The interface between aggregate and cement based matrices plays an important role in determining the properties of concrete. In an earlier study, a push-out test technique and a theoretical model were proposed to determine the stiffness, strength, and surface energy of the interface layer. To be valid material properties, these bond parameters should be independent of the geometry of the specimen, and this was confirmed in this study. The influence of neighboring sand and aggregate particles on the bond properties of the push-out aggregate was analyzed and found to be quite small. On the other hand, the addition of polymer latex and silica fume had major effects on the bond properties. All tests were performed in a closed-loop manner to obtain the load versus slip relationship. The microstructure of the interfacial zone was investigated with the use of back-scattered electron imaging. © 1995.