Experimental characterization of the post-cracking response in hybrid steel/polypropylene fiber-reinforced concrete

This paper presents and discusses the results of experimental tests performed on concrete specimens internally reinforced with polypropylene and steel fibers. Specifically, samples of five mixtures (plus a reference plain concrete), characterized by the same total volume of fibers, but different fractions of polypropylene and steel fibers, were tested under compression and in bending. This study was aimed to clarify the influence of different combinations of these fibers on the resulting fracture behavior of Hybrid Fiber-Reinforced Concrete (HyFRC). As expected, the results obtained from compression tests highlighted a negligible influence of fibers in terms of strength and, hence, FRC specimens exhibited a post-peak response more ductile than the reference ones. Conversely, the overall shape of the stress-crack-opening-displacement curves of HyFRC tested in bending was highly influenced by the type of fibers. On the one hand, FRC specimens made of only polypropylene fibers exhibited an excellent post-cracking toughness for the small crack opening ranges of relevance for the Serviceability Limit State, while an apparent decay was observed in terms of post-cracking response, especially at wide crack openings. On the other hand, a marked re-hardening response was observed in the post-cracking behavior for specimens with higher percentage of steel fibers; however, at the same time, the corresponding results showed a relevant scatter.