Plasticity modeling and experimental study of fiber-reinforced polymer (FRP)-confined concrete columns

Abstract

Fiber-reinforced polymer (FRP) has gained its popularity in the field of civil engineering for concrete column rehabilitation owing to its tremendous beneficial effect on strength and ductility enhancement. Hence, the mechanical behavior of concrete under FRP confinement has been extensively studied, both experimentally and theoretically, by researchers and engineers since the 1980s. However, there are still some open issues. One is that no consistent value has been achieved so far as to the efficiency factor of the confinement strain in FRP-confined concrete columns. Its accurate evaluation is essential for predicting the ultimate state of the confinement system. The other is that there is still no general and feasible constitutive model for confined concrete available in the literature. This constitutive model plays a crucial role in the non-linear analysis of FRP-confined concrete structures. Therefore, it is still highly desirable to conduct an experimental study and a theoretical analysis to clarify the inconsistent efficiency factor and to develop a modified plasticity constitutive model catering for FRP-confined concrete columns. First, an experimental program was designed and conducted in the Heavy Structures Testing Laboratory at the City University of Hong Kong. In this experiment, three batches of specimens (39 FRP-confined and nine controlling concrete columns) were tested with two variables, the strength of unconfined concrete and the number of FRP layers. Meanwhile, a coupon test of different FRP laminates and a scanning electron microscopic observation of fibers were conducted to investigate the difference in the physical properties of FRP jackets. Based on the test data, an extensive analysis was conducted to analyze the causes of the inconsistent efficiency factor, which has been generally reported in the range of 0.274 to 1.133. Unlike the current understanding, the curvature and multi-axis stress condition of FRP jackets do not significantly reduce the efficiency factor. A smaller efficiency factor could be induced by the difference in the strain measurement system used for the FRP jacket and flat coupon and in workmanship with which they are made. These differences can be overcome by normalizing the FRP fabrication method and strain measurement system. It has been found that with the proposed normalization scheme, a consistent efficiency factor of about 0.9 can be achieved for circular columns. Second, the Drucker-Prager (DP) plasticity constitutive model was applied to the non-linear analysis of FRP-confined concrete columns. The accuracy of capturing the stress-strain behavior is dependent largely on the involved material parameters that determine the yield criterion, hardening/softening characteristics and flow rule. Through the study on FRP-confined concrete columns at low and medium confinement levels, it was found that: (1) the plastic dilation angle is a function of the axial and lateral stiffness ratio; (2) the friction angle decreases slightly with an increase in plastic deformation; and (3) cohesion (or hardening/softening parameter) is governed by the plastic strains and lateral stiffness ratio. The explicit models for the three parameters were then established in this thesis. An extensive database of FRP-confined concrete columns, including the self-conducted and other tests, was built to modify the model parameters to widen the application scope of the DP plasticity constitutive relationship. The interrelationships among the three parameters were also discussed in terms of energy dissipation, which facilitates the classification of the confinement level. Through finite element analyses with the aid of the commercial software ABAQUS, the modified DP plasticity model was verified to be capable of predicting the stress-strain responses of FRP-confined concrete columns under low, medium and high confinement levels. Finally, some recommendations on the boundary value of the lateral stiffness ratio were proposed for the sake of providing efficient confinement for practical circular concrete columns.

Date of Award	3 Oct 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yufei WU (Supervisor)