Effect of confinement on shear dominated reinforced concrete elements

It has been demonstrated that transverse reinforcement not only provides the strength and stiffness for reinforced concrete (RC) members through direct resistance to external force demands, but also helps confine the inner core concrete. The confinement effect can lead to improved overall structural performance by delaying the onset of concrete fracture and allowing more inelastic energy dissipation through an increase in both strength and deformability of RC members.

The objective of this research was to evaluate the effect of confinement due to the transverse reinforcement on enhancing the shear performance of RC members. A new constitutive model of RC members was proposed by extending the Modified Compression Field Theory (MCFT) to incorporate the effect of confinement due to transverse reinforcement by adjusting the peak stress and peak strain of confined concrete in compression. The peak stress of confined concrete was determined from the five-parameter failure surface for concrete developed by Willam and Warnke (1974). The peak strain adjustment was carried out using a relationship proposed by Mander et al. (1988).

The proposed analytical model was compared with results from an experimental program on sixteen RC bent caps with varied longitudinal and transverse reinforcement details. Two-dimensional Finite Element Modeling (FEM) using the proposed constitutive model was conducted to numerically simulate the RC bent cap response. Results showed that the proposed analytical model yielded good results on the prediction of the strength but significantly overestimated the post-cracking stiffness of the RC bent cap specimens. The results also indicated that the confinement effect led to enhanced overall performance by increasing both the strength and deformability of the RC bent caps.

Two potential causes of the discrepancy in the underestimation of the RC bent cap deformations, namely the effects of concrete shrinkage and interfacial bond-slip between the concrete and main flexural reinforcement in the bent caps, were discussed. Parametric studies showed that the tension-stiffening in the proposed constitutive models to implicitly take into account the bond-slip between the concrete and main flexural reinforcement was the major cause of the overestimation of the post-cracking stiffness of RC bent caps. The explicit use of bond-link elements with modified local bond stress-slip laws to simulate the slip between the concrete and main flexural reinforcement led to good predictions of both strength and deformation.