A Study on Stress-Strain Relationship of Concrete with Uniform and Non-Uniform Confinement

Abstract

Strengthening and repairing of concrete columns by fiber-reinforced polymer (FRP) have received much attention from material scientists and researchers, and it has been widely applied in construction industries. Although the mechanical response of concrete members repaired and strengthened by FRP has been studied extensively via experimental and theoretical approaches, critical issues are yet to be addressed. In practical engineering, for example, more complicated multiaxial stress exists on the concrete structure. Furthermore, non-circular concrete structures such as square and rectangular sections with FRP confinements are usually implicated with non-uniform pressure and implicit in the experimental work. Existing studies used experimental results of concrete cube subjected to multiaxial active confinement for the calibration of concrete under passive and non-uniform confinement due to the lack of reliable triaxial test equipment that can generate passive confinement. On the other hand, a stress-strain model for damaged and subsequently repaired concrete is not available for the design of repair works. Therefore, extensive concentrically and eccentrically loaded non-uniform confined and repaired concrete tests, including versatile concrete grades, are required for developing constitutive models of non-uniform confined and FRP-confined predamaged concrete structures, respectively.

The primary research presented in this thesis begins with the stress-strain relationship of non-uniform confined concrete cubes. As of now, no studies have examined the effects of aggregate size and load eccentricity on the mechanical response of non-uniform confined concrete columns. This thesis thus presents analysis of the size effect through a study of the influence of aggregate size on the stress-strain behavior of non-uniform confined concrete columns. Experimental tests were conducted on concrete cubes with different aggregate sizes and fixed specimen dimensions. Aggregate size showed no significant effect of the mechanical response of non-uniform confined concrete columns but had a substantial impact on the transitional region of the stress-strain curve of non-uniform confined concrete columns. Based on the experimental results under concentric loading and Bazant’s size effect law, this study presents a new method for modeling the stress-strain relationship of non-uniform confined concrete columns allowing for size effect. Moreover, it was found that the stress-strain models developed from concentric loading tests are inappropriate for eccentrically loaded non-uniform confined concrete columns. Based on the test results of eccentrically loaded non-uniform confined concrete columns, a new stress-strain model was developed to simulate the mechanical response of non-uniform confined concrete columns under eccentric loading.

Another crucial issue is the stress-strain relationship for damaged and subsequently repaired concrete with FRP. An extensive experimental and analytical investigation was undertaken in this thesis to study the mechanical responses of concentrically and eccentrically loaded FRP-confined predamaged concrete. The existing literature, to the best of our knowledge, reaches no consensus on the effect of damage on the partial FRP confined concrete columns. Thus, the effect of damage and FRP strips are selected as test parameters. To accurately capture the dilation of the predamaged concrete, a digital image correlation (DIC) system was used. This thesis presents analysis of the effect of predamage on the stress-strain behavior of partial FRP-confined concrete using existing constitutive models. This study also revealed that the classical model of FRP-confined concrete by the previous studies is not only applicable to the original form of concrete structures but also applicable to its damaged state.

Moreover, it was found that stress-strain models developed from concentric loading tests of undamaged concrete are inappropriate for eccentrically loaded FRP-confined predamaged concrete cylinders because of the induced damage in the concrete and the effect of load eccentricity. An analytical study of FRP-confined predamaged concrete cylinders under eccentric loading was conducted based on the test results. A new model comprising damage degree and load eccentricity was developed to predict the stress-strain relationship of eccentrically loaded FRP-confined predamaged concrete cylinder.

Lastly, the energy balance method was introduced to model the ultimate strain of FRP-confined predamaged concrete cylinders. This new method can be used to predict the ultimate strain of FRP-confined predamaged concrete cylinders under both concentric and eccentric loadings. Because this model is established in a unified form without any restrictions to cross-sectional geometry, it applies to different shapes of repaired columns. On the other hand, although the energy balance method is unconservative for heavily damaged concrete, it exhibits a better performance with respect to the existing models when compared with the test result.

Date of Award	5 Aug 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	C W LIM (Supervisor)