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.

Owing to its enhanced strength, ductility, and resistance to harsh environments, increasing research attention has been paid to alkali-resistant glass fiber reinforced concrete (ARGFRC). This paper presents experimental studies concerning the effects of fiber content on the mechanical properties and microstructural characteristics of ARGFRC. The amount of glass fiber was considered at levels of 0.0, 0.3, 0.5, 0.8, 1.0, 1.3, and 1.5 of the concrete volume. The compression, flexural, impact resistance, scanning electron microscopy, and energy dispersive spectroscopy tests were conducted. The flexural load-deflection curve, flexural strength, flexural toughness index, flexural fracture energy, postcracking stiffness, postpeak stiffness, and impact resistance energy absorption were obtained. Then the changing law affected by fiber content on these mechanical properties was further analyzed, and the corresponding equation was fitted. When fiber content was 1.5, the flexural toughness index I₅, I₁₀, and I₂₀ values were 4.0, 5.9, and 8.9, respectively, and increased by 3.07.9 times. Glass fiber incorporation could increase the ductility and delay the brittle failure when the fiber content reached 0.8. The largest postcracking stiffness was calculated at 36.174kN/mm with a fiber content of 0.8. The higher the fiber content, the larger the postpeak stiffness of the tested beams. Impact resistance test results demonstrated that the optimum fiber content was 1.3. As the fiber content increased, the effect of the concrete grout on the fiber packaging decreased, according to the scanning electron microscopy analysis. The energy dispersive spectroscopy observation proved that adding a certain fiber content did not affect the concrete hydration reaction.