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2016SummerShotcreteEMag

result of rebound, it does not represent in-place shotcrete strength. The only way to know the shotcrete strength is to take a core from a shot sample panel. Typically, the compressive strength of shotcrete cores exceeds the compressive strength of molded cylinder samples of the shotcrete mixture as delivered in the truck. Because there are different shotcrete panels used for shotcrete sampling, the new guide describes both the difference and purpose of three different shotcrete panels: 1. Material panels; 2. Nozzleman/project qualification panels; and 3. Nozzleman certification panels. Admixtures—Advances in chemistry have improved admixtures and made dramatic impact on plastic concrete properties. In the past, shotcrete had to have a 2 to 3 in. (50 to 75 mm) slump. Today with admixtures, we are able to pump high-slump concrete through a small-diameter line long distances, and yet hang or stack the shotcrete as needed. The increased use of admixtures is one reason shotcrete contractors are competing and winning projects based on cost from traditional form-and-pour concrete contractors. Also, throughout the Guide, the committee recommends testing if a contractor is trying anything new. Shotcrete properties—Shotcrete properties have remained the same with the default compressive strength for structural shotcrete in ACI 506.2-13 as 4000 psi (28 MPa). Air content—Air content in shotcrete has been a source of friction between contractors and inspectors/engineers. Inspectors familiar with concrete become alarmed if the air content in the shotcrete mixture is greater than 6%. It has been repeatedly demonstrated that even with air content in the delivered concrete as great as 10%, the resultant in-place air content will be only 3 to 5%. Most concrete specifications call for 5 to 6% air content for concrete to provide resistance to frequent freezing-and-thawing cycling. Dry-mix shotcrete, however, has for years demonstrated excellent freezingand thawing resistance with only 2 to 3% in-place air content. Likewise, wet-mix shotcrete when shot with 5 to 6% entrained air has also demonstrated excellent freezingand thawing resistance, although the inplace air content of the as-shot shotcrete is only 3 to 5%. In practice, we find about half of the entrained air in concrete is lost during wet-mix shotcrete placement. Shotcrete, however, due to its low watercementitious materials ratio (w/cm) and the high level of compaction that occurs during placement has proven to be resistant to repeated freezing-and-thawing cycles. Boiled water absorption (BWA)— The BWA test can also cause controversy so clarification has been added. The BWA test and volume of permeable voids test is widely used in Canada. However, testing labs in the continental United States don’t have much experience with BWA testing, so erratic results have been reported and 18 Shotcrete • Summer 2016


2016SummerShotcreteEMag
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