Abstract
Size effects on the flexural strength of concrete have been experimentally known for a long time. Numerous studies have been conducted to explain the mechanism of such effects through various approaches including stochastic analysis of defects and internal restraint due to drying shrinkage. A fracture mechanics approach has shed new light on this mechanism in recent years.In the design of concrete pavement in Japan, a slab thickness to meet the service life requirement is determined by calculating the ratio of the stress resulting from various loads to the above-mentioned design flexural strength of concrete（stress ratio, S）and the fatigue degree determined from the design flexural fatigue curve and expected traffic. The design flexural fatigue curve used for this purpose is the one established based on the results of tests conducted in Japan using concrete specimens measuring 150 by 150 by 530 mm only.
However, the slab thickness of actual concrete pavement generally ranges from 250 to 300 mm for roads and around 450 mm for airports, being much thicker than specimens used for the above-mentioned flexural fatigue testing. In view of the clear existence of size effects on flexural strength, it is highly probable that flexural fatigue strength is also affected by size.
In this study, we investigated the size effect by using three different heights of concrete test specimens（150mm and 450mm）in the flexural fatigue test. As the result, it is known that the cycle of fatigue failure increases when the thickness of concrete is thickened. Furthermore, we made clear the size effect mechanism of flexural fatigue failure from the point of view of fracture mechanics.
