Ductility in reinforced concrete columns

Abstract

Uniaxial and triaxial test results are used to study the behaviour of concrete specimens. Based on existing test results in the literature, a new model was proposed for the axial stress versus strain response of normal and high strength concretes. This model is capable of predicting the concrete stress versus strain behaviour in confined conditions as well as in the unconfined state. The model predicts changes in concrete behaviour from low to very high confinement condition. This model is capable of predicting specimen size effect by incorporating compressive damage zone theory. Lateral strain of concrete is commonly used to predict the strength of reinforced concrete columns. Using existing literature a model is proposed to predict the lateral strain of unconfined and confined concrete. The model divides the concrete behaviour into three regions. A comparison between both models and test results was carried out showing the excellent accuracy of the models. Behaviour of reinforced high strength and normal strength concrete columns is studied in detail to predict the ductility of reinforced concrete columns. Methods to predict cover spalling and confinement efficiency of lateral reinforcement are reviewed and improvement to these methods is proposed. Using the proposed methods, a procedure is proposed to predict the behaviour of reinforcement concrete columns under static concentric axial loads. The proposed method is used to predict ductility level of reinforced high and normal strength concrete columns which are designed and detailed by AS3600 (2009). The comparison showed that, the requirement for column detailing in AS3600 (2009) provided the required ductility in low and normal strength concrete, however, as the concrete strength increases, the ductility decreases and the required ductility assumed by the code is not provided by the detailing required in AS3600 (2009). The code prediction of the squash load on a reinforced concrete column was also reviewed. The comparison showed that the code equation to predict the peak is very conservative for high strength concrete columns and needs to be updated to include the confinement enhancement of the core strength.