The effect of delay time in cold joints on the bonding and shear strength in concrete

This study investigates the effects of delay time in cold joints on the structural behavior of reinforced concrete (RC) beams and cubes with varying compressive strengths of 30 MPa and 50 MPa. The selected delay times (0, 60, and 180 minutes) were chosen to study their effect on shear strength and bond performance in the critical regions containing a cold joint. The shear strength at the critical regions that contain a cold joint was investigated in the beam without shear reinforcement tested under a four-point load. For the cubes, the bonding strength in the cold joint was indirectly investigated using the bi-surface shear test. Experimental results detected that cold joints significantly influence shear strength and failure modes, specifically for longer delay times. Beams without stirrups dominantly failed in shear, with diagonal cracks initiating near the support, while delay times exacerbated the weakening of bonds, altering stress distribution and crack patterns. Similarly, cubes subjected to bi-shear stress demonstrated varied failure modes, transitioning from cohesive to adhesive and mixed failures depending on the delay time and compressive strength. A higher compressive strength of 50 MPa enhanced the overall shear capacity compared to 30 MPa; however, it also contributed to increased brittleness in failure modes. The findings emphasize that the interaction between delay times and compressive strength plays a crucial role in influencing the bonding characteristics and shear behavior of concrete elements containing cold joints.