Experimental study on bond performance between UHPC and steel bars

This study presents the pullout experiments investigating the bond performance between steel bars and UHPC, which is becoming more significant due to the increasing utilization of UHPC as connection in precast concrete structures. Various parameters are considered in the experiments, including the bond length (l), steel diameter (d), and volume fraction of steel fibers in UHPC. Particularly, the current tests also consider the twin-steel bar configuration, which is commonly adopted in practice but scarcely incorporated in previous studies. The design of test specimens is introduced, followed by the test setup, measurement, and loading protocol. The observed damaged and measured responses are then presented, based on which the influence of each parameter is investigated. The results show that (1) the relative bond length of steel bars (n = l/d) is most substantial for the bond performance, with greater n leading to lower bond strength τu; (2) larger d causes reduced τu as well, due to lower ratios between ribs and diameter, as well as perimeter and area, of rebars; (3) the volume of steel fibers and steel configuration are not that significant factors on τu. Based on these results and reliability theory, the minimum bond length is finally suggested as 4d by referring to current design codes.