Modified regression model for bond-slip constitutive relationship between high-strength steel rebars and UHP-FRC based on experimental tests

Ultra-High-Performance Fiber Reinforced Concrete (UHP-FRC) has been widely applied due to its advantages over normal concrete. The bond strength between UHP-FRC and steel reinforcement, especially high-strength steel reinforcing bar, is the premise for an in-depth analysis of UHP-FRC's behavior at the component level. A series of 24 pull-out tests were designed considering key variables including embedment length (3d, 4d, 5d and 6d), the steel grades (HRB500 and HRB600) and diameter of rebars (16 mm, 18 mm and 20 mm) based on a mature UHP-FRC mix design. It was found that the bond properties of UHP-FRC with high-strength steel bars were much different from ordinary concrete. The constitutive relationship originally used to describe the bond-slip of ordinary concrete was no longer applicable. Based on the test data, a modified constitutive model was proposed based on the regression method and the fib Model Code 2010. Due to the excessively high bond strength, the effect of strain penetration that was originally presented only in bars with long embedment lengths was also considered in the new bond-slip relationship. The bond stress distribution of different locations was obtained by innerembedded strain gage and the position function which reflected the change of bond stress was deduced. Combined with the product of the average bond-stress-slip constitutive equation and the position function of UHP-FRC, a more accurate description of the bond-slip constitutive relation was presented. Good agreement with test data was achieved for both the proposed model and position function.