Investigation of the stress transfer mechanism of 180-degree rebar hooks embedded in concrete using 3D-RBSM

This study investigated the stress transfer mechanism of 180-degree rebar hooks embedded in concrete by varying bond conditions (bond and unbond) at the hook’s curved portion and the tail extension part, both with and without the hook tail, experimentally and numerically. A numerical model was developed using the Three-Dimensional Rigid Body Spring Model (3D-RBSM) and validated against experimental results from rebar hook pull-out tests. The Voronoi mesh and fiber beam element were used to model concrete and reinforcement respectively. The stress transfer mechanism is comprehensively discussed in the numerical results by evaluating the concrete stress and strain distributions, internal crack patterns, rebar strain distributions, rebar deformations, and failure modes. The results highlight the critical role of the bond along the hook curve part in resisting pull-out load. Once bond degradation occurs in the hook curve part, the tail extension becomes more active and can sustain a significant amount of stress, although it contributes less to the initial stiffness and peak strength. The local stress distribution is strongly related to hook deformation behavior; notably, the tail extension helps maintain the hook geometry and engages in confining the high-compression zone on the inner side of the hook curve. These findings contribute to a deeper understanding of the stress-transfer mechanism of 180-degree rebar hooks.