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2014SumShotcreteEMag

a pro forma exercise, but we jumped at the chance of its watertightness and structural longevity—not to show why a monolithic shell made using the to mention the superior bonding it allowed with shotcrete process would be superior in many finish materials. ways to a typical placed shell. (We were also Our presentation obviously spurred a response. motivated, of course, by the fact that we had the During the interview with the design group, in opportunity to build a top-quality competition fact, the engineering committee quizzed us on pool that would serve a high-profile institution what we were going to use as expansion or move- for decades to come.) ment joints, thereby enabling us to explore with We felt we were operating from a position of them one of the primary advantages of shotcrete: strength. History has shown time and again, for There are no expansion joints, nor is there any example, that cast concrete pools have issues with need for using bonding adhesive between areas expansion joints and watertightness that sooner of concrete placed on subsequent days. or later produce water losses that can never fully This bonding, we know, is a common concern be addressed. Our aim was to prove to the project among engineers, who often wonder how it’s team and Aquatica’s specification writers that possible to create monolithic structures when shotcrete was a viable alternative if only because material is placed over the course of many days. Digging In Now we were on a roll. Using the opening they gave us, we explained that transitioning applications from one day to the next included preparing the concrete in a con- struction-joint format that sets us a 45-degree angle treated with a gun or broom finish. Before new material is applied, any over- spray or miscellaneous dirt would be carefully removed from all exposed steel, and the already-applied material would be maintained in what is known as “saturated surface dry” (SSD) condition so there would be no moisture or liquid changes between already-applied and new material. We further explained that, by using proper velocity (that is, 375 ft3/min 10.62 m3/min) in the shotcrete process and thereby driving fresh cementitious material into the pores of the pre- vious day’s shoot, the result would be a tremen- dous physical and chemical bond attributable to the properties of cement paste. Once that issue was cleared away, they raised a second question about what we were going to add to the concrete as a water- or damp-proofing agent to prevent leaks or possible damage to finish materials. (We knew this would be an issue for them, because test holes dug previously indicated that the water table rose to a level 2 ft 0.61 m higher than the bottom of the dig elevation during certain seasons.) My team’s response was simple: If pursued correctly, the shotcrete process is such that there would be no call for any water- or damp- proofing agent. This was another great opening, enabling us to explain that we would be applying a high-density, low-permeability, low-porosity Optimal concrete strength is encouraged when surface evaporation is material; the shell mixture itself would be discouraged, so we do all it takes to keep the surface saturated during the designed, engineered, and installed to hold water shoot and throughout the 28-day curing period. Done properly, the on its own. integrity of the shotcrete surface is so high that the finish can be applied Now we called in our big guns, referring to directly to the substrate without bonding or damp-proofing agents three key documents published by the American 12 Shotcrete • Summer 2014


2014SumShotcreteEMag
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