Impact behaviour of fiber reinforced wet-mix shotcrete

The use of shotcrete for ground support in mines and tunnels has increased dramatically within the past ten years. It has long been used in the construction of dams, canals, and reservoirs, in the repair and rehabilitation of marine, highway and railway structures, and in rock stabilization work. Many of these applications are vulnerable to impact and impulsively applied loads. Traditionally, these loads were taken care of in design by providing welded wire-mesh reinforcement. Efforts are being made to replace conventional steel wire-mesh reinforcement with fibers to reduce the project completion time and cost, and to minimize sand pockets and shotcrete rebound. With this perspective, assessment of impact behavior of fiber reinforced wet-mix shotcrete assumes great significance and importance. In the first part of this study, plain and fiber reinforced shotcrete beams were tested under impact, and the data were compared with static test data on companion beams. It was found that wet-mix shotcrete is a highly strain-rate sensitive material. In general, it is stronger, stiffer, and more energy absorbing under impact than under static loading. This sensitivity is far more pronounced for plain shotcrete than for fiber reinforced shotcrete. Inclusion of fibers considerably enhances the energy absorbing capacity of shotcrete under impact load. This improvement in energy absorbing capacity is, however, not as pronounced as that under static conditions. Beams are essentially one-dimensional elements, subjected to uni-axial stresses. In mines and tunnels, shotcrete linings are invariably subjected to a bi-axial or tri-axial stress-state. To better simulate the multi-axial stress conditions, in the second part of this study plain and fiber reinforced shotcrete plates were tested under static and impact loads. Load vs. deflection data thus obtained was analyzed in a routine manner to calculate fracture energies absorbed at deflections of 2 mm, 5 mm, 10 mm, and 15 mm. As in the case of beams, peak loads and fracture energies of fiber reinforced shotcrete plates were compared with their unreinforced counterpart to get an idea of the effectiveness of fiber addition to the plain brittle shotcrete. Under static load, only steel fibers made a useful addition to the peak load supported by plates. At the high strain-rates associated with impact, however, none of the fibers enhanced the load-bearing capacity considerably. Under static load, all of the macro-fibers considerably improved the energy absorption capability, the prominent being hooked-end steel fiber, followed by flat-end steel fiber, polypropylene fibers, twin-cone steel fiber, and PVA fiber in that order. Similar trends were observed under impact. However, the improvements were much more pronounced under static conditions compared to impact. Plain shotcrete, micro-fiber reinforced shotcrete, and macro-fiber reinforced shotcrete all exhibited increased peak loads at higher rates of loading. Unlike macro fiber reinforced shotcrete beams, macro FRS plates are not sensitive to strain-rate, when it comes to energy absorption capacity. Finally beam data were compared with the plate data. Beams were found to be more stress-rate sensitive as compared to plates. This observation suggests that the stressrate sensitivity of shotcrete is geometry dependent. The roots of this dependence lie, probably, in the cracking process and failure mechanism of beams and plates.Applied Science, Faculty ofCivil Engineering, Department ofGraduat