Damage assessment and residual capacity of RC beams subjected to low-cycle fatigue

This paper presents the preliminary results of an extensive experimental campaign of investigation on the damage assessment and residual capacity of RC beams at the University of Canterbury using low-cycle fatigue loading approach. In the first stage, the structural behavior of an RC beam under constant-amplitude fatigue loading at ±2% was examined. This provided a basis to determine the total number of cycles required to failure (i.e., fatigue life), and also to assess the fatigue damage in terms of degradation in strength, stiffness and energy dissipation capacity over the fatigue life of the specimen. In the second stage, two companion RC beams were subjected to 70% and 90% of their fatigue life at ±2% drift, re-centered and then monotonically loaded up to failure. Furthermore, one reference intact beam was subjected to similar monotonic loading up to failure. To determine the residual monotonic flexural capacity of the beams, the lateral force-displacement curves of both cyclically pre-damaged specimens and the undamaged specimen were compared. The evolution of fatigue damage represented three different regions: initial degradation up to about 10% of fatigue life, followed by a gradual decay up to about 70% of fatigue life, and the third stage, consisting of significant drops due to fracture of longitudinal reinforcement. The study showed that 70% is the fraction of fatigue life that may be considered as the threshold beyond which the failure of the test specimen is inevitable. It was also observed that the application of 70% of the fatigue life degrades strength and secant stiffness in the subsequent residual monotonic behavior of the specimen by up to 38% and 72%, respectively but there is almost no degradation in deformation capacity up to 12.5% drift.