Water–Cement Ratio on High-Cycle Fatigue in the Theory of Critical Distances of Plain Concrete

The theory of critical distances (TCD) represents a new area of research on fatigue damage in concrete that is primarily used to characterise fatigue and fracture behaviours. Although TCD is accurate, it appears inconsistent when considering the water–cement ratio of concrete. Such inconsistency is related to the tendency to overlook the effects of the water–cement ratio owing to the small differences observed in tensile strength. The study’s main objective is to explore how sensitive fatigue characteristics in concrete are to different water–cement ratios. This research is important because information on concrete’s fatigue limits—especially regarding comparisons between different concrete mixes—is scarce. There is a lack of standard procedures for testing the fatigue and fracture behaviour of plain concrete, thus being inconsistent and slow. Thus, the current study has utilised the TCD concept to assess fatigue. However, TCD seems to be susceptible to changes in the water–cement ratio of concrete. Water–cement ratios of 0.3, 0.4, and 0.5 increased concrete’s fatigue limit to 2.883, 3.022, and 3.903 MPa, respectively. These increases were confirmed to be significant. Thus, this research will improve the current understanding of TCD’s value in fatigue analyses of concrete structures.