Below is a listing of frequently asked questions. When a Shotcrete article is referenced, please click on the Publications tab of this website and then click on the “Search Shotcrete Publications” link to download the issue. If a Concrete International article is referenced, please visit www.concreteinternational.com to obtain the article. If an ACI Publication is referenced, please visit www.concrete.org.

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The information provided below is 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.

1. What is the difference between shotcrete and Gunite?

2. How should I design joints for shotcrete?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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.

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

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.

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.

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?

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?

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.

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

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.

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?

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

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

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?

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?

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?

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?

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?

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. 

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?

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?

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.

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?

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?

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.

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?

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?

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

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? Can you recommend a contractor in the Denver, CO area for this type of work?

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?

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?

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

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?

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

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?

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

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?

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

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?

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?

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?

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

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?

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.

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?

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?

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

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?

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?

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.

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?

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

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?

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

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

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?

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?

69. Is there any specified finish for shotcrete?

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?

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?

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?

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?

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.

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?

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.

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.   (back to top)

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.   (back to top)

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. Training for ACI Certification is available through the ASA office. Contact Jessica L. Bournay, ASA Executive Assistant, for the roster of ASA Trainers. Certification exams are conducted by ACI-approved examiners in strict compliance with ACI certification policies.   (back to top)

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.    (back to top)

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.   (back to top) 

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.   (back to top)

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.   (back to top)

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.   (back to top)

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.   (back to top)

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.   (back to top)

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.   (back to top)

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.   (back to top)

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. (back to top)

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. (back to top)

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.   (back to top)

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.  (back to top)

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.   (back to top)

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.  (back to top)

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.  (back to top)

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.  (back to top)

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.  (back to top)

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.  (back to top)

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.)  (back to top)

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.  (back to top)

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.  (back to top)

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. (back to top)

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.  (back to top)

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.  (back to top)

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.  (back to top)

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?  (back to top)

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.  (back to top)

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

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!  (back to top)

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?