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

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 thesis 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 proportional 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. Based on the experimental test, a strength model was developed to predict the compressive strength of FRP-confined square columns. First, the existing models for predicting the compressive strength of FRP-confined circular columns were assessed by comparing them with the experimental test data collected from the open literature. A model was then developed for circular columns based on the database. A shape factor, which is expressed as a function of the corner radius ratio, was then suggested to be used directly in the model proposed for circular sections to predict the confined strength of square columns. The results predicted by this model showed a good agreement for specimens at lower confinement levels, although they were a little conservative at higher confinement levels. This model was proved to be suitable for both square and circular columns.