Confinement Characteristics of Rectangular FRP-Jacketed RC Columns

In this study, FRP jackets were investigated for their confinement effectiveness on rectangular RC columns. Thirteen reduced-scale short columns were tested to failure in axial compression. Variables investigated include: the type of fibers (AFRP, CFRP or GFRP), the thickness of the jacket, the aspect ratio of the rectangular cross section and the radii of the corners. For square columns, GFRP jackets were observed to increase the ultimate axial stress and strain more effectively than either AFRP or CFRP jackets. One explanation for this observation may be that glass fibers can sustain greater tensile strains than either aramid or carbon. Increasing the aspect ratio of the cross sections resulted in a decrease in ultimate strength, as did increasing the sharpness of the corners. INTRODUCTION Over the last few years, there has been a worldwide increase in the use of composite materials for the rehabilitation of deficient RC structures. One important application of this composite retrofitting technology is the use of FRP jackets to provide external confinement to RC columns when the existing internal transverse reinforcement is inadequate. Although a much better understanding of the structural behavior of RC columns retrofitted with FRP jackets exists now than 10 years ago, this technology remains in its developmental stages and much research is still needed. Thus far, the main thrust of research has been aimed at characterizing the behavior of columns with circular cross sections (Xiao and Wu 2000, Liu et al. 2000). The results of such research have wide applicability, particularly with regard to circular bridge piers. However, the vast majority of all columns in buildings are rectangular columns. Therefore, their strengthening and rehabilitation need to be given attention to preserve the integrity of building infrastructure. Jacketed rectangular columns warrant additional research efforts apart from circular columns because of additional complexities associated with their geometry. Circular RC columns experience uniform confining pressure around the circumference of their cross section, whereas for rectangular columns confining pressure is at a maximum at the corners of the cross section and less in between (Mirmiran et al. 1998). Also, for rectangular columns, the sharpness of the corners plays a role in the confinement effectiveness of the jacket (Picher et al. 1996), since stress concentrations at the corners can cause premature rupture of the FRP. In this study, the sharpness of the corners, the aspect ratio (ratio of the length of the long side of the cross section to that of the short side) of the cross section, the type of jacket fiber and Cole, C. and Belarbi, A., "Confinement Characteristics of Rectangular FRP-Jacketed RC Columns”, Proceedings of the Fifth International Symposium on Fiber Reinforced Polymer for Reinforced Concrete Structures (FRPRCS-5), Cambridge, UK, July 16-18, 2001, pp. 823-832. 2 the thickness of the jacket were investigated for their effect on the axial strength and axial strain of rectangular RC columns subjected to uniform compression. DESCRIPTION OF TESTING PROGRAM Specimen Description Third-scale RC columns were tested under uniaxial compression. Load was gradually applied to the specimens in a few load-reload cycles of increasing magnitude until failure. The approximately 21-MPa ready-mix concrete used when pouring the columns utilized 9.5 mm gravel as the course aggregate due to the small spacing between the reinforcing steel and the forms. All columns had rectangular cross section of 323 cm. The cross-sectional aspect ratios of the columns were 1.0, 1.5 and 2.0. Each specimen had a middle test region 914 mm long and two enlarged block-like ends with dimensions of 610 mm x 610 mm x 305mm, as shown in Figure 1. This configuration forced general failure to occur in the test region and prevented premature failure at the ends. The enlarged ends also served to stabilize the column during testing and simulate a slab or foundation connection at the bottom of the specimen. The columns were reinforced with four #4 Grade 40 longitudinal rebars, one located at each corner of the cross section. Transverse reinforcement consisted of 6.35 mm smooth dowel, fabricated with 76 mm extensions on 135 degree hooks. The corners of the columns were rounded with varying degrees of sharpness. The columns, having been chamfered during construction, were marked along their length with lines that were parallel to and offset from the edges of the chamfered corners. The columns were then ground so that the finished corners were an elliptical shape as shown by the dashed line in Figure 2. All of the cross sections received a 13 mm chamfer and a 9.5 mm offset, except for the two columns for which the corner radius was a test variable. One of these two columns received a 6.4 mm chamfer and a 4.8 mm offset, while the other received a 19 mm chamfer and a 14 mm offset. Corner rounding is a well-accepted procedure that is commonly used when retrofitting rectangular RC columns. The importance of the effect of the sharpness of the corners comes into play when one is considering the tradeoff between the expense of grinding larger, smoother corners and the increase in jacket performance that comes from this activity. Figure 1. Column Dimensions 305 mm