Steel rebar effect on tensile and cracking behavior of UHPFRC based on direct tensile tests and digital image correlation

Ultra-high performance fiber-reinforced concrete (UHPFRC) is usually used as the reinforcement and protection layer for structural members, and adding steel rebars to UHPFRC to form reinforced UHPFRC (R-UHPFRC) is widely employed in engineering practice. This paper focuses on the differences in tensile and cracking behavior triggered by the addition of steel rebars in UHPFRC, which significantly impacts its strengthening effect and durability performance. Six UHPFRC and six R-UHPFRC specimens are tested by direct tensile tests; the mechanical responses and the full-filed and full-process cracking behaviors are characterized by the digital image correlation (DIC) system. The fiber number, dispersion, and orientation are quantitatively obtained and evaluated based on image recognition, while their variations initiated by the addition of rebars are determined. Moreover, the correlations between fiber distribution, tensile mechanical response, and cracking behavior are analyzed, and the interaction mechanism in the full tensile process between the steel rebars and UHPFRC component in R-UHPFRC is studied. Test results show that the UHPFRC component in R-UHPFRC has deteriorated fiber distribution quality and lower load-bearing capacity than UHPFRC. With the addition of steel rebars in UHPFRC, the cracking mode changed from chain-like to full-field development, leading to better multi-cracking behavior. Meanwhile, the employment of steel rebars significantly improved the capacity of UHPFRC to control the crack width and weakened its susceptibility to the non-uniformity of fiber distribution. During the cracking process of R-UHPFRC, a peculiar load redistribution exists between the UHPFRC component and steel rebars. The experimental and analytical results illustrate that the axial tensile yield and peak load of R-UHPFRC can be evaluated based on the superposition principle, while this method may overestimate the stiffness in the service limit state.