Factors affecting bond between precast concrete and cast in place ultra high performance concrete (UHPC)

Abstract

The bond strength between precast concrete and cast in place ultra high performance concrete (UHPC) plays a significant role in the application of UHPC as a filling material for bridge connections or an overlay in the rehabilitation of bridges. This study examined several factors that affect the interfacial bond strength in order to cover wide range of conditions in the field. It investigated the effect of type of concrete substrate (normal and high strength concrete), aggregate type (crushed stone and river stone), surface preparation (dry and saturated surface dry), UHPC age, and testing method. The experimental program was conducted in two phases. In Phase I, the effects of concrete strength, UHPC age and type of aggregate were explored using direct tension test method, while in Phase II, the effect of surface preparation was investigated using four different test methods. The results indicate that the UHPC age after casting resulted in a marginal increase in the bond strength for normal strength concrete (NSC) and a significant increase in the bond strength of the high strength concrete (HSC) specimens. The HSC specimens, however, showed slightly higher bond strength than the NSC after 7 and 28 days of curing. Moisture at the interface increased the bond strength by 3%, 22.7% and 2.3% using direct tension, bi-shear and slant shear, respectively. Except for bi-shear test, the saturated surface dry (SSD) surface preparation reduced the variability of test results and the likelihood of the interfacial failure. The cohesion and shear friction coefficients were determined using several methods. The minimum cohesion value was 2.78 MPa (0.4 ksi) for dry and SSD conditions, while the friction coefficients were1.33 and 1.32 for dry and the SSD specimens, respectively. The minimum bond strength under tension was 3.39 MPa (0.49 ksi) and 3.49 MPa (0.51 ksi) for dry and SSD conditions, respectively. Both dry and SSD surface preparation resulted in superior bond and friction that exceeds the value reported by code provisions.