Fibre reinforced concrete beams under impact loading

Impact tests were carried out on small concrete beams reinforced with different volumes of both polypropylene and steel fibres. The drop height of the instrumented drop-weight impact machine was so chosen that some specimens failed completely under a single drop of the hammer, while others required two blows to bring about complete failure. It was found that, at volume fractions less than 0.5%, polypropylene fibres gave only a modest increase in fracture energy. Steel fibres could bring about much greater increases in fracture energy, with a transition in failure modes occurring between steel fibre volumes of 0.5% and 0.75%. Below 0.5%, fibre breaking was the primary failure mechanism and the increase in fracture energy was also modest; above 0.75% fibre pull-out was the primary mechanism with a large increase in fracture energy.