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2016SummerShotcreteEMag

Fig. 2: Crew shooting preconstruction test panel and inspected Core grading is a term that has been used for years for the evaluation of the concrete cores taken from preconstruction shotcrete test panels. This method has been linked to photos for acceptable and non-acceptable work taken from a now severely outdated version of ACI 506.2, “Specification for Shotcrete,” published over 20 years ago in 1995. The current version of ACI 506.2 was printed in 2013 and no longer includes core grading as an evaluation method due to its lack of consistency and the inability to relate a “grade” to actual structural performance. This antiquated “core grading” method is very subjective and interpretation can vary widely from one person to another. I have been in concrete testing labs that use a paper clip to measure voids. We need to remember shotcrete is concrete, and no concrete is perfect. The team approach is the best way to approach cores when evaluating the quality of in-place shotcrete in the pre-production test panel. By team, I mean the contractor and the engineer. Here are the steps I use to best evaluate the panels: • Start with a good mixture design and a qualified shotcrete team; • Submit shop drawings for all anticipated test panels; • Meet with the structural engineer responsible for evaluating the project and explain the shotcrete process (this is a must and I have been doing this for over 40 years)—it will be the best conversation you will have with the structural engineer, and he should have a much better appreciation for all the factors required for a successful shotcrete project. When the cores are ready for inspection, lay them out so you can get an overview of their sample locations in the test panel; and • Review the shop drawing—look at the consolidation of the concrete around the reinforcing where small voids or rock pockets may be, and report the percentage of embedment around bars. Please remember when larger 6 in. (150 mm) cores are taken through the full depth of the test panel, the surface area is much greater than Fig. 3: Preconstruction test panels have been stripped, cored, Fig. 4: Cores taken from thick wall preconstruction panels Fig. 5: Core drilling with a large barrel in a thick wall condition anticipated from the outdated ACI 506.2-95 grading system. The cores shown in Fig. 4 are from 12 in. (300 mm) and 24 in. (600 mm) thick test panels representing walls on the proposed project. All cores are 6 in. (150 mm) in diameter and drilled through the full depth of the panel. Figure 5 shows the core drill setup on the mockup panel for extracting the needed cores. These cores Shotcrete • Summer 2016 11


2016SummerShotcreteEMag
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