Investigation into Flexural Bond Strength Test Method to Evaluate Influence of Surface Roughness on Bond Characteristics of UHPC with Precast Concrete

The bond performance between UHPC and precast concrete was studied using three bond test methods—slant shear test, third-point flexural bond test and pull-off test methods. The influence of degree of surface roughness, prepared by sandblasting, on the bond between UHPC and precast was investigated. The surface roughness was quantified by two test methods—sand spread test and laser profiling. The test results showed that the third-point flexural bond test was a convenient and a realistic bond test to conduct. The increase in the duration of sand-blasting increased the roughness of the precast concrete surface which was quantified by both the sand spread test and the laser profiling. The laser profiling also reviewed that the mortar fraction was easier to be roughened than the coarse aggregate of the substrate precast concrete. Duration of sandblasting as short as 10 seconds was enough to achieve adequate bond between UHPC and precast concrete at the age of 7 days after casting UHPC, which was indicated by a failure in the precast concrete part and an ultimate load about same as the ultimate load of monolithic precast concrete specimen. Specimens without any surface roughened failed at the bond between UHPC and precast concrete, and the ultimate load was lower than that of monolithic precast concrete specimens. Moreover, the bond strength between UHPC and precast concrete cast on un-roughened sawn surface was higher than that cast on un-roughened molded surface.