Mechanical properties and microstructure of ITZs in steel and polypropylene hybrid fiber-reinforced concrete

To investigate the hybrid effect of steel-polypropylene fiber on the macroscopic mechanical properties, fracture properties and microstructure of the interfacial transition zones (ITZs) of concrete, 16 composite splitting tensile test and three-point bending test specimens with different volume fractions of steel and polypropylene fibers were tested by using the MTS and SEM. The results showed that the hybrid fibers significantly affected the interfacial fracture properties. The 0.4%PF+ 1.0%SF hybrid fiber composition is recommended, while a 30% decrease in interfacial fracture toughness and fracture energy occurs when the concrete contains 0.4%PF+ 1.5%SF. Fiber type affects the aggregate-mortar interface's ITZ micromorphology and has different influence mechanisms.