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2015SpringShotcreteEMag

Fig. 1: Steel fiber-reinforced dry-mix shotcrete concrete. Indeed, the mechanisms behind the formation of an air bubble during placement of dry-mix shotcrete as well as those involved in rebound are all leading to a final in­place compo- sition, where different factors (such as water- binder ratio, binder content, and air content) can have opposite effects on the resulting compressive strength. What is clear, however, is that even the highest air content measured allowed (4.8%) reached more than satisfactory compressive strength results. Discussion Why Do We Use Air Entrainment? Entire papers could be written and research conducted as to the effect of different shotcrete- Fig. 2: As-shot air content for dry-mix steel fiber-reinforced related parameters (including water content, shotcrete (air-entrained) velocity, and process) on the as-shot air content. However, the real question is whether or not we are able to generate a dense enough network of small air bubbles in the hardened shotcrete to protect it from freezing-and-thawing damage (that is, a small enough spacing factor). Unfor- tunately, the real answer comes from a test usu- ally conducted only when qualifying a specific mixture design (ASTM C457, “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete”); its relevance in QC is little because the test has to be conducted on hardened con- crete/shotcrete samples, usually after more than 28 days. When an air-entraining admixture is incorporated in the mixture, the as-shot air con- tent is an indirect measurement of the level of Fig. 3: Correlation between 28-day compressive strength and success we have in creating the dense small as-shot air content Shotcrete • Spring 2015 23


2015SpringShotcreteEMag
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