Bond performance of steel rebar embedded in 80–180 MPa ultra-high-strength concrete

Ultra-high-strength concrete (UHSC) has attracted attention from engineers because of its great capability on reducing the size of reinforced concrete columns. However, the bond performance of steel rebar embedded in UHSC has not been examined enough yet, although it is a fundamental information for structural design. So, this study comprehensively investigates the bond performance of deformed steel rebar embedded in high-strength concrete (HSC), very-high-strength concrete (VHSC), and UHSC with compressive strengths ranging from 80 to 180 MPa. Different bar diameters (12.7, 15.9, and 19.1 mm), embedment lengths (0.5, 1.0, and 1.5 × bar diameter), cover depths (1, 2, and 3 × bar diameter and center), steel fiber volume fractions (0, 0.5, 1.0, and 1.5%), and yield strengths of steel rebar (normal-strength vs. high-strength) were all considered. Test results indicate that the average bond strength increased significantly with an increase in the compressive strength of the concrete and decreased slightly with an increase in the bar diameter. Average bond strengths of steel rebars in HSC, VHSC, and UHSC were all increased by increasing the embedment length. The cover depth marginally influenced the bond strength when pullout failure was generated, and it significantly increased the bond strength when splitting failure occurred. The incorporation of steel fibers most effectively increased the bond strength in the UHSC mixture, compared with the HSC and VHSC mixtures. Lastly, the widely used prediction models for average bond strength were not accurate for concrete with a compressive strength greater than 80 MPa; thus, a new, appropriate model needs to be proposed in the near future.