Synergistic effects of steel fibres and expansive agent on steel bar-concrete bond

Previous studies have revealed that adding separately fibres or expansive agent to concrete can improve the steel bar-concrete bond. However, there have been few systematic studies on the possible combined addition of fibres and expansive agent to further improve the steel bar-concrete bond. Herein, it is envisaged that with fibres and expansive agent added together, the restraining action of the fibres against the autogenous expansion would produce active confining stresses to increase the bond. In this study, the effects of adding steel fibres (SF) and expansive agent (EA) on the bond behaviour of steel bar in concrete was investigated by using a new pull-out test method. A total of 72 concrete specimens cast from concrete mixes with varying water/binder ratio, SF content and EA content were tested. The test results showed that the combined addition of SF and EA would substantially improve the bond strength and bond stiffness. More importantly, such combined addition would offer the synergistic effects that the benefit of adding SF is larger in the presence of EA and the benefit of adding EA is larger in the presence of SF.