Theory of the Strength of the Bond between Rebar and Recycled Aggregate Concrete after Freeze–Thaw Cycles in a Stress State: Biaxial Lateral Compression

Recycled aggregate concrete (RCAC) has emerged as an environmentally friendly construction material that contributes to waste reduction and resource preservation. However, freeze–thaw damage can exacerbate the weak interface properties of recycled concrete aggregates (RCA), thus limiting their application in engineering. The anchorage section is a frequently damaged area, especially under complex lateral loads, such as biaxial lateral compression loads and biaxial lateral tensile loads. To investigate bonding properties of rebar in RCAC, this paper considered the number of freeze–thaw cycles, the biaxial lateral pressure ratio, and the RCA substitution rate as key variables. Bond-slip characteristics were analyzed, and a bond strength theory was introduced based on the stress distribution function and destruction surface stress analysis. The results reveal three failure patterns: splitting-pullout, pullout, and rebar yielding. When the average lateral compressive stress is lower than 0.12 fcu, the failure pattern is splitting and pullout, and when the average lateral compressive stress is greater than 0.21fcu, the failure pattern is rebar yielding; when the average lateral compressive stress is in between, the failure pattern is pullout. The occurrence of patterns and their corresponding strengths can be predicted using the proposed bond strength theory, which integrates a modified softened sleeve theory and an octahedral strength theory. These theories can not only describe the coupling influences of elastic-plastic sleeve characteristics, complex stress states, and the dynamic pullout process but also broaden the application of previous theories. The test results of the specific bond strength values are in agreement with the theoretical calculations.