Experimental Study Of FRP-confined Concrete Under Eccentric Compression

Fiber reinforced polymer (FRP) composites have been widely used in civil engineering due to their salient advantages, such as light weight, high strength,excellent corrosion resistance and ease of use in construction. One important application of FRP composites is strengthening of concrete columns through external wrapping as both the compressive strength and ductility of concrete can be significantly enhanced under confinement. In practice, concrete columns are inevitably subjected to eccentric loading. Even when the loading is nominally concentric, eccentric loading of the column still exists because of construction error and geometric imperfections of the column.To date, most relevant studies on eccentrically-loaded FRP-confined concrete have been conducted at the member behavior level and have proven that FRP confinement can effectively increase the load capacity of FRP-confined concrete columns. Only a very limited number of studies have further investigated into the stress-strain behavior level to clarify the possible effect of load eccentricity on the stress-strain relation of FRP-confined concrete. The few studies have shown that the lateral dilation of concrete is non-uniform in nature and varies over the entire cross section because of the presence of strain gradient due to eccentric loading, which causes a certain eccentricity effect on the stress-strain relation of FRP-confined concrete. However, the quantitative conclusions on the eccentricity effect of these studies are inconsistent, even contradictory, to each other.Against the above background, this thesis carries out an experimental study on eccentrically-loaded FRP-confined cylinders, aiming primarily to clarify the eccentricity effect. The main content and findings of the thesis are as follows:1) A total of 16 eccentrically-loaded and 4 concentrically-loaded FRP-confined concrete cylinders were tested with the major test parameters being the load eccentricity and FRP jacket thickness. The test results show that an increase in the load eccentricity decreases the load capacity but increases the ductility of FRP-confined cylinders; it also increases the ultimate axial strain of FRP-confined concrete, but little affects the FRP rupture strain. Test results also show that an increase in the FRP jacket thickness increases both the load capacity and ductility of FRP-confined cylinders; it also increases the ultimate axial strain of FRP-confined concrete, but little affects the FRP rupture strain.2) The results of section analysis show that the direct employment of Lam and Teng’s design-oriented stress-strain model developed for concentrically-loaded FRP confined concrete would underestimate both the load capacity and the ductility of tested specimens, and the degree of underestimation increases with the load eccentricity. To identify the eccentricity effect, the ultimate axial strain and the slope of the second branch of stress-strain curves of FRP-confined concrete are back calculated according to the experimental load-curvature curves as the basis to develop two empirical expressions. These two expressions are incorporated into Lam and Teng’s stress-strain model to extend the model’s applicability to the case of eccentric compression.3) A parametric study carried out using the revised Lam and Teng’s model reveals that an increase in the load eccentricity increases the ultimate axial strain of FRP-confined concrete but decreases the slope of the second branch of stress-strain curves of FRP-confined concrete. These two effects combined cause the compressive strength of FRP-confined concrete to increase in the initial eccentricity range but to decrease once the eccentricity exceeds a certain threshold.