Contribution of Concrete between Cracks at Inelastic Steel Strains and Conclusions for the Optimization of Bond

The bond betweem reinforcement and concrete should ensure high structural stiffness and small cracks in the serviceability limit state, generate small splitting forces and allow full utilization of the reinforcement ductility in the ultimate limit state. While bond behavior at service load and splitting behavior has been investigated intensively, bond behavior at large inelastic steel strains is not known very well. Therefore, in this paper the contribution of concrete between cracks at inelastic steel strains is investigated numerically based on a rational mechanical model and using realistic constitutive materials laws. The model predictions agree rather well with a large number of tst results. According to the results of the parametric study, after steel yielding the ratio of mean steel strain to the steel strain at the crack is mainly influenced by the reinforcement percentage and the shape of the steel stress-strain curve. It is much lower than at service load. Due to this lower ratio of mean steel strain to steel strain at the crack, the rotation capacity of plastic hinges and this the structural ductility is reduced significantly and may be very low if reinforcement with low ductility is used. Therefore an optimization of bond seems to be necessary. Corresponding extensive numerical and experimental studies are under way in Germany.