This study deals with the influence of the fiber concrete mesostructure on the size effect under compressive
fatigue loading. For this purpose, three series of cylindrical specimens of steel fiber–reinforced concrete were
fabricated. Before fatigue testing, all the cylinders were scanned and the main morphological, orientation, and
distribution parameters of the pores and fibers were checked. The images reveal that, near the cylinders’ walls,
the porosity is lower than that in the cores and is even lower for large sizes. Additionally, larger specimens are
more vulnerable to compressive fatigue loading, resulting in a reduction in fatigue life.
