Invited Lecture
Pages 17

The paper summarizes the latest results on the size effect in fracture of concrete and similar materials and the use of the size effect for determining material fracture properties, and presents new results on the influence of the loading rate on the size effect and determination of the time dependence of material fracture characteristics on the basis of the observations of this influence. Bazant's law for size effect is expressed in a shape - independent form in which the parameters are the fracture energy and the effective size of the fracture process zone. Linear regression of the maximum load data according to this formulation yields the values of these two basic material properties. The brittleness number of a structure is then analyzed, and calculation of geometry dependent Rcurves on the basis of the size effect is outlined. Furthermore, preliminary results of an ongoing project at Northwestern University dealing with the loading rate influence are presented. It is shown that the loading rate influence on the size effect can be used for determining the dependence of the material fracture properties on the rate of loading. The results confirm that the fracture energy decreases with decreasing rate of loading, and reveal an interesting result: the effective size of the fracture process zone decreases as the loading rate increases from the fastest static loading to very slow loadings in which the peak load is reached within several days. It appears that for such slow loadings the fracture process zone relaxes almost to a point, i.e. the effective length of the fracture process zone tends to zero. This would mean that the fracture behavior under long-time loading could be analyzed according to linear elastic fracture mechanics, taking of course into account the linear aging creep around the fracture process zone. The theoretical deductions are documented by experimental results.