Influence of rebar straightening on bond properties between steel and concrete

Bond between steel and concrete is a basic aspect for the behaviour of R/C structures both in service and ultimate conditions. It is involved into the structural definition of the resistant mechanism for each reinforced concrete element. In the past, a direct connection between bond behavior and the relative rib area (bond index) was investigated and minimum values of the latter were defined in order to ensure good bond properties; those values were based on a wide experimental campaign. In the recent years, some building codes reduced those limits. Furthermore building industry increased the use of de-coiled reinforcing steel and bending machine (de-coiler) that are used to straighten and shape de-coiled product for use on construction sites. The straightening produces a damage on the ribs that modifies their geometrical properties. It is essentially due to the action of passing through several rollers. In the present work, the experimental results of pull-out tests according to RILEM recommendation, are shown and discussed. Specimens with diameter of 8, 12 and 16 mm both with mechanical and manual straightening, taken from the same coil respectively, were tested. It is assumed that relative rib area, after the manual straightening process remains unchanged and then the differences in terms of bond-slip response are only attributable to the mechanical action of the machines.