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increase permeability and reduce durability. In addition, the fibers act as an internal curing aid, releasing water into the cement paste matrix as the concrete dries. It is believed that this, in conjunction with the higher modulus of elasticity and higher tensile strength of the hemp-based fiber, is what helps mitigate plastic shrinkage cracking in concrete and shotcrete. In addition to the previously mentioned beneficial attributes, the incorporation of the natural hemp-based fiber in wet-mix shotcrete was found to impart several other benefits to the application and performance of shotcrete, as detailed in the Laboratory and Field studies described in the article, which follows. LABORATORY STUDY Shotcrete Mixture Designs In the wet-mix shotcrete laboratory study, a comparative evaluation was conducted on the performance of shotcrete made with the natural hemp-based fiber (mixture designation WNF) compared to a plain control shotcrete without any fiber (mixture designation WP) and a shotcrete with a microsynthetic (polypropylene) fiber (mixture designation WSF). The fiber addition rates for the different mixtures are shown in Table 1. While the natural and synthetic fibers have the same fiber volume addition rate, the quantities added in kg/m3 (lb/yd3) differ because the natural fiber has a relative bulk density (specific gravity) of 1.48, while the synthetic fiber has a relative bulk density (specific gravity) of 0.92. Table 2 shows the concrete mixture proportions for each of the three shotcreted mixtures. The cement, fly ash, coarse and fine aggregate contents, and water added in all three mixtures was the same. The only differences in the mixtures were the type of fiber added and small variations in the amount of water-reducing admixture and air-entraining admixture added to provide the required slump and air content. The shotcretes were designed to meet CSA A23.1/23.2-2014 Class C1 exposure requirements—that is, structurally reinforced concrete exposed to chlorides with or without freezing-and-thawing conditions. Such concrete (or in this case, shotcrete) is required to have a maximum water-cementitious materials ratio (w/cm) of 0.40, a minimum compressive strength of 5100 psi (35 MPa) at 28 days, and be suitably air entrained. The base shotcrete mixture design in Table 2 has a fly ash content of 20% by mass of cementitious material and is typical of structural wet-mix shotcrete used in Western Canada and other parts of North America. The slightly higher waterreducing admixture dosage rate in the WNF mixture is attributed to water uptake by the natural fiber and hence the need to increase the admixture dosage to provide the required slump. Shotcrete Batching, Mixing, and Application The shotcrete mixtures for this laboratory study were pre-batched with aggregates in an oven-dry condition and supplied in 0.5 yd3 (0.38 m3) bulk bin bags. The dry bagged materials were discharged into a pan mixer with counter-rotating paddles, where the mix water and liquid air-entraining and water-reducing admixtures were added. Figure 2 shows dry-bagged material being discharged into Table 1: Wet-mix shotcrete mixtures Mixture type Shotcrete designation Fiber content kg/m3 lb/yd3 % volume Plain WP 0 0 0 Natural fiber WNF 2 3.4 0.15 Microsynthetic fiber WSF 1.35 2.3 0.15 Table 2: Wet-mix shotcrete mixture proportions Material Mixture WP: Plain shotcrete Mixture WSF: Microsynthetic fiber shotcrete Mixture WNF: Natural fiber shotcrete kg/m3 lb/yd3 kg/m3 lb/yd3 kg/m3 lb/yd3 Cement type GU (ASTM Type I) 360 607 360 607 360 607 Fly ash 90 152 90 152 90 152 Coarse aggregate (10-5 mm, SSD) 430 725 430 725 430 725 Fine aggregate (SSD) 1290 2174 1290 2174 1290 2174 Water, L 180 303 180 303 180 303 Water-reducing admixture, L* 1.15 2 1.05 2 1.3 2 NForce fiber — — — — 2 3.4 Microsynthetic fiber — — 1.35 2.3 — — Total 2351.15 3963 2352.4 3965 2353.3 3967 *Water-reducing admixture added during mixing at dosage required to achieve the maximum allowable w/cm and required slump www.shotcrete.org Spring 2017 | Shotcrete 37


2017SpringShotcreteEMag
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