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2016SpringShotcreteEMag

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: I just had a swimming pool built and everything I have read online says that the gunite shell has to be sprinkled with water for several days after the pour. My pool builder says they never do it, and, when I asked why, I’m just told that’s the way they do it. I told them I am worried because every other pool builder says to do that except the one I hired and I can get no answer as to why. Is this an acceptable practice? I am worried that years down the line I may have a problem. I live in Oviedo, FL, and the weather has been in the low 70s and the humidity not particularly high. They did hit the water table and have a pump running—would any of this have an impact? Answer: ASA recommends a minimum of 7 days curing for all exposed shotcrete surfaces. Wet curing is preferred to supply additional water to the concrete surface. If a spray-on curing membrane is used instead of water curing, the material should be applied at twice the manufacturer’s recommended rate for formed surfaces. Curing is important to allow the concrete to develop as much strength as possible and to help resist cracking from internal shrinkage of the concrete. Low humidity, wind, and exposure to sun will increase the need for proper curing. If the site is dewatered, the groundwater is below the concrete work, and not effective in curing the exposed shotcrete surface. The American Concrete Institute (ACI) has an excellent reference, ACI 308R-01, “Guide to Curing Concrete.” It appears your contractor is not following the industry standards as documented by ACI. Question: Our shotcrete mixture needs to be NSF 61 certified. We have been able to obtain certification of all components with the exception of reinforced fiber. Is there or does fiber reinforcing need NSF 61 approval? Answer: Each manufacturer of concrete constituents needs to have their products tested by NSF if they want NSF 61 certification. Whether the fibers need NSF 61 certification is an issue with the local authority having jurisdiction for exposure of components to potable water supply systems in your state. Generally, this is the state EPA-type agency, but maybe a federal agency if on a federal project. We don’t maintain a database of manufacturer products that meet NSF 61. However, you can readily identify contacts for the fiber manufacturers who are ASA corporate members with our Buyers Guide at www.shotcrete.org/pages/products-services/ Buyers-Guide/index.asp. When entering the Buyers Guide, you can select “Fiber + Reinforcement Sales” and the fiber type subcategory to get a list of our member fiber suppliers. Question: I am currently involved with the design of an unreinforced masonry building retrofit. Could you point me toward resources concerning the seismic behavior of a reinforced shotcrete masonry wall? I am interested in learning more about the force (shear) transfer between the masonry/shotcrete interfaces. Answer: Shotcrete is a placement method for concrete. Thus, seismic design for concrete is applicable to shotcrete placement. Here’s a link to an article in the Winter 2009 issue of Shotcrete magazine, titled “Seismic Retrofit of Historic Wing Sang Building,” that details the seismic retrofit of a brick building in Vancouver, BC, Canada: www.shotcrete.org/media/ Archive/2009Win_SCM01pg08-12.pdf. A second article from 1999, “Seismic Reinforcing of Masonry Walls with Shotcrete,” also gives some input on the design: www.shotcrete.org/media/Archive/1999Fal_Snow. pdf. In general, the structural engineer must evaluate the condition of the existing masonry structure and determine whether the added shotcrete sections will be supplementing the existing capacity or providing the full resistance to seismic loads. Question: I had a concrete pool shell installed using gunite (dry shotcrete method) in July 2013. It was never finished due to unfortunate circumstances and has been exposed to the elements of weather over the last 2 years, mostly filled up with water from rain and, in the colder months, frozen like a pond. We would like to finish the pool but were told by the pool company that the concrete looked odd and we should have it strength tested. We had core samples taken from the walls and floor from a certified testing lab. The results from the six samples ranged from 1700 to 2200 psi (12 to15 MPa). When the pool was blown on July 3, 2013, it was to achieve 4000 psi (28 MPa) compressive strength in 28 days. Is it normal for the shotcrete strength to have weakened so much? Answer: Properly produced concrete material shotcreted in place should gain strength over time, not lose strength. ASA recommends that concrete placed by the shotcrete method have a minimum compressive strength at 28 days of 4000 psi (28 MPa). Coring does damage the sample somewhat, so it is common to require core strength to meet 85% of the specified compressive strength. Cores should be no less than a nominal 3 in. (76 mm) in diameter for representative results because smaller cores (less than 3 in. 76 mm diameter) are more subject to damage from the core extraction, affecting the reported strength. Thus, at 85% of 4000 psi (28 MPa) the minimum should be 3400 psi (23 MPa). Based on the reported values, and assuming a 3 in. (76 mm) diameter or greater core, the concrete strength is well below ASA’s recommended strength, and the strength you originally specified in 2013. Question: We have a pool designed with the cast-in-place concrete construction method in mind. The project has been awarded to a dry-mix gunite contractor. To accommodate the 70 Shotcrete • Spring 2016


2016SpringShotcreteEMag
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