Fatigue response of concrete subjected to biaxial stresses in the compression-tension region

In this paper, the fatigue behavior of concrete subjected to combined stres ses in the compression-tension region of the biaxial stress space is studie d. Hollow cylindrical concrete specimens are subjected to combined stresses through torsional loading. The load-deflection responses of specimens subj ected to cyclic and constant amplitude fatigue loading are presented. Damag e imparted to the specimens during cyclic and fatigue loading processes was monitored using mechanical measurements and a nondestructive evaluation te chnique based on the measurement of structural resonance frequencies of vib ration. The complete load response of the specimen subjected to cyclic load ing was obtained by unloading the specimen at different points in the postp eak part (descending branch) of the quasistatic response. Changes in the re sonant frequencies during the loading procedure ware monitored. Fatigue tes ts were performed to failure with three different torsional load ranges. Th e decrease in rotational stiffness during fatigue tests was obtained from m echanical measurements, and the resonance frequencies are presented. It was observed that the decrease in rotational stiffness at failure for the cons tant amplitude fatigue loading was comparable to the corresponding load in the postpeak part of the quasistatic response. The number of cycles to fail ure is closely related to the rate of the reduction of stiffness of the spe cimen as well as the resonant frequencies in linear portion of the fatigue response. This relationship is independent of the applied load range. The f atigue failure of concrete subjected to torsional loading is a local phenom enon similar to failure for quasistatic loading; the damage is seen to loca lize to a crack in the first few cycles, and the subsequent fatigue behavio r is governed by the propagation of that crack. An approach for predicting the fatigue life and the stiffness of a pavement structure is finally prese nted using the results of this paper.