Cracked concrete structures under cyclic load

The safety of cracked concrete dams is fundamentally affected by their mechanical behaviour under seismic excitation. Such a load is far from being harmonic and is characterized by intermittent spikes. Therefore the sequence effect is analysed. Two widely accepted non-linear methods were used: the Cohesive Crack Model to analyse the evolution of the process zone and the Continuous Function Model (CFM) to analyse the local hysteresis loop. In its original formulation, CFM predicts that a higher preloading arrests the fatigue crack growth at a subsequent lower load level. This unrealistic and unconservative behaviour is due to the fact that the above mentioned model neglects the damage occurring during the so-called inner-loops. In other words the CFM assumes that inner loops are mere loading loops and not fatigue loops. This assumption causes an incorrect prediction of the sequence effect. For the same reason the CFM predicts an endurance limit which is higher than attested by experimental evidence. In order to obtain more realistic results, in the present paper the CFM was enhanced, introducing a damage mechanism for the inner loops too. In the new model proposed, as well as in the original CFM, the endurance limit is seen to be almost constant relative to structural size.