Confinement effect of fiber reinforced polymer WRAPS on circular and square concrete columns

Abstract

Effectiveness of confinement in concrete columns due to the variations of cross sectional geometry and sizes is studied by experiments. Confining pressures in the columns wrapped with fiber reinforced polymers (FRP) are engaged by the transverse dilation of concrete that in turn increases the axial load capacity. Concretes made of aggregates with higher porosity and lower density offer lower modulus of elasticity and higher Poisson’s ratio. This property in turn increases larger dilation effect. However, the magnitude of dilation effect is further related to the stress field of concrete section under load. In this context, the curvature and size of the columns are varied to investigate the effect of geometric parameters i.e. radius of curvature, column side to corner radius ratio, FRP volumetric ratio and rigidity ratio on dilation. To realize the experimental data, a total of 60 circular and 60 square short concrete columns made of stone, brick, recycled stone and recycled brick aggregate were cast and tested. Carbon-FRP (CFRP) and glass-FRP (GFRP) bonded to the surface of concrete with epoxy offered different confining effect on the specimens. Circular columns varied in their curvatures while the square columns had a constant curvature at the corners. Dilation effects of concrete are measured by using a simultaneous data acquisition system. The load and displacement histories of square and circular confined concrete columns obtained from the load cell of a computer controlled universal testing machine. The data are synthesized with the strain measurement results assembled from high speed (60 frames per second) video clips and high definition images. In this process, the confinement due to dilation of concrete for different aggregate types and cross-sections are evaluated and compared. To assess the Poisson’s ratio in concrete with different aggregates, numerical trials on nonlinear finite element (FE) platform are conducted using ANSYS 10.0. The dilation effect in brick aggregate concrete was found to be distinctly larger than the stone aggregate concrete. The Poisson’s ratio of stone aggregate concrete was also evaluated convincingly to be lower than brick aggregate concrete, recycled stone aggregate concrete and recycled brick aggregate concrete by FE analysis and the values were estimated at 0.25, 0.35, 0.37 and 0.40, respectively. Dilation of concrete in square columns is found to be larger than circular columns. Axial capacity increases as the dilation of concrete increases both for circular and square concrete columns made of different aggregates. The size and curvature effects on confinement are more pronounced for circular columns. There exists no such effect for square columns with the same corner radius except for ultimate axial strain of square columns. Results of FE simulations are found to be in reasonable conformities with the experimental result indicating validity of the FE models. The stress-strain model considering size and curvature of the columns, the Poisson’s ratios, dilation characteristics and modulus of elasticity is proposed for brick, stone, recycled brick and recycled stone aggregate concrete. The proposed general models for square and circular columns have shown a good agreement with experimental results. ACI 440 model (2002), Lam and Teng’s model (2003b), Kumutha et al. model (2007) and Wu and Wang model (2009) showed considerable similarities with the test results of square columns. ACI 440 model (2002) matched significantly with the test results for circular columns.