On the Effect of Ductility of Confining Material on Concrete Ductility

This paper presents the results of an experimental and analytical study on the improvements gained in the axial compressive behavior of confined concrete. Among several parameters affecting compressive ductility of confined concrete, including confinement mechanical volumetric ratio, shape and size effect, number of confinement layers, strength of plain concrete and ductility of confining material, the latter was found to play the most important role in determining the ultimate strain and post peak behavior of concrete. Axial compressive tests were performed on small-scale circular or square section concrete columns, which were retrofitted either by CFRP jacket or by two types of high-strength steel strips with ultimate ductilities of 1% and 7%. The axial stress-strain of confined concrete specimens was obtained by measuring axial and lateral displacements and strains. Great values of concrete ultimate compressive strains, in excess of 6%, were recorded by using ductile high-strength strips, which is much greater than the values reported in the literature. A database of results of compressive tests on concrete confined with various materials was collected from the literature. An analytical model was proposed based on results of this study and the collected database to determine the ultimate strain of confined concrete. The proposed model takes the confinement ductility into account and shows good agreement with the experimental results.