question 21:

I am looking for design information for shotcreting a steel sheet pile wall to create a composite structure for a lift station wet well. I can design the sheet piling, which would be driven into the ground in a plan circle of 12 feet diameter, followed by excavation. I need to know the practicality of then applying a layer of shotcrete, primarily as a means of sealing the joints of the sheet pile, protecting the sheet pile from the wastewater, and providing additional wall strength. The lift station will be above the water table during construction, but would be periodically below the water table under groundwater conditions.

Answer:

There are four common types of sheet pile sealing: 1.) all seams were welded to keep the ground water from seeping in, 2.) the sheet pile surface was sandblasted for bonding, 3.) wire mesh was tack welded to the sheet pile and 4.) rebar was tack welded to the sheet piles. This was done prior to the shotcrete layer. In each case the shotcrete is used as a coating to keep the water from touching the piles and in the third and fourth examples, it is used as a structural coating as well.

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Question 22:

What is the minimum thickness that shotcrete can be applied? We are currently using shotcrete on a restoration project and have a concern at the corner locations are returning to tight recessed steel framed windows. There is an exterior wood molding approximately 1 inch from the tight corner that needs to be preserved. Do you have any suggestions as to how we can address this? Do we need to provide caulking between the wood molding and the shotcrete?

Answer:

Thicknesses depend on the structure and surface (surface prep is the key to proper bonding of shotcrete) the shotcrete is being applied to. Depending on the application 1/4 flash coat to 1 inch thickness can be the minimum. As far as shotcrete up to the steel windows, you have to consider that cracking may occur off of each corner. This can be minimized by adding additional reinforcement at those locations. It is common to tool in a joint around the windows so that we could apply a caulk later. The caulking will assure a waterproof seal between the window and the concrete during temperature changes that may create some expansion and contraction. You do not have to depend on the trim work to create the weather and water tight seal the architect requires.

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Question 23:

When used on walls, can shotcrete be of equivalent strength as poured concrete?

Answer:

Basically, shotcrete is a method of placing concrete that does not require forms. As a matter of fact, shotcrete requires the concrete mix to be proper every time. With formed concrete walls, the ready mixed concrete going in can be substandard and still appear to be okay. Shotcrete also provides a more dense concrete less susceptible to water penetration. The most glaring difference will be the quality of the materials used. Most poured walls are designed for a compressive strength of 2500 to 3000 psi. Typically they are placed with a water/cementitious material ratio of 0.60 and higher. Curing is almost unknown in the poured wall sector. Protection only occurs in the coldest weather. By the very nature of the process, shotcrete will have a much lower w/cm ratio. This will produce a wall with higher compressive strength and have the attributes of lower w/cm ratio concrete, i.e. reduced permeability, less shrinkage, increased durability. With proper curing and protection, the shotcrete mixture will produce significantly better long-term performance. The shotcrete process should allow for easier addition of insulation to the walls as well. This is especially important if the basement is to be used for more than just storage.

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Question 24:

Can you provide any information on insulating gunite in spa installations?

Answer:

There are two ways to insulate the outside of concrete spas. The first way is to shotcrete the spa and then glue Styrofoam to the outside of the concrete shell or to spray the insulated foam to the outside surface. The second way is to use the ICF (insulated concrete form). You would only have to use one side of this form system. This system would act as the outside form so that the shotcrete could bond to the foam. This type of system has foam insulation thicknesses from 1 to 4 inches thick. Yes, it can be fitted to form circles. Each ICF system is different, so some research would be needed to see which system would work the best. Since most spas are formed up before they are shot, the ICF system would serve two purposes: forming and insulation in one step.

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Question 25:

Is it possible to put a texture on the application side of a shotcrete wall? I understand that I can shoot against a form, but what about the side that gets screeded?

Answer:

There are many textures that can be applied to the finish surface of the shotcrete. The least expensive is the natural nozzle finish which is rough and tends to absorb light as opposed to reflecting light and standing out. On the other extreme is carved and stained simulated rock as found in zoos and amusement parks. Stamping or rolling also creates a great finish. The broom finish is also very common. Color and textures are options and the owner or designer needs to decide on the value and effect he/she is looking for. Whatever finish, texture, pattern, color, stain, lump, bump, or crease that can be applied to concrete also applies here.

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Question 26:

I would like to get expert opinions regarding a proposal. I am reviewing from a contractor to replace precast concrete wall panels with shotcrete wall. The wall acts as a retaining wall and the precast panels were specified to span between the soldier piles (with tiebacks), driven and anchored into the rock at a spacing of 10 feet. Shotcrete walls over 3-inch wood lagging have been proposed to replace the precast panels and they have been designed exactly the same way as reinforced concrete walls. Using ACI Code working strength design for 4000 psi concrete, and fs= 24000 psi steel, the reinforcing in the shotcrete walls have been determined using value of a = 1.76 . ( As= M / 1.76. d ) I do not feel comfortable accepting the same equations and numbers for a shotcrete wall as for a cast-in-place or precast concrete wall with all the quality controls and rigid specifications per ACI 318 Code concerning mixing, formwork, placement, vibration and curing. Could you please provide an expert opinion on the matter? What would be the reasonable values of coefficient to determine the reinforcing in shotcrete walls?

Answer:

We often use shotcrete in lieu of cast in place concrete without using different design factors. Shotcrete is simply a method of placing concrete. Properly designed and constructed, the same reinforcing steel used for cast-in-place concrete or precast concrete should be able to be used with shotcrete constructed retaining walls. The only differences would be in the reinforcing detailing, in that the rebars should be tied in a configuration that makes them suitable for proper encapsulation with shotcrete. Avoid bundled bars or other conditions not conducive to proper shotcrete encapsulation. See "ACI 506R-90 Guide to Shotcrete" for guidance, except that it is possible to use much larger diameter bars than indicated in that document, as has been described in several articles. (See for example the article by James Warner on "Dealing with Reinforcing" in the Winter 2001 of Shotcrete magazine.)

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Question 27:

I am interested in constructing my home using shotcrete applied over polystyrene panels. There are several systems for this, but I'm most interested in avoiding "thermal bridging" that occurs when metal reinforcement passes from the inside of the home to the outside through the foam insulation. I am also interested in fabricating the panels myself, if possible. There was a system utilizing metal reinforcement grids on each side of the polystyrene panel connected by plastic components. Can you point me toward a company that offers this system in the US?

Answer:

ICS, 3-D panels are structurally reinforced styrofoam panels that, in conjunction with properly applied shotcrete, become a superior building system. This is a proven panel with a global track record and much experience among ASA members. They are located in Brunswick, GA.

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Question 28:

Do you have any publications on shotcrete curing, specifically in tunneling? How is shotcrete cured in tunnel constructions with the temperature and moisture problems?

Answer:

All concrete must be cured to ensure full and proper hydration of cementitious components control of shrinkage. Shotcrete is concrete placed pneumatically, therefore must be cured, as all concrete must be. The tunnel environment presents positive and negative conditions. The humidity in an underground space is generally high in humidity and constant in a moderate to cool temperature. Both conducive to slow egress of moisture from the concrete and "natural" curing. The negative in tunnel construction is ventilation air which is generally of high volume and high speed, which tends to dry the surface and "pull" important moisture out of the sprayed concrete. Most tunnels can tolerate extra water in the work space, therefore misting or spraying water onto the concrete surfaces, especially overhead, is the most practical method of curing. Sprayed on liquid membranes are effective as long as their interference with bonding of additional layers of concrete, sprayed or cast, is not an issue. Recommended reading: "Understanding and Controlling Shrinkage and Cracking in Shotcrete" by D.R. Morgan and C.Chan, published in the ASA Shotcrete magazine.

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Question 29:

I am trying to find an article on the bond strength between two layers of shotcrete. My company is placing a 22" thick shotcrete retaining wall and, at a later date, we are placing a small amount of shotcrete over the existing shotcrete wall. The Engineer thinks the shotcrete will just falls off over time. Is this true? Can you point me in a direction that might have information on the bond strength between two layers of shotcrete?

Answer:

There is a paper by Denis Beaupre about this issue in the May 1999 issue of Shotcrete magazine. The simple answer to bonding layers of shotcrete is the same as bonding layers of concrete in typical repair applications. Bonding agents are not recommended. The bond strength between shotcrete layers is generally superior to cast interface because of the impact of velocity and the matrices that form at the bond plane and provide a denser, therefore stronger interface. The key in any bonding situation is primarily dependent on the surface preparation before application of the next layer. The surface must be clean and free of latence and any other unsound materials and should be roughened or textured (gun finish is sufficient) to provide sufficient keying or mechanical locking as required. The surface should be SSD and overspray from progressive application should be controlled. ACI International and the International Concrete Repair Institute can provide direction for surface prep. AASHTO/AGC/FHWA Task Group 37 Report, "Guide for Shotcrete Repair of Bridges and Structures" contains spec and procedure information that should be useful.

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Question 30:

Can you provide input on the applicability of the shotcrete placement method for the structural repair of existing concrete walls? These walls (two) are conventionally reinforced, 31 feet in height and are parallel with a clear spacing of 5'-0". There length is 150 feet. Structural repair is required at many locations that have experienced spalled concrete with corroded reinforcing bars. Depth of repairs will range from 2" to approximately 6". Concrete substrate will have exposed aggregate with a significant amplitude. From a production and cost viewpoint, shotcrete appears to be more applicable than a form and pour or form and pump repair method.

Answer:

From the limited info given, it sounds like an ideal shotcrete application. But, with many caveats, such as: TOTAL deteriorated substrate removal, thorough removal of all aggregate/substrate that may have been fractured during removal of deteriorated concrete (heavy sandblasting and/or high pressure washing), using a replacement concrete mix with similar properties as the original, thorough cleaning or removal and replacement of corroded rebar, etc. It is suggested to discuss this with a shotcreter in the area that has experience with a similar application. It is also recommended to review related ACI and ICRI publications.

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Question 31:

We have a project that our subcontractor would like to change from concrete liner for a box culvert to a shotcrete liner it is a C.O.E. project. The C.O.E. has questions of durability. Could you help?

Answer:

If the shotcrete is applied correctly, the durability factor is better than cast in place concrete. The 506 and the ASTM documents have references on this subject. There have been papers written on durability and permeability. Countless culverts have been very successfully relined with shotcrete, not only concrete culverts but also brick lined and galvanized metal culverts. If you broaden the definition of culvert to include tunnels you would most likely be identifying where the largest volume of shotcrete is used as a rehabilitation method. To answer questions of durability, shotcrete should be thought of a process or method of placing concrete. Shotcrete in place is concrete. The higher cement content of shotcrete and the impaction of its placement mix design for mix design of other placement methods create a higher strength and more dense, thus less permeable concrete.

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Question 32:

We are building a home where some of the outside walls are bricked. Is there a way to use shotcrete over strand board (chipboard)? If so, how and what cost would there be approximately a square foot for the actual shotcrete installed?

Answer:

More information is needed before answering this correctly. Shotcrete will stick to strand board, but you need some type of reinforcement (wire mesh) to hold it all together. A good cement plaster mix at a lower velocity would be more economical. Contractors who have shotcreted a house in the past will tell you that it is too time consuming for the money involved.

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Question 33:

We are having a pool built with shotcrete. The pool company has asked us to change the contract to allow them to use the wet method instead of the dry method of shotcrete. I have read through your website and found it helpful in understanding the difference between the two, but I would like to know if one is better or more sound than the other.

Answer:

Pools are built with both processes. Some find it easier to shoot pools with the wet method. But, when properly done, there should be no difference in performance between wet and dry process shotcrete. Depending on the complexity of the pool, the wet method placement can be faster than the dry method. It comes down to the experience of the contractor and their crews, for a good quality placed pool shell. The nozzleman plays a key role in the placement of well placed shotcrete in both methods. The geographical area may determine the economics of which method is used. Curing of the in-place concrete shell is the same for both processes (water curing for 7 days). Wet concrete has a 90 minute window from the time it is batched at the plant until it placed. Temperature of the material and the air temperature can increase or decrease the set times of the concrete. Typically Ready-Mix companies hold back 10-15 gallons of water in the mix so that the contractor can adjust the slump of the concrete on site. Adding 1 gallon of water over the design mix (amount of gallons of water per yard of concrete) can decrease the strength of the concrete by 200psi.

If you have additional concerns, the following questions should be asked:

  • Does the contractor have a good track record of shooting pools with the wet method?
  • How many pools have they completed with the wet method?
  • Can you provide a list of past completed jobs?
  • How do they plan to incorporate the trimmed concrete into the shell? (The rebound and the trimmed concrete play a key role in the final quality of the pool shell.)
  • What concrete mix design do they plan to use?

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Question 34:

We have a design/build drainage channel project that requires a concrete lining over secant piles in which the secant piles form the main structural walls of a box culvert. The box culvert discharges into the ocean. We proposed a shotcrete concrete liner but there are concerns about the life service durability of shotcrete in a saline environment. Do you have any reference information on this matter that we could use to support our position?

Answer:

Please refer to the following articles:

Morgan, D.R. "Freeze-Thaw Durability of Shotcrete" Concrete International, Vol. 11, No.8, August 1989, pp 86-93.

Shotcrete magazine Vol. 4, No. 5, Fall 2002, pp. 32-38

Shotcrete magazine Vol. 5, No. 2, Spring 2003, pp. 30-37, “Freeze-Thaw Durability of Shotcrete,”

Gilbride,P., Morgan, D.R. and Bremner,T.W. "Deterioration and Rehabilitation of Berth Faces in Tidal Zones at the Port of Saint John", ACI, Concrete in Marine Environment, SP-109, 1988, pp.199-227.

Gilbride, P. Morgan, D.R. and Bremner T.W. "Performance of Shotcrete Repairs to the Berth Faces at the Port of Saint John", Third CANMET/ACI International Symposium on Performance of Concrete in Marine Environment,1996, pp 163-174.

Morgan,D.R., Rich L. and Lobo, A, "About Face-Repair at Port of Montreal", Concrete International, Vol. 20, No.9, September,1998, pp. 66-73.

The bottom line is that with a properly designed, air-entrained shotcrete, properly applied by qualified nozzlemen, you should be able to get a good quality product, with long-term freeze thaw durability every bit as good as a quality, air-entrained cast-in-place concrete.

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Question 35:

Our company is developing alkali-free accelerator, both powder and liquid types. Since our information and knowledge is limited, please answer the following questions:

  • 1. What is the formal definition of alkali-free in DIN, ASTM, or other specifications?
  • 2. What is the lowest pH value of alkali-free accelerator? In which pH value that the product won't harm to the human tissue or vascular system? Please also advise where we can find the related information.
  • 3. Is it acceptable to use Aluminum Sulfate as the main component of alkali-free accelerator?

Answer:

  • 1. - Na2O (sodium oxide) equivalent, below 1.0%
  • 2. – 3 is the lowest; anywhere between 3 and 10, most European specifications state a range between 3 and 8 for better performance.
  • 3. - Yes

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Question 36:

I wish to request expert advice from ASA in regard to the Gunite Contractor's Association method that we are using to make test cylinders (i.e. 6" diameter and 12" high shot into a form of 3/4" square mesh hardware cloth). Since we are currently in the process of guniting a silo and have today received 3,250 psi rather than the mix designed 4,000 psi 7-day strengths, we would appreciate your prompt response.

Answer:

The method of using 6" diameter by 12" long wire mesh cylinders has not been used regularly in several years. The most accepted means of taking samples is as specified in ACI 506 documents which generally require a sample panel of approximately 18"X18" by 4" thick from which cores are taken. The cores should be taken at a minimum distance from the edge of the thickness of the panel to yield fair test results. ACI 506.4R-94 references under testing of shotcrete, ASTM C 1140-03 (Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels. Also ASTM C42/C 42M-03 (Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. Standard 18"X18"X4" panels are typically made. ASTM C 1140-03 states a 24"X24"X4", cores are to be taken 1 core diameter plus one inch from any side of the test panel.

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Question 37:

I have a special request for a shotcrete mix design. My company has been using shotcrete for about three years, here in Alaska. I have recently had a request to shotcrete a 60'x50' duck pond to make it waterproof. The problems I am running into are that moose keep walking into the pond, and the pond is on the side of a hill with built up edges around the outside. The mix design I am looking for needs to have an epoxy or some kind of adhesive to help stop the water from running out the cracks. Last, are there any fabric or plastic materials that I could lay down and spray the wet shotcrete on to put on the sides of the pond?

Answer:

This inquiry involves a lot more than just mix design. First, additives to the mix by themselves will not keep the shotcrete from cracking. To minimize leakage for the proposed application, he will have to use either a waterproofing membrane on top of the shotcrete, or plaster like would be used on a swimming pool. Putting a membrane behind the shotcrete would only serve to keep ground water from entering the pond through the back side. The other aspect to be addressed is the fact that all concrete shrinks, and that is what causes the cracks. So anything that can be done to minimize shrinkage should help. To name just a few items: avoid shooting on a windy and or low humidity day; use aggregates in the mix that have a good record regarding shrinkage; avoid excessive cement content in the mix; use reinforcing steel (mesh or rebar); synthetic fibers help reduce early plastic shrinkage; proper curing is absolutely essential!

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Question 38:

We are shotcreting our first wall and the contractor tells us that in shotcrete, the lapping of the bars is not done by putting the bars alongside each other as in conventional pouring of concrete but rather a gap is left between the bars in order to avoid voids behind bars bundles. A two-inch gap is being used on our job. Is there a publication that deals with reinforcing steel placement in shotcrete in general and one that deals with bar laps in particular?

Answer:

The ACI 506R-90 Guide to Shotcrete, Section 5.4.2 is the publication you are looking for. Amongst other things it states: "If the design allows, lapping of the reinforcing splices should be avoided. Lapped bars should be spaced apart at least three times the diameter of the largest bar at the splice". If laps are not permitted by the design, then it is best to lap the bars one on top of the other (relative to the shooting orientation), rather than side-by-side, to facilitates proper encapsulation with shotcrete.

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Question 39:

I'm looking for information as to the thickness design of shotcrete for ditch slope lining purposes. Can you direct me?

Answer:

Typically, the thickness is a minimum of 3 inches and slope lining in the 6 to 8 inch range is often installed. The reinforcing is also variable with the lightest sections with no reinforcing or a low dosage of polyfibers or light welded wire fabric and the heavier sections with rebar. Basically, a lot of different designs can be used. We are not aware of any widely used standards.

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Question 40:

Our development has 8 recirculating water ponds of various sizes. All are vinyl liner under concrete construction. Some ponds have developed leaks due to cracking of the concrete. Will shotcrete provide an adequate seal to stop the leaks for an appreciable time?

Answer:

When trying to find a contractor in your area, please visit the Corporate Member page of this website. When constructing water ponds, the liner is always under the concrete just in case the concrete cracks not on top. Master Builders makes a product called Master Seal 345 which is designed to waterproof the concrete before the shotcrete is placed. Using a macro synthetic fiber for strength, flexural and to control shrinkage cracking will help. It comes down to proper prep work prior to placement and curing of the concrete (7 days of water) to control cracks. Bentonite shotcrete could be a possibility or perhaps plastic shotcrete (cement and bentonite shotcrete).

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Question 41:

We are a construction company and are currently executing a cathodic protection work for the reinforced concrete pile caps of a jetty. After the application of concrete repairs and placement of CP system over the R/C surfaces, we are to cover the concrete surfaces with a waterproofing material. The engineer of the project recommends the shotcrete application with a thin layer in order to provide with the protection of the buried anode strips and as well as waterproofing of the surface. We use strip type CP anodes and we place them into the sound/repaired concrete by saw cutting the surface. Saw cuts are 1/4" width by 1" depth and located top, mid and bottom sections of the 40" depth vertical pile cap surface. Our questions are:

  • 1. Can we apply a thin layer of shotcrete over the repaired concrete surfaces without having any reinforcement and would it be a good solution as far as the stability of the shotcrete is concerned?
  • 2. Would it be a safe solution to apply the shotcrete over the repaired surfaces just to provide with the protection of the CP anodes placed in saw cuts as described above?
  • 3. Would it be enough to make a waterproof coating instead of applying shotcrete to the whole concrete surface so that the waterproofing of the surface shall be provided?

Answer:

When involved with The High Level Bridge in Fairmount, WV we had several aspects of shotcrete repair on this project. This also included the largest installation of cathodic mesh on top and bottom of each arch span which was then covered by shotcrete. Several cathodic design issues affected the shotcrete application. Surface profiling had to be conducted prior to mesh installation for bonding purposes. The anchor spacing had to be drastically reduced because of the small gauge wire and the vibration it caused during shooting. A thin layer of silica fume dry shotcrete (Gunite-MS) from the Quikrete Company was applied over the mesh. A natural gun finished was chosen over a trowel/broom finish because of the delaminations it created during the finishing. Curing of this thin layer was very important in preventing it from delaminating from the surface.

  • 1. Yes, but the surface needs to be profiled for bonding of the shotcrete to the old surface.
  • 2. Yes, but more information is needed. Repair all bad areas, profile the entire surface that will receive the cathodic system, shotcrete the entire surface, waterproof the entire surface.
  • 3. With out seeing the job or design, it would be hard to make any suggestions on this subject. You may need to review what the manufacture for the cathodic system recommended?

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Question 42:

We are currently in the process of doing a seismic upgrade to one of our parking structures using shotcrete. During this process, the murals that are painted on the interior walls are being removed and will be repainted at a later date. How long do I wait before it is cured enough to begin painting?

Answer:

The easy answer is that shotcrete material is the same as concrete material and that the same rules or guidelines would apply to shotcrete as to concrete. We usually tell our customers to present this question to the painters. The curing process and chemical reactions are greatest in the first 28 days. Generally a paint or coating is not applied until after the curing of the shotcrete is complete, or mostly so, and the moisture content of the shotcrete is below a point specified by the coating manufacturer.

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Question 43:

I am trying to find out if there is any research or literature regarding the drying shrinkage of shotcrete. Can you help?

Answer:

See ACI 506R, Sec. 1.7 (ACI document). Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete, mostly due to less and/or smaller coarse aggregate in the shotcrete mix.

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Question 44:

I've been a pool builder all my life and I use your magazine as a technical source and I really enjoy it. I found a conflict: In Shotcrete Summer 2004, page 30, the answer to the second question suggests the use of 8% as batched air content with max sized coarse aggregate of 3/8 inch. The conflict I have is that a) won't 8% as batched drop to 1-2% after wet gunning? and b) previous articles suggested the use of 15-22% air as batched to help get it through the hose and to achieve 8% in place. Can you clarify?

Answer:

For over 30 years in Canada we have been designing wet mix shotcrete for exterior exposure (rock-slope stabilization, tunnel portals, canals and beams, infrastructure rehabilitation, etc.) to have air content at the point of discharge into the pump to be in the 7 to 10% range. Pumping and the impact on shooting reduces the air content in the in-place shotcrete by about half. i.e. we find the in-place air content in the shotcrete to consistently be in about the 3.5 to 5.0% range. (Only about 1 to 2% air content is lost in pumping; the rest is lost in impacting on the receiving surface).

The air content is measured either by digging out the in-place shotcrete (or dig it out of a shot test panel) and reconsolidating it in the base of the air pressure meter in the ASTM C231 test and conducting the test. Alternatively the shotcrete can be shot directly into the air pressure meter base. It provides virtually the same value as obtained with dug-out shotcrete (as described above), provided the nozzle is held perpendicular to the air pressure meter base, and at the appropriate distance for proper consolidation of the shotcrete.

Testing on numerous projects has demonstrated that shotcrete with 3.5 to 5% in-place air content has a good air voids system ( air content, spacing factor and specific surface), when analyzed in the ASTM C457 test. Such shotcrete has been demonstrated to have good freeze/thaw durability in the ASTM C666 test and deicing salt scaling resistance in the ASTM C672 test. More importantly, feedback from the field demonstrates that such air entrained shotcrete with many thousands of cycles of freezing and thawing in the field over several decades display good durability. There are many research and case-history examples in the published shotcrete literature to support these observations. (See references 1 and 2 below)

With respect to the use of very high air contents at the pump (15-22%), this has been more of a research initiative, used on only a few projects in Quebec, and is not common practice, nor in this writer's opinion, necessary.

There is another benefit which accrues from the use of air entraining admixtures to get 7-10% air content in the shotcrete discharged at the pump. As any concrete user knows, as the air content increases, the slump goes up. For shotcrete mixes (which have high cementitious contents and low rock contents compared to concretes) this makes the mix easier to pump and shoot. Thus it is common to shoot air entrained wet mix shotcrete at 100 to125mm (4 to 5 inch) slump. On impacting on the receiving surface, as the air content is reduced by about half, the slump of the in-place shotcrete is also instantaneously reduced by about half. (This can be demonstrated by digging the shotcrete out of the in-place material, or a test panel and conducting a slump test on it). We refer to this phenomenon as the "slump killing "process and have used it to advantage on many shotcrete projects. With a good air entrained shotcrete mix design (particularly when silica fume is used) we commonly shoot vertical sections as much as 500mm (20in) thick at 100 to 125mm (4 to 5 inch) slump in a single pass with no problems of sagging or sloughing (fall-out), without having to resort to the use of accelerators.

Finally, there are a few situations where 7 to 10% air content in the shotcrete at discharge into the pump may not work. These are situations where excess air content reduction could occur during shotcrete conveyance, such as dropping shotcrete down a pipe from the surface in an underground mine and catching it in a kettle or remixer unit. In this case, air, if needed, is best added underground in the remixer. Also, pumping shotcrete long distances (particularly pumping shotcrete downhill) may result in excessive loss of air content in the line, which could cause a slump reduction in the line and possible pumping problems. Other than for situations such as these, we always use 7-10% air content in the shotcrete at the point of discharge into the pump (even if it is not needed for frost resistance reasons) because of its enhanced pumping and "slump killer effects".

Reference 1: Morgan, D.R., “Freeze-Thaw Durability of Shotcrete”, Concrete International, Vol. 11, No. 8, August, 1989, pp 86-93

Reference 2: Morgan, D.R., Kirkness, A.J., McAskill, N. and Duke, N., “Freeze-Thaw Durability of Wet-Mix and Dry-Mix Shotcretes with Silica Fume and Steel Fibers”, ASTM Cement, Concrete Aggregates, Vol. 10, No. 2, Winter 1988, pp 96-102.

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Question 45:

As a specifier, should I specify which process—dry or wet—should be used on my projects? What are the significant differences?

Answer:

The application of shotcrete can be done successfully with either method. The dry-mix shotcrete process tends to be more favorable for lower volume placements. It is also a more flexible method, allowing for more frequent relocations of equipment. Equipment is more easily cleaned at the end of the placement. The nozzleman must exercise great care in adding the necessary amount of water while shooting.

The wet-mix shotcrete method is more favorable for larger volume placements. Rebound is substantially less than in the dry-mix shotcrete process. The nozzleman does not have to be concerned with controlling the water addition. This method is less efficient when there is a requirement for frequently starting and stopping placements. The wet shotcrete mixture has a limited “pot-life.”

Remember, shotcrete is not a special product. It is a method of placing concrete. All the recommended practices for concrete placed by any other method, such as curing and protection, also apply to shotcrete.

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Question 46:

My firm is a general contracting entity that frequently uses shotcrete subcontractors. When project specifications are not clear on testing, I have been relying on the advice of my shotcrete subcontractors on the frequency of taking tests for compliance with strength requirements. We always shoot a test panel prior to starting construction. How much testing should we be doing during construction?

Answer:

ACI 506.2, “Specification for Shotcrete,” recommends that a test panel be produced for every 50 yd3 (38 m3) of shotcrete placed or one per day, whichever is less. A minimum of three cores are to be cut from the test panel for compressive strength testing in accordance with ASTM C 42, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” Testing must be performed in accordance with ASTM C 1140, “Standard Practice for Preparing and Testing Specimens from Shotcrete Panels.” The average of the strength results from the cores must be at least 85% of the specified strength with no individual core less that 75% of the specified strength.

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Question 47:

Is there a U.L. (Underwriters Laboratories) certification for shotcrete?

Answer:

No. Shotcrete is a method of placing concrete. Therefore, any applicable certifications would apply to concrete regardless of the method of placement.

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Question 48:

I have a client who may be interested in using shotcrete for walls in a radiosurgery unit requiring radiation shielding. Could you please tell me the typical density of shotcrete?

Answer:

Shotcrete made with normalweight aggregates will have a density of approximately 145 lb/ft3 (2323 kg/m3).

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Question 49:

Are there specific benefits in using silica fume in shotcrete beyond reduced permeability in the hardened shotcrete?

Answer:

Shotcrete containing silica fume will tend to be more adhesive (sticking to substrate surfaces) and cohesive (adhesion to itself). This will result in quicker build-up (greater thicknesses per pass) and possibly reduced need for acceler­ators. Silica fume additions also result in dramatic reductions in rebound, particularly with the dry-mix process.

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Question 50:

The Park District Department of our city is in the process of designing a new swimming pool. One of the prospective bidders made a presentation in which they said they would use shotcrete instead of conventional cast in place concrete. Their design is to use 6 in.-thick walls instead of the 12 in.-thick walls as proposed for the cast in place design. They claim that 6 in. of shotcrete is as strong as 12 in. of formed concrete. Is this a true statement?

Answer:

If this statement was true, there would be a lot more shotcrete projects! The truth is that shotcrete is a method of concrete placement, not a special material. The materials, mix designs, and mix proportions may vary between the shotcrete method and the conventional concrete form and pour method, but the thickness and reinforcing of the structure will be very similar.

There is a subtle difference between the two methods that might affect thickness requirements. Shotcrete is generally placed directly onto the undisturbed soil, joining with the soil to provide the shell for the pool. To use the form and pour method, over-excavation would be required to accommodate two-sided forming. The walls would then have to withstand the forces of backfilling. This may result in a thicker wall requirement. The final decision regarding wall thickness, however, should be made by a structural engineer.

Shotcrete is widely used for swimming pool construction. In some areas it is virtually the only method used. Successful shotcrete swimming pool construction is a result of having an appropriate design, selecting a qualified contractor with certified nozzlemen, selecting appropriate materials and shotcrete mixture design, and following industry recommendations for placing, finishing, and curing.

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Question 51:

I am working on repairing some mildly deteriorated walls in a drinking-water treatment plant. There are no chlorides used in the treatment process. I would like to apply a 1 in.-thick shotcrete layer over the existing concrete utilizing a mix containing silica fume, which will achieve a compressive strength of 5000 psi at 28 days. I am having difficulty formulating a mix to meet those requirements that also has a water soluble chloride content of less than 0.10 % chloride ion concentration by mass of cement. I cannot get the chloride ion concentration below 0.15%. What adjustments can I make to get to my goal of 0.10% or less?

Answer:

There are areas that have no problem getting values lower than the most stringent ACI requirement of 0.06% for prestressed concrete with no special adjustments. It would be prudent to test each of the proposed shotcrete constituents to determine their soluble chloride ion content. The most likely suspects are the aggregate and water sources. Typically portland cement and silica fume would contribute little, if any, detectable chloride ions. Assuming this would be a dry-process application, the only admixture other than the silica fume might be an air entraining agent, which would not provide any chloride ions. This leaves only the aggregates and water as the sources. At a minimum, the aggregates and water should be tested by a qualified laboratory for soluble chloride ion content. Alternate sources of aggregates and water may be required based on the laboratory results.

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Question 52:

Our firm is working as a consultant for a project. We have very little experience with shotcrete. What is the life span for a shotcrete wall?

Answer:

Shotcrete is a method of concrete placement, not a product. Therefore, concrete placed by the shotcrete method will exhibit the same characteristics as concrete placed by other methods. Mixture designs and proportions for shotcrete are modified for high-velocity placement. The high velocity provides some performance improvements over conventional cast-in-place methods when properly placed.

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Question 53:

What is the best reference when specifying aggregate gradations for shotcrete projects?

Answer:

ASTM C 33 contains a variety of aggregate gradations. Gradations recommended for shotcrete applications can be found in ASTM C 1436, Standard Specification for Materials for Shotcrete, or ACI 506, Guide to Shotcrete. Note that ACI 506 includes the caveat that “aggregates failing to comply with gradations shown in Table 2.1 may be used if preconstruction testing proves that they give satisfactory results or if acceptable service records are available.”

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Question 54:

I want to apply a 3.5 in. (89 mm) veneer of shotcrete over an existing cast-in-place wall. I am concerned about how well the shotcrete will bond to the existing wall. This wall is 50 ft (15.25 m) in height. What are the keys to doing this work successfully?

Answer:

This is a common use for shotcrete. The key elements are as follows.

  • 1. Proper surface preparation. To establish suitable surface roughness, use heavy-duty sandblasting, high-pressure water blasting, or mechanical methods such as scabblers or scarifiers, followed by sandblasting or high-pressure water blasting to remove the “bruised” surface material. Refer to ICRI Guideline No. 03732, concrete surface profile Chip 6 (CSP 6), or greater.
  • 2. Provide mechanical connection between the shotcrete and concrete by installing L-bar anchors (epoxy or portland cement grouted) on a systematic pattern, with reinforcing bar (or heavy-duty mesh) spanning vertically and horizontally between the anchor bars. Size and spacing of the bars to be determined by the structural engineer. Position anchors and reinforcing bar to ensure adequate shotcrete cover to them. Nonmechanically connected veneers are not recommended.
  • 3. Wash concrete surface with clean water to remove dust or any other contaminants to achieve a good bond and presaturate concrete. Allow concrete to dry back to a saturated surface dry (SSD) condition immediately prior to shotcrete application. If concrete dries excessively, bring back to SSD condition with fogging. (A 3000 psi [21 MPa] water pressure sprayer works well for this purpose).
  • 4. Apply the shotcrete from the bottom up, taking care not to entrap rebound/hardened overspray. Use proper shotcreting techniques to encase reinforcing bar and anchors. Use 45-degree construction joints (do not construct long tapered joints).
  • 5. Use shooting wires, guide forms, or other suitable methods (for example, rods with alignment bubbles) to establish proper line and grade. When the shotcrete has stiffened sufficiently, trim it to line and grade with cutting rods and then finish using fresnos or floats to provide the desired surface texture (wood floats for more textured finish, rubber/sponge floats or magnesium floats for intermediate texture finish, or steel floats with steel toweling for smooth finish). Note: very smooth finishes are not recommended as they tend to show imperfections from hand-finishing procedures. Avoid over-finishing of shotcrete or procedures/timing which could pull tears or sags/sloughs/delaminations in the fresh shotcrete.
  • 6. Cure the freshly placed shotcrete using one of the methods prescribed in ACI 506R-90. Our preferred method is fogging/misting until the shotcrete has reached initial set, followed by wet curing for 7 days using presaturated plastic-coated geotextile fabric (for example, Transguard 4000), which is kept wet with soaker hoses. Curing compounds are a (second best) alternative, but should not be used if a paint or coating is to be applied, unless they are approved by the coating/paint supplier for such purposes.

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Question 55:

We are having a swimming pool built with shotcrete. Our question is, what is the required curing time for shotcrete prior to exposure to heavy rain? We are trying to plan the shotcrete installation when the weather looks most favorable.

Answer:

Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following final set, it should be wet cured for at least 4 days, preferably 7 days if possible. The exposure to rain would prove beneficial as the rain would assure the presence of moisture for continued curing.

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Question 56:

I will be shotcreting an existing structure that has some diesel fuel and oil stains on the existing concrete. How should I treat them before shotcreting?

Answer:

There are a number of ways to treat these stains. Successful treatment will depend on the specific material in the stain and the depth of the stain. The first step would be to try to draw out the material from the surface by applying a poultice of finely ground kitty litter, cement powder, or talc and allow the surface to dry. Repeat this application if necessary.

Next, try a scrubbing a nominally dry detergent powder into the surface. Allow the powder to dry and rinse off the surface. Follow this treatment with a liquid detergent scrubbed with a bristle brush into the surface. Allow the liquid to remain in the surface for 1 to 2 days, then rinse thoroughly. Should the staining persist, you may want to try a proprietary stain remover specifically intended for use on concrete.

Muratic acid is also an option. However, muratic acid can have deleterious affects on the concrete if not thoroughly removed. Because of its potential to attack concrete aggregates and mortar, along with the hazards inherent with applying and removing acid, muratic acid should only be used with the guidance of an experienced consultant. Following a thorough power washing, the surface should be mechanically roughened to ensure proper bond with the shotcrete.

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Question 57:

Our firm is preparing to use the shotcrete method on a project for the first time. What type of prequalification work should we be specifying?

Answer:

There are four basic reasons to require preconstruction qualification testing:

1. To prove the suitability of the fresh shotcrete mixture design for the intended use;

2. To verify the proposed mixture will produce the required strength and any other specified hardened shotcrete properties;

3. To prove the ability of the nozzleman (and blowpipe oper­ator, if required) to place dense, homogeneous shotcrete completely encasing the reinforcing steel under field conditions; and

4. To prove the desired surface finish can be achieved.

This testing must be discussed in detail with the shotcrete contractor in advance with a clear understanding of the expected outcomes and the process for any required adjustments. Requiring ACI Nozzleman certification is an important requirement in screening for qualified nozzle operators. However, it is not a guarantee that the nozzleman has applied shotcrete under the same conditions to be encountered on your project. Therefore, a preconstruction plan is an important part of critical projects. Other prequalification testing may be necessary depending on the nature of the work.

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Question 58:

We are hearing a lot of discussion about performance versus prescription specifications? What do we need to know about this discussion?

Answer:

The short version of this discussion is that performance specifications provide a list of desired results. The contractor takes this list and selects materials and methods to produce the desired results. The contractor assumes responsibility for results. Prescriptive specifications are very specific as to what materials, proportions, and methods of installation are to be used. The specifier assumes responsibility for the results. The contractor must be able to demonstrate compliance with the specification. Which method is better? The answer to this question is highly dependent on the nature of the project. However, in general, performance specifications produce a higher probability of achieving the desired results as the contractor is better able to use his expertise as it applies to project conditions.

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Question 59:

I am doing a wet-process shotcrete project. The shotcrete mixture is being delivered by a ready mixed concrete company. Recently we had some delays on the site. The inspector told us that any concrete not unloaded within 90 minutes of arrival on the site would be rejected. Where does that rule come from?

Answer:

ASTM C 94, “Standard Specification for Ready Mixed Concrete” states that concrete must be unloaded within 90 minutes of contact between water, cement, and aggregates, or before the mixer drum has revolved 300 revolutions—whichever comes first. This limit, however, may be waived by the purchaser if the concrete has sufficient workability that it can be placed without the addition of water. In hot weather, the 90-minute limit may be reduced by the purchaser.

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Question 60:

I am bidding a tunnel project and am uncertain about part of the specifications. Are specifications for shotcrete temperature different for the wet and dry processes? Are there separate requirements for the shotcrete, ambient, and surface temperatures? Can you refer me to industry standards?

Answer:

The requirements for material temperatures are the same for both wet and dry shotcreting. Refer to Sections 8.7 and 8.8 of ACI 506R-90, “Guide to Shotcrete,” for recommended shotcrete temperatures during placement. Additional information is available in ACI 506.2-95, “Specification for Shotcrete,” in the sections on hot and cold weather shotcreting. Generally, concrete mixtures should be maintained at temperatures above 50 °F (10 °C) and below 100 °F (38 °C). Ambient temperatures should be maintained in a similar range.

Regarding surface temperatures, concrete should never be placed on a frozen substrate. Practical experience in Canadian mines has lead to a suggested minimum temperature of 40 °F (4 °C) for the rock receiving the shotcrete. Without special measures, cold temperatures will cause the shotcrete to set more slowly and result in slower strength development. Remember that in thin sections, the shotcrete will lose its heat more quickly in cold conditions.

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Question 61:

What wire size and opening are recommended for repair of bridge substructures? We realize the mesh would not be for restoring or improving structural capacity, merely to help control cracking.

Answer:

The inclusion of wire mesh must be considered on a case-by-case basis, depending on the thickness and orientation of the shotcrete. Thin sections may well not have any wire mesh. In aggressive environments, at least 2 in. (50 mm) of shotcrete must cover the mesh. The mesh size should be at least 2 x 2 in. (50 x 50 mm) and preferably 4 x 4 in. (100 x 100 mm) to allow for proper encapsulation. Overhead shotcrete usually includes wire mesh for thicknesses greater than 2 in. (50 mm) in case the shotcrete debonds from the substrate. The mesh must be mechanically anchored.

Some designers are eliminating wire mesh and relying on synthetic fiber reinforcement for shrinkage crack control. The use of synthetic fiber eliminates the concern over cover and corrosion in aggressive environments. Specific recommendations on the amount and type of fiber should come from the manufacturer.

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Question 62:

Our general contracting firm is working on a project with a very tight schedule and significant penalties for missing the completion date. It has been suggested that we consider using shotcrete for the below-grade foundation walls. We have been told that we can save significant time by using shotcrete instead of cast-in-place construction. These walls are heavily reinforced. Has this been done successfully elsewhere?

Answer:

Yes. Heavily-reinforced shotcrete has been used in California for over 50 years in response to the need to retrofit structures to resist earthquake damage. The shotcrete contractor must demonstrate his ability to shoot test panels with the same reinforcement as designed into the project. By using an experienced and qualified shotcrete contractor, it is possible to achieve cost savings of almost 30% and time savings approaching 50%.

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Question 63:

Is a bonding agent recommended when placing shotcrete on an existing substrate?

Answer

A bonding agent is not required or recommended. A properly prepared substrate in a saturated surface-dry condition (SSD) is the optimum condition for application of shotcrete. Bonding agents may act as a bond breaker in some circumstances.

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Question 64:

My firm just completed a 2 in. (51 mm) overlay of shotcrete in an existing storage tank. Almost immediately after the shotcrete was applied, we noticed spider web cracking on almost the entire surface. The weather was very hot during shotcreting, and we suspect this caused the cracking. The project engineer is concerned about permeability and is thinking of having the shotcrete removed. Is removal really required or can we live with this cracking?

Answer:

Removal is probably not called for in this situation. Spider web cracking usually is an indication of crazing, a form of plastic shrinkage cracking. Crazing generally occurs when the combination of temperature and humidity creates a rate of evaporation at the surface of the concrete that is higher than the rate of bleed water exiting the concrete. Because the surface has very little, if any, tensile strength at this time, crazing cracks start to form. The good news is that crazing is an aesthetic problem. It affects only the very top surface and does not extend deeply into the concrete. Crazing cracks are more apparent when the surface is damp.

To avoid or limit crazing, be conscious of the weather conditions during placement. If there will be high temperature, low humidity, and moderate to high winds, measures such as fogging and/or erection of windbreaks may be required during placement. Synthetic fibers will help inhibit the formation of crazing cracks. Curing must begin as soon as possible, especially in these conditions.

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Question 65:

What is the recommended core size for shotcrete? Are there unique characteristics of shotcrete cores?

Answer:

Regarding sample size for compressive strength, the core length-to-diameter ratio should be in the range of 1:1 to 2:1, with length-to-diameter core strength correction factors applied as per the requirements in ASTM C 42, Clause 7.9.1. Shotcrete test panels are typically between 3.5 to 5 in. (89 to 127 mm) deep. Thus, either 3 or 4 in. (76 to 102 mm) diameter cores should be drilled for compressive strength testing, depending on test panel thickness. We would also suggest referring to ASTM C 1604/C 1604M for securing and testing cores of shotcrete. This new test method allows smaller core diameters for shotcrete in an effort to provide for increased length-to-diameter ratios. Care should be taken when interpreting the compressive strengths using smaller-diameter cores because of the possible presence of voids, which may result in compressive strengths that are not representative of the actual in-place shotcrete.

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Question 66:

Is the core grading scale in the ACI CP-60(02) manual used as an acceptance tool on projects?

Answer:

According to ACI 506R-05, the core grading method in ACI CP-60(02) is only to be used for nozzleman evaluation. (This is typically done in ACI Shotcrete Nozzleman Certification sessions and/or in preconstruction testing.) The core grading method should not be used to evaluate structures.

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Question 67:

Our construction management firm is relatively new in allowing shotcrete on our projects. In the most recent issue of Shotcrete magazine, there was a discussion of cores taken from shotcrete in the FAQ feature. Is there additional critical information we should be aware of when determining our coring plan?

Answer:

ASTM C 1604, Standard Test Method for Obtaining and Testing Drilled Cores, covers cores that are obtained for determination of length, compressive strength, or split tensile strength. In addition to discovering the thickness of the applied shotcrete and its strength, a visual assessment can be made to evaluate the shotcrete quality, workmanship, shotcrete-to-substrate bond, and condition of the reinforcement. Shotcrete core strength is affected by core orientation relative to the direction of the shotcrete application. Therefore vertical, sub-horizontal, and overhead application of the same shotcrete may show variability. If obtaining cores for determination of compressive strength, cores containing wire mesh or reinforcing bars may not be used. Also, if a sample has been damaged in the process of removal, it cannot be used for strength determination. Cores must have a diameter of at least 3.0 in. unless otherwise permitted by the specifier. Cores with diameters less than 3.0 may demonstrate somewhat lower strengths and have greater variability. They may also be more sensitive to length-diameter ratio. Cores with length-diameter (L/D) ratios greater than 2.1 must be sawed to produce a capped or ground specimen with a L/D ratio between 1.9 and 2.1. Strength results from cores with L/D ratios less than 1.75 must be corrected as detailed in ASTM C42. A core having a length of less than 95% of its diameter before capping or a length less than its diameter after capping or grinding shall not be tested unless otherwise directed by the specifier. To avoid introducing the effects of moisture gradients of wetting and drying, extracted cores are to be stored in a sealed plastic bag at all times except during end preparation and a maximum of 2 hours prior to capping. Prior to capping, it is a good idea to determine the density of each core. Reported results should include the following: length of the core as drilled reported to the nearest ¼" (5 mm); length of the test specimen before capping or grinding reported to the nearest 0.1 in. (2 mm) and average diameter to the nearest 0.01 in. (0.2 mm); compressive strength reported to the nearest 10 psi (0.1 mpa) if the diameter is reported to the nearest 0.01 in. (0.2 mm) or nearest 50 psi (0.5 mpa) if the diameter is reported to the nearest 0.1 in. (2 mm); direction of the application of the load with respect to the horizontal plane of the shotcrete as placed; moisture conditioning history; date and time of test; nominal maximum size of the shotcrete aggregate; if determined, the estimated density; and any deviation from the stated test method and the reason for the deviation.

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Question 68:

We have a large pond (12,000 ft2 [1115 m2]) 12 ft (4 m) deep with 2-to-1 sloped sides. It currently has an old PVC liner that is ripped and cannot be repaired. We have no shotcrete experience and wonder if shotcrete would be a better option than installing a new PVC liner?

Answer:

Shotcrete is used extensively for zooscapes, water parks, museum exhibits, swimming pools, and spas. A shotcrete water feature, although more expensive than PVC liner, would provide a long-term, more aesthetically pleasing alternative to a new PVC pond liner. Shotcrete is very versatile and can be shaped to replicate natural rock ledges or boulders. A properly designed and built water feature would provide a low-maintenance, durable solution.

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Question 69:

Is there any specified finish for shotcrete?

Answer:

There are several different finishes that are specified for shotcrete. One is a natural gun finish, which is the natural finish as sprayed (often used in slope protection). Another is a cut-down finish, which is cut-to-grade with the edge of a trowel or cutting rod (this finish is often flashed and sealed with a light gun finish to seal and texture the surface). Often in concrete repair, a trowel finish is specified where the shotcrete is cut down with the edge of a trowel or cutting rod to grade after the initial set of the material, and the surface is lightly flashed and toweled. Several different finishes can be achieved with shotcrete, but it should not be pushed or floated with the flat part of the trowel, as is done with poured concrete. It is important to wait for the initial set of the material and to use the edge of the trowel to cut the high points or shave the surface to achieve the grade or effect desired. Several excellent articles describing shotcrete finishes and finishing techniques are available as free downloads from the ASA website: www.shotcrete.org. One article, Technical Tip: Technical Tips on Shotcrete Finishes, written by Denis Beaupre, describes the different finishes that can be applied to shotcrete. Another article of interest would be Finishes for Retaining Walls by Marcus H. von der Hofen. Go the Publications section of the ASA website, click on "Click here to search the archive of Shotcrete Publications" and type "Shotcrete Finish" in the search window.

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Question 70:

I am a project engineer. Recently I received a mixture design for a shotcrete project that included limestone coarse aggregate. This is a first for me. All other shotcrete mixtures I have seen have had pea gravel as a coarse aggregate or no coarse aggregate at all. Is limestone commonly used in shotcrete?

Answer:

A limestone coarse aggregate will generally be harder and more angular than what you are used to seeing in shotcrete mixtures. It really shouldn’t be a problem to use. In dry-process gunning, it is considerably more abrasive so there is more wear and tear on equipment, such as hoses, bowls, and wear plates, but it generally guns fine. In wet-process gunning, a sharper aggregate may not flow as easily through the hoses as smoother sand and pea gravel aggregates would. These are issues that the shotcrete contractor will have to address. They should have no effect on the quality of the in-place shotcrete.

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Question 71:

We are concerned about the compressive strengths of shotcrete recently placed on one of our projects. The specification calls for 8000 psi (55 MPa). Test results indicate we are only at 5200 psi (36 MPa) at 28 days. Ambient temperatures are constant at about 45 °F (7 °C) at the point of placement. Should we be considering removal of the shotcrete?

Answer:

Shotcrete, like any other concrete mixture, will continue to gain strength as long as there are unhydrated cement particles present along with sufficient temperature and moisture. Strength development will generally be quite slow at the ambient temperature reported. The inclusion of supplementary cementitious materials in this mixture is a benefit in this instance as strength will increase as long as calcium hydroxide is available from the hydration of the cement. The specified strength should eventually be attained as long as the ambient temperature does decrease further and some form of moisture is available to the shotcrete.

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Question 72:

I am in the process of designing a 6" shotcrete overlay for an existing wall that is approximately 1,250 square feet. The shotcrete subcontractor has proposed to use a dry-mix shotcrete. What are the advantages and disadvantages to the dry-mix process? The design includes dowels on 24" centers and 4x4 W4xW4 wire mesh. Can the entire 6 inch thickness be placed at one or will it require a number of different lifts to build up to the 6 inch thickness?

Answer:

The overlay can be placed successfully with either a dry-mix or wet-mix shotcrete process. The preference of the shotcrete subcontractor is likely related to his/her past experience and what they are best suited doing. The advantages of dry-mix process are beyond the scope of a simple answer. The process is well described in ACI 506R Guide to Shotcrete. The entire 6 inch thickness can be placed in one layer using the bench gunning technique. The number of vertical lifts would depend upon the height of the wall and the nature of the surface that the shotcrete is being placed against.

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Question 73:

We are currently designing a retaining wall, sloped at 1H:0.5V, 5.5 high. We want to use shotcrete for this 12 inch (300mm) thick structural wall. For strength requirements, we are able to use a 10mm mesh, however this does not satisfy for crack control requirements. For crack control, it is required that we us 1/2 inch (12mm) individual rebars. Obviously for cost and ease of construction, the mesh is a favorable choice for reinforcing. Is there a typical section for this type of application? Will shotcrete shrink less than poured concrete?

Answer:

Each retaining wall needs to be engineered for the specific job conditions. However it is fairly common to see two layers of reinforcing bars in a wall of this thickness. In addition to reinforcing the wall, the steel would help support the shotcrete during placement. If drying shrinkage crack control is an issue, synthetic fibers may be added. Shrinkage in shotcrete mixes may be higher than a poured concrete with a 1" (25mm) maximum sized coarse aggregate content, and higher cement/cementitious material content. This may be partially offset by a slightly lower water/cementitious material ratio in a shotcrete mixture.

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Question 74:

We are looking at lining an existing 20 ft (6.1 m) diameter brick sewer with shotcrete that is 15 in. (0.4 m) or more thick and fairly heavily reinforced. Can this be done? The existing sewer is about 3 mi (4.8 km) long and 100 years old. Would shotcrete be a suitable method of rehabilitation? The rehabilitation is not just a liner, but the owner wants the shotcrete designed as a replacement pipe inside the existing brick sewer, designed for all earth and other superimposed loads as though the brick sewer were not there.

Answer:

Yes, this can and should be done in shotcrete. Shotcrete has been used to successfully line brick sewers for 75 years. Shotcrete has been used to line over $40 million worth of brick sewers in Atlanta alone. Large brick sewers have been lined with shotcrete in most of the major midwestern cities. All of them were designed using the existing sewer as a one-sided form. Properly designed and constructed, shotcrete will provide the owner with a new concrete pipe or permanent tunnel lining and the associated expected longevity.

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Question 75:

We are currently designing a retaining wall, sloped at 1H:0.5V, 18 ft (5.5 m) high. We want to use shotcrete for this 12 in. (300 mm) thick structural wall. For strength requirements, we are able to use a 0.4 in. (10 mm) mesh; however, this does not satisfy for crack control requirements. For crack control, it is required that 1/2 in. (12 mm) individual reinforcing bars are used. Obviously, for cost and ease of construction, the mesh is the favorable choice of reinforcing. Is there a typical section for this type of application? Will shotcrete shrink less than placed concrete?

Answer:

Each retaining wall needs to be engineered for the specific job conditions. It is fairly common, however, to see two layers of reinforcing bars in a wall of this thickness. In addition to reinforcing the wall, the steel would help support the shotcrete during placement. If drying shrinkage crack control is an issue, synthetic fibers may be added. Shrinkage in shotcrete mixtures may be higher than placed concrete with a 1 in. (25 mm) maximum-sized coarse aggregate due to smaller coarse aggregate size in shotcrete mixtures, higher fine aggregate content, and higher cement/cementitious material content. This may be partially offset by a slightly lower water-cementitious material ratio in a shotcrete mixture.

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Question 76:

We have a 6 in. (152 mm) thick tilt-up concrete wall that needs to be upgraded to achieve a 4-hour fire rating. We would like to add shotcrete to achieve that rating. What is the hourly rating per inch of shotcrete? We were hoping that 2 in. (51 mm) of shotcrete would provide the desired rating.

Answer:

Shotcrete is a method of concrete placement, not a special type of concrete. The fire-rating of a concrete wall constructed by shotcreting or pouring will be the same. The important consideration is the requirements of the Underwriters Laboratory (UL) Fire Resistance Directory. The directory will provide guidance. UL ratings provide the most widely accepted criteria.

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Question 77:

We would like to apply a 2 in. (50 mm) layer of shotcrete on 10 ft (3 m) diameter steel pipes including wire mesh. Is this practical? If so, how do we do this successfully?

Answer:

This type of application is very common. Either wet- or dry-process shotcrete can be used successfully. The mixture should contain a minimal amount of coarse aggregate and be rich in cementitious material to minimize rebound. Generally either 2 x 2 in. (50 x 50 mm) 14 gauge or 2 x 4 in. (50 x 100 mm) 12 or 14 gauge welded wire fabric is used. The wire fabric needs to be spaced off the surface of the steel pipe to allow the shotcrete to encase the wire properly. This can be accomplished by welding studs or nuts on the pipe surface and securing the wire to them.

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Question 78:

Can brackish or salt water be used to make shotcrete for a pool and will it have any negative effect on the quality of a shotcrete pool?

Answer:

As a general rule of thumb, brackish or salt water should not be used as shotcrete mixing water. High chloride ion contents can cause rapid setting of the shotcrete (which can make finishing difficult) and longer-term reinforcing steel corrosion-induced cracking, delamination, and spalling. Other components of brackish water can also be damaging to the fresh and hardened shotcrete. For a detailed statement on what constitutes acceptable contents of various dissolved chemicals for concrete/shotcrete mixing water, refer to the Portland Cement Association publication Design and Control of Concrete Mixtures, Chapter 4, "Mixing Water for Concrete." One could also consult ASTM C1602/C1602M for limits on the composition of nonpotable water for use in the production of shotcrete.

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Question 79:

We are constructing a canopy for a mine entrance. We need to attach some type of wire mesh to the wood fillers to give the shotcrete some surface to bond to. What type of wire would be the best for this application? The mine canopy is self-supporting and the shotcrete is strictly to be used as a sealant.

Answer:

AA typical wire mesh for such applications is 2 x 2 in. (51 x 51 mm) by 12 or 14 gauge; 3 x 3 in. (76 x 76 mm) by 11 gauge; or 4 x 4 in. (102 x 102 mm) by 10 gauge. It is important that the mesh be secured such that it does not move during the shotcrete placement. The mesh will tend to be pushed away from the back surface by the pressure of the shotcrete application.

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Question 80:

What can we add to dry-process shotcrete mixtures for cold weather operations?

Answer:

Successful cold weather placements require more than just modifying a mixture. The mixture temperature, condition of the substrate, and the placing and curing environment are also important considerations. Generally, one is discouraged from trying to apply shotcrete if substrate temperatures are too cold and the ambient temperature is at 40 °F (5 °C) and falling. There are, however, exceptions for extreme situations such as shotcreting in permafrost ground conditions, where it is not possible (or advisable) to heat up the substrate. In such conditions, special accelerated dry-mix shotcretes (in conjunction with the use of heated materials) have been successfully used. This type of work is highly specialized and not recommended for the novice.

Accelerators can be added to shotcrete mixtures to help overcome cold weather conditions. The accelerator can be either a liquid accelerator added with the mixing water at the nozzle or a dry-powdered accelerator in prebagged dry-mix shotcrete. Caution is advised when using accelerators containing calcium chloride, as the use of these materials may accelerate corrosion of reinforcing steel. More information can be found in ACI 306R, "Cold Weather Concreting," available from the American Concrete Institute, www.concrete.org.

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Question 81:

How can I maintain a 2 in. (50 mm) thickness of shotcrete in a rock excavated tunnel?

Answer:

There are many ways of maintaining the thickness of shotcrete. When placing shotcrete over a rough rock excavation, the thickness will vary with more material filling in the voids than covering the high points. Some methods of checking or maintaining the thickness are as follows: stabbing the plastic shotcrete with a depth gauge; preinstalling pins to the desired thickness; and using groundwires or shooting wires that would create an even plane over the length of the wires.

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Question 82:

Can shotcrete be applied to a slope to act as a retaining wall without a moisture barrier? If a moisture barrier is recommended, what type should we use?

Answer:

Most shotcrete slopes are placed without moisture barriers and are constructed to ensure that water pressure does not build up behind the slope and create hydrostatic pressure on the backside of the shotcrete. This is generally done with drainage material and weep holes or vents near the base of the shotcrete slope. Please bear in mind that shotcrete slope paving alone is not generally considered as a retaining wall. If shotcrete slope paving is to be used as a retaining structure, it is generally done in conjunction with soil nailing, tie backs, or some type of structural footing. If the shotcrete is intended to be used as a structural wall, a structural engineer must be consulted to be sure all structural issues are addressed.

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Question 83:

We are having a swimming pool constructed. The pool consultant is concerned about cold joints during construction if walls and the floor are shotcreted on different days. The shotcrete subcontractor states that there is no problem as the next layer of shotcrete will knit itself to the previous placement and form a solid bond. Is the shotcrete subcontractor correct?

Answer:

Yes, if care is taken to prepare the receiving surface properly. The receiving shotcrete edge must be sound (no loose or unconsolidated material), clean (no traces of laitance or gloss), rough, and dampened to a saturated surface-dry condition. If these steps are followed, there should be no concern about the soundness of the joint.

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Question 84:

What is the maximum thickness for shotcrete used for shear walls? Can we use more than 12 ft (3.7 m) if we use a double layer of reinforcing?

Answer:

There is no stated maximum thickness for shotcrete used in shear walls or any other type of wall. Walls have been successfully placed to a thickness of 36 in. (914 mm) for some time. The two main concerns are the heat of hydration and proper encapsulation of the reinforcing steel. Because shotcrete mixtures typically contain more cement per cubic yard or cubic meter than formed and poured placements, there will be more heat generated by the shotcrete mixture. The ability of the nozzleman to encapsulate the reinforcing will be a function of proper mixture design, proper selection of shotcrete equipment, and the skill level of the nozzleman and the crew.

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Question 85:

We would like to get approval to use shotcrete on the perimeter walls of an existing laboratory building. We would be shooting against a waterproofing membrane and shoring lagging. The project engineer is concerned that the shotcrete will damage the membrane, resulting in leaking into the occupied space. Are there any examples where this type of shotcrete placement has been used?

Answer:

This is a commonly used technique in the Western U.S. and Canada, and has been used successfully from Stanley Hall at the University of California at Berkeley, Berkeley, CA, to the Baltimore Hilton Convention Center near Camden Yards, Baltimore, MD. There are a number of suppliers of waterproofing materials to choose from for this application. In selecting a supplier, be sure there is field service available to inspect the project before placement of the shotcrete.

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Question 86:

I am repairing a concrete masonry unit (CMU) block wall that was partially damaged when a portion of the roof collapsed. The engineer on the project is proposing to apply shotcrete to one side of the wall to help structurally reinforce the wall. I would like to know if there is a way to finish the wall so it is cosmetically pleasing, especially since this is on the inside of an existing building with the other walls being a painted CMU. Also, were can I get some conceptual pricing for applying the shotcrete?

Answer:

Shotcrete can, and often is, finished to provide nice printable wall surfaces. To be the same general texture of the concrete block wall, you should specify a wood or rubber float finish. You can access the ASA Buyers Guide at www.shotcrete.org/pages/products-services/Buyers-Guide/index.asp to locate organizations regarding budget or conceptual pricing.

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Question 87:

I have an unfinished (dirt) basement with a stacked stone and mortar foundation. Can I shotcrete the existing dirt walls and floor with shotcrete MS (micro silica enhanced) and have it adhere to the dirt portion of the basement? If so, what method would be best?

Answer:

Shotcrete would work well for the overlay of the walls. In most cases, floors are placed by a conventional cast-in-place method. Either the wet- or dry-mix procedure would work well for the walls. To ensure good bond of the shotcrete to the walls, the walls should be cleaned and prepared to assure that the shotcrete is bonding to sound material rather than contaminates such as dirt or weathered material. You might also want to consider reinforcing the walls, but you should consult with an engineer on how to do this and with what material. If you were to use the wet-mix process, you could use the same equipment to place the floor as you are using for the walls.

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Question 88:

I am lining a below-ground conical shaped excavation with shotcrete. Dimensions are approximately 90 ft (27.4 m) diameter by 45 ft (13.7 m) depth. Sand will be moved in and out of the container daily. Temperature range is 590 to 740 °F (310 to 393 °C). Can you tell me if a mixture is available that can meet the following specific conditions:

  • Withstand the temperature ranges noted above without spalling, cracking, etc.; and
  • Resist abrasion assuming hot sand is flowing over the surface area daily?

Answer:

You certainly have adverse conditions to work with! There are products on the market based on calcium aluminate cements that will tolerate the temperatures you mention and are durable. These products can be placed using the shotcrete process. A list of companies who supply this product can be found at www.shotcrete.org/pages/products-services/Buyers-Guide/index.asp.

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Question 89:

I am reconfiguring the interior of a spa and am wondering if drains and jets can be relocated without compromising the overall structure and getting cold cracks. Can the entire interior be re-shot to maintain the monolithic form and guarantee against failure? Is there an independent professional who could conduct an on-site inspection and recommend a next step?

Answer:

We are not able to advise you on the structural integrity of a remodel of a spa or any other structure and would suggest you consult with a local engineer who is familiar with pools and spas. Shotcrete is often used to overlay or patch structures and the success of such overlays and patches is highly dependent upon the quality of the surface preparation prior to the application of the shotcrete. With respect to referrals of independent professionals, we would suggest that you use the directory of Corporate Members in the ASA Buyers Guide.

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Question 90:

I am a civil engineer looking to use shotcrete in a culvert rehabilitation project. Due to flow constraints, we are forced to have a maximum wall thickness of 3 in. (76 mm). For the typical 96 in. (2438 mm) precast concrete culvert, the walls are approximately 9 in. (228 mm). What can I do to obtain a near similar product with only 3 in. (76 mm) of wall thickness? Can shotcrete be applied at higher compressive strengths, 10 psi (0.07 MPa), or is it better to use fiber-reinforced shotcrete? The intent of the retrofit is to at least obtain a 10-year service life to this temporary solution.

Answer:

This is an engineering question, not an application question. Precast pipe is sized for multiple uses and services. Depending on this service (depth of cover or loads), creative reinforcing bar placement and higher compressive strengths can reduce the wall thickness significantly. For example, success has been realized using elliptical steel to reduce concrete section thickness. Fiber reinforcement is secondary reinforcing and is not a suitable replacement for reinforcing steel. Given the short life required of the culvert, and assuming fairly equal loading on the circumference, a 3 to 4 in. (76 to 101 mm) section with judiciously placed reinforcing bar, and silica fume (8 to 10% of cement for higher strength up to 10,000 psi (69 MPa)]) would be sufficient. The resulting culvert’s life would probably be much longer than 10 years. In the end, an engineering call should be made, but the material will perform.

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Question 91:

Is it feasible and economical to construct floodwalls approximately 5 ft (1.5 m) high with shotcrete?

Answer:

Yes, it is feasible and economical to construct structural walls such as a 5 ft (1.5 m) high floodwall. Shotcrete is a method of placing concrete and has similar, if not identical, properties after placement. As you can imagine, shotcrete needs to be shot against something such as a one-sided form, gabion baskets, earth, expanded metal lath, or just about any structurally sound thing you can think of. The economy of the system is dependent upon the site conditions and the ingenuity of the contractor. An example of a similar structure is on the east side of I-880, south of Dixon Landing Road in Milpitas, CA. This project, a flood control channel, involved trapezoidal channel sections, vertical wall sections, and a combination of sloped walls with a vertical extension. If you have further interest, you should contact an organization with experience in this area. An excellent source is the directory of Corporate Members in the ASA Buyers Guide.

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Question 92:

I am a pool builder who favors dry-mix shotcrete. I have a project requiring: a) cast-in-place concrete retaining walls, where there will be exposed downhill faces (that are not necessarily meant to be seen). Should my shotcrete contractor be able to finish the exposed face in some sort of reasonable finished appearance? and b) placing a pool house foundation (about 4 ft [1.2 m] high). Would I be able to shoot these? I am thinking not because there is no place for the rebound to go.

Answer:

a) Shotcrete can be finished in a wide variety of ways. It can be left with anywhere from a very rough to a very smooth finish and a huge variety of other finishes. We suggest you visit ASA’s Web site, click on the tab for Shotcrete magazine, and search the previous articles for finishes and swimming pools. You will find a lot of photos of great-looking walls. Not all shotcrete contractors are proficient in providing these attractive finishes. You need to discuss this with your current shotcrete contractor and/or interview other shotcrete contractors to make sure the chosen contractor can provide what you are looking for. We also suggest you look at work these contractors have previously completed. You can also locate contractors online at ASA’s Buyers Guide, www.Shotcrete.org\BuyersGuide. b) If the pool house foundation is a footing trenched into the ground 4 ft (1.2 m) deep, dry-mix shotcrete would not be a good solution. If the foundation is 4 ft (1.2 m) above grade, then it could be done with shotcrete against a one-sided form. This would be considered structural shotcrete and not all shotcrete contractors are qualified to place shotcrete for structural walls. Again, we suggest you ensure the chosen contractor is qualified to do the work.

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Question 93:

Our client has a retaining wall that has experienced movement in the precast concrete panels and has asked us to research a product that could be applied to give a smooth look to the retaining wall. Is shotcrete a possible option? I would also like information on the recycled content of shotcrete.

Answer:

Shotcrete is basically concrete that is pneumatically applied. Shotcrete can be used as an overlay for an existing wall to provide structural strengthening and a smooth look. Again, we suggest that you visit ASA’s Web site and search previous Shotcrete magazine articles for finishes. Before the shotcrete is applied, the wall must be stabilized from any anticipated future movement. Relatively thin layers of shotcrete or concrete will not withstand future wall movements without distress and cracking. The recycled content of most shotcrete mixtures is limited to the substitution of fly ash or other pozzolans for a percentage of the cement in the mixture. To properly place shotcrete, this substitution is generally limited to approximately 25% of the cement content.

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Question 94:

We recently stained a shotcrete wall. After we placed the staining on the wall, the stain came out in different shades across the wall, in effect bring out the different curing of the concrete. What can be done to eliminate this inconsistency?

Answer:

It is not unusual to have variations in the tone of color for shotcrete or concrete walls that have been stained due to variation of the texture or density of the surface being stained. An acid-based stain typically results in more consistent shading. When anticipating that a wall will be stained, extra care needs to be taken in the curing process. It is generally recommended that walls to be stained should be water-cured to avoid any interaction between a curing compound and the stain material. If a curing compound is used, it must be completely removed prior to applying the stain material. Consult the stain supplier for more information.

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Question 95:

I have a seawall with a gunite (dry-mix shotcrete) outer layer. The gunite layer has cracked in multiple locations on the seawall resulting from years of exposure to the harsh environment. The original gunite was not part of a soil nail system. I am considering a re-coat of shotcrete probably 3 to 4 in. (76 to 100 mm) thick with wire mesh and L-anchors on a 2 to 3 ft (0.6 to 0.9 m) grid. I know the importance of surface treatment for bonding, etc., but I am not sure if I should remove the original gunite layer (which is still sound in some places) or apply the re-coat. The new overlay needs to be structurally effective. I know that a soil nail system is the most dependable solution, but cost is a major concern. Do you have any suggestions?

Answer:

The new shotcrete layer can be added to the existing shotcrete or installed after the existing shotcrete is removed. The decision to remove or not remove the existing shotcrete is beyond the scope of what we can comment on. If the existing shotcrete is left in place and overlayed, it should be thoroughly cleaned and roughened to create a good bonding surface. Because this is in a marine environment and you are considering the use of wire mesh, you need to make the new layer thick enough to have sufficient cover on the reinforcing steel. Alternately you could consider the use of fiber-reinforced shotcrete and silica-fume-enhanced fibrous shotcrete. Please note that there are many types of fibers on the market. We recommend that you review some of the Shotcrete magazine articles on fibrous shotcrete and on shotcrete in a marine environment on the ASA Web site. We suggested two papers for reference. The first is by Gilbride, Bremner, and Morgan on the Port of Saint John, and the other is by Morgan on the use of fibers that cover marine repairs. You mentioned using "L-anchors" at 2 to 3 ft (0.6 to 0.9 m) spacing. The use of grouted anchors with a reasonable embedment is quite common, but the design of such anchors is again beyond the scope of what we can advise.

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Question 96:

We will be tiling a pool. The pool’s shotcrete walls and floor were placed approximately 10 days ago. What is the earliest we can begin gauging the pool walls and floors?

Answer:

It is generally good practice to let the shotcrete cure for the full 28 days before attempting to apply coatings or overlays. We would recommend you get a recommendation on the cure time from the manufacturer of the gauging product before doing the work.

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Question 97:

We are considering the use of bentonite in a blind-side waterproofing situation to waterproof a basement with shotcrete as the confinement material. The basement has a 8.2 ft (2.5 m) head of water permanently against it (approximately 6.5 ft [2 m] higher than the slab/shotcrete wall construction joint).

In brief, we intend to construct as follows:

  1. Pump the area dry;
  2. Place secant piles, and then apply shotcrete over the piles. The shotcrete will be troweled to accept the bentonite;
  3. Apply the bentonite sheet membrane to the troweled shotcrete;
  4. Tie two rows of reinforcing steel at 11.8 ft (300 mm) centers in each direction;
  5. Shoot shotcrete through the steel onto the bentonite tanking; and
  6. Turn the pumps off once the curing period is complete.

We have been advised this will be effective. Any advice on this system would be greatly appreciated, as we believe using shotcrete rather than cast-in-place concrete as the confinement material would result in significant cost savings. We know little, however, of the confinement properties of shotcrete.

Answer:

The use of shotcrete over waterproofing in blind-side applications is not uncommon; and, as you note, it is generally very efficient from a cost and schedule standpoint. It should be noted that the shotcrete applicator (shotcrete contractor) should be very experienced in high-quality structural shotcrete work. The application of shotcrete in tunnels, canals, channels, or swimming pools is very different from the application of shotcrete for structural walls. The experienced structural shotcrete contractor will use experienced and knowledgeable tradesmen including a certified ACI nozzleman.

There are many types of waterproofing material including sodium bentonite, as you mentioned. Other membrane material and additives can be added to the shotcrete mixture as delivered. It is not within our scope to comment on the choice of these materials. You can contact the various manufactures or engage a waterproofing professional to give you the proper advice. The ASA online Buyers Guide is a great starting point in locating qualified professionals.

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Question 98:

I have reviewed ACI 506R, "Guide to Shotcrete," and 506.5R, "Guide for Specifying Underground Shotcrete," but was unable to find specific criteria pertaining to shotcrete protection for reinforcing steel. Would the clear cover then be based on ACI 318 Section 7.7.1 for cast-in-place concrete? For underground structures, would 3 in. (76.2 mm) of clear cover from ground be required?

Answer:

Shotcrete is concrete, and therefore if designing structures based on the ACI 318 Code, cover for conventional reinforcing steel should be those suggested in ACI 318 for concrete against ground. If the shotcrete is a "temporary" support, with further placement of "final" support, then these requirements do not apply.

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Question 99:

I have a question on cold weather shotcreting. I have heard that for shotcrete operations, the ambient temperature has to be 40°F (4.4°C) and rising. I am on a job, and the inspector said it only needs to be 35°F (1.67°C) and rising. The high for the day is expected to be around 45°F (7.2°C), then fall back into the high 20s F (–4 to –1.67°C). What would be your advice?

Answer:

Shotcrete is concrete and the same rules apply with respect to cold weather applications. Cold weather is defined in ACI 306R, "Cold Weather Concreting" as "a period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40°F (4.4°C) and 2) the air temperature is not greater than 50°F (10°C) for more than one-half of any 24-hour period." ACI 306R is an excellent reference that provides recommendations for cold weather concrete placement and protection. A copy of ACI 306R can be purchased online at ACI’s Web site, www.concrete.org, from the Bookstore and Publications tab. You can also download articles regarding cold weather placement from ASA’s Web site, www.shotcrete.org—click on "Shotcrete magazine," go to the article search page, and type in "cold weather."

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Question 100:

One of our clients has a 65.6 ft (20 m) tall mechanically supported earth (MSE) wall (to dump the ore from the mine into the crushers). The wall is about 984.25 ft (300 m) long and has approximately 30-degree slopes on both ends, like a pyramid. These slopes have eroded over the last 8 years of operation and some of the wall reinforcing is exposed. We want to stop the erosion and stabilize the slopes. The instructions issued to the contractor are: level the slopes; fill the voids; compact; apply shotcrete (maximum 1 in. [25 mm]). The area in question is 6.6 x 65.6 x 131.2 ft (2 x 20 x 40 m). Is shotcrete application in this case appropriate? Can you forward information on experts we could consult on?

Answer:

Shotcrete is well suited to the application you have described. You need to determine the characteristics that you want from the shotcrete (strength, toughness, freeze-thaw durability) and include these in the specification. The 1 in. (25 mm) seems very thin for a long-term installation. Please be aware that the material costs (in most cases) will be a small part of the total cost of the installation. You should also make sure that you have a good specification for surface preparation. If the existing surface is not properly prepared, the added shotcrete will not bond well and the installation will not last very long.

The ASA Online Buyers Guide (www.Shotcrete/BuyersGuide) is an excellent source to locate members within the field of shotcrete whom are listed as shotcrete consultants.

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Question 101:

Type CA and FA shotcrete are two classifications listed in ASTM C1480. What is the application of these two types of shotcrete?

Answer:

Type FA shotcrete uses a fine aggregate meeting the requirements of ASTM C1436 Gradation #1. Type CA shotcrete uses a combined aggregate gradation meeting the requirements of ASTM C1436 Gradation #2. The decision on which type to use depends on the application, shotcrete thickness, specification requirements, and perhaps the shotcrete equipment to be used, that is, wet- or dry-process. For example, one may want to use Type FA if using dry-process equipment and placing thin sections, or when a smooth finished surface texture is required. For thicker sections, Type CA shotcrete may provide the best properties for the application. The choice of which to use depends on the application, equipment, and experience of the contractor.

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Question 102:

How soon after shooting a pool shell can formwork be removed? How soon can tiling begin?

Answer:

Vertical formwork can generally be removed the day following the shotcrete installation. If the formwork is supporting a load like a soffit form, the form should not be removed until the shotcrete has attained full strength such that it can support the weight of the member.

Your question regarding the installation of the tile should be directed to a professional who installs tile.

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Question 103:

We have an approximately 9500 ft2 (882.6 m2) pool that was built and finished in midsummer. Four weeks later, the pool has developed "spider web" cracking in the bottom. We need to have a compressive strength test done. Our crew is on site now and is going to pull a 4 in. (101.6 mm) core sample for testing. I need to know what procedure to follow and where to send the sample for testing.

Answer:

Consult with a local engineering firm that is qualified to develop a coring plan, obtain cores, and perform testing in accordance with ASTM C42/C42M or ASTM C1604/C1604M. Please refer to ASTM C42/C42M for further guidance.

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Question 104:

We have demolished two radioactively contaminated buildings down to their concrete slabs. One of the slabs has a concrete pit that is 26 ft (8 m) deep. The slabs have not been removed because the soil beneath the slabs is contaminated and we’re using the slabs as a cover to protect the spread of contamination in the soil until the soil remediation begins. We’d like to use shotcrete to temporarily (up to 5 years) fix the contamination on the slabs and the 5 ft (1.5 m) area surrounding them. The questions we have are: 1) Will shotcrete adhere to the concrete slabs and pit walls for up to 5 years without special preparations? (Portions of the radioactively contaminated concrete are painted and it is dirty from demolition activities); and 2) What is the minimum thickness of shotcrete needed to last for 5 years in this type of application? We do not want to use any wire or fabric mesh as it would require personnel to work in a radiologically controlled environment to install the material.

Answer:

Shotcrete, like concrete, likely will not adhere to surfaces that are painted and dirty from the demolition activities. There should be no issue to the time durability. Shotcrete is pneumatically placed concrete and has great long-term durability characteristics if placed properly.Shotcrete has been installed in many adverse environments at a thickness of 2 in. (50 mm) with fibrous reinforcement and provided a long service life. Many irrigation districts line their canals with shotcrete and it has provided decades of great service in freeze-thaw exposures.

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Question 105:

What is the recommended cure time for shotcrete pools and spas so that shrinkage cracks in finished tile work can be avoided?

Answer:

Concrete, when applied using the shotcrete process, or cast-in-place, needs to cure for 7 days. Water is the best curing method (7 continuous days). Curing compound can be applied, but the membrane film that is formed will have to be removed by sand or water blasting (5000 psi [34.5 MPa] is recommended) before the plaster or tile can be set (it will create a bond breaker if not removed). There are curing compounds with a dissipating resin, which means after about 30 days in the sun, the material will break down. In either case, it is a good practice to pressure wash the concrete surface to remove the grit and dust out of the pours so that the plaster and tile will have a good bond. This is normal, everyday concrete curing practice that helps to prevent shrinkage cracks. The concrete being applied should have a water-cement ratio (w/c) of 0.35 to 0.45. Having the w/c at 0.40 at a 2 to 3 in. (50 to 75 mm) slump will keep the water demand low to help minimize the shrinkage. Wet-fogging freshly placed concrete before the curing process begins will also help prevent shrinkage cracks.

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Question 106:

We are designing underground support for a hydropower tunnel. I want to know whether wire mesh-reinforced shotcrete or steel fiber-reinforced shotcrete will be better and more economical. What are the advantages and disadvantages of both of these types of reinforcement if used for supporting a tunnel for hydropower? Also, for slope protection work, which type of shotcrete is better in terms of reliability, durability, and cost?

Answer:

There are really two questions here: 1) Underground fiber-reinforced versus mesh reinforced; and 2) slope protection fiber reinforced versus mesh reinforced.

  1. Underground fiber reinforced versus mesh reinforced: it is not clear what the alternatives are that you are considering, but sprayed concrete has a good, solid track record for ground support. If it is a simple comparison of steel mesh versus steel fiber reinforcement, then the issue is one of a design approach.

    Wire mesh and bolts have a longer history and are simple to design as a rigid structure. To install mesh and bolts, however, requires working under unsupported ground. Mechanized spraying of concrete is done with the operator under supported ground and therefore is intrinsically safer.The design of fiber-reinforced sprayed concrete as ground support is approached differently. The sprayed concrete is allowed to deform to a certain extent before coming to rest with the ground forces finding a new equilibrium. The extent of this deformation depends on the energy absorption of the sprayed concrete structure, which is provided for by the fibers.Steel fiber-reinforced sprayed concrete is by far faster to place and therefore has economic benefits. As the fibers are discontinuous, there is merit in considering this structure less susceptible to corrosion and consequential durability issues. We recommend consulting ACI 506.1R and ACI 506.5R.
  2. Slope protection fiber reinforced versus mesh reinforced: for slope protection, both fiber-reinforced and wire-mesh-reinforced shotcrete work well and are durable, reliable, and cost effective if done properly. Care must be taken with wire mesh reinforcing to ensure that it is maintained in the middle of the section and not on the ground where it is not effective. Wire mesh can also be difficult to install on an irregular surface and require more shotcrete material to cover the area and the mesh. The wire mesh can be an asset to the installer in providing a grid to support a scaffold system. In many applications, the choice of wire mesh or fibers should be left to the installer with the engineer specifying the minimum requirement for each.

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Question 107:

We are repairing a culvert in Dallas, TX. The concrete wall of the structure is pre­maturely disintegrating. We are considering a process to temporarily support the ceiling, remove the wall, place a form on one side, and use shotcrete to replace the wall. Does this sound like a reasonable use for shotcrete? What kind of specifications should be used?

Answer:

Yes, this sounds like a good use of the shotcrete process. Your sequence sounds like a good plan. A sample Structural Shotcrete Specification is available from the Shotcrete magazine archive on the ASA Web site (www.shotcrete.org).

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Question 108:

We’re looking at adding approximately 4 in. (100 mm) of shotcrete to an existing 8 in. (200 mm) wall to meet new load requirements. What’s the minimum cover between the rein­forcement and existing wall for proper encapsulation of the reinforcement?

Answer:

A minimum clearance for the reinforcment off the existing surface should be 0.75 in. (19 mm) or one bar diameter, whichever is greater, to allow a good flow of material around the reinforcing steel.

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Question 109:

I am working on a water feature formed out of cast-in-place reinforced concrete with a hot-fluid-applied waterproofing system over the concrete. To protect the waterproofing, we plan to install shotcrete over it. What minimum thickness of shotcrete is required? Would welded wire fabric or fiber mesh be required as well?

Answer:

In general, we would recom­mend a minimum of 2 in. (50 mm) of shotcrete. Either fibers or wire mesh or both should be used in this application. Please note that there are different types of synthetic fibers (microsynthetic and macrosynthetic). Refer to ACI 506.1R for information on fiber-reinforced shotcrete.

If the surfaces are steep or vertical, wire mesh should be used, but provisions need to be included to stabilize the wire mesh. This would likely require attachment points through the waterproofing system.

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Question 110:

Can a shotcrete mixture be designed using crushed washed sand instead of natural washed sand?

Answer:

The grading of fine aggregates, natural or manufactured, should be in compliance with the combined aggregate gradations in ACI 506R or ASTM C1436. Using crushed washed sand will be more difficult than using natural washed sand due to the more angular particle shapes. Due to the more angular particles, crushed sand will likely require a higher paste content to successfully convey it through the shotcrete hose.

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Question 111:

We are proposing a project that will use shotcrete on an existing metal bin wall to match recently constructed soil nail walls with shotcrete facing. What is the proper way to prepare the bin-wall surface? Also, what type of reinforcement would you recommend and what is the suggested method of attaching the reinforcement to the bin wall?

Answer:

The surface should be cleaned using a high-pressure water blaster or sandblasting to remove any loose material and rust. If the metal bin material is thick enough, you might want to consider welding metal studs or nuts to the bin to secure the reinforcing steel or mesh. The amount and type of reinforcement is beyond the scope of our organization and we suggest getting guidance from a qualified engineer. You may gain some insight from the design of the reinforcing used in the soil nail walls.

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Question 112:

I am working on an existing slope with a ratio of 3:1 (horizontal:vertical) and a total height of 6 ft (1.8 m). The slope has been surfaced with asphalt concrete. Will shotcrete adhere to the asphalt concrete surface, or should the asphalt concrete be removed prior to applying shotcrete?

Answer:

Shotcrete will adhere to properly prepared asphalt concrete. Shotcrete bond is generally related to the preparation of the surface that you want to bond with. If the surface is dirty, the shotcrete will not bond very well.

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Question 113:

Our current project is a pier with severe corrosion of reinforcement and obvious spalls. The work will all be overhead with the surface 18 in. (457 mm) above the mean tide level and, for a variety of reasons, dry-mix is not an option. We are looking for a good, dense, wet-mix design for saltwater marine exposure. Compressive strengths need to be in the mid-range of 7000 to 8000 psi (48.3 to 55.2 MPa).

Answer:

For a potentially suitable wet-mix shotcrete mixture design for marine structure repair, go to the ASA Web site (www.shotcrete.org). Click on Shotcrete magazine and search for "Shotcrete Classics: Deterioration and Rehabilitation of Berth Faces in Tidal Zones at the Port of Saint John." This mixture design worked well for over 1.2 miles (2 km) of ship berth face repair over a 10-year period. Note: Because of high freezing-and-thawing exposure, the shotcrete was required to be air entrained. While the original mixture design called for 7% air content as shot, it was subsequently modified to require an air content of 7 to 10% as batched (at the point of discharge into the shotcrete pump) and an air content of 5 ± 1.5% as shot (into an air pressure meter base). Such shotcrete has provided good freezing-and-thawing resistance. You should be aware that your local materials (coarse and fine aggregates and cement) may have different properties in the concrete mixture, however, as compared to the mixture discussed in the article. It is recommended that a local engineer, testing laboratory, or concrete supplier be retained to develop a concrete mixture using local materials that meets the performance requirements of the mixture design mentioned in the article. Also, test panels constructed with the mixture, nozzlemen, and equipment to be used in the shotcreting are highly recommended to verify the strength performance of the shotcrete.

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Question 114:

We are currently working on a Request for Deviation to use shotcrete in lieu of cast-in-place concrete. The engineer is requesting additional information/confirmation. The application locations are structural, using No. 6 and No. 8 reinforcing bars on 1.5 ft (0.5 m) thick walls approximately 40 ft (12.2 m) high. The engineer’s comments refer to detailing construction joints, curing, and plastic shrinkage gaps (work done in July). We have also requested a slump to be reduced to 2 ± 1 in. (51 ± 25 mm) and the use of 3/8 in. (9.5 mm) aggregate. What methods would you suggest to address each issue?

Answer:

The project as described sounds very feasible for a structural shotcrete application. As we understand, the concerns are:

  1. Detailing construction joints—Please refer to ACI 506R, "Guide to Shotcrete," Paragraph 5.7, Joints. Typically, shotcrete joints are beveled to increase the surface area of the bonding surface and reduce the likelihood of trapping rebound. Other considerations for construction joints should follow the principles of cast-in-place concrete. Shotcrete is a method of placing concrete.
  2. Curing—Shotcrete is concrete consisting of smaller aggregates and generally higher cement content. Good curing practices should be followed as they should be with cast-in-place concrete. Considerations should include the temperature and humidity when evaluating a curing program. High temperatures with low humidity will require significantly more effort than high temperature with high humidity. The key is to ensure that sufficient moisture is available to hydrate the cement during the curing period.
  3. Plastic shrinkage gaps or cracking—The shrinkage characteristics of shotcrete are similar to cast-in-place concrete. Shotcrete is composed of smaller particles and higher cement but generally places at a low water-cement ratio (w/c) or less than 0.45. Shotcrete is somewhat more prone to plastic shrinkage cracking due to the surface not being protected by a form in its early stages. If the finished surface is subjected to high ambient temperatures, low humidity, or high winds, it will tend to dry quickly on the surface and exhibit more plastic shrinkage cracking. In these environmental conditions, fogging of the exposed shotcrete surfaces soon after shotcreting may help to reduce or eliminate the plastic shrinkage cracks. Plastic shrinkage cracks are generally superficial in nature and can be repaired if necessary.
  4. Slump to be reduced to 2 ± 1 in. (51 ± 25 mm)—This is a good range if measured and treated properly. It is important to ensure that the shotcrete material has enough slump at the nozzle to properly encapsulate the reinforcing steel and is stiff enough to stay in place without sloughing or sagging. The slump at the nozzle is far more relevant than the slump at the pump.

The important factors influenced by slump are maintaining the proper water-cementitious material ratio (w/cm) and consistency at the nozzle to ensure good placement. The most important consideration is to ensure that you have an experienced shotcrete contractor who has a history of success with similar projects with respect to the size and complexity of the installation. You can locate shotcrete contractors on the ASA online Buyers Guide at www.shotcrete.org.

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Question 115:

I will be placing a large amount of concrete via the shotcreting process onto a river bed. There are minimal forces and the only reason I need reinforcing is for temperature and shrinkage. If I add fibers to the mixture design, what percent of steel will I still need (if any) or, in other words, how much fiber do I need to include so that any other form of mesh or reinforcing bar is not required to meet temperature and shrinkage requirements? In addition, will too much fiber have any unwanted effects?

Answer:

Fibers are typically added to shotcrete linings for canals, channels, and other water structures in lieu of conventional reinforcing, such as welded wire reinforcement (WWR). For your "large amount of concrete via the shotcreting process," we assume that you are using the wet-mix shotcrete process.

Temperature/shrinkage reinforcement is typically placed in thin sections governed by the structural concrete provisions of ACI 318 at a rate of 0.15 to 0.18%. Please be aware that if the lining is intended to be liquid-tight and has movement joints spaced at greater than 40 ft (12 m) apart, a reinforcement ratio of at least 0.50% is recommended by ACI 350 for concrete liquid-containing structures. Assuming that the section does not need to be liquid-tight and using the ACI 318 requirements, let’s consider the tensile capacity of a conventionally reinforced section and provide an equal or greater tensile capacity with fibers. Assuming a 3 in. (75 mm) thick lining with an assumed 28-day compressive strength of 4000 psi (28 MPa), using a WWR of 6 x 6 x W2.9/W2.9 in this section provides a percentage of steel of 0.161% and a tensile capacity of 3770 lb/ft (5610 kg/m). (Asfy = 0.058 in.2/ft [0.12 mm] x 65,000 psi [448 MPa] = 3770 lb/ft [5610 kg/m].)

Then, we assume that the direct tensile strength is 75% of the flexural strength (modulus of rupture [MOR]). For 3770 lb/ft (5610 kg/m) in a section 3 x 12 in. (75 x 300 mm), we have 3770/(12 × 3) = 105 psi (0.72 MPa). Then, we need an average residual strength (ARS) (ASTM C1399) of 105/0.75 = 139.6 psi (0.963 MPa) = 140 psi (0.965 MPa).

Using a macrosynthetic fiber, one can achieve these results using 4 to 5 lb/yd3 (2.4 to 3.0 kg/m3) in wet-process shotcrete. Fiber manufacturers will provide exact dosages to meet the ARS requirements.

Using steel fibers, approximately 43 lb/yd3 (25.5 kg/m3) will provide an equivalent area of steel to the WWR of 6 x 6 x W2.9/W2.9 in a 3 in. (75 mm) thick concrete section. Using steel fibers, however, may require a flash coat to cover the fibers that will protrude and rust over time. The corrosion of the fibers will only reach a carbonation depth of 0.05 to 0.10 in. (1 to 2 mm) but may result in staining the lining.

These calculations assume a thickness and strength. You must adjust for your conditions.

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Question 116:

I am searching for a sample specification that calls for the use of a polymer-modified cement mortar in lieu of one that does not have the polymer additive. My thought is that this material would be more durable. I am also wondering if it would have greater bond to the old substrate.

Answer:

Most of the industry does not endorse the use of polymer-modified additive in shotcrete. Please refer to ACI RAP Bulletin 12 and ACI 506R for further information and insight from the American Concrete Institute (ACI) at www.concrete.org. Shotcrete applied by competent contractors to properly prepared surfaces exhibits excellent bond characteristics to the substrate. Additionally, a good shotcrete mixture that is properly applied will yield a durability equal to or superior to cast concrete. There are many examples discussed in various articles of Shotcrete magazine at www.shotcrete.org/archivesearch/ArchiveSearch.asp.

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Question 117:

Are there any guidelines/regulations as to how close in proximity the installer/nozzle person can be to the receiving surface? I have a chimney job (existing chimney repair) that has an opening of 3.5 x 3.5 ft (1.1 x 1.1 m) and the interior is calling for a gunite (dry-mix) liner to be installed. Is there an issue with splash-back or any other concern with using gunite in such a confined space?

Answer:

When gunning in tight areas, you have to allow for the bend in the hose and the length of the nozzle, which will require at least 2 to 2.5 ft (0.6 to 0.8 m). An area 3.5 ft (1.1 m) wide is a very tight area to gun in, but it can be done. In areas that restricted, it’s not possible to always maintain a 90-degree shooting angle, so you will get much more rebound from the deflection when shooting at less than desirable angles. Also, with the dry process, you have to reduce the air pressure and volume to keep from blowing the material off the walls. Ideally, you would like the nozzle to be at least 3.5 to 5 ft (1.1 to 1.5 m) from the receiving surface, depending on the nozzle you use. For a tight area like you are proposing, in addition to reducing the air pressure and running it slowly, we would recommend using a double-bubble nozzle, as it has a wider spray pattern, allowing the nozzleman to be closer to the receiving surface and still get an adequate spray pattern. A double-bubble nozzle is also flexible, which will help in extremely tight areas. You can locate organizations that sell a range of nozzles by visiting the ASA Online Buyers Guide at www.shotcrete.org/pages/products-services/Buyers-Guide/index.asp.

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Question 118:

I would like to add fibers to a shotcrete mixture. Many research articles discuss steel fibers and sometimes synthetic. I’d like to consider glass or synthetic fibers because the exterior wall will be visible and I don’t want to see the corroding steel fibers toward the surface of the concrete. What are the pros/cons of glass fibers versus steel fibers and how much should I add to the mixture design to achieve a product that can be submerged in water and experience as few cracks as possible? Is there reference material for these questions?

Answer:

Refer to ACI 506.1R-08, "Guide to Fiber-Reinforced Shotcrete," at www.concrete.org for guidance on fiber types and dosages. Glass fibers are seldom (if ever) used in shotcrete because they tend to break under the high velocity required for shotcrete. Steel or macrosynthetic fibers should be used at about 0.4 to 0.5 volume percent to control hardened shotcrete cracking, 50 to 66 lb/yd3 (30 to 39 kg/m3) for steel (specific gravity (SG) of 7.85), and 6 to 7.5 lb/yd3 (3.6 to 4.5 kg/m3) for macrosynthetic polypropylene (SG of 0.91). Fiber suppliers can provide more technical guidance for their products. You can locate fiber suppliers by visiting the ASA Online Buyers Guide at www.shotcrete.org/pages/products-services/Buyers-Guide/index.asp.

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Question 119:

I have been in the swimming pool industry for 30 years and I deal with a lot of different engineers on my commercial projects who want a wet test to verify water tightness before the finish is applied to the pool. In my experience, air-entrained shotcrete tends to be porous and leak. Are there any engineering specifications that state that air-entrained shotcrete is porous and will leak if the surface is not trowel-finished?

Answer:

Properly added and mixed air-entraining admixture in concrete will actually reduce the permeability of concrete. This is because the small, well-formed air bubbles from air-entraining admixtures are not interconnected as larger, entrapped air bubbles may be in non-air-entrained concrete. Thus, the reported higher permeability of the air-entrained shotcrete is not a material flaw but must be from poor shotcrete application. Air entraining from 4 to 7% air is advantageous for enhanced resistance to the freezing-and-thawing cycles of saturated concrete and should be specified by the designer in areas subject to significant numbers of freezing-and-thawing cycles annually. The reported high permeability and resultant failure to pass a water-tightness test could be investigated by taking cores of the "porous" material and conducting a petrographic analysis of the core. Based on the reported results, I strongly suspect that the in-place shotcrete has major issues with sand pockets, overspray, and rebound.

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Question 120:

We recently contracted with a shotcrete company to install a shotcrete structure for a swimming pool. After the pool was completed and filled with water, rust stains began emerging through the plaster surface. When we broke out a section of the pool structure, we found that there was little to no coverage of shotcrete over the steel reinforcement. The shotcrete company’s excuse is that they shot the pool to maintain the desired finished depths and widths and there was little to no coverage because the steel was set too high (even if that were the case, they never alerted anyone during the installation). This sounds like an excuse to me. Shouldn’t the shotcrete company we hired make sure that the concrete coverage met or exceeded what the structural engineer called for? Is there any credibility to their explanation of why they didn’t cover the reinforcing bar enough? What is the standard practice for shotcrete installation?

Answer:

In short, the shotcrete contractor is responsible for maintaining proper cover over the reinforcing steel. The reinforcing bar installer should set the steel in the proper location for achieving the required cover corresponding to the final desired shape. If the shotcrete contractor finds that he cannot maintain proper cover with the reinforcing as placed, however, he needs to communicate to the designer/owner/general contractor that the reinforcing needs to be fixed before he shoots the section in place. There is no excuse for placing shotcrete with less than the specified cover, as shooting it with reduced cover will obviously create a section that has much less durability than intended by the designer.

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Question 121:

What is the impact force on formwork resulting from a shotcrete application? I am designing the formwork for a wall to be placed via shotcrete and need to know the forces imposed on the wall forms.

Answer:

In structural applications, most of the impact force from nozzling shotcrete is directed toward compacting the shotcrete in place rather than against the formwork. This was the subject of a study conducted by Marc Jolin of Laval University, Quebec City, QC, Canada, and reported in the Fall 2007 issue of Shotcrete magazine. There is virtually no hydrostatic pressure on the forms from the application using the shotcrete process. A copy of this study can be viewed on the ASA Web site at www.shotcrete.org/archivesearch/ArchiveSearch.asp.

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Question 122:

Can shotcrete be applied on wet shale rock? How well does shotcrete bond to shale?

Answer:

Shotcrete is routinely used to seal shale after excavations. It is typically done as soon as possible after the excavation because the shale will deteriorate when exposed to the air. When shotcreting, it is considered good practice to wet the receiving surface prior to gunning to create a saturated surface-dry (SSD) condition so the substrate will not draw moisture from the newly placed shotcrete. A good SSD condition is where the surface is wet without any standing water on it. Gunning over wet shale should not be a problem unless the water seeping from the shale is moving. If that is the case, we would recommend installing weep holes with plastic pipe at the locations where the water is seeping from and using an accelerator to flash-set the material immediately around the weep-hole pipe. It is also a good idea to install weep holes at regular intervals along the excavation or exposed hillside. It is important to use a qualified shotcrete subcontractor for this or any high-quality shotcrete installation. A qualified shotcrete contractor will use ACI-certified nozzlemen and should provide you with a résumé of similar, successfully installed projects, along with the up-to-date contact information of representatives from the owners or engineers involved in those projects. The ASA Buyers Guide (www.shotcrete.org/pages/products-services/Buyers-Guide/index.asp) is an excellent source of shotcrete contractors.

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Question 123:

We are removing up to 0.75 in. (19 mm) of the existing scaled concrete on a fire-damaged concrete wall. The architect has asked if shotcrete is applicable for a vertical 0.75 in. (19 mm) application. Also, the walls are circular and the working distance from the wall is no more than 36 in. (0.9 m). Is this enough room to apply shotcrete?

Answer:

Yes, shotcrete can be applied in a 36 in. (0.9 m) area. Keep in mind, however, that it’s difficult to get as nice a gunning pattern as you would like when you are that close to the receiving surface. When you cannot back off from the wall, there is a tendency for a more irregular gunning surface, which would require more cutting and screeding to get an aesthetically pleasing result.

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Question 124:

I would like advice about spraying shotcrete on the exterior walls of a house I am building. In constructing the exterior walls of the house, I plan to shoot approximately 0.75 in. (19 mm) on Day 1 and shoot 1.25 in. (31 mm) on Day 2 for 2 in. (50 mm) thick walls. I have hung 14-gauge wire mesh spaced at 1 in. (25 mm) over all the walls and am planning to use a 3000 psi (20.7 MPa) mixture. I am greatly concerned about cracking. Is my planned technique a good way to mitigate cracking or are there better approaches? Should I consider upping the strength of the concrete?

Answer:

In structural applications, most of the impact force from nozzling shotcrete is directed toward compacting the shotcrete in place rather than against the formwork. This was the subject of a study conducted by Marc Jolin of Laval University, Quebec City, QC, Canada, and reported in the Fall 2008 issue of Shotcrete magazine. There is virtually no hydrostatic pressure on the forms from the application using the shotcrete process. A copy of this study can be viewed on the ASA WebIt is fine to place shotcrete in two layers on 2 consecutive days, although simply placing two layers on 2 consecutive days won’t prevent long-term drying shrinkage cracking. For the best bond, the surface of the shotcrete on Day 1 should be given a rough broom finish to provide a rough texture for the Day 2 shotcrete to bond to. On Day 2, before shooting, wet the surface of the Day 1 shotcrete to prevent a hot, dry surface from absorbing water from the fresh shotcrete. Please note that the surface needs to be dampened but allowed to dry to an SSD condition. A surface that is too wet can inhibit good bonding. It is essential to moist-cure the shotcrete as soon as it has finally set to help reduce early-age shrinkage cracking. On a hot, windy day, you may need to fog the surface soon after placement with a pressure washer using a fogging nozzle to reduce the rapid evaporation of water from the surface of the shotcrete. Wet curing with a wetted burlap overlay or drip system for at least 3 days (preferably 7 days) is recommended to help reduce the potential for longer-term drying shrinkage cracking. Using macrosynthetic fibers in the shotcrete mixture will also help reduce early-age shrinkage cracking. Because you are in Florida, unless you are shooting in the dead of winter, you may also want to consider using a concrete mixture with up to 20 to 25% fly ash. This will slow down the hydration of the cement and resultant set time to give you some more time to finish the surface and get proper curing started. Fly ash also helps reduce the concrete permeability and increases the long-term strength and is generally less expensive than portland cement. If you use a concrete mixture with silica fume (also called microsilica), it will increase the water demand of the mixture during hydration and has a greater tendency for early-age plastic shrinkage cracks. Thus, if you use silica fume, you will need to pay close attention to keep the surface wet through fogging and then wet curing as soon as it is practical. As previously mentioned, a 2 x 2 or 3 x 3 in. (50 x 50 or 75 x 75 mm) wire mesh would be preferred to reduce congestion of the reinforcement. Stay away from rolled mesh, as it is very difficult (even nearly impossible) to get to lay flat. Sheets of welded wire mesh are recommended. ASA recommends a minimum 28-day compressive strength for shotcrete of 4000 psi (27.6 MPa). A 3000 psi (20.7 MPa) mixture will have a higher water-cement ratio (w/c); therefore, there is more water in the mixture, which will significantly increase the potential for drying shrinkage cracking in the final surface. A 4000 psi (27.6 MPa) mixture is easily achieved with current portland cements and normal supplemental cementitious products such as fly ash. Finally, you mentioned that you will be shooting the surface of a house. You haven’t provided any details about what you are shooting the shotcrete on, but the substrate must be rigid enough to not vibrate when shotcrete hits the surface. If it is not rigid enough, the vibration of adjacent areas of freshly shot plastic shotcrete could cause cracking. This would be more of a problem in the Day 1 coat of shotcrete, but cracks that form in the Day 1 shotcrete would create a weaker section and increase the likelihood of mirrored cracking in the Day 2 layer. Again, please note: While it is appropriate to wet down the Day 1 shotcrete prior to application of the Day 2 shotcrete, it is important to let the wetted Day 1 shotcrete dry back to an SSD condition before application of the Day 2 shotcrete. If the Day 2 shotcrete is applied to a wet substrate (with liquid water on the surface), it will fail to meet the specified 150 psi (1 MPa) bond pulloff strength requirements for the project.

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Question 125:

What are the requirements for selection of the shotcrete lift height and delay between successive layers? ACI 506R describes only a general approach.

Answer:

Although some have tried, there are not and should not be specific recommendations for lift height or time between lifts of layers. Shotcrete is a method of placing concrete, and concrete properties vary with many parameters, such as admixtures, ambient temperature, concrete temperature, slump, and age of concrete, to name a few. The lift height is also influenced by the surface on which you are shooting (rough, porous, smooth, dense, and so on); the orientation being applied (vertical, sloped, or overhead); and the size and density of the reinforcing steel, if it is present. Regardless of the period of time between lifts or layers, the receiving surface must be clean and moisture-conditioned to create a good bond between lifts or layers. As you can see, there are too many variables to spell out recommended guidelines or rules of thumb for lift heights or time between lifts or layers. The goal is to place the lifts or layers in heights or thick­nesses that do not slough or sag. The time between lifts or layers is the time required for the initial lift or layer to support the subsequent lift of layer. These decisions must be made on the job on a daily and hourly basis by a properly trained and experienced nozzleman and shotcrete foreman. These decisions may vary during the day to meet the current situation. It is critical that the shotcrete is placed by a shotcrete contractor with trained and experienced crews who is experienced and successful in the type of work being installed.

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Question 126:

We want to shotcrete a porous rock wall to stop water leakage out of a small pool that is home to an endangered fish. The wall is quite rough and uneven. The pool will be drained to do the work. How long should we allow the concrete to cure before refilling the pool with water? Does this sound like a good application for shotcrete?

Answer:

This is a great application for shotcrete. Once drained, the existing surfaces should be cleaned by water blasting or sandblasting to provide a good bonding surface. The shotcrete can be submerged within a few hours or upon reaching the final set (hardened). One factor to be concerned about is the chemical reaction and alkalinity of the area around the shotcrete during the curing period. A good solution would be to submerge the pool for a period of at least a week, drain and waste the water, flush the shotcrete surfaces, refill the pool, and test the pH before reintroducing the fish. This should eliminate the danger of the alkalinity to the fish.

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Question 127:

Can you provide any information or research on the sound absorption performance of shotcrete?

Answer:

We are not aware of any testing done specifically for the sound absorption performance of shotcrete. Shotcrete is a method of placing concrete and, once hardened, it should have similar parameters as cast concrete. With shotcrete, one has the ability to use many different finishes, which might influence the sound absorption characteristics. A smooth troweled shotcrete wall would be the most similar to a formed cast-in-place wall. On the other end of the spectrum, a nozzle-finished wall would likely absorb far more sound. A recent design of a concert hall at a major university was to be built with oval concrete or shotcrete perimeter walls covered with fabric curtains for sound purposes. The ceiling was to be suspended nozzle-finished shotcrete.

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Question 128:

We are rehabilitating a limestone-brick masonry storm sewer by lining it with shotcrete. The sewer is approximately 7 ft (2.1 m) tall with an arch ceiling and walls that are 8 ft (2.4 m) apart. The limestone surface is fairly rough, but the brick portions are not. While the existing structure shows no signs of needing to be reinforced for structural support, we are reinforcing to prolong the service life of the culvert. Is there a recommended minimum shotcrete thickness and reinforcement?

Answer:

Shotcrete has been used to successfully rehabilitate sewers for over 50 years.

The thickness to be used is an engineering issue and beyond the scope of our association. We would recommend a 2 in. (50 mm) minimum thickness reinforced with either polypro­pylene fibers or a light-gauge welded wire reinforcement. The surfaces must be cleaned thoroughly to remove grease, oils, and other substances deleterious to good bond. Bonding to brick is not a problem.

Finish is another consideration. The added liner thickness will reduce the size of the culvert. If capacity is not an issue, it is recommended to leave the new shotcrete lining with a nozzle finish. If capacity might be a problem, then a float or trowel-smooth finish may be necessary.

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Question 129:

Is it possible to use a penetrating sealer, such as those used on driveways, to make shotcrete repel moisture? If so, will the sealer improve the shotcrete’s freezing-and-thawing performance?

Answer:

It is possible to use a penetrating sealer on shotcrete in the same manner as cast-in-place concrete. We are not aware of research on the durability of such a sealer and do not know if it would enhance the freezing-and-thawing characteristics. A high-quality shotcrete mixture that is properly placed will exhibit excellent freezing-and-thawing characteristics with or without a sealer.

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Question 130:

Often, steel fiber-reinforced shotcrete (SFRS) linings are applied in underground construction. In some areas of high tensile stresses, it is necessary to use additional ordinary reinforcement (reinforcing bar/mesh). It may be inefficient to switch to non-fibrous shotcrete for these regions. Are the shadowing problems to be expected in that case (SFRS with additional ordinary reinforcement) more severe and how can they be resolved?

Answer:

It is not uncommon to encapsulate lattice girders or steel sets in fibrous shotcrete. The skill of the nozzleman, the size and density of the reinforcing, and the characteristics of the mixture and the accelerator are the most important factors in achieving good encapsulation of reinforcing bar or these more complicated applications around lattice girders or steel sets. With welded-wire reinforcement, you should have a 4 x 4 in. (100 x 100 mm) or greater spacing. With reinforcing bar, you should use the minimum diameter possible at a minimum spacing of around 6 in. (150 mm). Preconstruction mockups should be considered to prove the competency of the nozzleman and the mixture. Please note that the best nozzleman cannot succeed without a good, workable mixture.

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Question 131:

We are currently working on a tunnel that will cross through a drinking water protection zone in the alluvial aquifer. Do shotcrete technology and materials exist that can be used on groundwater-sensitive areas?

Answer:

Shotcrete is the same as concrete when evaluated as a material and its exposure to potable water. In the U.S., many admixtures and cements for concrete have been tested and certified to meet the NSF 61 standards for materials exposed to potable water. In my experience, potable water stored in concrete tanks with direct exposure to the concrete (no coatings) has not exhibited any significant rise in alkalinity. Exposure of a tunnel in a groundwater aquifer would seem to have much less contact area per volume of water contained in the aquifer, such that any rise in alkalinity would be miniscule. Because concrete in the U.S. is universally accepted for the storage and transport of potable water, I’d assume that the use of shotcrete in your tunnel would be perfectly acceptable.

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Question 132:

We are considering the use of shotcrete to line a 3600 ft (1097 m) channel that is approximately 15 to 20 ft (4.5 to 6 m) wide. The purpose of the lining is to cap impacted sediments in the channel bottom. What is the suitability of shotcrete for this type of application, and can you provide a conceptual/budgetary estimate for the implementation of this approach?

Answer:

Shotcrete is a method of placing concrete and therefore the material has the same basic characteristics of concrete. Shotcrete is often used for canal, channel, and ditch lining. It is important with shotcrete (concrete) that the subgrade the material is placed over be compacted and stable. The thickness, strength, and reinforcing needs to be designed and specified by a professional engineer familiar with this type of structure or pavement. For budget numbers, you should contact one of our contractor members, who can be found in the Buyers Guide on the Web site at www.shotcrete.org.

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Question 133:

We are currently placing a shotcrete wall in a tunnel. The wall has a minimum thickness of 8.25 in. (210 mm) and is placed against secant piles. Our specs called for a wet cure. To minimize shrinkage cracking, what is the minimum amount of time to allow after shotcrete placement before the wet cure is applied?

Answer:

There is a difficult balance between wet curing too early or too late. You should not add water too early (before the material sets), as this would increase the water-cement ratio (w/c) of the material on the surface. You also do not want to add water during the finishing process, as this would also work the water into the surface and increase the w/c at the surface. Good practice would be to use an evaporative retardant, which generally also serves as a finishing aid during the finishing process, and then get the wet cure set up as soon as possible.

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Question 134:

We are studying a repair to an existing large-diameter corrugated metal pipe. The owner requires that the repair meet the fifth edition of the AASHTO LRFD Bridge Design Specifications with 2010 Interim Revisions. We want the owner to consider shotcrete as opposed to installing a new carrier pipe. I have pipe dimensions, depth, and so on, but need some help deciding if this is practical.

Answer:

Shotcrete has been used in many cases to repair, rehabilitate, and strengthen pipes, culverts, and tunnels. It is not uncommon to use shotcrete to strengthen a culvert under a highway or roadway section. Shotcrete is a method of placing concrete at a high velocity. The shotcrete placed inside the existing pipe can be designed for strengths from 4000 to 10,000 psi (27.5 to 69 MPa), depending on the amount you are willing to spend on the shotcrete products. We cannot speak to the acceptance by the governing body, but it has been done successfully and often in the past. It is vitally important that the shotcrete contractor be competent and experienced in installing the lining. Your specification should require evidence of similar previously completed projects with current references.

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Question 135:

We are constructing a new custom residence on the Gulf Coast of Texas using a Monolite insulated concrete form (ICF) system. The ICF system is basically a "sandwich" system with an expanded polystyrene (EPS) panel with a wire cage and shotcrete on both sides. Because of the storm surge and high humidity of the region, we are looking for a mixture formula for a waterproof shotcrete for the exterior coating to help prevent moisture migration to the interior. What can you suggest?

Answer:

The insulation itself should provide a vapor barrier. Various additives can be used with the shotcrete to improve its permeable properties, such as silica fume or a commercial waterproofing additive. It is also not uncommon to use a plaster coat over the shotcrete to provide improved water resistance and an architectural finish. The density and uniformity of the shotcrete can be influenced by the competency of the shotcrete applicator. It is always advisable to engage a competent and experienced shotcrete contractor to ensure the best possible results. You can search for a contractor with certified shotcrete nozzlemen from our Buyers Guide at www.shotcrete.org/pages/products-services/Buyers-Guide/index.asp or submit a bid request through our Online Bid Submittal Tool at www.shotcrete.org/pages/secured/ProjectBidRequest.aspx.

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Question 136:

When replacing welded-wire reinforcement with micro- or macro-synthetic or steel fibers, how is the "equivalent dosage" of fibers determined?

Answer:

The equivalent dosage of fibers to replace embedded steel reinforcement needs to be evaluated by the design engineer for the specific project or application. Guidance for the designer is available in ACI 506R-05, "Guide to Shotcrete," and ACI 506.1R-08, "Guide to Fiber-Reinforced Shotcrete."

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Question 137:

Is there a recognized standard addressing pass/fail criteria for abrasion testing of cement mortar shotcrete-lined corrugated steel pipe?

Answer:

We are not aware of any recognized standard for abrasion testing or acceptance specifically for this application. ASTM International has several abrasion tests for concrete and mortar that include:

  • ASTM C418-05, "Standard Test Method for Abrasion Resistance of Concrete by Sandblasting";
  • ASTM C779/C779M-05(2010), "Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces";
  • ASTM C944/C944M-99(2005)e1, "Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method"; and
  • ASTM C1138M-05(2010)e1, "Standard Test Method for Abrasion Resistance of Concrete (Underwater Method)."

Also, ACI 350-06, "Code Requirements for Environmental Concrete Structures," Section 4.6, has requirements for protection against erosion.

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Question 138:

We are considering a shotcrete application over a weathered rock outcrop (consisting of sandstone, siltstone, and clay stone) in our backyard to prevent further erosion and unstable conditions. Does the outcrop need to be prepared as described in your previous Shotcrete FAQs (loose material removed, saturated surface-dry [SSD]) if a mesh that is anchored to the outcrop will be used? Also, will the shotcrete need to have fibers in the mixture? Do we need joints?

Answer:

It is always a good practice to scale off the loose material from the rock face, particularly when dealing with shale or clay stone, as they degrade when exposed to the air. In addition to scaling the rock face, it should be washed down with air and water prior to gunning. As for expansion joints, they are not normally used when gunning over natural rock. The shotcrete is typically gunned continuously across the hillside without any expansion joints, with a natural gun finish following the natural contours of the rock face. With an anchored mesh in place, the use of fibers is not necessary. In many applications, fibers can be used in place of or in addition to mesh.

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Question 139:

I am considering the use of shotcrete as an alternative to grouted riprap for slope stabilization. The project involves a basin with varying slopes and easily erodible soils. Water will cascade down the side slopes. I was going to specify shotcrete with welded-wire reinforcement but am now considering fiber-reinforced shotcrete. Is fiber-reinforced shotcrete the better choice and, if so, is 3 in. (76 mm) thickness sufficient?

Answer:

Structurally, using proper quantities of either welded wire or fibers should work well. If fibers are used, they should be specified by an engineer who has the experience to specify the type of fiber and either performance requirements or dosage levels. The advantage of fibers is that they are uniformly distributed through the section, whereas the welded-wire reinforcement can be difficult to maintain in the proper location within the pavement section. The proper thickness should also be determined by a qualified engineer, as soil and groundwater pressures can impact the required thickness.

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Question 140:

Can shotcrete be used to repair a wall made of cement and fly ash? If so, should the wall be prepared for the shotcrete application?

Answer:

Structurally sound concrete that contains up to 20% fly ash in the total cementitious materials should not present any problems for subsequent bonding of shotcrete. Concrete with fly ash contents up to 30% have been used in recent years without any reported problems with strength and bond. Although concrete mixtures with higher levels of fly ash (up to 55%) have been proposed, we don’t have direct experience with their bonding characteristics. We suspect it would be fine as long as the base concrete develops adequate compressive and tensile strength. This could be confirmed by a simple bond strength test of shotcrete on the concrete substrate in question.

The existing surface needs to be properly prepared, removing all soft or deteriorated material back to sound concrete. For extensive defects in the existing concrete, chipping hammers may be required. For removal of light surface carbonation or laitance, a strong, high-pressure water blast or sand/bead blasting may be adequate. Depending on the thickness of the shotcrete, reinforcing may be required in the overlay. Specific details of the repair are best developed by an engineer experienced in shotcrete repair.

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Question 141:

Is shotcrete used as a canal liner?

Answer:

Shotcrete has been used for canal lining throughout the United States. The Bureau of Reclamation published a study on Canal Lining Test Sections constructed in the Bend, OR, area and studies the durability at 5 and 10 years. Shotcrete is a very viable means of placing canal linings. Basically, shotcrete is a method of placing concrete. Care should be taken to ensure that the mixture is designed to withstand the local environmental conditions, such as using air-entraining admixtures to ensure durability due to exposure to freezing and thawing. ACI 506R-05, "Guide to Shotcrete," contains a lot of useful information in evaluating and using shotcrete in a variety of applications, including canal linings. If liquid-tightness and long-term durability of the canal lining are important, provisions of ACI 350/350R-06, "Code Requirements for Environmental Engineering Concrete Structures and Commentary," should also be considered in the design of canal lining reinforcement, cover, and joints.

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Question 142:

Are there tolerance standards for the use of shotcrete in pool construction? For example, in regard to the pool depth, what is the accepted variation from the depth specified?

Answer:

We are not aware of specific tolerances for shotcrete in swimming pools. Shotcrete is a method of placing concrete and the cover over reinforcing steel should be the same as that for cast concrete. With respect to the depth of the pool, this would be a building code issue, not a shotcrete issue.

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Question 143:

An artist we have commissioned will be using gunite for the creation of a large-scale geode-inspired sculpture. There is some concern from the community about vandalism, specifically graffiti. Do you recommend sealing or applying anti-graffiti coating to gunite? If so, what brand of sealant or coating do you recommend?

Answer:

The ability to clean graffiti from the surface will, to some extent, depend on the finish texture. A rough texture will be difficult to coat successfully with a sealer or paint. Commercially available anti-graffiti paints have been used very successfully on shotcrete tanks with relatively smooth float finishes. We do not have any recommendations on the brand of sealer or coating.

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Question 144:

In placing shotcrete in layers, what is the recommended thickness of each layer?

Answer:

Appropriate thickness of the shotcrete layers is impossible to generalize because it depends on many factors, including:

  1. The type of shotcrete (wet- or dry-mix);
  2. The texture and stiffness of the receiving surface;
  3. The physical properties of the fresh concrete used, including a) w/cm ratio; b) slump; c) use of accelerator; d) type of supplementary cementitious materials used in the mixture (microsilica, fly ash, and slag); e) fibers used in the mixture; and f) mixture temperature;
  4. Weather conditions—Is it hot or cold, dry or wet, and/or windy or calm?;
  5. The shotcrete equipment used: a) type of nozzle; b) distance from the receiving surface; and c) air pressure and air volume;
  6. The orientation of the shotcreting (vertical/sloped/overhead)

Experienced shotcrete contracting firms using ACI Certified Nozzlemen have a wealth of experience in evaluating all these factors to achieve the proper results. You may consider subcontracting the shotcrete work to an ASA member contractor with experience in this type of work. You can submit your project details for bids from our ASA Corporate Members using the Web form at www.shotcrete.org. For further reference, ACI 506R-05, "Guide to Shotcrete," provides some general discussion of the shooting techniques that may be appropriate. Retaining an engineer or shotcrete consultant experienced in shotcrete application may be of value to assist in evaluating your specific factors and recommend the best solution.

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Question 145:

Can shotcrete be recycled?

Answer:

Shotcrete is concrete applied using the shotcrete process. Therefore, any recycling potential that applies to concrete would apply to shotcrete.

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Question 146:

We will be shotcreting the inside of a tunnel entrance. The plan is to apply 3 ft (0.9 m) of shotcrete on the walls and ceiling to support a large section of limestone rock 22 ft (6.7 m) high, 30 ft (9.1 m) wide, and 20 ft (6.1 m) deep above the tunnel at the entrance that has moved and is wedged and supported with an existing steel structure. We would like to test the shotcrete and are wondering what type and quantity of tests you recommend and which testing labs are in our area that would be able to conduct the testing?

Answer:

The article "Shotcrete Testing—Who, Why, When, and How" in ASA’s Summer 2011 issue of Shotcrete magazine should help answer most of your questions on testing of shotcrete. ACI 506R, "Guide to Shotcrete," and ACI 506.2, "Specification for Shotcrete," also have helpful information on shotcrete testing. Most competent testing labs should be able to test the compressive strength of cores extracted from shotcrete panels or sections, as they are very similar to concrete cylinder tests. If conducting more advanced testing, you may want to consider selecting a lab experienced with shotcrete.

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Question 147:

We are building 6 and 8 in. (150 and 200 mm) thick cast-in-place concrete retaining walls with No. 4 (No. 13M) reinforcing bar at 18 in. (450 mm) on center each way. We would like to change to shotcrete as an alternate method of construction. Does the reinforcing bar design have to change for shotcrete application?

Answer:

Shotcrete would be a great substitute for the retaining wall. Design-wise, the shotcrete is equivalent to concrete because shotcrete is really just a way of placing concrete.

The No. 4 (No. 13M) at 18 in. (450 mm) on-center spacing is not a problem. A No. 4 (No. 13M) bar can be easily encased by a qualified, experienced nozzlemen using either wet- or dry-mix shotcrete. In longer walls, or anywhere where reinforcing bars are lap spliced, the lap splice bars should be spaced apart. ACI 506R-05, "Guide to Shotcrete," Section 5.4, provides some good guidance on optimizing reinforcing bar layouts for shotcreting. On the issue of lap splices, it says: "If the design allows, direct contact of the reinforcing splices should be avoided. Non-contact lapped bars should have a minimum spacing of at least three times the diameter of the largest bar at the splice."

Thus, with No. 4 (No. 13M) bars in a lap splice, you should have the reinforcing bars spaced 1.5 in. (38 mm) apart at the splice to allow the shotcrete material to flow around the bar during shooting.

Chapter 8 of ACI 506R-05, "Guide to Shotcrete," also provides a lot of guidance on proper shooting techniques for a variety of applications, including walls.

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Question 148:

I have a question regarding the oscillator on a shotcrete rig. When applying shotcrete, does the oscillator serve any purpose other than uniform application? I’m searching for the main reason to use an oscillator and am wondering if the integrity of the shotcrete would be compromised if it were disabled?

Answer:

Assuming this is an oscillator on a robotic arm, it should not be disabled. Good nozzling technique, for either wet or dry, requires the nozzle to be moved in a constant overlapping circular pattern. This allows for better encapsulation of reinforcing bar and produces a more uniform surface; and, particularly for dry process, it is required for final mixing of materials that occurs on the surface.

Without proper nozzle technique, which requires oscillation, you will not get uniform, homogeneous shotcrete.

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Question 149:

We will be blasting close to a shotcrete wall. Can you suggest any precautionary measures or offer lessons learned when blasting next to a recently shotcrete wall?

Answer:

It is very common in drill and blast operations to blast shortly after the application of shotcrete. There are certainly risks involved, but a knowledgeable and experienced mining crew working with or for a knowledgeable, experienced contractor would not have any problem with this type of application.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

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Question 150:

We are considering a shotcrete lining of a new corrugated metal pipe to improve the smoothness and hydraulic capacity. What is the minimum thickness over the corrugations and should we be looking at any reinforcement or studs to support the shotcrete?

Answer:

This has been done in the past to improve hydraulic capacity and provide better wear resistance in the invert. Typically, the minimum cover over the corrugation is 2 to 3 in. (51 to 76 mm) with a welded-wire fabric either welded or otherwise attached to the corrugated pipe. The cover could likely be reduced with the use of structural fibers of either steel or synthetic material. Steel fibers and wire mesh should not be used together. Care must be taken to specify the required finish. This application would likely benefit from a smooth trowel or light broom finish. A light broom finish is preferable from a safety standpoint, as a trowel finish creates a very slippery surface both during construction and for the maintenance at a later date. In addition to the hydraulic and wear characteristics, once shotcreted, the entire pipe will become a composite section with improved structural characteristics. It should be noted that the pipe must be large enough for workers to work in safely and productively. This would mean an absolute minimum of 48 in. (1219 mm) and preferably larger.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

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Question 151:

We are a local agency considering the repair of a number of older culverts with shotcrete. Like most agencies, we are trying to be creative about maximizing our funds. We work with a federal agency when it is determined that a "new" culvert is needed. In other words, the agency will not pay for maintenance repairs but will pay for "new" culverts. We are wondering if the shotcrete method has ever been viewed as a means of creating a "new" culvert. Could the existing culvert be considered as merely a form for the new culvert? Could you also speak to the life expectancy of shotcrete (life cycle) versus a new concrete culvert?

Answer:

Shotcrete has been used extensively for the purpose of relining existing culverts. You are correct to visualize the existing culvert as a form for building a new structure. Because it is a stay-in-place form, it may actually act as a composite structure. Shotcrete is a method of placing concrete and will have similar, if not better, durability and life span if installed professionally with good mixtures. You can locate numerous past articles on durability of shotcrete that have appeared in Shotcrete magazine in the magazine’s archive on the ASA website, www.shotcrete.org. Similar work has been done in California, Colorado, and other states. This approach is currently being used as permanent tunnel lining in many places, including many of the current New York Transit projects.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

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Question 152:

I have been asked to recommend repairs to a fire-damaged brick wall. The wall is 12 in. (30.5 mm) thick and 14 to 16 ft (4.25 to 4.9 m) high. The fire caused spalled brick—3/8 in. (10 mm) deep—and soft mortar joints. The damaged side of the wall is exposed to weather. I plan to recommend tuck-pointing the mortar joints but am wondering if shotcrete is appropriate to repair the spalled brick. The brick could be cut out and replaced, but shotcrete would seem to offer the advantage of repairing and reinforcing the brick wall.

Answer:

Shotcrete would be an excellent process to repair or overlay your wall. You are correct in saying that it could not only repair but also reinforce and enhance the strength of the wall system. It is important to remove all deteriorated brick and sandblast or water-blast the surface if you are looking for a good bond between the shotcrete and the existing brick. Dowels epoxied or grouted into the existing brick are often used to mechanically tie the shotcrete overlay to the brick wall and also stabilize the new reinforcing steel in the shotcrete overlay.

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Question 153:

Which method of placing concrete provides a longer service life—traditional cast-in-place concrete with two-sided forms or shotcrete?

Answer:

Shotcrete is a placing method for concrete. Wet-mix shotcrete will be very similar in density to fully consolidated concrete when the concrete mixture designs are similar. Properly mixed and shot, dry-mix shotcrete may have a slightly higher density. Properly designed, placed, and cured, both concrete and shotcrete will give an excellent service life.

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Question 154:

I have a swimming pool that appears to have shrinkage cracks in the floor. I have tried to inject an epoxy, but the cracks are too small. Do you have any suggestions?

Answer:

There are a wide variety of epoxies and polyurethanes used for crack injection. Smaller crack widths would require a lower-viscosity material to penetrate the crack. You should contact an engineer or injection specialist experienced in shotcrete and cracking issues to evaluate the cracking and make a specific recommendation for repair. Proper concrete mixture design, placement techniques, and early water fogging and curing can help to reduce plastic shrinkage and drying shrinkage cracking in the future.

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Question 155:

To keep shrinkage cracking sufficiently tight so as not to cause reflective cracking in the plaster layer, what is the proper depth of reinforcing steel from the shotcrete surface in swimming pool applications?

Answer:

This question should be addressed by a qualified engineer with experience in designing swimming pools and well-versed in shotcrete technology. Shotcrete is a method of placing concrete and the parameters that work for concrete cover work for shotcrete.

Good practices for placing shotcrete or concrete include:
1. Predampening the soil that the concrete/shotcrete is placed against;
2. Ensuring that the reinforcing bar temperature is not too high; and
3. Early curing of the shotcrete surface and maintenance of curing for the specified duration. If no duration is specified, a 7-day wet cure is recommended.

Using fibers in the shotcrete/concrete can also help control surface early-age plastic shrinkage cracking.

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Question 156:

Can shotcrete be effectively used to repair holes in an old 8 ft (2.4 m) diameter storm sewer tunnel constructed of stone/brick/mortar? One of the holes is completely worn through to the earthen backfill material. The other two holes are missing the innermost layer of stone masonry, but the outer layer of masonry is still in place.

Answer:

The friction coefficient n of well-finished shotcrete for use in Kutter’s equation (and, more streamlined, Manning’s equation) is generally used as 0.012. Shotcrete is used not only to improve flow characteristics of brick, corrugated metal, or any other pipe construction but can also be conventionally reinforced as a structural liner to eliminate the need for liner plates or other pipe-lining alternatives.

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Question 157:

A circular concrete tank built in the 1980s is to be resurfaced due to cracks in the exterior shotcrete lining. It has been proposed that the existing surface will be hydroblasted. Is there a concern that the shotcrete may contain asbestos?

Answer:

No, there is no reason to assume that the shotcrete would contain asbestos. Shotcrete linings typically contain sand and cement. Asbestos was commonly used for pipe insulation and high-temperature industrial uses and not for shotcrete.

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Question 158:

We are a shotcrete contractor in Gold Coast, Australia. We have noticed that in the United States, you use different types of tools for cutting the shotcrete. How do you maintain a plum wall with the shotcrete rods? When a project requires a smooth finish or steel trowel finish, what are the tools and processes that are typically used? Finally, for a structural wall, what is the typical psi (MPa) and size of aggregate used?

Answer:

The face or surface of shotcrete walls as described are typically established with ground wires or screeds, which assist the person using the shotcrete rod in cutting the wall to the proper plane. The tools typically used to achieve a troweled surface are the shotcrete rod, wood floats, and steel trowels. Typically, shotcrete walls are a minimum of 4000 psi (27.6 MPa) 28-day compressive strength and the aggregate varies from sand only to a blend of sand and 0.375 to 0.5 in. (9.6 to 13 mm) aggregate. Consult our Shotcrete magazine archives for examples at www.shotcrete.org.

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Question 159:

In regards to the strength of shotcrete and weather is there a Mine Safety and Health Administration (MSHA) regulation requiring our paste cylinder sample results to be above a certain psi (MPa)?

Answer:

Shotcrete is normally expected to meet or exceed 4000 psi (27.6 MPa). We are not aware of any specific MSHA requirements. We would suggest you consult a tunnel or mining engineer who is well-versed with shotcrete. Cores taken from field-shot test panels are generally used for the evaluation of compressive strength of shotcrete (ASTM C1140/C1140M-11).

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Question 160:

We plan to use fiber-reinforced (polypropylene fibers) shotcrete as a brown coat for stucco (three-coat stucco) over a concrete shear wall. Does a maximum thickness of 1.5 in. (38 mm) of shotcrete require any mechanical anchor/connection, or is the bonding strength of the shotcrete layer to the concrete shear wall substrate sufficient?

Answer:

The addition of fiber will not increase the bond of shotcrete to the concrete shear wall. A 1.5 in. (38 mm) thick layer of properly designed and applied shotcrete should have adequate bond to a properly prepared concrete substrate without additional mechanical anchors. However, exposure conditions, geometry of the wall, shrinkage potential of the shotcrete mixture, application technique, and curing—as well as the age and quality of the shear wall concrete substrate—may affect the bond. These factors should be considered by an engineer experienced with shotcrete overlays in deciding whether additional anchoring is advisable.

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Question 161:

What is the standard method for steel fiber-reinforced shotcrete (SFRS) testing? Are you supposed to core test panels or do you only do that for plain shotcrete? Additionally, is round panel toughness testing on SFRS standard today in the United States?

Answer:

SFRS is routinely cored from shotcrete test panels or in-place shotcrete linings without difficulty. The shotcrete should, however, have a minimum compressive strength of about 10 MPa (1450 psi) at the time of coring.
Round panel testing of flexural toughness of fiber-reinforced shotcrete to ASTM C1550 is often specified and used for quality control (QC) purposes in tunneling and mining projects in North America and elsewhere (for example, mines in Australia) virtually every day of the year.

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Question 162:

We have a client with a 6 1/2 in. (165 mm) thick reinforced concrete roof slab, the underside of which is in need of repair. There are places where the concrete has spalled, exposing reinforcing bar that has a 3/4 in. (19 mm) cover. There is efflorescence, and there is spalling that does not expose reinforcing bar and some at the steel supporting the concrete slab. In addition, there are hairline cracks and rust spots.
Is shotcrete a feasible overhead repair for this situation? What holds the shotcrete to the slab? What is the minimum thickness of shotcrete we should specify? Should we specify shotcrete to be used only at the spalls, cracks, and efflorescence or the whole underside of the slab? Do you have a shotcrete repair procedure that we can put in our specification?

Answer:

This type of repair is commonly done using the shotcrete process. The extent of the repair is an engineering issue, not a shotcrete issue—shotcrete can and is used for patches and overlays. The shotcrete will adhere to the properly prepared existing concrete. It is installed such that the weight of the plastic shotcrete does not;exceed the adhesion to the existing surfaces; if additional material is needed, it is added at the initial layer or layer set up. The minimum thickness is related to the material used for the repair and the need to establish cover on the existing or added reinforcing. Some repair mortars can be placed as thin as 1/2 in. (13 mm).
Please find a link to a paper on “Concrete Repair by Shotcrete Application".
The success of the shotcrete repair will be highly dependent upon using a qualified shotcrete contractor and doing an excellent job of preparing the surfaces. Where the reinforcing is exposed, you should require that it be chipped out the entire perimeter allowing for a space of 3/4 in. (19 mm) behind the reinforcing bar so that the repair material can completely encase the reinforcing.

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Question 163:

Is shotcrete a viable option to encase galvanized steel beams at a coal unloading facility to protect them from impact and abrasion? Will the galvanizing on the steel inhibit bonding?

Answer:

Yes, shotcrete would be suitable for this application. A well-installed shotcrete lining will be durable and protect the steel from impact, abrasion, and from the acid attack that occurs from sulfur in the coal. Shotcrete is used to cover both the steel hopper walls and to encase the steel beams. Calcium aluminate cement is typically recommended for coal bunkers because of the mild acid condition that occurs that can attack the steel. Whether or not the steel beams are galvanized or not is irrelevant because the shotcrete will not bond well enough to any steel surface without welded studs and mesh to hold it in place. The beams will need to have studs welded and mesh installed around the beams for the shotcrete placement. With galvanized steel it is often necessary to grind off a spot of the galvanized coating at the spot of each stud weld location to properly weld the studs.

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Question 164:

We are working on a geotechnical project in the northwest to repair an existing rockery retaining wall. The wall is around 750 ft (229 m) in length and up to 12 ft (4 m) in height. The issue is that some of the basalt boulders within the wall are weathered soft and falling apart. The total weathered rocks that are falling apart comprise approximately 7% of the wall. Can we use shotcrete on the weathered rocks to give them more stability as a repair process? If not, is there a process we can use with shotcrete to repair the wall without having to rebuild the entire wall?

Answer:

Shotcrete has been used in the Northwest to strengthen and overlay existing rockery walls. The need to remove the weathered material is dependent upon the need for the overlay to bond with the existing wall, which is an engineering issue and not a shotcrete issue. Shotcrete can and is shot successfully against soil and other weathered surfaces.

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Question 165:

I’m planning to add 6 in. (152 mm) of shotcrete to an existing 12 in. (305 mm) wall of a below-surface concrete tank to accommodate the removal of an existing middle support slab. The soil grade is approximately near the top of the existing tank wall. I’ve been told that since the existing wall is preloaded with soil, adding shotcrete will not increase the strength of the thickened wall and that the only way the wall will act as a whole (based on 18 in. [457 mm] thickness) is if the retained soil load is removed, then the shotcrete is added, and then soil is put back in place. Is this assessment accurate? Is there a way make this wall work as 18 in. (457 mm) without removing the existing soil?

Answer:

Stress distribution from external loads through the tank wall with the shotcrete lining will depend on the geometry of the tank and the structural function of sections to be removed. A professional engineer experienced in shotcrete and concrete tank design should be consulted to ascertain the structural capacity of the completed wall. It would certainly be important to create a good bond plane by roughening the surface and removing any loose or fractured materials and using sufficient drilled dowels to make the existing 12 in. (305 mm) wall and new 6 in. (152 mm) overlay work well together. Also, it might help to specify the use of a shrinkage reducing admixture.

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Question 166:

When was the 4000 psi (28 MPa) standard set for shotcrete?

Answer:

ASA has taken the position that structural shotcrete is shotcrete that meets or exceeds a compressive strength of 4000 psi (28 MPa). Looking at pertinent ACI Codes related to watertight concrete, as we would expect in a pool, we find ACI 318-95, “Building Code Requirements for Structural Concrete,” introduced a provision in 1995 that required: “Concrete intended to have low permeability when exposed to water shall have a Minimum f´c of 4000 psi (28 MPa)”. Similarly, ACI 350-01, “Code Requirements for Environmental Concrete Structures,” first issued in 2001 required: “Concrete intended to have low permeability when exposed to water, wastewater, and corrosive gases shall have a Minimum f´c of 4000 psi (28 MPa)”. Since ACI 350 is more directly applicable to water-containing structures, the 2001 date is probably the most relevant, though ACI 318 introduced the concept in 1991. We do, however, see shotcrete specified at lesser levels for different types of uses.

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Question 167:

We have a unique situation where we need to apply shotcrete around a steel plate that is surrounding a beam supporting a floor. Can you provide any UL listings for applying shotcrete to a steel beam, column, or plate?

Answer:

UL designs are typically for the hourly fire proofing ratings on structural steel members such as I-beams, wide flange beams, and vessel skirts. The beams and columns are tested for specific fireproofing products, beam sizes, and configurations. The thickness of the steel and other considerations factor in the evaluation; therefore, there is no blanket UL design number that you can use for steel plate. You can get guidance on the cover needed for different fire ratings in ACI 216.1-97/TMS 0216.1-97, “Standard Method for Determining Fire Resistance of Concrete and Masonry, Construction Assemblies.”
Remember that shotcrete is a process for applying concrete. You may also consider looking for a similar concrete UL design and submit it for consideration. Applying the shotcrete at a greater thickness to compensate for any variances should be proposed and presented to engineer or the owner for consideration.

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Question 168:

I need to specify a shotcrete cover to some structural steel in a coal dump hopper. The idea is to provide abrasion and impact protection to the steel beams. However, the client cannot afford to have the hopper out of service for an extended period. Is there a “high-early-” strength option for shotcrete as there is for cast-in-place concrete?

Answer:

There are prepackaged materials commercially available for impact and abrasion resistance. Please contact material suppliers from “ASA’s Buyers Guide” for product information: www.shotcrete.org/Buyers Guide.

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Question 169:

Is it critical for the early and intermediate compressive strength at 3 and 7 days, respectively, to be met for shotcrete applications for a rock fall face if the 28-day compressive strength is met?

Answer:

Compressive strength at 1, 3, and 7 days can be important to all for subsequent operations. In general, 7-day strengths are a good indicator of the ultimate 28-day strength. The need for early strength is an engineering and construction sequence issue, not a normal or typical shotcrete requirement.

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Question 170:

Can you send me a document with ASA specifications for gunite coverage of reinforcing bar for swimming pools, please?

Answer:

ASA does not have such a document. The concrete cover for embedded reinforcing steel is subject to the local Building Codes and may be increased by the structural plans and specifications produced by an Engineer or Architect for a specific project. ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” covers concrete structures intended for water containment and would be applicable to pools. The following is a link to ACI’s bookstore: www.concrete.org/BookstoreNet/ProductDetail.aspx?itemid=35006.

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Question 171:

I would appreciate if you could comment on a city of Los Angeles shotcrete code that requires that shotcrete lifts not exceed 3 ft (1 m) and that 3 hours must pass before the second lift can be applied.
First of all, if you waited 3 hours between lifts, you would have full-length cold joints along the whole length of the wall. You’d also have to wash out the pump after every lift or the concrete would harden in the pump and hoses. Second, you can’t leave a 4000 to 5000 psi (28 to 34 MPa) mixture sitting in the truck for 3 hours! Does it make any sense to you?

Answer:

This provision has been an issue for shotcrete contractors in the region for many years. ASA and ACI Committee 506, Shotcrete, do not endorse the concept stated in the “Los Angeles Bulletin.” Unfortunately, this provision has shown up in other areas around the country.
A good shotcrete practice is to limit lift height to that which can be placed without sloughing or sagging and to place subsequent lifts at such a time that the previous lift is sufficiently firm to support the subsequent lift. ACI 506R-05, “Guide to Shotcrete,” Section 8.5.8, specifically addresses this point. The following is a link to ASA’s bookstore: http://shotcrete.org/BookstoreNet/ProductDetail.aspx?itemid=506R-05.

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Question 172:

I am a Civil Engineer working on a hydropower project. Is it possible to place shotcrete at a thickness of 24 in. (600 mm) inside a tunnel that will be used as a water tunnel to generate power?

Answer:

Yes, it is possible to shoot 24 in. (600 mm) thick tunnel linings. There are various ways of doing this, depending on the reinforcing steel configuration. One method we have successfully used for shooting tunnel linings this thick with a double mat of reinforcing bar (1 in. [20 mm] diameter bars at 6 in. [150 mm] on center, vertically and horizontally) is to bench gun shoot the walls up to the spring line with a wet-mix silica-fume-modified shotcrete (without accelerator) and then ribbon-shoot (2 ft [0.5 m] wide strips) overhead using the same mixture but with the option of using an alkali-free accelerator added at the nozzle.
If the shotcrete requires a smooth finish (equivalent to a cast-in-place concrete finish), then the initial shotcrete is shot to within about 1 in. (30 mm) of the final shotcrete thickness and allowed to set and harden. Following that, a final non-accelerated finish coat can be applied that can be trimmed to shooting wires with a cutting rod, closed up with a darby, and then trowelled with either a magnesium or steel trowel, depending on the required finished surface texture.
Such work can be done with a remote-control manipulator arm (robot) or, for more precision, with hand nozzling out of a basket on a manlift (provided the tunnel floor is sufficiently smooth for operation of a manlift). The bottom line: hire a contractor who has experience in conducting such work.

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Question 173:

I would like to know the standard operating procedures for cleaning out shotcrete hoses with air and/or water and, in particular, how to keep the hose from whipping when using air.

Answer:

Shotcrete hoses can be cleaned out using either water or air. In many instances, the site conditions make cleaning with water not feasible. When cleaning with air, the free end or discharge end of the hose should be secured to something to ensure that the hose does not whip as the material and cleaning ball or rag discharges.

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Question 174:

We are working with an architect in New York City on an unreinforced masonry (URM) building where they want to remove brick to provide a larger storefront opening. I would like to use the remaining walls to resist lateral forces but the brick is insufficient. We would like to remove one width of brick and apply 4 in. (102 mm) of reinforced shotcrete in its place. Can you tell me where I can find applicable code and design guidelines for this application?

Answer:

Your proposed solution is certainly reasonable and is used regularly. Shotcrete has been used to strengthen both URM and tilt-up panels to accommodate enlarged openings. Shotcrete is a method of placing concrete and the in-place properties would be the same. The applicable code would be the code you would use if you were to strengthen this wall with concrete. Designs using the ACI 318 Building Code and Commentary are fully applicable to shotcrete placement, although compressive tests for acceptance are secured using cores from shotcrete test panels per ASTM C1140/C1140M and C1604/C1604M, rather than cast cylinders.

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Question 175:

Is there a specification with regard to cold joints when using shotcrete?

Answer:

Generally, the interface between sequentially placed layers of shotcrete is not considered a cold joint because the shotcrete abrasion, velocity of impact, and high paste content make excellent bonding conditions. Cores taken through layers of shotcrete on shotcrete often show that it is virtually impossible to ascertain one layer of shotcrete from the next. Please refer to ACI 506R, “Guide to Shotcrete,” for information on joints in shotcrete.

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Question 176:

We are in the process of designing retaining walls that will be supported by either concrete piers or steel piles. We would like to see some typical details on how the reinforcing is secured to either the piers or piles.

Answer:

For concrete piers, the reinforcing steel is generally secured to the piles with reinforcing bar grouted dowels. For steel piles, the reinforcing bar is generally secured with Nelson studs.

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Question 177:

I am evaluating a community in central Colorado that contains shotcrete slope reinforcement ranging from 14 to 44 ft (4 to 13 m) in height. Assuming the installation met all required guidelines, what should I anticipate as a useful life for this product?

Answer:

Shotcrete is a method of placing concrete and properly placed shotcrete should have a service life similar to cast concrete. Generally, concrete structures in normal environmental exposures are expected to have service lives from 50 to 100 years. With particular attention to materials and construction methods, some concrete structures, such as the new San Francisco Bay Bridge, have been designed for a service life up to 150 years. The first step in achieving a long-lasting, high-quality installation is to engage a highly qualified and experienced shotcrete contractor. There are many other factors that influence service life, including using the right mixture design for the anticipated exposure conditions.

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Question 178:

Is there is a manufactured depth gauge that would be glued/nailed to the form to allow the nozzleman to physically see how much concrete is being applied to the surface? We have a condition where there will be two or three applications on the same surface, and I am concerned that the correct depth is not being applied in each pass.

Answer:

ACI 506R-05, “Guide to Shotcrete,” Section 5.6, on Alignment Control (refer to ASA Bookstore: http://shotcrete.org/BookstoreNet/ProductDetail.aspx?itemid=506R-05) gives specific guidance on proven methods to establish the line and grade of the surface, as well as proper material thickness and cover. Common methods are use of ground wires, guide strips, depth gauges, and depth probes. Please refer to “ASA’s Buyers Guide” (http://shotcrete.org/BuyersGuide) and contact one of our members who provides supplies to the shotcrete industry

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Question 179:

We are working on a renovation of an existing shopping plaza where some of the existing walls are split face block. Would it be an acceptable application to resurface the block with shotcrete to achieve a smooth finish? If so, what is the thinnest we would be able to go?

Answer:

Shotcrete could be used for this application. The thickness of the overlay would be dependent on the material used. A potential concern would be the lines of the existing block showing on the new surface. We would suggest that you search for and review various ASA Shotcrete magazine articles as well as ACI 506R-05, “Guide to Shotcrete”

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Question 180:

We have a two-story shotcrete wall enclosing an indoor community pool. We are specifying a board-form finish for the interior and the exterior will have a parge finish coat. Are there any issues with the consistent moisture from the pool that should be addressed in the concrete mixture or topical sealant? How should we deal with the exterior versus interior finishes in regards to water intrusion protection and allowing the green concrete to “dry out” over time?

Answer:

Shotcrete is a method of placing concrete and the characteristics of shotcrete are those of cast concrete. Although the enclosed swimming pool will increase the interior humidity, the high humidity should have no detrimental effects on the exposed shotcrete, and may even be beneficial in reducing long-term drying shrinkage of the wall.
Both cast-in-place and shotcreted concrete are commonly used for construction of water tanks with constant exposure to water under significant hydraulic pressure. Using good construction techniques with good-quality concrete to build the tank’s walls produces walls with no moisture evident on the exterior face of the tanks. Simply having a high-humidity atmosphere is a much less severe exposure and should not result in interior air moisture being transmitted into and through the shotcrete wall. Any coatings considered for aesthetics should follow the manufacturer’s recommendations for drying time of concrete before application. If there is a concern about the permeability of the shotcrete wall, a premium shotcrete mixture including silica fume might prevent some issues on this application.

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Question 181:

What difference would there be in the density of shotcrete before and after shooting? Is there any shotcrete mixture-design software in SI units available? Or any document of shotcrete mixture design in SI units for optimizing shotcrete design?

Answer:

Shotcrete is simply a placing method for concrete. Thus, the mixture design and material properties are the same as concrete. We are not aware of any software specific to shotcrete in any units. ACI 506R-05, “Guide to Shotcrete”, has guidance on desirable mixture characteristics (aggregate grading, supplemental cementitious material [SCM], and so on) that would be helpful in developing a concrete mixture design for shotcrete placement.

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Question 182:

I am a structural engineer working on underground structures such as tunnels and caverns. I would like to know the permissible shear strength of shotcrete to be taken for M30 Grade SFRS (M30 = 30 MPa [4350 psi] at 28 days). I would like to know more about its other properties, as well.

Answer:

Shotcrete is simply a placing method for concrete. Thus, the in-place material properties are essentially the same as cast concrete. A specific value for the shear is beyond the scope of our Association because many design and material properties can affect the shear capacity. We would suggest you engage a Professional Engineer who specializes in Underground Shotcrete. You should consult our Buyers Guide to find such a consultant. ACI 506R-05, “Guide to Shotcrete”, would be a helpful primer to learn more about shotcrete.

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Question 183:

I have a new construction project where I want to apply shotcrete to cast-in-place concrete columns and an elevated, post-tensioned concrete slab as a finish material. The finished application is intended to be in varying depths from 3 to 12 in. (76 to 305 mm) or more. The desired end result is a smooth, curvilinear, sculptural form. Is this type of application achievable?

Answer:

Shotcrete can and has been used to increase the size of columns and thicken overhead slabs while providing great-looking linear or curvilinear finishes. Examples of curvilinear finishes can be found in past Shotcrete magazine articles. You can search the Shotcrete magazine archives.

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Question 184:

We just shot a wet-mix swimming pool for a customer. The shallow end depth starts at 39 in. (991 mm) to the top of the beam and over 10 ft (3 m) linear slopes down to 54 in. (1372 mm). From there we maintain our 1 to 3 ft (0.3 to 0.9 m) slope down to 8 ft (2 m) for the diving end of the pool.
The customer would like to raise the entire shallow end pool floor up to the 39 in. (991 mm) depth. We prefer to use wet-mix shotcrete. The overlay would be tapered from the 39 in. (991 mm) start to 15 in. (381 mm) thick at the 54 in. (1372 mm) depth. What would you recommend for this overlay to bond and not “pop loose” or cause crack transfer to pool plaster?

Answer:

The proposed overlay will be similar to any repair where shotcrete is placed over existing concrete. Proper surface preparation is essential for allowing good bond. Guidance on surface preparation can be found in ACI 506R-05, “Guide to Shotcrete”. It also appears you are suggesting tapering the thickness from 15 to 0 in. (381 to 0 mm). Feathering thickness down to 0 in. (0 mm) is not encouraged, and a minimum thickness should be established. Because the overlay section will be quite thick and experience differential shrinkage from the previously shot material, the overlay will require additional reinforcement to accommodate temperature and shrinkage stresses. You should consult with an engineer experienced in shotcrete design to establish the proper amount of reinforcement. The required reinforcement and cover over the reinforcement will control your minimum overlay thickness.

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Question 185:

I am interested to know if any shotcrete contractors have shot a magnesium phosphate material (dry-process) before and, if so, could you detail the special requirements necessary in placing such a unique product?

Answer:

Phosphate-bonded refractory materials were routinely shot in cyclone boilers in the 1970s. These phosphate-bonded materials don’t have a cement bond, but achieve a chemical bond when heat is applied. Without knowing the precise formulation of the mixture and grain sizes involved, we cannot tell you definitively that your specific material can be shotcreted. However, there is a long history of successful past experience with phosphate-bonded refractory materials being shot with the dry-mix process. You may want to consider a field trial before construction to verify your specific mixture works with your dry-mix shotcrete equipment.

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Question 186:

I am an engineer working on a project involving shotcrete and earthwork. The shotcrete that was placed has some expansion cracks, which we expected. I would like to know the best way to repair them. Is there some type of waterproof coating/grout that can be applied between the cracks? Part of the cracks will be continuously under water. The shotcrete is the surfacing material for a diversion ditch at a mine, and we need to recommend some remediation solutions to our client.

Answer:

There are many products in the marketplace for repairing cracks. Because shotcrete is simply a method for placing concrete, any method for concrete crack repair would be applicable. It would be wise to use a product that filled the cracks and is able to tolerate thermal movement in the future (not a brittle product). Many injectable polyurethane grouts can accomplish this. Surface-applied coatings would need an adequate thickness and elasticity to tolerate moving cracks. We suggest that you contact one of our corporate members who is familiar with your area and get their specific advice. Please refer to “ASA’s Buyers Guide”.

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Question 187:

How might one add fibers to a gunite (dry-mix) application? I have heard of some companies adding them by hand at the base of the auger and others who poured them over their sand and mixed them in with a loader before loading it into the truck. Is there a more efficient way to add them to a dry mixture so that they are distributed evenly throughout?

Answer:

Many of our members add them by hand at the mixer and have had good success when using an adequate mix time. Another method is to have the mix blended at a bag mix plant with the fibers.

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Question 188:

We have a 17 mile (28 km) long TBM tunnel for water that will drive our underground powerhouse. Is there a recommended shotcrete surface texture we could use? Our contractor is using 0.31 in. (8 mm) aggregate, but they are getting an undulating surface. Can you provide some clarity as to what we should ask our contractor to try and achieve?

Answer:

Shotcrete can be applied with many different textures. The nozzle finish shown is very rough, even for a natural gun finish. Nozzle finishes can be done smoother than this. Another technique would be to use a broom to make it smoother after it is shot. Other finishes include wood float, rubber or sponge float, broom, and smooth trowel finishes. There are many examples of finishes shown in articles in Shotcrete magazine.

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Question 189:

I am an engineering technologist working on a landslide project where shotcrete had been applied to stabilize the sandstone head scarp at the crest of the slope. The shotcrete was applied in 1998. After a recent inspection, it was noted that the surface of the shotcrete had some cracking in some sections. How can this be repaired? Can the cracks simply be filled with a grout/mortar mixture of some sort or do the cracked sections have to be removed entirely and shotcrete be reapplied?

Answer:

Shotcrete can and has been used to overlay previously installed shotcrete or concrete that has cracked over time. It would be advisable that you engage an engineer knowledgeable in geotechnical engineering and concrete properties to formalize a solution. It is important that the cause of the cracks be determined and adequate reinforcing be designed to ensure that the cracks do not propagate through the overlaid shotcrete.

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Question 190:

I am a structural engineer and I am supposed to design structures for shotcrete applications. Should I calculate and check its stability by the “working stress method?” Or, could I use the “ultimate limit design?” Are there regulations or specifications about the application of method on ACI? Finally, is elastic coefficient different between normal concrete and shotcrete?

Answer:

Shotcrete is a method for placing concrete. Thus, the concrete placed by the shotcrete method has the same physical properties as cast concrete with the same mixture proportions. Either working stress or ultimate strength methods used for concrete design are applicable. Local building codes may require a particular design approach.

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Question 191:

I have a customer who would like to place 2 in. (51 mm) of shotcrete onto our geotextile canal liner, which has been used for many years with 2 to 4 in. (51 to 102 mm) of shotcrete. In all of these previous projects, contraction joints were installed. For this project, the customer is asking whether this is an absolute requirement, as the geocomposite canal liner beneath is the water containment component. Does it make a difference in terms of cracking and joints whether the shotcrete is 2 or 4 in. (51 or 102 mm) thick? What is the typical finishing that is done on canal projects?

Answer:

Long expanses of concrete canal lining exposed to the sun and weather would experience significant internal tensile drying shrinkage stresses. Regular contraction joints help to relieve the internal tension created by concrete shrinkage. If no contraction joints are provided, shrinkage will still occur and the concrete lining will produce its own contraction joints, better known as “cracks.” Unfortunately, the resulting cracking will be random and can vary significantly in size and length. Thus, contraction joints are a good approach to help induce cracking at regular, controlled locations. If the client doesn’t want contraction joints, they need to understand that cracking will be much more extensive and likely more noticeable.
Theoretically, with the same percentage of embedded reinforcement, cracking between a 2 or 4 in. (51 or 102 mm) should not be substantially different. Of course, the 4 in. (102 mm) thick shotcrete section would require twice the concrete material and twice the embedded reinforcement to maintain the same percentage of reinforcement. A 2 in. (51 mm) thick section could have some difficulty in maintaining adequate cover over embedded reinforcing bars. The designers could also consider using fiber-reinforced shotcrete to help control shrinkage and temperature stresses, although fairly high dosages are needed for effective elimination of reinforcing bars. More guidance on fiber-reinforced shotcrete is available in ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete”. A 2 in. (51 mm) overlay is absolutely the least possible and 3 or 4 in. (76 or 102 mm) is far more normal in practice.
Canals are generally specified to have a natural gun finish, a rough broom finish, or a light broom finish.

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Question 192:

I have been asked to come up with a 5000 psi (35 MPa) in 24 hours shotcrete mixture, using cement, fly ash, silica fume, and fine aggregate. I need some advice on a mixture.

Answer:

Design of a concrete mixture to be placed by the wet-mix shotcrete method is essentially the same as normal cast-in-place concrete mix design. The major differences with shotcrete mixtures are:
• The maximum coarse aggregate size is generally limited to about 3/8 in. (9.5 mm);
• They use a fairly low water-cementitious material ratio (w/cm) and slump to allow shooting on vertical surfaces without sloughing;
• The potential to use an accelerator that can be added at the nozzle; and
• The pumpability is an important workability characteristic.
Since you desire a high-early-strength mixture, using fly ash as a supplemental cementitious material (SCM) wouldn’t be recommended because it slows set and strength gain at early ages. Microsilica may be beneficial for early strength gain. Consideration should be given to using accelerator added at the nozzle. There is some guidance on concrete mixture design in ACI 506R-05, “Guide to Shotcrete"; however, because local materials (aggregates, cements, SCMs) can vary significantly, you should consult with an engineer or concrete testing laboratory familiar with shotcrete to produce and test a mixture design to meet your requirements.

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Question 193:

Can shotcrete be used to help seal a leaking pond? We have a 1.5 acre (6070 m²) pond that we are in the process of completing. We spread 90,000 lb (40,823 kg) of bentonite in, but the bentonite washed off the steep banks and now we are stuck with a half-full pond. Would shotcrete be a practical solution for our problem?

Answer:

Properly designed shotcrete (both concrete materials and reinforcing are important in the design) placed by an experienced shotcrete contractor can certainly be used to provide a somewhat watertight lining for your pond that will be serviceable, durable, and require little to no maintenance for decades to come. We would suggest you consult with an engineer or shotcrete contractor experienced in this type of shotcrete work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

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Question 194:

We are in the process of building a shotcrete pool and are required to wet-test the pool before set, waterproofing, and tile. What is the expected water-loss percentage? We are required to achieve 1%.

Answer:

We are not aware of a specific standard for pools. However, ACI 350.1-10, “Specification for Tightness Testing of Environmental Engineering Concrete Containment Structures,” specifies a volume loss of 0.05% of volume per day conducted over a 72-hour test period for “hydrostatic tightness testing of open liquid containment structures. Specifics of conducting the test can be found in the ACI 350.1-10 document.
It should be noted that the pool should be filled and allowed to saturate for 3 days before beginning the measurements.

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Question 195:

We are working on a project with a wall that requires additional capacity due to increased loading requirements. We are contemplating shotcrete with additional reinforcing to provide additional thickness for the wall.
Is it possible to achieve a composite wall to design for a thicker section for bending, using the bond of the existing concrete and shotcrete along with a reinforcing bar hook anchor epoxied into the existing wall? Any information you can provide would be appreciated.

Answer:

Shotcrete is often used in similar applications. The question of bending is a structural engineering question. Shotcrete is a method of placing concrete and the properties of shotcrete are similar if not the same as cast concrete. To achieve a composite wall, you must ensure that the existing surface is properly prepared to maximize the potential bond between the overlay shotcrete and the existing wall. Shotcrete placed against a properly prepared existing wall should achieve great bonding strength without the use of bonding agents. Drilled and grouted dowels also contribute to the system, working as a composite wall.

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Question 196:

I am looking for some technical assistance concerning temperature guidelines for shooting gunite pools. Is there a suggested range of air temperature and humidity that is recommended? Thanks for your help!

Answer:

Shotcrete is a method of placing concrete. The basic guidelines for placing concrete or shotcrete can be found in documents ACI 305R-10, “Guide to Hot Weather Concreting,” and ACI 306R-10, “Guide to Cold Weather Concreting.” There are some basic “rules of thumb,” such as 40°F (4°C) and rising for starting concrete/shotcrete operations and 40°F (4°C) and falling for stopping concrete/shotcrete operations. With proper planning and procedures, concrete or shotcrete can be placed at below-freezing temperatures and at very hot temperatures, but only with proper planning, procedures, and likely at some cost.

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Question 197:

What is the fire rating information for shotcrete?

Answer:

Shotcrete is concrete, pneumatically applied. So the same fire ratings for concrete would apply to shotcrete. The standard is ACI 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

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Question 198:

I am doing an owner/builder pool. The shotcrete company I hired left an approximately 4 x 4 in. (102 x 102 mm) hole in the deep end of the pool on the side wall. What is the proper way to patch this hole prior to plastering? (The shotcrete was applied a month ago.)

Answer:

The normal repair is to pressure wash with at least 3000 psi (21 MPa) of water pressure to remove any dirt and laitance on the surface of the concrete. Given the rather small size of the hole, it can be hand-patched with a nonshrink hydraulic cement with at least 4000 psi (28 MPa) 28-day compressive strength to plug the hole. After the patch is completed, roughen the surface that will receive the plaster.

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Question 199:

The Los Angeles City Bulletin states that no bars over No. 8 (No. 25) shall be used. The structural engineer has No. 10 (No. 32) bars in the columns. I am being told the test panel will get this approved but my City Inspector is balking a little. Is there a publication or code somewhere that allows the test panel to supersede the LADBS Bulletin?

Answer:

The International Building Code (IBC), Section 1913, allows for larger bars as long as it is demonstrated in a Preconstruction Test Panel. However, the Local Building Code likely takes precedence over the IBC. You may want to present IBC Section 1913, which requires anything over a No. 5 (No. 16) bar to be proven in a Preconstruction Test Panel.
There have been many projects shot in Los Angeles County subject to the LADBS with bar sizes larger than No. 8 bars. ASA is not in a position to give you project references, but perhaps our local members can.
Properly encasing No. 10 (No. 32) bars can be challenging, and should only be attempted by qualified contractors using ACI Certified Nozzlemen who have previous successful experience doing this type of work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

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Question 200:

What are the common standard test methods to measure or assess the permeability of shotcrete?

Answer:

Boiled absorption and volume of permeable voids testing (ASTM C642) may be required for structures that need enhanced liquid-tightness or resistance to aggressive environmental exposures. The test is sometimes used to provide an overall indication of the quality of the shotcrete mixture, particularly in dry-mix. However, many factors, including admixtures and aggregate, as well as shotcrete placing, can affect the porosity of shotcrete, so it should not be considered an absolute measure of shotcrete quality. When required, the mean average of tests on three specimens from a test panel, or from in-place shotcrete, should be less than or equal to the specified boiled absorption and/or specified volume of permeable voids limits at the specified test age with no single test greater than the specified boiled absorption plus 1%.

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Question 201:

We are evaluating a school building with a 4 in. (102 mm) thick dome roof with a diameter of 120 ft (37 m) bearing on a 5 in. (127 mm) thick perimeter shear wall. The roof is constructed by anchoring a membrane to the top of an exterior finish wall, inflating the membrane, shooting foam insulation to both the interior wall and membrane surfaces, and then shotcreting both the wall and roof structural elements. The contractor specifies to shotcrete only half of the concrete thickness of the walls and roof, install the reinforcing, and then shotcrete the remaining wall and roof to finish thickness.
a) Will the shotcrete elements installed in separate layers over the entire roof and perimeter wall system act as effectively as elements that have been shotcreted in one layer?
b) Will bonding between layers under gravity be a concern during the curing process under the dome roof?
c) Would alternate types of construction joints be more advantageous in fabrication of the wall and dome roof?
d) Would any admixtures be recommended for this application?

Answer:

a) The shotcrete will act as a single layer when it is finished. The bond between rough layers of shotcrete is very good. This has been documented in research done at Brigham Young University.
b) No, bonding will not be a concern because, as indicated previously, the shotcrete bond between layers is excellent.
c) No, other types of construction joints are not really viable. This is the best procedure to construct a shotcrete dome.
d) No admixtures are specifically required. Use of silica fume (microsilica) as a supplemental cementitious material may be advantageous in shooting overhead. An experienced shotcrete contractor would identify whether use of silica fume or accelerator are appropriate for their materials and equipment.
You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

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Question 202:

I have a newly constructed in-ground pool in which shotcrete was used. The pool has been holding dirty water since just after the shotcrete cured. Does the shotcrete have to be cleaned and/or treated before an overlay is applied?

Answer:

To ensure a good bond between the shotcrete shell and the overlay, the surface should be cleaned and allowed to dry before application of the overlay material.

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Question 203:

We are using wet-mix shotcrete for culvert linings, with an existing corrugated steel plate pipe stream culvert. The pipe is 96 in. (2438 mm) long and deteriorated. There is a water diversion, but there is a pressure gradient forcing water through the voids. Any ideas on leak repair procedures?

Answer:

Installing a shotcrete lining requires a somewhat dry substrate and certainly is not compatible with running water. The water needs to be blocked or diverted.
A means of blocking the inflow is to inject a swellable urethane grout through the openings in the existing pipe. The grout, if done properly, will expand upon contact with water and seal the outside of the pipe. Another means of diverting the water is to install drainage material over the inflowing area to collect the water and remove it from the pipe. The shotcrete can then be applied over the drainage material.

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Question 204:

I need to find the reference in ACI standards indicating the technical and practical reasons why thermal expansion joints and contraction settings are eliminated in the stabilization of nonstructural slopes covered with shotcrete and steel fiber. Can you help?

Answer:

Shotcrete is a method of placing concrete. Fibrous shotcrete will have very similar, if not identical, properties as fibrous cast concrete. Expansion and contraction joints should be similar in shotcrete to those needed in cast concrete. ACI 224.3R-95, “Joints in Concrete Construction,” covers joints in many different applications. The closest relevant document for eliminating joints is ACI 360R-10, “Guide to Design of Slabs-on-Ground,” where, in Section 8.3, it states:
“To eliminate sawcut contraction joints, a continuous amount of reinforcement with a minimum steel ratio of 0.5% (PCA 2001) of the slab cross-sectional area in the direction where the contraction joints are eliminated is recommended.”
This 0.5% reinforcement is consistent with the provisions of ACI 350-06, “Code requirements for Environmental Engineering Concrete Structures and commentary,” for the minimum reinforcement for temperature and shrinkage without contraction joints.
You can refer to ACI 506 series documents regarding shotcrete, and possibly the FHWA SA-96-069R “Manual for Design, Construction, and Monitoring of Soil Nail Walls” for additional guidance. Copies of the ACI 506 series documents are available in the ASA Bookstore.

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Question 205:

What is the R-value per inch for shotcrete without any integrated insulation?

Answer:

Shotcrete is concrete, pneumatically applied. So the same R-value fire ratings for concrete would apply to shotcrete. The standard is Joint ACI - TMS 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

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Question 206:

My company has been using the gunite process (dry-mix shotcrete) for years now. What I have been finding lately is that a lot more questions are being asked by outside safety services, neighbors to our facility, etc., about the health effects of the shotcrete process. I believe that with the new proposed laws dealing with silica, everyone is paying more attention to products with sand and cement, and shotcrete has both. To try to educate myself and to answer these questions I am on the search for enlightenment and am coming up short.
Here is where I am falling short: there is no (or I haven’t found a) general material safety data sheet (MSDS) on shotcrete. Most MSDSs I have found on the Internet are for proprietary mixtures. I have yet to find an MSDS or any safety info on just plain sand and cement mixture.

Answer:

Shotcrete is a method for placing concrete, so an MSDS for concrete or its constituent components would be appropriate. MSDS sheets for cement, aggregates, and concrete are readily available from cement manufacturers, aggregate suppliers, and concrete producers, as evidenced by a simple web search. This is the type of issue that can be discussed and effectively addressed by networking with other contractors and suppliers in the shotcrete industry. This is a primary benefit of actively participating in ASA—you or your organization should join ASA and attend committee meetings. ASA meetings are held three times a year. Check our Calendar for the next available meeting.

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Question 207:

We are designing a 41 x 60 ft (12 x 18 m), 2.5 to 4.5 ft (0.8 to 1.4 m) deep swimming pool. The walls and floor are 8 in. (203 mm) thick with No. 4 (No. 13) bars. We are specifying shotcrete to build the pool. What should be the minimum spacing for the expansion joint?

Answer:

You need to address this question to a professional engineer who is knowledgeable in the characteristics of shotcrete and concrete. Shotcrete is a method of placing concrete and the same parameters used in concrete design apply to shotcrete placements. Many pools of this size are designed and built without expansion joints, but it is beyond the scope of our association to provide further guidance.

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Question 208:

We are developing a tunnel. At the tunnel portal (entrance), we have high walls around the portal about 60 ft (18 m) tall. They will have an inner structural shotcrete layer (4 in. [102 mm]) and outer architectural shotcrete (12 in. [305 mm]). Between the structural shotcrete and rock/soil, we have a drainage system to handle the groundwater. At the same time, we may have water at the top ground surface that will drain from top to bottom of the wall.
The owner didn’t want to make the water flow as a sheet over the wall surface. We proposed an inlet and vertical 6 in. (152 mm) pipe drop from the top to bottom and band to a ditch at the base of the wall. Can we locate the 6 in. (152 mm) pipe between the structural shotcrete and the architectural shotcrete?

Answer:

The Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls” should address this issue. Many soil nail wall systems incorporate a drainage ditch at the top of the wall that catches the runoff and takes it to the ends of the wall. Your concept of a catch basin and drain between the layers is not something we have seen in the past and we are not qualified to express an opinion on this. We have seen systems with catch basins at the top of the wall and the drains behind the initial layer of shotcrete requiring notching the subgrade. To answer your question, yes, a 6 in. (152 mm) pipe can be fully encased in shotcrete between the layers. Complete encasement of an embedment of this size needs an experienced shotcrete nozzleman with properly sized equipment, appropriate concrete mixture design, and a trained shotcrete crew. The issue of appropriateness of the approach is better answered by a licensed professional engineer familiar with soil nail systems or retaining walls, and shotcrete/concrete.

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Question 209:

I am currently involved in the design of several long retaining walls. One option under consideration is the use of soil nails with shotcrete reinforced by welded wire fabric (WWF) and the other is the use of mechanically stabilized earth (MSE) reinforcement (geogrids) with shotcrete reinforced by WWF. What is the best method (or product) to anchor each system to the shotcrete, and how are shotcrete-to-shotcrete (gunite) anchors treated in an MSE wall?

Answer:

There are many ways to attach a shotcrete facing to a soil nail shoring system or an MSE wall system. For the attachment to a soil nail wall system, you could review the Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls.” For MSE wall systems, you should consult with the MSE wall system vendors. Shotcrete facing systems are commonly used on both types of walls, but it is beyond the scope of our association to provide further guidance.

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Question 210:

I’m looking for information on the quantity of rebound expected when applying shotcrete against soil. We have a W4 4 x 4 in. (102 x 102 mm) layer of mesh 2 in. (51 mm) from the soil face that is covered by a 4 in. (102 mm) initial layer of shotcrete. Is there a general ballpark figure that can be used, such as a percent of the total shotcrete placed?

Answer:

Your question does not indicate the orientation of the application. If the shotcrete is being applied to a sloped surface for a channel or slope the rebound should be incidental. If shooting a vertical wall, the amount of rebound is relative to the skill of the nozzleman, the quality or nature of the mixture, the shotcrete process being used (wet-mix or dry-mix), the stability of the wire mesh, and other parameters. The range could easily vary from 5 to 20% on vertical walls relative to the aforementioned listed parameters.

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Question 211:

I have been hired to design a large concrete pit for a fertilizer plant. The pit will need to be approximately 13 ft (4 m) deep by 55 ft (17 m) long by 15 ft (5 m) wide. The pit will contain water at varying depths and will support grating covering the pit that will support equipment. The state is requiring the pit slab and walls to be a monolithic pour. Could shotcrete be used in this situation and be considered a monolithic pour?

Answer:

If the directive from the state is to cast (or shotcrete) both the slab and the walls monolithically, this would be a difficult task with either shotcrete or cast concrete. If the directive is to cast the floor monolithically, and then the walls monolithically, shotcrete could certainly be used and would be considered a monolithic placement. Once the state’s intent is clarified, this question should be posed to a shotcrete contractor who might be the actual contractor on the project for their input.
As this is a fertilizer plant, there may be additional considerations due to the potentially aggressive nature of the fluids introduced into this pit.

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Question 212:

We own a 200-year-old house with a rubble foundation. The foundation is structurally sound, but needs to be repointed, and some of it has no mortar at all. We would like to seal it to make it watertight and keep out radon. Could shotcrete be applied directly to the interior of the rubble wall (which includes small, loose stones; large gaps; and cracks), or would we have to first have the walls repointed and smoothed over?

Answer:

Yes, shotcrete would be an excellent method to fill the voids, open mortar joints, and gun an overlay over the irregular stone foundation. The use of shotcrete would be dependent on the access and ability of the applicator to safely place the shotcrete. A tight or low crawl space would make it difficult. We would suggest cleaning out loose materials with compressed air and water prior to the shotcrete placement. We recommend installing either a 2 x 2 in. (51 x 51 mm) 12-gauge or a 3 x 3 in. (76 x 76 mm) 11-gauge wire mesh over the stone foundation and gunning the shotcrete in place to fill in the mortar joints, creating a shotcrete overlay over the entire stone surface.

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Question 213:

We will be shotcreting a pool and the designer has put an expansion joint in the pool going from the top of one wall through the floor to the top of the other wall. It also shows a 9 in. (229 mm) polyvinyl chloride (PVC) waterstop in this joint. I have seen this used with cast-in-place concrete, but not with shotcrete. I was wondering if there are any guidelines on shooting around a PVC waterstop.

Answer:

This detail is normally only used on very large competition pools on the order of 164 ft (50 m) in length. It takes a lot of skill, technique, and care to properly encapsulate the waterstop and it should only be attempted by a shotcrete contractor with experience in this application. The successful encapsulation of the waterstop is more challenging with the dry-mix process than when using wet-mix shotcrete. The techniques for the proper encapsulation are generally developed by the individual shotcrete contractor and there is no specific “guideline” available for encasing waterstops.

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Question 214:

We are, and have been, designing and constructing permanent soil nail and shotcrete retaining walls. Typically, our designs consist of a primary nozzle-finished shotcrete facing to shore during our top-down construction, followed by a secondary shotcrete facing that is shot and sculpted once the full height of the wall has been excavated, drilled, and shot with the primary facing.
We had a comment recently that only the secondary facing thickness can be used in our design for the wall’s flexural capacity because the shotcrete layers may delaminate. Our general practice is to pressure-wash the primary nozzle-finished shotcrete facing before our approved and experienced nozzlemen place the secondary layer. From our experience, this procedure has been very effective and we have not experienced any delamination between shotcrete layers on any of the millions of square feet of shotcrete we have placed this way.
If installed correctly with our general practice, is there any reason the shotcrete layers would delaminate? If not, have any studies been done to prove this to our reviewer?

Answer:

All of your points are valid, but the Engineer of Record or the owner makes the final decision on recognizing a composite system or ignoring the value of the initial layer. As your experience shows, shotcrete provides an excellent bond between freshly placed layers and properly prepared concrete or shotcrete substrates. There are many articles available in the Shotcrete magazine archives—found on our website, —that may provide the designer or owner more information to allow them to make their design decision.

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Question 215:

I have been experiencing slow curing times (early set times). Every year during the wet season, my shotcrete curing times go from 1 MPa (145 psi) in 2 hours to 1 MPa (145 psi) in 8 hours. I believe that there is a change in the materials when the groundwater comes up. I have had water tests done, but I’m not sure what to be looking at. The recycled water that was being used had a pH of 5.7. We changed water, the problem was still there, and the pH is now 9.7. What effects does the pH level have?

Answer:

A pH of 5.7 is slightly acidic, while 9.7 is quite alkaline. According to PCA’s “Design and Control of Concrete Mixtures,” most inorganic acids have no adverse effect on concrete. Organic acids (such as tannic acid) can significantly reduce strength when present in higher concentrations. Some alkaline materials, such as sodium hydroxide, in higher concentrations may cause a quick set. However, because this occurs in the rainy season, another factor that may have an impact is an increase in dissolved solids. PCA states that solid contents exceeding 50,000 ppm can increase water demand, accelerate set, lower compressive strength, and increase permeability of the hardened concrete. The appropriate test for acceptable non-potable concrete mixing water is ASTM C1602/C1602M, “Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete.”

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Question 216:

We are working on an historical renovation project where the existing structure has been reinforced with shotcrete. We need to attach structural studs/furring members to the face of this shotcrete. Is there any difference between standard concrete and shotcrete when it comes to fastener embed depth? We are considering powder-actuated fasteners (Hilti-type) or Tapcons.

Answer:

The embedment depth of anchors in shotcrete would be the same as it would be for conventionally formed and placed concrete. Shotcrete is essentially a method of placing concrete and the same rules would apply. As with any anchoring system, it is important to make sure that you are anchoring to sound material.

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Question 217:

Our company is carrying out a tunnel project in rather poor geological conditions, including water seepage and poor rock, with wire mesh and two layers of steel mat. What is the reasonable rebound percentage in such conditions?

Answer:

Shotcrete rebound varies for many different reasons, many of which you mention in your question. The water seepage must be controlled or the shotcrete will likely not adhere to the surface and will slough off as the water saturates the fresh shotcrete. Accelerator will help, but it is difficult, if not impossible, to achieve good results against a seeping surface. ACI 506R-05, “Guide to Shotcrete,” estimates approximate range of shotcrete losses from 10 to 30%. Some other factors affecting the percentage of rebound are:
Mixture design
• Shotcrete process (wet- or dry-mix)
• Concrete mixture design and materials (for example, microsilica will tend to create less rebound; more than 30% coarse aggregate can cause more rebound)
• Plastic concrete properties (air content, slump)
• Nozzleman competence
• Vertical placement generally has less rebound than overhead
• Thickness of buildup per layer
Reinforcing grid
• Size and spacing of reinforcing
• Stability of reinforcing grid

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Question 218:

I had wallpapered over a cement interior basement wall years ago. Recently, When I removed the wallpaper and the liner beneath it, the shotcrete came off with the paper. Is there any way I can repair these spots? Can the shotcrete process cover a garage floor that is heavily pitted, has a few cracks, and has some dirt and road salt marks? Will it hold up to road salt and prevent further deterioration?

Answer:

You mention that you had originally wallpapered over a cement interior basement wall. It is not clear that the cement interior wall was placed using the shotcrete process. If it was installed with the shotcrete process, then the application was flawed due to improper surface preparation or application. There are many concrete repair products on the market which could be used to repair the surface. Many of these are troweled on by hand or sprayed. Check with a local building supply company or on the Internet.
With respect to the garage floor, we would not recommend the shotcrete process for a thin overlay on a horizontal surface. Again, there are many products on the market that are designed for resurfacing floor slabs. Check with a local building supply company or on the Internet for potential products.

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Question 219:

I am interested in any information or suggestions you may have regarding practical working space requirements for shotcrete applications. Shotcrete is a common approach for sewer pipeline and storm-water culvert rehabilitation projects. My concern relates to the space requirements necessary to best ensure a quality installation—for pipelines, this boils down to the question: What is the smallest diameter pipe that can be used for this method? Technical specifications that I have come across call for a minimum of 3 ft (1 m) between the surface being covered and the application nozzle. To me, this means that pipes that are much smaller than 6 ft (1.8 m) would create some difficulty. Similarly, for applications between vertical walls, how much room does a nozzleman need between the wall receiving the shotcrete and the wall at his/her back?
Are there robotic means or other methods in use that would allow shotcrete applications without a hands-on nozzleman? Are there any other workspace limitations or controls that should be considered when determining feasibility of shotcrete application methods?

Answer:

In the case of installing a lining inside of an existing pipe, there are robotic methods available, such as spin lining, where the cementitious material is cast from a rotating head as the carrier is moved along the pipe. For pipe smaller than 42 in. (1067 mm) diameter, the spin lining is likely the best method.
For pipe larger than 42 in. (1067 mm) and up to 6 or 8 ft (1.8 or 2.4 m) diameter, either hand shotcrete nozzling or spin lining are applicable. For pipes much larger than 6 to 8 ft (1.8 to 2.4 m), hand nozzling is likely the best solution.
In the case of clearance between a wall to be shotcreted and an obstruction, 3 ft (1 m) is a good rule of thumb, but a qualified and experienced shotcrete contractor can use modified equipment to place quality shotcrete in tighter spaces. A recent article on shotcreting in confined spaces can be found here.

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Question 220:

We have a vertical shaft that is (right now) 70 ft (21.3 m) deep and we do blasting every 5 ft (1.5 m) after applying shotcrete to the vertical surface for protection. My concern is that if we have less than 48 hours between successive blasting, is it allowable? How does one measure if the shotcrete reaches the required percentage of strength?

Answer:

The best guidance on this subject can be found in ACI 506.5, “Guide for Specifying Underground Shotcrete” (available through the ASA Bookstore), and some articles from past issues of Shotcrete magazine might be of interest:
"Shotcrete Spraying Machines for Immediate Support in Tunnels"
"Slope Stabilization in an Open Pit Mine"
"Where Are We Now with Sprayed Concrete Lining in Tunnels?"
"The Danger of Fallouts in Overhead Shooting"
"Incline Tunnel—S&S Quarries, Inc."
"Reaching 20 MPa (2900 psi) in 2 Hours is Possible"
"Rapid-Setting Cement in Shotcrete"
With properly qualified nozzlemen, a good shotcrete mixture, and high-quality accelerator added at the nozzle, the re-entry time can be minimal—normally 24 hours.

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Question 221:

I know that there are many factors that affect the distance that shotcrete can be pumped. For a dry-mix process, is there a rule of thumb for a maximum recommended horizontal pumping distance?

Answer:

The best information on this subject can be found in ACI 506R-05, “Guide to Shotcrete,” and likely in past articles in Shotcrete magazine. The distance that can be pumped is a function of too many parameters to fit a rule of thumb. The distance that can be pumped is influenced by the equipment being used, the vertical lift, the available compressed air, and other factors. We would suggest that you consult with one of our corporate members (www.shotcrete.org/BuyersGuide) in the area of the project and get their input.

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Question 222:

We would like to place 4 in. (100 mm) thick shotcrete reinforced with welded wire reinforcement and anchoring bolts in a water pressure tunnel. The water velocity would be between 10 and 16.4 ft/s (3 and 5 m/s). We would like to know if there is a possibility of erosion or cavitation of the shotcrete at this range of velocity.
It is mentioned in our concrete manual that cavitation and destructive erosion begin when water velocities reach about 40 ft/s (12 m/s). Because the roughness of the shotcrete surface is higher than the concrete surface, is erosion more likely to occur? Do you know what may be the maximum water velocity acceptable for reinforced shotcrete?

Answer:

Shotcrete is a method of placing concrete and the surface finish can be as smooth as that of cast concrete. Even with a nozzle finish, shotcrete erosion or cavitation should not be an issue at the stated velocities. Examples of smooth shotcrete surfaces can be found in many Shotcrete magazine articles and in particular ("Restoring the Century-Old Wachusett Aqueduct").

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Question 223:

We are shooting 5000 psi (34.5 MPa) shotcrete. Because of rising temperatures, the mixture is getting too stiff to pump, and the inspector will not let us add water. What should be done in this situation??

Answer:

At the point at which concrete/shotcrete temperature is starting to rise and the mixture is stiffening up, adding water should not be allowed. Water should only be added when the mixture is stable and only up to the water specified in the approved mixture design. In warm or hot conditions, retarders, set stabilizing admixtures, or ice may be needed to keep the mixture stable for the period of time to transport and pump the load.

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Question 224:

Is there any documentation showing that it is okay to tie off to a man-lift basket? I have never found any. How do other contractors deal with ACI requirements of an air lance, knowing that OSHA has contradicting standards of air wand pressure?

Answer:

OSHA requires that the personnel in aerial man baskets be tied off with the appropriate harness and lanyard. When you are in a JLG or other type of man lift, the only place to tie off to is to the basket or boom bracket. This question may be better answered by studying current OSHA documents.
We cannot recall any of our members being cited for using an air lance or blow pipe.

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Question 225:

Can you refer me to the standards for adding water to ready mixed shotcrete?

Answer:

Wet-mix shotcrete is a placement method for concrete. Ready mixed concrete used for wet-mix shotcrete needs to meet the requirements of ACI 506.2-13, “Specification for Shotcrete.” ACI 506.2 specifies concrete materials shall meet ASTM C94/C94M, “Standard Specification for Ready-Mixed Concrete.” ACI 506.2 also requires you shall batch, mix, and deliver wet-mixture shotcrete in accordance with Specification C94/C94M, or Specification C1116/C1116M if fiber-reinforced. Further guidance may be found in ACI 506R-05, “Guide to Shotcrete,” and ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete.”

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Question 226:

Could you provide information regarding the appearance of efflorescence on a newly constructed 2 million gal. (7.57 million L) holding tank? The tank was constructed correctly and has held water for over 6 months. A leak test shows no water loss over a 72-hour period, and no moisture has been seen on the surface, but efflorescence has been noted.
The tank was painted after the shotcrete was properly cured. (The applied paint was inspected by a NACE inspector and found to be approximately 7 mils [0.2 mm] and meets the specification.) At what point will this stop and what is the best practice to prevent it from happening again? Would covering the areas where it has occurred with additional paint seal the cracks??

Answer:

Efflorescence is common on many exposed concrete and cement mortar applications. Generally it is seen when cracks in concrete or mortar are exposed to water rather than accumulating within the crack. The basic mechanism creating efflorescence is when concrete is exposed to water for a long time; excess free lime (calcium hydroxide) in the cement paste goes into solution with water (leaches). Then when that water eventually leaves the crack and dries on the surface, the white residue of calcium hydroxide creates what is termed “efflorescence.”
It is very common to see efflorescence on brick structures where the mortar joints are exposed to rainwater that leaches out the calcium hydroxide and the resulting white efflorescence is highlighted on the dark-colored face of the brick. In concrete tanks, it is often found in cracks that can accumulate water for a sufficient time to leach the calcium hydroxide. The bottoms of vertical cracks or low areas in horizontally oriented cracks often show the greatest buildup of efflorescence. These can be surface cracks that are exposed to rainwater or through wall cracks that are exposed to water contained within the tank.
Although the tank was cured properly to help deal with long-term drying shrinkage, surface cracking on shotcrete often results from early-age plastic shrinkage cracks. These are shallow cracks that form within hours (or minutes, in extreme conditions) of placement due to rapid evaporation of water from the exposed surface of fresh concrete (common in exposed floor slabs or in your case the fresh shotcrete wall surface).
To answer your question regarding when it will stop, the answer is it won’t unless the cracks are sealed, or water is prevented from getting into the cracks. Cement-rich shotcrete has more than enough free lime to continue the leaching for decades. Although surface-applied coatings may initially span small cracks, as the walls of tanks expand and contract due to filling and emptying, and undergoing daily and seasonal thermal changes, the surface cracks will open and close slightly and eventually mirror through the coating. Coatings designed to tolerate moving cracks would likely be much thicker than the 7 mils used on your project. If the cracks are through-wall cracks that are seeping from the contained water, the crack will need to be sealed, most commonly by injection of polyurethane grout or interior surface coatings.
To answer your question on how to prevent this in the future, early-age plastic shrinkage cracks can be reduced by fogging the fresh shotcrete surface to keep the surface humidity high and reduce evaporation of the water at the surface of the concrete. Also, using fibers in the shotcrete can help reduce plastic shrinkage cracking. In hot or windy climates, placing the final layer of shotcrete during the coolest or calmest time of the day may help, too.
To answer the question if additional paint would seal the cracks, simply coating with an additional 7 mil (0.2 mm) coating would provide a temporary seal, but more than likely the crack will mirror through after some period of exposure. A coating designer would need to evaluate the crack widths and potential movement to design a coating system that would provide a long-term seal.
Finally, the efflorescence caused by exposure to rainwater is generally only a visual defect and doesn’t affect the long-term structural integrity or durability of the tank. Many owners tolerate efflorescence on the tanks and simply clean it off when it becomes objectionable.

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Question 227:

We have a large project involving shotcreting soffits in an underground parking garage. The shotcreting overhead is not the problem; the problem is properly screeding the excess shotcrete from the ceiling leaving a semi-smooth finish.

Answer:

Properly screeding and finishing overhead shotcrete is very challenging. The contractors who do this type of work properly have very well-trained and skilled tradesmen throughout the crew, including the nozzlemen, rodman, and finishers. Shotcrete that is not screeded and finished properly will likely suffer bonding and other issues.

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Question 228:

We are involved in the design of a hydro project in a section of a water-conveyance power tunnel; we are considering using shotcrete reinforced with welded wire reinforcement as a final liner. In this particular section, the tunnel is under an internal water pressure of 189 psi (1.3 MPa) and water velocities in the range of 16.4 ft/s (5 m/s) can be expected. We have not found any examples of such a design/use at this water velocity and are concerned about long-term durability and potential erosion of the shotcrete and entrainment of fragments into the turbine/powerhouse.
Would you have any information regarding the ability of shotcrete to resist water erosion, particularly at 16.4 ft/s (5 m/s)? (Any examples would be appreciated.) What additive can be used to reduce the porosity of the projected mixture?

Answer:

Shotcrete is simply a placement method for concrete, so characteristics of concrete that are resistant to erosion are equally applicable to shotcrete. ACI 210R-93, “Erosion of Concrete in Hydraulic Structures,” has guidance on flow characteristics that lead to erosion of concrete. Also, ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” Sections 4.6.2 and 4.6.3, also provide guidance on concrete mixture characteristics helpful for protecting against cavitation erosion. Properly designed shotcrete mixtures can easily meet the ACI 350 4.6.3 concrete requirements.
In 2000, Rusty Morgan compiled a list of some 37 water supply tunnels that had been lined with shotcrete (a copy of the data sheet can be supplied upon request). Shotcrete was not the final lining in all of these tunnels and not all the inverts were lined with shotcrete. The evaluation does not document the water velocity in these tunnels, but could be ascertained by contacting the project owners.
It should be noted, however, that the 16.4 ft/s (5 m/s) water flow rate is not particularly fast. The water velocity needs to be in excess of 39.3 ft/s (12 m/s) before cavitation erosion can be expected (refer to A. M. Neville, Properties of Concrete) and cavitation would be the most likely cause of erosion of the concrete surface.
Supplemental cementitious materials including microsilica, fly ash, and slag will generally reduce the porosity of the hardened concrete. Microsilica is used in many shotcrete mixtures, as it helps to reduce rebound, as well as gives the fresh concrete better adhesion and cohesion that can allow for thicker or overhead placements.

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Question 229:

I have a client with an old, soft-stone masonry building of approximately 150 years of age. The mortar is badly deteriorated and the stone is quite friable. I am advocating the use of shotcrete as an application to the interior face of the walls that will restore both in-plane and out-of-plane strength to the building walls.
My client has expressed concern that there may be incompatibility issues between the stone masonry and the shotcrete both from a structural stiffness perspective as well as from a moisture intrusion perspective. (We have successfully used shotcrete over stone masonry in the past.)
Do you have any information you can share with me on this topic? Do you have either examples of incompatibility or successful use of shotcrete over stone masonry?

Answer:

As you have noted, shotcrete has been used extensively to reinforce unreinforced or under-reinforced masonry walls and rock walls. It has been used on the Crater Lake Lodge to strengthen and stabilize a rock wall foundation and any number of other projects. In California, shotcrete has been used to strengthen or repair walls since the 1933 Long Beach Earthquake. It was used to strengthen the California State Capitol (3 ft [0.9 m] thick brick walls) in the late 1970s and all of the older unreinforced masonry walls for the San Francisco School District. To the best of our knowledge, there have been no failures of shotcrete strengthening on the West Coast in the past 80 years.

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Question 230:

Why is there not more extensive use of fiberglass reinforcing bars? It seems like it would be a natural choice for most projects involving shotcrete in wet applications, as well as conventionally placed concrete, especially in the types of jobs we do, such as the rehabilitation of existing concrete channels that usually contain acidic waters. I understand that anything other than steel is more expensive, but isn’t prevention now cheaper than remediation later?

Answer:

Although similar in dimensions, fiberglass reinforcing has distinctly different structural properties when compared to conventional steel reinforcement. This is a question better answered by the fiberglass reinforcing industry or the structural engineering community. As the American Shotcrete Association, we do not get involved in the engineering design of structural sections. However, it should be pointed out that properly designed and applied shotcrete provides a very corrosion-resistant environment around embedded steel reinforcement, providing excellent long-term durability in normal exposure conditions.

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