Effect of active and passive concrete confinement on the bond stress-slip response of steel bars in tension

Abstract

This paper summarizes the results of an experimental investigation undertaken to evaluate the effect of different types of concrete confinement on the bond strength and the local bond stress-slip behavior of steel bars in tension. Thirty-two pullout specimens were tested. The investigated parameters included type and level of confinement, and ratio of concrete cover to steel bar diameter (c/db). Two categories covering four different types of confinement were evaluated: (1) active confinement applied as external transverse pressure and (2) passive confinement using: (i) conventional transverse steel ties, (ii) carbon fiber reinforced polymer (CFRP) jackets, (iii) a combination of CFRP jackets and transverse steel ties, or (iv) a combination of CFRP jackets and CFRP anchors. The specimens developed three distinct modes of bond failure: splitting mode associated with splitting of the concrete cover, pullout mode associated with pullout of the steel bar from the concrete, and a combination of pullout and splitting modes. Confining the concrete with any of the confinement methods used increased significantly the local bond strength, altered the failure mode from splitting to pullout or mixed splitting/pullout, and improved the ductility of local bond stress-slip response when compared to unconfined concrete. These improvements were highly dependent on the type and level of confinement used as well as the ratio of concrete cover to bar diameter c/db. The failure modes, the maximum local bond strength, and the local bond stress-slip behavior corresponding to the different types of confinement were compared and discussed vis-à-vis unconfined concrete. The intrinsic shapes of the experimentally measured local bond stress-slip responses were compared with the prediction of available analytical bond constitutive relationships.