Effect of corner radius on the performance of CFRP-confined square concrete columns: Test

Abstract

Jacketing confines concrete, and hence increases the strength and ductility of jacketed columns. A significant amount of research has been devoted to circular columns that have been retrofitted with fiber-reinforced polymer (FRP), but much less is known about FRP-confined rectangular/square columns in which the concrete is non-uniformly confined and the effectiveness of confinement is much reduced. Some studies suggest that a sharp corner (i.e., a zero corner radius) offers no confinement. However, most published research indicates that a certain degree of effective confinement is provided by a jacket with sharp corners. To further study this problem, this study undertook compressive testing to investigate the effect of corner radius on the strength and ductility of FRP-confined concrete columns. This paper reports a series of tests on 108 carbon FRP (CFRP) confined short concrete columns. The primary variables in the investigation were the corner radius, transverse jacket stiffness, and concrete grade. The test results demonstrated that the corner radius ratio is in direct proportion to the increase in confined concrete strength. Furthermore, it is revealed and explained that confinement provided by a jacket with sharp corners is insignificant in increasing the strength of columns but significant in increasing the ductility of columns.

Introduction

In recent years, the use of externally bonded fiber reinforced polymer (FRP) composites has become increasingly popular for the repair and retrofitting of concrete structures. The popularity of FRP composites is due to their well-known advantages, including a high strength-to-weight ratio and excellent corrosion resistance. One important application of FRP composites is as a confining material for the retrofitting of existing reinforced concrete (RC) columns with FRP jackets.

The results of earlier research that was conducted by Mirmiran et al. [1] suggest that the confinement effectiveness of FRP jackets in concrete columns depends on several parameters, namely, concrete strength, types of fibers and resin, fiber volume and orientation, jacket thickness, shape of cross section, length-to-diameter (slenderness) ratio of the column, and the interfacial bond between the concrete core and the jacket. Thereafter, extensive investigations with different parameters have been conducted to study the confinement effectiveness of FRP jackets. Although the available experimental data, particularly on large-scale columns, is limited, e.g.,  [2], [3], [4], recent studies that involve mostly small-scale specimens have shown that confinement which is provided by FRP wraps results in a remarkable enhancement of the strength and ductility of concrete [5], [6], [7], [8].

Externally bonded FRP laminates have to be bent when they are wrapped around columns. Bending affects the performance and efficiency of the FRP laminate, and the corresponding confinement action depends on the curvature of the corners [9], [10], [11], [12]. The tests that have been reported in the literature have established that the corner radius significantly affects the confinement effectiveness. Mirmiran et al. [1] suggested that square sections of FRP-confined concrete specimens are less effective in confining concrete than are their circular counterparts. The effectiveness of a square jacket was proposed that is measured by a modified confinement ratio (MCR), which is a function of the corner radius and the hoop strength of the jacket. The MCR is defined as $\text{MCR}=\left(\frac{2r}{b}\right)\frac{f_r}{f_{co}^{\prime }}$, where $r$ denotes the corner radius, $b$ is the breadth of the column, $f_{co}^{\prime }$ is the compressive strength of the unconfined concrete, and $f_r$ is the confinement pressure that is given by $f_r=\frac{2f_j{t}_j}{b}$, where $f_j$ is the hoop strength of the tube and $t_j$ is the tube thickness. No enhancement in the compressive strength of confined concrete should be expected if the MCR is less than 0.15. Rochette and Labossière [9] stated that the confinement effect is directly related to the shape of the section for a given number of wraps around the column. The ratio of the corner radius to the side length of a square section, which takes values between 0 and 0.5, can be used to estimate how much confinement can be potentially provided to concrete by the composite wraps.

The test that was conducted by Yang et al. [13], [14] indicated that a smaller corner radius can significantly reduce the ultimate strength of the FRP laminate due to the stress concentration around the corner area. The stress concentration factor increases when the corner radius decreases. According to the latest research conducted by Wu et al. [15], the corner radius ratio, which is defined as the ratio of the corner radius to the half breadth of the column, is the single most important factor that affects the confinement across the cross-section. The corner radius ratio that is employed should be as large as possible to achieve a better confinement.

A significant amount of research has been devoted to circular columns that are retrofitted with FRP, and FRP wrapping of existing circular columns has proven to be an effective retrofitting technique. In contrast, relatively limited data have been reported on FRP-confined rectangular or square columns, even though rectangular or square columns in need of retrofit are very common. Mirmiran et al. [1] stated that a sharp corner (i.e., a zero corner radius) offers no confinement. However, most published studies state that a certain degree of effective confinement is provided by a jacket with sharp corners [9], [12], [13], [14], [16], [17]. To further investigate this problem, Wu et al. [15] theoretically analyzed the corner radius problem and proved that no confinement is offered by flexible jackets with a zero corner radius. Their preliminary testing, which was based on a small number of specimens, also indicated that the strength gain of the confined concrete is in direct proportion to the corner radius ratio.

To further substantiate the interesting findings of the previous work [15], a comprehensive experimental investigation that involved 108 test specimens was undertaken in this study to investigate the confinement effect on short square concrete columns with a full range of corner radius ratios and variations in the transverse jacket stiffness and concrete strength.