Shear experiments were conducted on 18 Z-shape specimens from 6 different groups to
investigate the shear properties of the interface of new and old concrete. The effects of different
interfacial techniques,i. e., smearing cement paste, directly casting new concrete, and smearing
commercial interfacial agents, on the shearpropertieswere investigated. The experimental results show
that the interfacial techniques have a great effect on the shear properties of a new-to-old concrete
interface. The shear strength of specimens is greatestwhen the technique of smearing cementpaste is
used in the three interfacial techniques. Furthermore, reinforcing steel bar planting can improve the
shear properties of the interface remarkably. The experiments show that the depth of reinforcing steel
barplanting should be greater than ten times the diameter of a reinforcing steel bar in the repair and
strengthening of structures.