Fatigue behavior analysis and life prediction of CFRP-strengthened reinforced concrete structures / Zou Chuanlong

Zou , Chuanlong (2024) Fatigue behavior analysis and life prediction of CFRP-strengthened reinforced concrete structures / Zou Chuanlong. PhD thesis, Universiti Malaya.

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Abstract

Infrastructures are frequently vulnerable to sustained cyclic loads and structural vibration. The accumulated cyclic stresses will induce fatigue in the structures and contribute to their inadequate service lifespan. Consequently, analyzing the present structural health status by the structural stiffness measurement is crucial. Meanwhile, a significant issue in structural health monitoring is identifying fatigue damage in concrete structures that have been retrofitted with Carbon Fiber Reinforced Polymer (CFRP). Although the finite element (FE) method has significantly contributed to modeling the damage mechanisms of concrete, reinforcement, and CFRP, further research is still needed in this area. For instance, reinforced concrete (RC) slabs reinforced with externally bonded CFRP (EB-CFRP) technology, and RC beams strengthened with side-bonded CFRP (SB-CFRP) technology due to narrow soffit constraints, must be evaluated and predicted for high-cyclic fatigue and progressive damage. Therefore, this study aims to investigate the fatigue performance and dynamic progressive damage behavior of strengthened RC structures under high-cyclic fatigue loadings using nonlinear FE analysis. Initially, a new model was recommended for predicting the concrete's residual strength of unstrengthened RC slabs. The accuracy of the suggested model and the FE simulation was validated by comparing the predicted natural frequencies, mode shapes, residual strength, and crack characteristics of specimens with the experimental results. Then, a novel model for determining the dynamic stiffness of RC slabs was developed for strengthened RC structures such as the RC slabs with EB-CFRP and RC beams with SB-CFRP. Finally, this research developed a cyclic-dependent CFRP concrete interface degradation model that could be utilized to analyze the dynamics of fatigue RC structures and find damage in structures that have prestressed EB-CFRP reinforcement. Results showed that the cumulative degradation of the natural frequency, stiffness, and damage development of steel rebars and concrete increased as the fatigue loading cycle increased, indicating that the dynamic response and fatigue damage for RC slabs in the later stages of high-cyclic fatigue loading were more severe. Validation of the dynamic stiffness model demonstrated its capability in predicting the stiffness of fatigued and damaged RC slabs. It was discovered that prestressed EB-CFRP-strengthened RC slabs can significantly affect the vibration response of specimens. When the prestressing and fatigue loads were high, the prestressing altered the damage distribution at the CFRP concrete interface, leading to debonding of the CFRP near the ends of the RC slabs. Fatigue failure modes in non-prestressed and low-prestressed SB-CFRP-strengthened beams primarily involve rebar rupture, while high prestress levels lead to CFRP delamination. Properly placed end U-shaped wrapping effectively reduces CFRP-concrete interface damage caused by highly prestressed SB-CFRP, extending the structure's service life. In addition, a simple model is proposed to predict the fatigue life of externally bonded CFRP reinforced structures, considering combined load and prestress levels. The results provide practical insights for analyzing the dynamic behavior of existing structures by considering the nonlinear progressive damage and may improve the efficiency of structural damage detection.

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