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2017SpringShotcreteEMag

New Hemp-Based Fiber Enhances Wet-Mix Shotcrete Performance By Dudley R. (Rusty) Morgan, Lihe (John) Zhang, and Mike Pildysh An innovative processed natural hemp-based fiber has been developed for use in concrete and shotcrete (wet- and dry-mix shotcrete processes) in lieu of conventional synthetic fibers, which are derived from hydro-carbon (oil and/or gas) feedstock. A driving force behind this development has been the desire to produce a truly sustainable green fiber with dramatic reductions in CO2 emissions into the atmosphere during fiber production, compared to the synthetic (mainly polypropylene) fibers currently being used in the concrete and shotcrete industries. Synthetic microfibers have mainly been used in concrete and shotcrete to help mitigate early-age plastic shrinkage cracking. In the absence of adequate protection and initial curing, plastic shrinkage cracking has been an issue in the shotcrete industry. Figure 1 shows an example of shotcrete used for slope stabilization at a project in California that developed severe plastic shrinkage cracking in unfavorable environmental conditions (high ambient temperatures and winds and a low relative humidity). Initial laboratory and field trials in Calgary, AB, Canada, indicated that natural hemp-based fiber was very effective in mitigating plastic shrinkage cracking. Consequently, the fiber manufacturer decided to commission a comprehensive study to evaluate the performance of the natural hempbased fiber in cast-in-place concrete and wet- and dry-mix shotcretes, compared to plain concrete control mixtures without any fibers and mixtures with a synthetic (polypropylene) microfiber. These studies, conducted in Vancouver, BC, Canada, demonstrated that when used at equivalent fiber-volume addition rates, not only was the natural hempbased fiber more effective in mitigating plastic shrinkage cracking than synthetic microfiber but it also provided many additional benefits in the plastic and hardened concretes. This was particularly true for the wet-mix shotcrete process, where some marked benefits to the shotcrete application and finishing processes were found. This article provides a summary of the findings from the wet-mix shotcrete laboratory study. It also provides observations from a subsequent full-scale field application of wet-mix shotcrete with the natural hemp-based fiber, with ready mixed concrete batching, mixing, and supply and conventional pumping, shooting, and finishing of structural shotcrete walls at a project in California. INTRODUCTION Natural fibers have been used in building products such as brick, mortar, and plaster since ancient times for the known benefits they provide to such products. One of the strongest and most durable of natural fibers has been hemp-based fiber, the same product widely used in marine ropes. Up until now, such fiber has, however, not found any significant use in portland cement-based products. This has been in part because of concerns about the durability of such fiber in a highly alkaline portland cement environment. The manufacturers of the natural hemp-based fiber used in this report have, however, developed a process for production and surface treatment of the fiber, which makes it suitable for use in an alkaline portland cement environment. The hemp-based fiber has a higher modulus of elasticity (E-value) and tensile strength than high-quality polypropylene synthetic fibers. Also, unlike synthetic fibers, which are hydrophobic (repel water), the treated hemp-based fiber is hydrophyllic (absorbs water). This is a highly beneficial attribute for port land cement-based products such as concrete, as it promotes development of both mechanical and chemical bond of the paste to the fiber. It also reduces bleeding in concrete, thus reducing the formation of continuous b Fig. 1: Plastic shrinkage cracking in shotcrete slope stabilization leed channels, which 36 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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