Effects of Relative Rib Area on Bond Behaviour

Steel-to-concrete bond is a basic aspect for the behaviour of R/C structures both at service and ultimate states. When bond rules were originally developed, experimental results were mainly obtained on normal strength concrete and a minimum relative rib area (bond index) was required by building codes to ensure good bond properties. The arrival into the market of high performance concretes and newer structural needs may require different bond indexes.
In the present paper, the experimental results of pull-out tests on short anchorages are presented. More than 90 pull-out tests on ribbed bars, embedded in normal and high strength concrete, have been carried out in order to better understand the influences of the relative rib area on the bond strength and stiffness as well as on the splitting crack width generated by the wedging action of ribs. Tests were performed on bars with three different diameters.
Experimental results show that the bond strength increases with the relative rib area and that the bond response depends on bar diameter (size effect). The width of splitting cracks that develop around ribbed bars increases with the bond index.