Experimental research on bond behaviour between high-strength steel bars and ultra-high-performance fibre-reinforced concrete

The ability of high-strength steel bars (HSSB) and ultra-high-performance fibre-reinforced concrete (UHPFRC) to work together depends on the bonding properties of the two materials. In this study, pullout tests were performed on thirty UHPFRC specimens and three normal concrete specimens to examine the bonding performance of the HSSB embedded in UHPFRC. The parameters investigated included the steel fibre volume content, bar diameter and relative cover thickness, relative anchorage length, stirrup ratio, and concrete type. The bond failure pat-terns, pull force, and slip of the bars were measured. The test results indicated that three types of failure modes (splitting, pullout and splitting-pullout) were obtained from the tests. The specimen strain field was analysed using digital image correlation (DIC) technology to determine the strain development at the respective stages of the UHPFRC specimen based on the two failure modes. Compared with normal concrete specimens, UHPFRC specimens exhibit better ductility and bonding performance owing to the fibre-bridging effect, which limits the development of cracks. The ultimate bonding strength was enhanced with an increase in the steel fibre volume content, relative cover thickness, and stirrup ratio, whereas it decreased with an increase in the relative anchorage length. In addition, a formula for calculating the ultimate bonding strength between the HSSB and UHPFRC was developed. Good agreement was identified by comparing the calculated results with the experi-mental results and existing data in the literature. A calculation formula for the critical anchorage length is also proposed, which can be used in structural design.