Liu, Xiangsheng
(2025)
Shear performance of reinforced concrete (RC) beams strengthened with mortar-based composites under monotonic and fatigue loading.
PhD thesis, University of Nottingham.
Abstract
Reinforced concrete (RC) beams form the backbone of modern structures, yet their performance is increasingly compromised by aging, environmental degradation, outdated design standards, unauthorised modifications, increased load demands, impact damage, poor construction quality, and corrosion. These challenges have significantly heightened the demand for effective structural maintenance and strengthening strategies. While fibre-reinforced polymers (FRPs) are widely adopted due to their high strength-to-weight ratio and design flexibility, their limitations—such as poor fire resistance, environmental toxicity, and incompatibility with concrete substrates—restrict their applicability. In this context, mortar-based composites, including Steel-Reinforced Grout (SRG) and High-Performance Fibre-Reinforced Concrete (HPFRC), have emerged as promising alternatives for enhancing the shear capacity of RC beams. Despite their potential, research on SRG and HPFRC systems remains limited, particularly under cyclic and fatigue loading conditions.
This study aims to evaluate the application of SRG and HPFRC jacketing for the shear strengthening of RC beams. The research begins with a comprehensive literature review and the establishment of a database containing 218 samples of RC beams strengthened with mortar-based composites. This database facilitates the analysis of key design parameters influencing shear strengthening performance and assesses the accuracy of traditional empirical models for shear capacity prediction.
Subsequently, experimental investigations evaluate the static and fatigue performance of SRG-strengthened beams, with comparative analyses including Carbon Fabric Reinforced Cementitious Matrix (CFRCM) and Steel-Reinforced Polymer (SRP) systems. Unlike these systems, HPFRC, which lacks textile reinforcements, is studied independently to account for its unique mechanical properties. Key parameters, such as shear span-to-depth ratio (a/d), textile density, jacket configuration, and mortar properties, are systematically explored. Results confirm the effectiveness of all strengthening systems in enhancing shear capacity, with fully wrapped SRG systems uniquely capable of transforming failure modes from brittle shear to ductile flexural behaviour. Predictive models for shear capacity and fatigue life were developed for SRG and HPFRC systems based on experimental findings.
In addition, nine machine learning (ML) models were developed to predict the shear capacity of FRCM-strengthened beams, with XGBoost achieving the highest accuracy and stability. Shapley Additive Explanations (SHAP) and Partial Dependence Plots (PDP) were employed to enhance model interpretability and identify key factors influencing shear capacity, such as beam depth, concrete compressive strength, and mortar thickness. A novel finite element analysis (FEA) model for SRG systems was also proposed, addressing limitations in existing methods by independently modelling the behaviours of mortar and textile components. This innovation enables accurate simulation of premature delamination in high-density SRG systems, providing a robust framework for future design optimization.
This research validates the efficacy of mortar-based composites for shear strengthening of RC beams, advancing understanding and application in both static and fatigue contexts. The findings bridge critical knowledge gaps in the performance of SRG and HPFRC systems, enhance the predictive accuracy of design models, and offer innovative tools and methodologies to improve the resilience of aging infrastructure.
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