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benches because it will be covered up by a layer of “good” shotcrete and won’t affect the structural integrity or watertightness of the pool shell. Fact: When shotcrete impacts a surface, the material that bounces off is called “rebound.” This rebound material is mostly aggregate (sand and rock) and much less paste than in the shotcrete mixture design. Thus, rebound material is substantially weaker and more porous than the shotcrete mixture supplied. When rebound is incorporated in any final shotcreted section, one is introducing a layer of weakness within the concrete section. The section will then not act monolithically, as the designer intended; plus, it gives a weak plane in the shell that will encourage cracking and delaminations when the pool is exposed to wetting/drying and seasonal temperature variations. Rebound and overspray must always be removed and not incorporated in any of the structural pool shell. Inquiry: An engineer working on a large freeform concrete fountain basin called and asked, “Our shotcrete contractor said we needed to use wet-mix shotcrete because dry-mix is porous and needs extra coatings or plastering to make the fountain basin watertight. Is this true?” Misconception: Dry-mix is more porous than wet-mix, and not acceptable for liquid-containing pools or structures because it would require additional coating to provide the desired serviceability. Fact: Dry-mix shotcrete produced with quality materials, good mixture design, proper equipment, and experienced nozzlemen will produce concrete in-place equal to concrete produced using a wet-mix process. Dry-mix will actually tend to have a lower w/cm. The perception that dry-mix is more porous may occur because drymix water content is controlled by the nozzleman. An inattentive nozzleman or inadequate water pressure may allow dry spots in the work. These areas are definitely more porous, but should not be present in quality shotcrete. Inquiry: Here’s an interesting inquiry: “The plaster color installed in my pool was the wrong color. The plaster has been chipped out. My concern is damage to the shotcrete shell in the process. There are deep holes, gouges, and there was water seepage in a few areas behind the shotcrete. There is also evidence of honeycombed areas in the shotcrete as well as some other shotcrete concerns since reading up on the shotcrete process. I’m being told that they will just plaster over these concerns. However, the plasterer says that plaster thickness should not exceed 7/8 in. (178 mm) thickness, but can be a little thicker around plumbing fixtures (refer to Fig. 1 and 2). Fig. 1 Fig. 2 Misconception: Shotcrete can be shot rough and without close attention to full compaction because the surface will be covered with plaster and provide the final surface finish. Fact: ASA has published a position statement, “Watertight Shotcrete for Swimming Pools.” The position statement stresses that shotcrete can and should be built as an essentially watertight structural shell. Further, shotcrete can be finished to very uniform surface tolerances and finishes. A relatively thin, consistent layer of plaster is desired. Properly shotcreted sections should not have any significant voids or sandy, porous, or low-strength sections. From the images, it appears the contractor did not properly place the shotcrete for the pool. Plaster should not be used to fill substantial voids because it has significantly different mechanical properties (strength, thermal expansion/contraction, and shrinkage), and will not provide significant supplemental strength if the shotcreted shell has low strength or porous areas. Inquiry: “Our pool sat over the winter, and we noticed many areas where the concrete looked sandy or porous. We had cores taken and tested and strengths resulted in 2500 psi (17 MPa). The contract indicated the shotcrete should be 4000 psi (28 MPa). We asked the contractor about the discrepancy, and he said that’s normal. Concrete loses strength over time. Is that correct? Misconception: Concrete loses strength over time. Shotcrete • Fall 2016 13


2016FallShotcreteEMag
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