Application of Drucker-Prager Plasticity Model for Stress-Strain Modeling of FRP Confined Concrete Columns

Department of Building and Construction, City University of Hong Kong, China
Procedia Engineering 01/2011; 14:687-694. DOI: 10.1016/j.proeng.2011.07.088

ABSTRACT Existing research works have identified that Drucker-Prager (DP) plasticity model is capable of modeling the stressstrain behavior of confined concrete. However, the accuracy of the model largely depends on the adequate evaluation of its parameters that determine the yield criterion, hardening/softening rule and flow rule. Up to date, most research works mainly focus on the first two criteria. The plastic dilation angle is the major parameter that governs the DP flow rule. This paper addresses the plastic dilation properties of concrete for FRP confined circular concrete columns under the theoretical framework of DP model in the commercial software ABAQUS. Through careful analyses of test results for FRP confined concrete columns, it is found that the plastic dilation angle is a function of axial plastic strain and the lateral stiffness ratio. A simple model for the plastic dilation angle is subsequently developed. With the implementation of this model, the finite element analysis results fit well with the experimental stress-strain curves for columns with both low and high confinement.

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    ABSTRACT: This paper presents the results of numerical studies carried out on concrete columns confined by Carbon Fiber Reinforced Polymer (CFRP) and loaded in uni-axial compression. A numerical simulation of confined columns experimentally tested by [1] is performed for relatively small concrete specimens of constant size but with different cross-section corner radius. The experimental results, reported in terms of axial stress–strain relationships and failure modes, constitute a useful database for the calibration of numerical models. These results clearly demonstrate that CFRP confinement is much less effective in square than in circular cross-sections. Therefore, the influence of the corner radius on the non-uniform stress distribution due to confinement, is numerically investigated using the proposed microplane-based model that takes into account the effect of multi-axial stress states in concrete. The experimental results are used to calibrate and verify the prediction capability of a three-dimensional finite element code (3D FE) that is based on the microplane constitutive law for concrete [2]. In the finite element model carbon fibers are modelled using nonlinear truss elements, while epoxy resin as well as concrete are modelled using microplane-based constitutive law and 3D finite elements. Given that experimental results for unconfined and confined configuration are available for each specimen, the 3D FE concrete model has been preliminarily calibrated on unconfined concrete specimens and then used in the analysis of a confined specimen. It is demonstrated that numerical models can predict behavior of confined concrete columns from the experimental investigations, confirming the predictability of the numerical microplane-based approach used.
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    ABSTRACT: Concrete dilation is one of the main parameters that controls the stress–strain behaviour of confined concrete. Several analytical studies have been carried out to predict the stress–strain behaviour of concrete encased in fibre-reinforced polymer (FRP), which is crucial for structural design. However, none of these studies have provided a simple formula to determine the dilation parameter that is always required in the finite element (FE) material modelling of concrete. This paper presents a simple empirical model predicting the confined concrete dilation parameter within the theoretical framework of a Karagozian and Case type concrete plasticity model. A set of 105 FRP-confined specimens with different unconfined concrete strengths (f′c) and confinement moduli (E1) was analysed using the LS-DYNA program. The model predictions of the confined ultimate strength (f′cc), confined ultimate axial strain (ℰcc) and confined ultimate hoop strain (ℰh) were compared with the corresponding experimental database results for each specimen. In addition, the model axial and hoop stress–strain curves of each specimen were developed and compared with the corresponding experimental ones. The proposed model was able to predict stress–strain curves of the test specimens quite well .The proposed model was able to predict f′cc with mean errors (M) and standard deviations (SD) of 2.6% and 10.7%, respectively. Similarly, the model predicted ℰcc with M and SD values of 0.3% and 29.0%, respectively. Finally, the model was less successful in predicting ℰh with M and SD values of 13.7% and 26.3%, respectively.
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    ABSTRACT: The effects of confinement on concrete elements subjected to axial load are well known: the resulting axial stress–strain relationship shows higher strength and ultimate strain with respect to the unconfined one. Several studies have been devoted to the understanding of the behavior of circular concrete sections while comparatively fewer studies have dealt with square and rectangular ones. Regarding square and rectangular sections, the so called “arching effect” is usually considered in order to define an equivalent reduced area, which is supposed as efficient as a circular section. Inside the equivalent area the stress induced by the confinement is considered uniformly distributed, even if it is not so, as already discussed in [17] where further on a strength criterion has been proposed for predicting the material strength increase, based on a revision of the classical strength criterion reported in [18]. That strength criterion, along with the methodology here proposed, are the basis of a procedure meant to develop a mechanics-based stress–strain relationship for FRP-confined concrete which stems from a step-by-step analysis of the behavior of FRP-confined square concrete sections.
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