Shotcrete is often subjected to impact and other dynamic loads in service. These loads occur due to excavation blasting, traffic, sudden ground movements, rock bursts, and seismic activity. Sufficient anecdotal evidence exists of the ability of fiber reinforcement to enhance the resistance of shotcrete to such dynamic loads, but only limited real data exists in this regard.In this study, impact resistance of wet-mix shotcrete reinforced with ten different types of fibers was investigated using instrumented impact tests and compared with their quasi-static response. Both beam and plate specimens were tested. In Part 1 of this two-part paper, results with beam specimens are given. In Part 2 of the paper, data with plate specimens will be presented and correlated with the beam data reported here. Based on the beam tests, it is clear that fiber reinforcement is highly effective in improving the fracture energy absorption and toughness under impact loading. However, it is also apparent that the toughness or energy absorption capability of fiber reinforced shotcrete under impact loading is substantially different from that predicted by the standardized toughness tests carried out at slow, static rates of loading. Among the various types of fibers investigated, steal fibers were found to be the most effective in improving the resistance of shotcrete to impact, but the improvements depend on the geometry of the fiber. In the case of synthetic fibers, while polymeric macro-fibers of either polypropylene or polyvinyl alcohol adequately improve the resistance of shotcrete to impact loads, pitch-based carbon micro-fibers are relatively ineffective.

RI~SUMI~In Part 1 of this paper, data on impact resistance of fiber reinforced wet-mix shotcrete obtained by testing beam specimens was presented. Ten different types of commercially available fibers were investigated. It was argued that since shotcrete in many applications is loaded dynamically, slow-rate static tests are not sufficient for a comprehensive characterization of its in-place performance. It was also demonstrated that shotcrete is a highly stress-rate sensitive material, and its performance under impact is very different from that predicted by the slow-rate, quasi-static tests.From a standardization point of view, proper characterization of toughness and energy absorption capability of fiber reinforced shotcrete remains a challenge before the industry. Both beam and plate specimens are prescribed, but little understanding exists of the correlation, if any, between these two specimen geometries. In this paper, the same ten fiber types, tested in Part 1 using beam specimen, were investigated using plate specimens. As before, both static and impact tests were performed. Plate tests showed similar toughness enhancement due to fiber reinforcement, but the relative improvements between fiber types are not necessarily in agreement with those indicated in the beam tests. Plate specimens were also found be less sensitive to changes in the rate of loading from static to dynamic. Considering these issues, and realizing that in practice shotcrete is often loaded in bi-axial bending, the use of plate specimens rather than beam specimens for the characterization of toughness of fiber reinforced shotcrete is recommended.

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