The effect of concrete type and rebar diameter on bond strength in reinforced concrete structures

Concrete is the most widely used building material in recent decades, and researchers are innovating in the production of this material to improve some of it’s properties such as strength and durability. A part of these innovative efforts has led to the production of a new class of concrete called ultra-high performance concrete (UHPC). UHPC has the unique mechanical features that provide opportunities for the development of new techniques of construction, repairs and renovation of concrete structures. While most of the recommended relationships in the standards are based on normal strength concrete (NSC), it is necessary to know more about ultra-high performance concrete behavior. This study aimed to investigate bond behavior (the interaction between rebar and concrete) because it directly affects the performance of concrete structures. 24 cubic concrete specimens (20 cm and 25cm wide) with a centered rebar and two bond lengths, using UHPC and NSC, were subjected to pull-out tests. Concrete’s compressive strength and relative cover, bond length, and yield stress of the rebar significantly influence the failure mode. UHPC reduces the embedded length of rebars by increasing bond strength up to 5 times that in NSC. Regarding to the bond-slip curves, plain UHPC (used in this study) had less ductility than NSC. In NSC, increasing the bond length increases the maximum bond stress, while in UHPC, for increasing bond length, the maximum bond stress decreases. New relationships have been proposed to predict bond-slip behavior.