Numerical Study on the Influence of Unbonded Reinforcement on Strengthened Reinforced Concrete Beams

Date of Award

6-27-2025

Date Published

August 2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Riyad Aboutaha

Keywords

finite element analysis, reinforced concrete, strengthening, unbonded reinforcement

Subject Categories

Civil and Environmental Engineering | Civil Engineering | Engineering

Abstract

Reinforced concrete (RC) beams are fundamental elements in structural systems owing to their cost-effectiveness and high load-bearing capacity. Nevertheless, various factors—such as changes in usage requirements, design deficiencies, structural modifications, and material degradation—may necessitate strengthening interventions. In particular, environmental effects (e.g., freeze-thaw cycles, chloride ingress, carbonation), cyclic loading, and fatigue can accelerate deterioration, compromising structural performance over time. To address these challenges and enhance the load-carrying capacity or extend service life, external strengthening techniques using materials such as carbon fiber reinforced polymer (CFRP) and steel plate (SP) are widely adopted. Although externally strengthened, RC beams containing internal steel reinforcement that is partially unbonded from the surrounding concrete may still exhibit reduced flexural strength and durability, especially in critical regions subjected to repeated or excessive loading. This study investigates the effect of partially unbonded reinforcement on the structural performance of strengthened RC beams, with a particular focus on how reinforcement debonding—defined here as the partial loss of bond between internal steel bars and surrounding concrete—affects flexural response and failure characteristics in CFRP- and SP-strengthened beams. A finite element analysis (FEA) model was developed to simulate different degrees of reinforcement debonding and examine their influence on stress distribution, internal force transfer, and ultimate strength. The model was validated against experimental data from previous studies, showing strong agreement with test results. A total of 296 RC beams were analyzed using the validated FEA model to evaluate the effects of five key parameters on flexural behavior: reinforcement ratio (ρ), shear span-to-depth ratio (a_v/d), debonding degree (λ), type of strengthening material (CFRP/SP), and thickness of strengthening material (t_str). To investigate how these factors influence the flexural behavior of externally strengthened RC beams with unbonded reinforcement, an extensive parametric study was carried out. Based on the FEA results, two analytical models were proposed to predict the remaining flexural capacity of strengthened RC beams with unbonded reinforcement. To support model development, a new dimensionless parameter, Ψ, was introduced, representing the ratio between the equivalent plastic region length (L_0) and the neutral axis depth (c). Separate multivariable quadratic regression models were generated for each strengthening approach using IBM SPSS Statistics 27.0.1. The resulting models for CFRP- and SP-strengthened beams demonstrated good alignment with the FEA results, confirming their effectiveness in estimating residual flexural performance. Owing to the complexity of these analytical models, their direct application in engineering practice is challenging. Therefore, simplified analytical models were developed to estimate the residual ultimate moment capacity of strengthened RC beams with unbonded reinforcement. Comparisons with FEA results confirmed the high accuracy of the simplified models in predicting the residual flexural capacity for both CFRP- and SP-strengthened beams. Insights gained from this work improve the understanding of how unbonded reinforcement affects the behavior of strengthened RC beams, with implications for enhancing strengthening, rehabilitation, and repair practices. The proposed models enhance the accuracy of residual flexural capacity predictions, supporting the development of effective strategies for structural assessment and design. Future research should focus on the experimental validation of simplified models and the exploration of alternative strengthening approaches to further mitigate the adverse effects of reinforcement debonding in strengthened RC beams.

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