Stress-strain behavior of FRP-confined concrete column

Abstract

Existing reinforced concrete structures without adequate transverse reinforcement are vulnerable to above moderate earthquakes. Existing columns do not have the high ductility required for withstanding a seismic event and fail prematurely at low flexural ductility. Therefore retrofitting columns is especially important for rehabilitation of existing structures. Extensive research works have been conducted in the last three decades in this area. Theoretical and experimental studies have already demonstrated that external jacketing is highly effective for improving the strength and ductility of Fibre-reinforced polymer (FRP)-confined circular concrete columns. External jacketing has also been found very effective in enhancement of ductility of square and rectangular columns, though less confinement is provided compared with circular columns. Experimental works have been undertaken to study the effect of FRP confinement of square and rectangular columns. Nevertheless, a more extensive understanding of how the mechanism works is still needed. Another important and challenging part of external jacketing is the strain-softening behavior. It has been generally accepted that localization occurs in concrete columns under compressive loading. Compressive fracture energy in localization and fracture zones has been investigated. Also, theoretical analyses of concrete column under uniaxial and tri-axial confinement have been reported. The results show a reasonable and accurate representation of the softening behavior. With lower FRP confinement, the stress-strain behavior of FRP-confined columns results in a strain-softening behavior, which should also be explained by using compressive fracture energy. Such investigations cast some light on the strain-softening behavior of FRP-confined concrete column. In this research work, both theoretical and experimental research were used to study strength and ductility of FRP-confined concrete columns. Particular attention was focused on the effect of cross-sectional aspect ratio on the strength of CFRP-confined rectangular concrete columns and the strain-softening behavior of low FRP-confined concrete column. Qualitative as well as quantitative experimental works were conducted. Firstly, CFRP-confined rectangular concrete columns were studied for effect of the aspect ratio. Most of the research works reported in extant literature have concentrated on the behavior of FRP-confined circular and square columns. This work focuses on the behavior of CFRP-confined rectangular concrete columns. Clear conclusions are drawn on how the aspect ratio affects the strength and ductility of CFRP-confined rectangular concrete columns. And then, a unified stress-strain model of concrete for circular, square and rectangular columns confined by FRP jackets is proposed. Through the unification, variation of parameters is found to be continuous and consistent, in contrast with the inconsistency or discontinuity in results from some other existing models. The unification also improves the performance of the stress-strain model by greatly extending the range of the parameter space, thus allowing a clearer picture of the trends and variations in the results. Then, circular columns with different heights were used to investigate the strain-softening behavior with low FRP confinement. Results of theoretical analysis of concrete column under un-axial and tri-axial confinement show a reasonable and accurate representation of the softening behavior. With lower FRP confinement, the stress-strain behavior of FRP-confined columns results in a strain-softening behavior, which should also be explained by using compressive fracture energy. In this research, a large series of FRP-confined concrete columns were tested under concentric loading to investigate the strain-softening behavior. Digital Image Correlation (DIC) technique was used to measure the strain along the column height and to obtain the localized strain. Measurement of platen-to-platen by LVDT is compared with the DIC results and a correction method of strain measurement is proposed. New models are proposed to predict the strain-softening behavior. The effect of the ultimate residual stress is discussed. Based on the definition of the ultimate residual stress, the area of the post-peak inelastic displacement is calculated. After that a parametric study is conducted to analyze the influence factors of the proposed function. New equations are proposed to predict the compressive fracture energy and the length of the fracture zone. By incorporating the compressive fracture energy, a stress-strain model is proposed to predict the strain-softening behavior. The performance shows good agreement with experimental results.

Date of Award	14 Feb 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yufei WU (Supervisor)