Size effect on fatigue in bending of concrete

This paper presents a semianalytical method to predict fatigue behavior in flexure of concrete in the case of maximum flexural moment larger than that of the first crack moment based on force equilibrium in the critical cracked section. The model relies on the cyclic bridging law, the so-called stress-crack-width relation under cyclic tensile load as the fundamental constitutive relationship in tension. The structural size effect on fatigue in bending of concrete beams is studied by the present model. Eight series of beams, with heights from 50 to 800 mm, are analvzed. The model results show that the fatigue performance in bending, normally expressed in terms of the maximum flexural stress versus fatigue life diagram (S-N) is strongly dependent on the structural size, even when the same material parameters are used in the model. Under the same cyclic flexural stress levels, the smaller the beam height, the longer the fatigue life is. Besides this, the deformation characteristics, such as fatigue crack growth history as well as the final fatigue crack length, are also size dependent. Finally, the model predictions are compared with experimental results. Good agreement between model predictions and experiments has been obtained.