International Concrete Abstracts Portal

Some of the most widely referenced models for stress-strain relationship of confined concrete columns with square cross section are reviewed, and the performance under monotonic compressive loading are evaluated based on a database in which compressive strength ranges from 40 to 140 MPa and yield strength of transverse reinforcement ranges from 400 to 1400 MPa. It is found that majority of the models examined can predict the strength of confined concrete reasonably well, however, the predictability to the deformation is rather low. A model for predicting deformability of confined concrete column subjected to monotonic compressive loading is proposed in terms of strain energy provided by confining reinforcement up to the peak point and the point of 85% of the maximum stress beyond the peak point. Predictions based on the strain energy are found to be in good agreement with experimental results.

This paper presents an analytical procedure for determining the stress-strain curves of concrete under flexure. The procedure is used to derive the stress-strain curves of confined concrete from tests of thirteen HSC columns confined by transverse reinforcement and subjected to different strain gradients. The effect of strain gradient to the stress-strain curves of confined concrete is observed and an analytical model for confined concrete that account for this effect is proposed.

A key of seismic design of ductile frame is to provide the adequate flexural ductility to potential plastic hinge regions. This is realized by limiting the amount of tension reinforcement index, providing transverse reinforcement and others. For columns, the application of transverse reinforcement to potential plastic hinge region is essential, that is, the compressive ductility of concrete is improved and results in larger flexural ductility. In the 1980s, a new RC project was carried out as a Japanese National Project to establish the design and construction guidelines for high-rise buildings up to 200 meters high. For columns at the lower part of high-rise buildings, the use of high-strength concrete (HSC) is required. However, HSC fails in brittle manner and results in small flexural ductility of potential plastic hinges. Therefore the new RC project gave an opportunity to re-recognize the importance of lateral confinement to concrete. This paper presents the recent research works on confined concrete in Japan, mainly for HSC. Some experimental works and idealizations of stress-strain curve of confined concrete are introduced. Maximum compressive strength covered in this paper is 176 MPa.

Concepts in ductile design have led to an increased interest in understanding the role of confinement in improving the seismic performance of reinforced concrete members. While transverse reinforcement is regarded as a form of passive confinement in RC members, the observed increase in the strength of confined concrete is typically a function of the axial strain levels. Confinement models have been developed by numerous researchers to describe the stress-strain behavior of concrete as a function of certain key parameters that are related to the amount and type of transverse reinforcement. Accurate constitutive models of confined concrete are necessary for direct use in fiber-model based discretization of RC components or for indirect use in hysteresis based phenomenological models. This paper examines the relevance and importance of accurate confinement modeling in predicting the inelastic behavior of well-confined concrete columns. In particular, the influence of incorporating confinement effects in predicting the monotonic and cyclic response of RC columns is investigated. It is analytically demonstrated that the role of the longitudinal reinforcing bars play a more significant role in determining the overall force-deformation behavior of RC components. Detailed fiber-based discretizations that rely entirely on constitutive models are incapable of reproducing post-yield softening and deterioration because of their inability to incorporate complex large deformation behavior of both the longitudinal and the confining reinforcement. Approximate phenomenological models will continue to see widespread use in inelastic analysis of RC structures until these limitations of constitutive-based element models are overcome.

This paper summarizes results from a comprehensive research program that aims at developing rational guidelines for the design of confinement reinforcement in concrete columns. The first part of the paper briefly introduces an analytical model for confined concrete in tied columns. The model is based on the results of testing 24 square columns with various tie configurations under concentric compression. The second part presents results from square columns tested under cyclic flexure and shear, and constant axial load simulating earthquake loads. The specimens tested included normal-strength concrete (NSC) and high-strength concrete (HSC) columns confined by steel and NSC columns confined by fiber-reinforced polymers (FRP). Performance-based procedures for the design of confinement reinforcement in these columns are proposed in light of the experimental results and analytical models. The design procedures incorporate various ductility parameters that include energy dissipation capacity, ductility factors, and cumulative ductility indices in addition to the type, amount, and configuration of the confinement reinforcement and the level of axial load. The areas in which further research is needed are also discussed.