Bond strength theory between rebar and recycled aggregate concrete after freeze-thaw cycles under stress state Ⅰ: Uniaxial lateral compression

Abstract

In recent years, the utilization of recycled aggregate concrete (RAC) as an environmental-friendly material in the construction industry has been actively promoted. A sufficient bond between rebar and RAC is crucial for the load carrying capacity and long-term durability of RAC structure. However, many connection sections between rebar and RAC may be destructed, especially in cold regions where freeze-thaw cycles can worsen the deterioration of structures, under uniaxial compression. To investigate the coupling effect of freeze-thaw cycles and uniaxial lateral pressure on bonding properties of rebar and RAC, variables, such as number of freeze-thaw cycles (F), uniaxial lateral pressure stress ratio (ξ), and recycled aggregate replacement ratio (R), are considered in this study. 117 specimens were fabricated and divided into 39 groups for pull-out test. Different bond-slip curves were obtained, and corresponding characteristics were analyzed. Moreover, this paper proposed a novel bond strength theory by combining a superposed stress field distribution function with a splitting surface stress analysis. The distribution function consisted of the stress distribution of uniaxial lateral pressure loading state without pullout and pullout process without lateral pressure, while the stress analysis amended the softened sleeve theory with a spatial intensity model. The results show that both bond strength and peak slip are positively correlated with uniaxial lateral compression but negatively correlated with the accumulation of freeze-thaw damage. The recycled aggregate (RA) replacement ratio has a minor effect on bond strength and peak slip. The factor of uniaxial lateral pressure is dominant and mitigates the adverse effects of freeze-thaw damage and RA substitution on bonding properties. Notably, the methods of decomposition and stress synthesis break through the limitations of traditional bond strength theory, resulting in a wider scope of application. The bond strength theory applies not only to elastic sleeve bonding part but also to softened sleeve bonding part in reinforced concrete building structure. This theory can more accurately predict the failure morphology and ultimate bond strength in freeze-thaw conditions under uniaxial lateral pressure load.