Evaluation of Bond Strength of Joints in Hybrid UHPC and SCC Members

Abstract

One of the major barriers to the use of UHPC (ultra-high performance concrete) in the precast industry is the high cost relative to conventional concrete. Often, the properties of UHPC are not fully utilized throughout a member. One solution is to create a hybrid member with UHPC in zones of high or unpredictable stresses while filling the rest of the section with a conventional concrete. Constructing such a member will require innovative techniques to achieve a successful bond between the two different materials. This paper is unique in that it investigates the bond behavior when both materials are in the fresh state. Several specimens of 2 × 2 × 17 inches (50x50x431 mm) were fabricated for direct tension tests. These specimens were fabricated with half UHPC and half SCC (self-consolidating concrete). The interface between the two types of concrete was perpendicular to the tensile force so that the bond was in direct tension. All materials were placed immediately after mixing and were separated by a removable barrier. Time dependency of the bond strength was controlled by removing the barrier at different times after placing. The quality of the bond strength between fresh UHPC and fresh SCC was compared to the tensile strength of the SCC. The results were used to determine a maximum allowable time by which both concretes can be combined while maintaining an acceptable bond strength.