Stress-Strain Behavior of FRP-Confined Concrete Column under Cyclic Loading

Abstract

The application of Fiber-Reinforced Polymer (FRP) composites as the external jacket for the seismic strengthening or retrofit of Reinforced concrete (RC) columns has been studied extensively over recent years. Theoretical and experimental studies have already demonstrated that external jacketing is highly effective for improving the strength and ductility of concrete columns. The majority of this research, however, has been focused on its monotonic behavior. Little attention has been paid to the application subjected to cyclic loads. For the safe and economic design of FRP jackets, the stress-strain behavior of FRP-confined concrete under cyclic compression must be properly understood and modelled. This study was carried out to investigate the cyclic behavior of FRP-confined concrete columns and determine key factors that govern the stress-strain curve.

First, based on the existing literature and databases, a cyclic stress-strain model is proposed for passively confined (FRP-confined) concrete with a post-peak strain-hardening curve. This model employs an existing monotonic stress-strain relationship as the envelope curve. The cyclic load paths are formed by using a new and simple algebraic function. The concrete strength and confinement stiffness ratio are selected as the main factors that govern the parameters of the unloading and reloading paths. The available experimental results are used to evaluate the model parameters. The model uses more appropriate mathematical forms and includes significant factors that were previously ignored by others, which makes it not only more rational and accurate but also simpler than other existing models reported in the literature.

Second, a literature review has concluded that the majority of experimental studies investigating the cyclic stress-strain behavior of passively confined concrete featured post-peak strain-hardening behavior. Research on the cyclic response of passively confined concrete featuring post-peak strain softening is rare, and the modelling of this type of response is still in the early stage. Hence, this work addressed this gap through experimental testing and analytical modelling. Sixty FRP-confined concrete cylinders were tested under cyclic compression, half of which exhibited a post-peak strain softening. Concrete strength and confinement pressure were selected as the main test variables. The test results indicated that the confinement level and concrete grade have a more pronounced effect on the strain-softening type of stress-strain curves regarding the overall shape of the stress-strain curve, unloading paths, reloading paths, and plastic strain compared with the strain-hardening type of curves. The test results provided a valuable database for the development of a general stress-strain model that features both strain-hardening and strain-softening curves.

Third, partial unloading and reloading are more common and general load patterns than completed cycles in practice. The key parameters, in terms of tangent unloading stiffness, reloading stiffness, and plastic strain, may be affected by different partial reloading stress levels and effective partial load history. Hence, this study also conducted a series of tests on concrete cylinders (a total of 14 specimens) with random partial unloading and reloading. The test results indicate that partial reloading modulus are dependent on the stress level; only an effective cyclic load history can cause accumulative damage on a plastic strain and reloading modulus, and the partial effective cyclic factor is determined to be 0.8 based on the test results.

Fourth, summing up the experimental work including the self-conducted and other tests in the open literature, a more extensive database of FRP-confined concrete columns including random partial cyclic loading was built to modify the model parameters and widen the application scope of the previously proposed cyclic model. By considering the influence of strain softening on the main cyclic shape factors in the previous model developed for only post-peak strain hardening, the improved model is capable of and more accurate in predicting the cyclic behavior of FRP-confined concrete with post-peak softening compared to the other existing models. In addition, by considering effective accumulative damage on plastic strain and reloading stiffness, this proposed model is capable of modelling the stress-strain behavior of partial unloading and reloading with good accuracy.

Subsequently, a comparison study between passive confinement (FRP-confinement) and active confinement (hydraulic pressure or steel confinement) of a concrete cylinder subjected to cyclic loads was conducted. The monotonic stress-strain relationship of actively confined concrete has been used as the base model to establish an analysis-oriented stress-strain model of FRP-confined concrete. This approach relies on the assumption that the axial stress and strain of FRP-confined concrete are the same as those of actively confined concrete under the same confinement pressure and lateral strain. In this study, a series of tests was conducted to verify this assumption for concrete subjected to cyclic loading. A total of 31 actively confined and FRP-confined concrete cylinders were tested. The test results indicate that this hypothesis is inapplicable to concrete under cyclic loading, and a gap was found between the envelope curves of the two types of confined concrete. Moreover, the test results also reveal that the confinement pressure significantly affects both reloading modulus and plastic strain, which are the main factors controlling the cyclic behavior of confined concrete.

Finally, a mechanical model that predicts the cyclic stress-strain behavior of FRP-confined concrete under uniaxial compression is proposed. This model consists of simple rheological elements (springs and blocks) and provides a realistic description of the complex behavior of confined concrete subjected to monotonic and cyclic axial compression. In this system, the concrete cohesion and friction are clearly presented and correspond to elastic deformation and plastic deformation. Moreover, this model simulates well the main known characteristics of the confined concrete response to cyclic loadings, such as a nonlinear unloading curve, a sudden change in stiffness between unloading and reloading, bilinear reloading behavior, and a smooth transition zone. The capability and accuracy of the proposed mechanical model are also validated by comparisons between the predictions of the proposed model and test results.

Date of Award	28 Oct 2016
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yufei WU (Supervisor)