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2016FallShotcreteEMag

Misconceptions about Shotcrete—True Stories from ASA Technical Inquiries As Executive Director and Technical Director for ASA, I get the unique opportunity to tackle a number of technical inquiries every month about shotcrete. Over the last couple of years, we’ve seen a sharp increase in inquiries from engineers, architects, and owners about pool and water feature installations. Likely part of this increase is the publication on our website of pool position statements (www.shotcrete.org/pages/ products-services/shotcrete-resources.htm). A few of these inquiries were simply questions about best practices, but many have resulted from issues during or after construction. These are true stories. Believe me, I couldn’t make some of these up. Inquiry: A pool owner called and asked, “My shotcrete contractor had some delays while shooting our pool. The concrete got too hard to pump through the machine, so they took the concrete out of the truck in wheelbarrows, and then hand packed the concrete into the remaining open spots in the floor, steps, and benches. The contractor said they do this all the time. Is this OK?” Misconception: Concrete intended to be shotcreted doesn’t need to be physically shot. Fact: Shotcrete by its definition is concrete pneumatically placed at high velocity. These velocities average from 50 to 80 mph (80 to 125 kmh). The energy released by impact of this high-velocity material provides compaction and consolidation of the concrete. Without the velocity and impact of shooting the concrete, alternative methods of consolidation must be used to eliminate voids and densify the concrete. Hand packing of very stiff, unpumpable material does not provide the compaction effort needed, so likely the concrete in-place has random voids and poor bond to adjacent concrete. Overall, these hand-packed sections will have less strength, reduced durability, and because this is a pool—greater permeability. Inquiry: A pool owner in Arizona called after cracks formed in their new pool shell. The contractor had told them they needed to “water” the By Charles Hanskat pool twice a day to prevent cracks, which they did, but cracks still formed. Misconception: Adequate curing of shotcreted concrete surfaces is just dampening the surface occasionally. Fact: Proper curing means keeping the concrete surface continuously damp. Curing for 7 days is recommended for shotcrete. Wet curing with water is preferred to using spray-on curing membranes. ACI 308R-01, “Guide to Curing Concrete,” states “The objectives of curing are to prevent the loss of moisture from concrete and, when needed, supply additional moisture and maintain a favorable concrete temperature for a sufficient period of time.” Curing is essential to allow the cement in the concrete to continue to hydrate. Hydration of the cement is how concrete builds strength, reduces permeability, and improves durability. Because shotcrete has a relatively low water-cementitious materials ratio (w/cm) compared to most placed concrete in floors with w/cm of 0.50 or greater, it would benefit greatly from supplying additional curing water. ACI 308.1-11, “Specification for Curing Concrete,” requires for wet curing: “Keep the concrete surfaces continuously wet. Do not allow alternate wetting and drying of concrete surfaces.” One would expect that the hot, dry, and potentially windy conditions prevalent in Arizona would create high evaporation rates and wetting the pool surface once or twice a day would not provide a continuous supply of supplemental water. Inquiry: Another pool owner called about cracking in the coves, benches, and steps in their new pool. Investigating further, they found substantial delaminations and voids below the surface in many of these areas. I asked, “Did you watch the shotcreting procedure?” They answered “Yes.” I then asked, “Did you ever see any concrete or material shoveled out of the pool?” Answer: “…No.” Misconception: Some shotcrete contractors feel shotcrete rebound and overspray is able to be left in the floor and coves, or shoveled up into the 12 Shotcrete • Fall 2016


2016FallShotcreteEMag
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