The influence of the bond between concrete and reinforcement on tension stiffening effect

Abstract

This paper deals with the analysis of reinforced concrete structures with special emphasis on modelling of the bond influence on the tension stiffening effect. This effect is the reduction in deformation caused by concrete in tension after cracking and has great significance for calculation of serviceability limit states and nonlinear analyses. The most popular models for consideration of tension stiffening are based on semi-empirical formulae. It seemed that the bond concepts would make significant progress in this subject, but obtained results are rather disappointing. A new model for the bond phenomenon is proposed. It is based on the bond functions, which take into account not only the relative slip between a steel bar and surrounding concrete but also the position of the cross-section. They allow the direct calculation of the bond influence on crack spacing and width and tension stiffening. The form of these bond functions was positively verified on the specially designed specimens. The theoretical results obtained from the proposed model are very interesting and important for real-life situations. The cracking stage of loading determines the further behaviour of the member. The loss of stiffness just after cracking is very sharp and depends mainly on the steel/concrete ratio. For its small value, the stiffness after cracking is about only 12% of the uncracked value. Contrary to the commonly held belief, the bond itself has no significant influence on the changes in stiffness at this stage of loading. Similar conclusions are also valid for members under bending, but the degree of stiffness drop is smaller in terms of the compression zone and the moment distribution along a beam.