Digital imaging monitoring of fracture processes in hybrid steel fiber reinforced concrete

Ten hybrid fiber reinforced concrete mixtures are designed to investigate the effect of three different steel fibers and their combination on the Mode I fracture of concrete. The fracture process and damage characteristics are studied by measuring the full-field displacement and strain on the surface of the specimen by the digital image correlation technique. A horizontal strain standard deviation method is proposed for determining the tip of the fracture process zone (FPZ). The FPZ length before the peak load and the damage degree factor (Ddf) are measured. The results indicate four stages during the fracture process development: microcrack propagation, fracture formation, rapid fracture expansion, and post-peak propagation. The critical FPZ length increases from 21.6 mm to 79.1 mm when the fiber volume fraction increases from 0 % to 2 %. The hybridization of the reinforcement has a negligible effect on the growth trend of the FPZ length and the critical FPZ length. Within the reinforcement content of 0 %–1.5 %, the inhibiting effect on the Ddf produced by the hybrid fibers is enhanced with increasing volume fraction.