This paper studies the size effect on flexural fatigue in concrete. In particular, the main objective is to evaluate how the addition and content of steel fibers affect this size effect. For this purpose, four types of concrete have been produced: plain and reinforced concrete with fiber contents of 0.3%, 0.6% and 1%. Two prismatic specimen sizes were considered: 75×75×300 mm, named S, and 150×150×600 mm, named L. All specimens were subjected to 3-point bending fatigue at the same relative stress levels up to failure. The results reveal very interesting conclusions. The fibers mitigate the size effect on fatigue life, going from a difference of three orders of magnitude in PC, to practically zero in SFRC. Furthermore, it is observed that fibers do not necessarily improve fatigue life; in fact, the trend changes depending on specimen size. Finally, it is shown that the secondary crack opening rate dCMOD/dn has a good correlation with fatigue life, explaining the dispersion of N in general and the size effect in particular.
