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Shotcrete FAQs not exceed 3 ft (1 m) and that 3 hours must pass before the harden. Following that, a final non-accelerated finish coat can be second lift can be applied. applied that can be trimmed to shooting wires with a cutting rod, First of all, if you waited 3 hours between lifts, you would have closed up with a darby, and then trowelled with either a magnesium full-length cold joints along the whole length of the wall. You’d or steel trowel, depending on the required finished surface texture. also have to wash out the pump after every lift or the concrete Such work can be done with a remote-control manipulator would harden in the pump and hoses. Second, you can’t leave a arm (robot) or, for more precision, with hand nozzling out of 4000 to 5000 psi (28 to 34 MPa) mixture sitting in the truck a basket on a manlift (provided the tunnel floor is sufficiently 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 subse- quent 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. Question: 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 accel- erator 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 Shotcrete • Summer 2013 71


2013SumShotcreteEMag
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