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Shotcrete FAQs be determined and adequate reinforcing be designed to ensure (www.shotcrete.org/BookstoreNet/ProductDetail. that the cracks do not propagate through the overlaid shotcrete. aspx?itemid=506108). A 2 in. (51 mm) overlay is absolutely the least possible and 3 or 4 in. (76 or 102 mm) is far more Question: I am a structural engineer and I am supposed to normal in practice. design structures for shotcrete applications. Should I calculate Canals are generally specified to have a natural gun finish, and check its stability by the “working stress method?” Or, a rough broom finish, or a light broom finish. could I use the “ultimate limit design?” Are there regulations or specifications about the application of method on ACI? Question: I have been asked to come up with a 5000 psi Finally, is elastic coefficient different between normal con- (35 MPa) in 24 hours shotcrete mixture, using cement, fly ash, crete and shotcrete? silica fume, and fine aggregate. I need some advice on a mixture. Answer: Shotcrete is a method for placing concrete. Thus, the Answer: Design of a concrete mixture to be placed by the concrete placed by the shotcrete method has the same physical wet-mix shotcrete method is essentially the same as normal properties as cast concrete with the same mixture proportions. cast-in-place concrete mix design. The major differences with Either working stress or ultimate strength methods used for shotcrete mixtures are: concrete design are applicable. Local building codes may • The maximum coarse aggregate size is generally limited to require a particular design approach. about 3/8 in. (9.5 mm); • They use a fairly low water-cementitious material ratio Question: I have a customer who would like to place 2 in. (w/cm) and slump to allow shooting on vertical surfaces (51 mm) of shotcrete onto our geotextile canal liner, which has without sloughing; been used for many years with 2 to 4 in. (51 to 102 mm) of • The potential to use an accelerator that can be added at the shotcrete. In all of these previous projects, contraction joints nozzle; and were installed. For this project, the customer is asking whether • The pumpability is an important workability characteristic. this is an absolute requirement, as the geocomposite canal liner Since you desire a high-early-strength mixture, using fly beneath is the water containment component. Does it make a ash as a supplemental cementitious material (SCM) wouldn’t difference in terms of cracking and joints whether the shotcrete be recommended because it slows set and strength gain at is 2 or 4 in. (51 or 102 mm) thick? What is the typical finishing early ages. Microsilica may be beneficial for early strength that is done on canal projects? gain. Consideration should be given to using accelerator added at the nozzle. There is some guidance on concrete Answer: Long expanses of concrete canal lining exposed to mixture design in ACI 506R-05, “Guide to Shotcrete” (www. the sun and weather would experience significant internal shotcrete.org/BookstoreNet/ProductDetail.aspx? tensile drying shrinkage stresses. Regular contraction joints itemid=506R-05); however, because local materials (aggre- help to relieve the internal tension created by concrete gates, cements, SCMs) can vary significantly, you should shrinkage. If no contraction joints are provided, shrinkage will consult with an engineer or concrete testing laboratory still occur and the concrete lining will produce its own contrac- familiar with shotcrete to produce and test a mixture design tion joints, better known as “cracks.” Unfortunately, the to meet your requirements. resulting cracking will be random and can vary significantly in size and length. Thus, contraction joints are a good approach Question: Can shotcrete be used to help seal a leaking pond? to help induce cracking at regular, controlled locations. If the We have a 1.5 acre (6070 m²) pond that we are in the process client doesn’t want contraction joints, they need to understand of completing. We spread 90,000 lb (40,823 kg) of bentonite that cracking will be much more extensive and likely in, but the bentonite washed off the steep banks and now we more noticeable. are stuck with a half-full pond. Would shotcrete be a practical Theoretically, with the same percentage of embedded solution for our problem? reinforcement, cracking between a 2 or 4 in. (51 or 102 mm) should not be substantially different. Of course, the 4 in. Answer: Properly designed shotcrete (both concrete mate- (102 mm) thick shotcrete section would require twice the rials and reinforcing are important in the design) placed by concrete material and twice the embedded reinforcement to an experienced shotcrete contractor can certainly be used to maintain the same percentage of reinforcement. A 2 in. provide a somewhat watertight lining for your pond that will (51 mm) thick section could have some difficulty in maintaining be serviceable, durable, and require little to no maintenance adequate cover over embedded reinforcing bars. The designers for decades to come. We would suggest you consult with an could also consider using fiber-reinforced shotcrete to help engineer or shotcrete contractor experienced in this type of control shrinkage and temperature stresses, although fairly high shotcrete work. You may use our online Buyer’s Guide dosages are needed for effective elimination of reinforcing (http://shotcrete.org/pages/products-services/Buyers- bars. More guidance on fiber-reinforced shotcrete is available Guide/index.asp) to find an ASA corporate member con- in ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete” sultant or contractor to assist you. Shotcrete • Fall 2013 77


2013FallShotcreteEMag
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