Experimental analysis of twist-off and pull-off testing methodologies to measure the bond strength between GFRP and concrete

The strength of the bond between overlay and concrete substrate is traditionally measured with a pull-off test, which indicates whether the tensile strength of the bond between the overlay and concrete is stronger than the tensile capacity of concrete itself. In many instances, it may be more useful to measure the bond shear capacity of the interface because shear strength is what controls the composite action between overlay and substrate. Twist-off testing can measure the bond shear capacity. In order to compare the two methodologies of pull-off and twist-off, a parallel group of tests was conducted on two sets of specimens: the first was an FRP reinforcing overlay on concrete substrate while the second was concrete. The results from the experimental program were used to study the effect of the twist-off shear stress distribution on the deter-mination of bond strength