Effect of CFRP confinement on bond-slip behavior of steel rebar in low-strength concrete: Experimentation, prediction and parametric study

The bond strength between steel rebar and low-strength concrete (LSC) is critical for the structural integrity of reinforced concrete structures, particularly in seismic regions where weak bonding can compromise load transfer and overall stability. To address this issue, an experimental study was conducted on 216 unique pullout specimen designs, incorporating concrete compressive strengths of 5, 10, 15, 20, 25, and 30 MPa, rebar diameters of 12, 16, and 20 mm, concrete cover sizes of 25, 35, 45, and 55 mm, and CFRP confinement conditions (unconfined, and confined with one or two CFRP layers). Using the LASSO optimization algorithm, a closed-form solution was developed to predict bond strength, integrating key influencing parameters. The results demonstrated that CFRP confinement significantly enhanced bond strength, with a maximum improvement of 184 % for specimens with two CFRP layers. The increase in concrete cover led to a rise in bond strength, with a maximum recorded bond strength of 24.72 MPa in CFRP-confined specimens. Conversely, increasing rebar diameter from 12 mm to 20 mm resulted in a bond strength reduction of up to 27.7 %, highlighting the inverse relationship between rebar size and bond performance. The parametric sensitivity analysis revealed that compressive strength had the most substantial effect on bond strength (81.8 %), followed by concrete cover (23.5 %) and CFRP confinement (13.6 %), while rebar diameter had a negative impact (-18.9 %). The developed predictive model provides a practical and reliable tool for estimating bond performance, demonstrating that CFRP confinement can enhance the bond strength of LSC to levels comparable with higher-strength unconfined concrete. These findings offer valuable insights into the design, assessment, and strengthening of reinforced concrete structures, contributing to the development of efficient retrofit strategies for improving structural resilience.