Laboratory Tests of Adhesion of Steel Bars to Ordinary and Frozen Concrete

The issue of adhesion of steel to concrete is one of decisive features characterising reinforced concrete. Adhesion constraints to concrete are simplistically presented with shear stress evenly distributed on the side surface of reinforcement. However, an exact value of this stress depends on many factors and its estimation is very complicated. There is a lot of laboratory methods aimed at testing adhesion of the reinforcement to concrete but they frequently give different results. Sometimes, they are even contrary.

In this paper, the state of knowledge on the steel-concrete adhesion, mechanism of its loss, and factors influencing it were reviewed. Own tests were conducted with two methods, pull-out and push-on, aimed at reflecting the operation of reinforced concrete structure and specifying the chemical adhesion, as well as the friction force of the reinforcement to concrete. The impact of the freeze-thaw cycle on this process was also studied. The interfacial transition zone on the steel-concrete boundary was analysed as well. The structure of boundary layer around the reinforcement in concrete is very similar to the zone of aggregate separation from paste. In the Polish and foreign literature, the thickness of the transition zone is estimated as 50-100 μm but only 10-20 μm of this zone has variable mechanical properties. The microstructure of the boundary line depends on the kind of concrete and reinforcement, presence and kind of additions, conditions of curing, and many other factors. In order to describe this phenomenon more precisely, own tests of the steel-concrete interfacial transition zone involving analysis of SEM images and identification of chemical composition of this EDS area were conducted. (paper)