Abstract
The bond stress-slip behaviour of steel bars embedded in concrete is an import parameter in the ultimate limit state (ULS). Currently, design codes in the UK seek to ensure a robust and stable design in reinforced concrete (RC) structures, and it is important to understand the both pre- and post-peak bond behaviour at the element level has potential implications for the safety of people inside of structures subject to extreme behaviour. In this study, the bond stress slip behaviour has been measured (under displacement control) as a function of the displacement at values approaching three times the rib spacing. The influence of bar corrosion on this behaviour has also been investigated. A local bond stress-slip model has been proposed by the Task Group on Bond Models of Structural Concrete (FIB) that assumes the bond stress-slip behaviour of pull-out tests is linear when tested under confined conditions. However, in this study the observed global behaviour of the bond stress, as a function of the bar slip, showed a previously unreported fluctuation behaviour (measured by LVDT) when tested under displacement control. The fluctuation behaviour of the bond stress-slip relationship was found to be both reproducible and consistent and appears to be related to the rib spacing of the deformed bar. The fluctuation behaviour of the bond stress-slip relationship of both uncorroded and corroded steel bar was independently verified by measuring the strain changes along the bar using bonded ER strain gauges. The pull-out behaviour of ribbed steel bars was investigated as a function of the degree of corrosion - applied using an accelerated corrosion method. Steel pickling of steel bars extracted from the failed pull-out specimens indicate that the degree of corrosion affects the peak and residual bond stress differently in that the peak stress was found to exhibit a maxima value whilst the residual strength consistently decreased. This implies that when using bond stress-slip models to predict the effect of corrosion on the bond stress it is necessary to use different ratios of the peak bond stress and residual bond stress at any given degree of corrosion. Correlation between the behaviour measured experimentally for 16 mm diameter ribbed steel bar and that predicted by a 2-D FE model, based on a proposed 3 component (in parallel) delamination interface model, has been shown to be reasonable. A single delamination interface element was used to verify the proposed FE bond model of pull-out test against existing data from the literature and found to give reasonable agreement over displacements equivalent to one rib spacing.