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2013FallShotcreteEMag

final aspect of the concrete produced. Unfortu- research. Unfortunately, there are absolutely no nately, when glass is crushed in particles larger studies available about the effect of glass powder than 75 µm, it induces alkali silicate reaction in either dry- or wet-mix shotcrete. This subject (ASR) in concrete.11 The ASR, often-called “con- is, therefore, the focus of a study recently under- crete cancer,” results in the creation of an expan- taken in the Shotcrete Laboratory at Laval sive (swelling) gel that produces internal cracking University in Québec City, QC, Canada. In this in the aggregates and the hardened cement paste study, various levels of cement replacement with matrix. Concrete suffering from ASR exhibits glass powder (with and without silica fume) are reduced mechanical strength and often severe to be evaluated in both shotcrete processes. cracking, which can contribute to accelerated Obviously, the hardened shotcrete properties corrosion of embedded steel reinforcement. will be evaluated and compared to reference Luckily, when the recycled glass is ground into finer particles, it does not create ASR (some studies even suggest that ASR is reduced in the presence of reactive aggregates12). Even better, studies have shown that glass powder exhibits pozzolanic activity in which it reacts to form a higher-quality (densified) hydrated cement matrix. Some cases have even shown improved compressive strengths.2 Other encouraging studies revealed that glass powder enhances the properties of concrete consisting of fly ash or silica fume. The compressive strengths are higher when glass powder is used with silica fume than when silica fume is used alone.2 Similar observations have been reported for concrete made with fly ash and glass powder; in this later case, dura- bility was also noticeably enhanced.13 Finally, another interesting effect is the water-reducing effect produced when using glass powder. Indeed, despite the high surface area of the glass powder, its surface properties are such that it does not attract water in fresh concrete, leaving more water available for improved workability.14 In real life, visionaries have already accepted partial replacement of cement by glass powder. In Montreal, QC, Canada, the designers of the Centre of Sustainable Development have used glass powder for some of their building’s con- crete. Using this approach helped the building reach a LEED® Platinum certifi- cation.15 The double impact of dimin- ishing the amount of cement in the concrete and avoiding dumping the glass in a landfill has an important impact on sustainability throughout the concrete industry in Canada. the Use of recycled Glass in Shotcrete With its obvious potential, the use of glass powder in shotcrete appears to be an extremely interesting avenue of Shotcrete • Fall 2013 15


2013FallShotcreteEMag
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