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2016FallShotcreteEMag

Sustainability results when combined with 10% replacement of cement by silica fume (SF). In general, the fine particles (0.04 to 4 mils 1 to 100 μm) of glass powder allow higher water content in the shotcrete without creating stability issues once on the receiving surface. This is of great interest, as it improves the plasticity of the material, resulting in a lower rebound and a better reinforcing bar encasement (ACI Committee 506 2016; Beaupré and Jolin 2001). An increase in the water-binder ratio (w/b) of the shotcrete, however, can impact the mechanical strength and the overall service life of the structure (Fig. 2). In fact, some very novel observations were made in the placement phase of GP-shotcrete and can possibly be explained either by a shear thinning or thixotropic behavior created by the glass powder in the cement paste (ACI Committee 238 2014). Such behaviors are fairly new in the drymix shotcrete industry and have to be carefully evaluated. In fact, using this kind of material and accommodating such behavior could completely challenge our approach to shotcrete. Once these materials and behaviors are understood and mastered, they could help us significantly improve the quality and performance of dry-mix shotcrete. More recently, other waste materials have been evaluated in dry-mix shotcrete mixtures, but in this case as replacement for natural aggregates (Gagnon 2016). First, plastic aggregates have been produced from collected plastic containers crushed into small particles (Fig. 3). In general, plastic can be recycled in a sustainable way, but the plastic used in this research comes from the portion of plastic that cannot be properly sorted in the plant, making it unsuitable for recycling and thus for resale. Secondly, rubber aggregates made from shredded used tires have been used (Fig. 4). Car and truck tires are consumed in large quantities every year, but there are still not many ways to reuse them after their initial life. Therefore, rubber powders have become available in high volumes and are particularly cheap. Both alternative aggregates have shown workability issues in the case of cast-in-place concrete (Nacif et al. 2013; Saika and Brito 2012). This is why the dry-mix process is the most suitable method to use these products without sacrificing workability. Plastic and rubber have been tested in mixtures at 20% replacement of the total volume of aggregates as a substitute for sand (Fig. 5). The results of this study have shown, as expected, a reduction in mechanical strength due to the poor mechanical properties of these new aggregates. However, the quality of the shotcrete Fig. 2: Behavior of dry-mix shotcrete containing glass powder and silica fume as cement replacement Fig. 3: Plastic aggregates (0.04 to 0.2 in. 1 to 5 mm) from crushed plastic containers Fig. 4: Rubber aggregates (0.04 to 0.12 in. 1 to 3 mm) from shredded tires Shotcrete • Fall 2016 29


2016FallShotcreteEMag
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