Keywords: carbon fibre reinforced polymer, confined column, plastic dilation angle, friction angle, cohesion, Drucker-Prager plasticity model.

During a previous study on fibre reinforced polymer (FRP) confined cylinders, it was discovered that the concrete plastic volumetric deformation presents apparent variations for columns with changing FRP confinement levels [1,2]. The accuracy of the deformation prediction accounts for the passive confinement system, as the lateral pressure results from the lateral dilation. This phenomenon can be captured well by the plastic dilatation angle which is one of the three major parameters governing the Drucker-Prager plasticity model.

In a previous study, a similar trend was observed in all curves that relate the plastic dilation angle to the axial plastic strain [1,2]. However, there is a lack of sufficient test data on highly confined concrete columns where the lateral stiffness ratio is larger than 20. Therefore, this encourages doubt about the accuracy of the prediction. This paper addresses an experimental study which involves 39 cylinders with one to five different plies of carbon fibre reinforced polymer (CFRP) sheets. The lateral stiffness ratio ranges from 14.90 to 145.22. In the test observations, the three controlling values, the maximum plastic dilation angle, its corresponding axial plastic strain, and the ultimate plastic dilation angle, reveal an apparent trend when the lateral stiffness ratio is greater than 20 and it can be numerically expressed with a high correlation coefficient. The explicit model for the plastic dilation angle has been extended further for concrete columns that are highly confined by FRP sheets. The model can capture the plastic deformation with a change of the FRP properties which was not considered in the Karabinis and Rousakis model [3]. It has overcome the limitation of Oh's asymptotic model [4] with respect to the confinement ratio. It has advantages over the constant value derived by Rousakis et al. [5] which cannot describe the nonlinear deformation trend.

In addition, this paper addresses the modeling of FRP confined concrete using the framework of the modified Drucker-Prager plasticity model. Simple models for friction angle and cohesion are subsequently adopted [2], to verify the performance of the modified plastic dilation angle model. Using the commercial software ABAQUS, the finite element analysis results fit well with the experimental stress-strain behaviour of concrete columns with FRP confinement for a large range of lateral stiffness ratios. The good agreement with test results and the predicted stress-strain response curves demonstrate the accuracy and effectiveness of the proposed model.

1

J.F. Jiang, Y.F. Wu, X.M. Zhao, "Application of Drucker-Prager Plasticity Model for Stress-strain Modeling of FRP Confined Concrete Columns", In "Proceedings of The Twelfth East Asia-Pacific Conference on Structural Engineering and Construction", Hong Kong, China, 26-28 January 2011. doi:10.1016/j.proeng.2011.07.088

2

J.F. Jiang, Y.F. Wu, "Identification of Material Parameters for Drucker-Prager Plasticity Model for FRP Confined Concrete Columns", International Journal of Solids and Structures, submitted. doi:10.1016/j.ijsolstr.2011.10.002

3

B. Oh, "A plasticity Model for Confined Concrete under Uniaxial Loading", PhD Thesis, Department of Civil Engineering, Lehigh University, 2003.

4

A.I. Karabinis, T.C. Rousakis, "Concrete Confined by FRP Material: a Plasticity Approach", Engineering Structures, 24(7), 923-932, 2002. doi:10.1016/S0141-0296(02)00011-1

5

T.C. Rousakis, A.I. Karabinis, P.D. Kiousis, R. Tepfers, "Analytical Modelling of Plastic Behavior of Uniformly FRP Composites", Part B: Engineering, 39(7-8), 1104-1113, 2008. doi:10.1016/j.compositesb.2008.10.001

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