Property Assessment of Hybrid Fiber-Reinforced Ultra-High-Performance Concrete

The purpose of this study was to determine the effect of steel/polypropylene hybrid fibers on the mechanical properties and microstructure of ultra-high-performance concrete (UHPC). Tests were carried out on UHPC without and with fibers (steel and/or polypropylene in amounts of 0.25–1%). In this study, granite or granodiorite coarse aggregate with a grain size of about 2/8 mm was employed. The three-point bending tests displayed prolonged post-peak softening behavior. In addition, increasing the content of polypropylene fibers reduced the fracture energy. Moreover, the SEM results illustrated that adding a certain amount of fibers to concrete considerably changes the microstructure. It was observed that the smallest micro-cracks in the interfacial transition zone between the paste and aggregate occurred in the concrete containing steel fibers.