Effect of corner radius on the performance of CFRP-confined square concrete columns: Test
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 , where denotes the corner radius, is the breadth of the column, is the compressive strength of the unconfined concrete, and is the confinement pressure that is given by , where is the hoop strength of the tube and 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.
Section snippets
Specimen design
All of the specimens were 150×150 mm in cross section and 300 mm in height but had different corner radii. The primary variables in this investigation were the corner radius, the thickness of the CFRP, and concrete grade. Although the size effect is an important factor that closely relates to the confinement effectiveness of the column, it was not considered in this work. Two series of concrete with a grade of C30 and C50 were tested. In both series, the specimen varied with a corner radius of
Failure modes
The failure of the specimens could be divided into three modes: (i) tensile rupture of the CFRP jacket, usually near a corner; (ii) delamination of the CFRP jacket; and (iii) a combination of delamination and tensile rupture of the CFRP jacket. The typical failure modes of the specimens with the corner radius varying from 0 to 75 mm are given in Table 2 and illustrated in Fig. 7.
In the C30 series, for the concrete specimens with a small corner radius, i.e., , and 30 mm, the failure was
Conclusions and recommendations
A total of 108 CFRP-confined concrete short columns were tested in this study. The main purpose was to investigate the effect of corner radius on the effective confinement that is provided by CFRP jackets, especially for sharp-edged columns. The test variables included the corner radius ratio, the thickness of the CFRP jackets, and the concrete grade.
The test results indicate that corner radius is of great importance in relation to the level of confinement. The confinement effectiveness of the
Acknowledgement
The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 1113/04E).
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