MODE-I FRACTURE OF CONCRETE - DISCONTINUOUS CRACK-GROWTH AND CRACK INTERFACE GRAIN BRIDGING

In the present paper results are presented of vacuum impregnation tests using single-edge-notched concrete plates subjected to uniform boundary displacement. A realistic fracture model for cement-based composites should be based on correct micro-structural observations of cracking and localization. Macroscopic structural effects such as non-uniform opening and non-uniform drying out have a pronounced influence on the specimen behaviour. The results of the vacuum impregnation tests reveal that the fracturing of the specimens is a three-dimensional growth process. Cracking starts from the outer surfaces of a specimen, most likely due to non-uniform drying out and the related tensile eigenstresses near the specimens surface. The macrocracks are highly discontinuous cracks with debonding near larger aggregates and intact material bridges between them. The load carring capacity of a tensile specimen for average crack openings larger than 50-mu-m can be explained from distributed crack interface grain bridging. The failure of the grain bridges is a process involving bending and frictional pull-out. Next to this an 'intact core' was observed in some specimens loaded up to average axial deformations between 50 and 100-mu-m. The effects of maximum aggregate size on softening was significant for the range of specimen sizes and aggregate sizes tested. The increased post-peak load carrying capacity for coarse grained mixes can be explained from the larger size (and thus the increased flexural capacity) of the crack interface grain bridges.