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2013FallShotcreteEMag

Shotcrete FAQs As a service to our readers, each issue of Shotcrete will include selected questions and provide answers by the American Shotcrete Association (ASA). Questions can be submitted to info@shotcrete.org. Selected FAQs can also be found on the ASA website, http://shotcrete.org/pages/products-services/technical-questions.htm. Question: We are working on a renovation of an existing shop- Answer: Shotcrete is simply a placing method for concrete. ping plaza where some of the existing walls are split face block. Thus, the mixture design and material properties are the same Would it be an acceptable application to resurface the block as concrete. We are not aware of any software specific to shot- with shotcrete to achieve a smooth finish? If so, what is the crete in any units. ACI 506R-05, “Guide to Shotcrete” (avail- thinnest we would be able to go? able here: www.shotcrete.org/BookstoreNet/default.aspx), has guidance on desirable mixture characteristics (aggregate Answer: Shotcrete could be used for this application. The grading, supplemental cementitious material SCM, and so thickness of the overlay would be dependent on the material on) that would be helpful in developing a concrete mixture used. A potential concern would be the lines of the existing design for shotcrete placement. block showing on the new surface. We would suggest that you search for and review various ASA Shotcrete magazine articles Question: I am a structural engineer working on underground (www.shotcrete.org/pages/products-services/shotcrete- structures such as tunnels and caverns. I would like to know magazine.htm) as well as ACI 506R-05, “Guide to Shotcrete” the permissible shear strength of shotcrete to be taken for (available here: www.shotcrete.org/BookstoreNet/default. M30 Grade SFRS (M30 = 30 MPa 4350 psi at 28 days). I aspx). would like to know more about its other properties, as well. Question: We have a two-story shotcrete wall enclosing an Answer: Shotcrete is simply a placing method for concrete. indoor community pool. We are specifying a board-form finish Thus, the in-place material properties are essentially the for the interior and the exterior will have a parge finish coat. same as cast concrete. A specific value for the shear is Are there any issues with the consistent moisture from the pool beyond the scope of our Association because many design that should be addressed in the concrete mixture or topical and material properties can affect the shear capacity. We sealant? How should we deal with the exterior versus interior would suggest you engage a Professional Engineer who finishes in regards to water intrusion protection and allowing specializes in Underground Shotcrete. You should consult the green concrete to “dry out” over time? our Directory of Members to find such a consultant (www. shotcrete.org/pages/products-services/Buyers-Guide/ Answer: Shotcrete is a method of placing concrete and the index.asp). ACI 506R-05, “Guide to Shotcrete” (available characteristics of shotcrete are those of cast concrete. Although here: www.shotcrete.org/BookstoreNet/default.aspx), the enclosed swimming pool will increase the interior humidity, would be a helpful primer to learn more about shotcrete. the high humidity should have no detrimental effects on the exposed shotcrete, and may even be beneficial in reducing Question: I have a new construction project where I want to apply long-term drying shrinkage of the wall. shotcrete to cast-in-place concrete columns and an elevated, post- Both cast-in-place and shotcreted concrete are commonly tensioned concrete slab as a finish material. The finished applica- used for construction of water tanks with constant exposure tion is intended to be in varying depths from 3 to 12 in. (76 to to water under significant hydraulic pressure. Using good 305 mm) or more. The desired end result is a smooth, curvilinear, construction techniques with good-quality concrete to build sculptural form. Is this type of application achievable? the tank’s walls produces walls with no moisture evident on the exterior face of the tanks. Simply having a high-humidity Answer: Shotcrete can and has been used to increase the size of atmosphere is a much less severe exposure and should not columns and thicken overhead slabs while providing great-looking result in interior air moisture being transmitted into and linear or curvilinear finishes. Examples of curvilinear finishes through the shotcrete wall. Any coatings considered for can be found in past Shotcrete magazine articles. You can search aesthetics should follow the manufacturer’s recommenda- the Shotcrete magazine archives at www.shotcrete.org/pages/ tions for drying time of concrete before application. If there products-services/shotcrete-magazine.htm. is a concern about the permeability of the shotcrete wall, a premium shotcrete mixture including silica fume might Question: We just shot a wet-mix swimming pool for a cus- prevent some issues on this application. tomer. 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 Question: What difference would there be in the density of 54 in. (1372 mm). From there we maintain our 1 to 3 ft (0.3 to shotcrete before and after shooting? Is there any shotcrete 0.9 m) slope down to 8 ft (2 m) for the diving end of the pool. mixture-design software in SI units available? Or any document The customer would like to raise the entire shallow end pool of shotcrete mixture design in SI units for optimizing floor up to the 39 in. (991 mm) depth. We prefer to use wet-mix shotcrete design? shotcrete. The overlay would be tapered from the 39 in. Shotcrete • Fall 2013 75


2013FallShotcreteEMag
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