Investigation of vertical shear strength in unreinforced interfaces between concrete cast at different times

This thesis presents an experimental study of vertical interface shear strength between layers of concrete cast at different times. Current design provisions allow only for load to be transferred across reinforced interfaces. However, previous research has suggested that unreinforced interfaces can be manipulated to sustain significant load transfer. The first part of this project was to identify key factors affecting interface shear strength through a review of previous research. The key factors identified were interface roughness, differential stiffness, and differential shrinkage. To quantify the effects of these factors, an experimental program consisting of twenty-seven specimens was conducted in two phases. Phase one consisted on specimens with a precast shell and a cast-in-place core, while phase two consisted of a cast-in-place shell with a precast core. The variables tested were smooth interfaces compared to rough, sandblasted interfaces, conventional concrete compared to shrinkage reducing concrete, and circular compared to square interfaces. Interface slip data was also recorded to better characterize the behavior of the interfaces under loading. Interface strengths observed ranged from 41 psi to 236 psi. It was found that roughened, circular interfaces tended to be the strongest and circular smooth interfaces with shrinkage reducing concrete were the weakest. Results indicated that circular interfaces were preferable compared to square interfaces. Further, the use of shrinkage reducing admixture greatly reduced interface shear strength, which was an unexpected result. Finally, it was concluded that interface roughening, even in a modest amount, can provide significant strength gains independent of concrete strength or stiffness. This research also suggests a modified version of the AASHTO interface shear transfer equation to allow for future discussion on the use of unreinforced interfaces in design applications.