Fracture Mechanics and Size Effect of Concrete in Tension

It is found that strength of concrete structures generally decreases with increasing structure size before reaching a limiting value. Fracture mechanics is used to predict such size effect. However, most previous fracture models predict that the strength drops to zero after structures become very large. The size effect is studied with the two-parameter fracture model (TPFM). A general formulation for predicting the strength is first proposed. Two types of specimens, three-point bend beams and split-tension cylinders, which are frequently used to experimentally measure the strength, are then examined. For three-point bend beams, TPFM predicts that the nominal strength decreases with increasing beam size, but approaches to a minimum constant value when sizes of the beam become very large. For split-tension cylinders, the model indicates that load-distribution width is an important parameter on the strength. In the presence of the distributed load, the nominal strength initially decreases with increasing cylinder size, and rises up slowly after the lowest limit. These observations are in accord with the experimental data.