Analysis of flexural load-bearing capacity of damaged concrete beams reinforced with CFRP

Abstract This study addresses the analysis of the flexural bearing capacity of CFRP-reinforced damaged reinforced concrete beams. Considering the influence of secondary loading on the initial tensile strain at the tension edge of concrete, combined with stress–strain relationships, it is established that the stress distribution in the compressed zone of concrete forms a triangular shape. The calculation and analysis are provided for two failure modes during the yield stage: one where compressive reinforcement has not yielded and another where it has yielded. Additionally, calculations for two ultimate failure modes—CFRP rupture and crushing of compressed concrete after yielding of tensile reinforcement—are presented regarding the height coefficient in the compressed zone. The CFRP-reinforced reinforced concrete beams are categorized into three damage levels based on load conditions: before cracking ( M i < M k ), from cracking to yielding ( M k ≤ M i < M y ), and from yielding to ultimate load ( M y ≤ M i < M u ). When M i ≥ M k i ndicates varying degrees of cracks present in unreinforced beams, according to ‘Code for Design of Concrete Structures’ (GB50010-2010), an approximation can be made regarding internal moment arms by taking ηh 0 for tensile reinforcement’s contribution to flexural capacity in reinforced beams. An approximate calculation method for flexural bearing capacity during yield stages is proposed specifically when M y ≤ M i < M u under unloading conditions post-yielding. In this context, we consider both enhanced strength due to steel reinforcement and insufficient utilization of actual tensile strain capacities offered by carbon fiber composites through a strength utilization factor affecting post-CFRP strengthening flexural capacity analysis. Experimental validation is conducted, comparing results with those derived from our proposed calculations for CFRP-reinforced damaged concrete beams’ flexural bearing capacities; findings indicate a mean value of 1.015182 and a variance of 0.001455, demonstrating high accuracy.