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Shotcrete FAQs If installed correctly with our general practice, is there any Question: Our company is carrying out a tunnel project in reason the shotcrete layers would delaminate? If not, have any rather poor geological conditions, including water seepage studies been done to prove this to our reviewer? and poor rock, with wire mesh and two layers of steel mat. What is the reasonable rebound percentage in such conditions? Answer: All of your points are valid, but the Engineer of Record or the owner makes the final decision on recognizing Answer: Shotcrete rebound varies for many different reasons, a composite system or ignoring the value of the initial layer. many of which you mention in your question. The water As your experience shows, shotcrete provides an excellent bond seepage must be controlled or the shotcrete will likely not between freshly placed layers and properly prepared concrete adhere to the surface and will slough off as the water saturates or shotcrete substrates. There are many articles available the fresh shotcrete. Accelerator will help, but it is difficult, if not in the Shotcrete magazine archives—found on our website, impossible, to achieve good results against a seeping surface. www.shotcrete.org/pages/archive-search/archive-search.asp— ACI 506R-05, “Guide to Shotcrete,” estimates approximate that may provide the designer or owner more information to range of shotcrete losses from 10 to 30%. allow them to make their design decision. Some other factors affecting the percentage of rebound are: Question: I have been experiencing slow curing times (early • Mixture design set times). Every year during the wet season, my shotcrete curing ○ Shotcrete process (wet- or dry-mix) times go from 1 MPa (145 psi) in 2 hours to 1 MPa (145 psi) ○ Concrete mixture design and materials (for example, in 8 hours. I believe that there is a change in the materials microsilica will tend to create less rebound; more than when the groundwater comes up. I have had water tests done, 30% coarse aggregate can cause more rebound) but I’m not sure what to be looking at. The recycled water that ○ Plastic concrete properties (air content, slump) was being used had a pH of 5.7. We changed water, the problem ○ Nozzleman competence was still there, and the pH is now 9.7. What effects does the ○ Vertical placement generally has less rebound than pH level have? overhead ○ Thickness of buildup per layer Answer: A pH of 5.7 is slightly acidic, while 9.7 is quite alka- • Reinforcing grid line. According to PCA’s “Design and Control of Concrete ○ Size and spacing of reinforcing Mixtures,” most inorganic acids have no adverse effect on ○ Stability of reinforcing grid concrete. Organic acids (such as tannic acid) can significantly reduce strength when present in higher concentrations. Some Question: I had wallpapered over a cement interior basement alkaline materials, such as sodium hydroxide, in higher con- wall years ago. Recently, When I removed the wallpaper and centrations may cause a quick set. However, because this occurs the liner beneath it, the shotcrete came off with the paper. Is in the rainy season, another factor that may have an impact is there any way I can repair these spots? Can the shotcrete an increase in dissolved solids. PCA states that solid contents process cover a garage floor that is heavily pitted, has a few exceeding 50,000 ppm can increase water demand, accelerate cracks, and has some dirt and road salt marks? Will it hold up set, lower compressive strength, and increase permeability of to road salt and prevent further deterioration? the hardened concrete. The appropriate test for acceptable non- potable concrete mixing water is ASTM C1602/C1602M, Answer: You mention that you had originally wallpapered over “Standard Specification for Mixing Water Used in the Produc- a cement interior basement wall. It is not clear that the cement tion of Hydraulic Cement Concrete.” interior wall was placed using the shotcrete process. If it was installed with the shotcrete process, then the application was Question: We are working on an historical renovation project flawed due to improper surface preparation or application. where the existing structure has been reinforced with shot- There are many concrete repair products on the market which crete. We need to attach structural studs/furring members to could be used to repair the surface. Many of these are troweled the face of this shotcrete. Is there any difference between on by hand or sprayed. Check with a local building supply standard concrete and shotcrete when it comes to fastener company or on the Internet. embed depth? We are considering powder-actuated fasteners With respect to the garage floor, we would not recommend (Hilti-type) or Tapcons. the shotcrete process for a thin overlay on a horizontal surface. Again, there are many products on the market that are designed Answer: The embedment depth of anchors in shotcrete would for resurfacing floor slabs. Check with a local building supply be the same as it would be for conventionally formed and company or on the Internet for potential products. placed concrete. Shotcrete is essentially a method of placing concrete and the same rules would apply. As with any anchoring system, it is important to make sure that you are anchoring to sound material. Shotcrete • Spring 2014 59


2014SprShotcreteEMag
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