Comparison of fracture behavior of set concretes based on natural and crushed aggregates

The studies were carried out to diagnose the effects of coarse aggregate type on the mechanical behavior of plain concretes under incremental loading. During the studies mechanical parameters including compressive strength (f(cm)) and splitting tensile strength (f(ctm)), as well as fracture parameters involving critical stress intensity factor (K-Ic(S)) and critical crack tip opening displacement (CTODc) were evaluated. The aim of the study was to accurately assess the impact of the type of mineral aggregates on the fracture toughness of ordinary concretes. In order to obtain the most reliable comparison of the obtained test results, mineral aggregates, both natural and crushed, from the same geographic area (most commonly used in plain concretes) were used in the experiments. Hence, crushed aggregates covered: basalt (BA), granite (GT) and limestone (LM) and natural pebble gravel aggregate (GL) were used in the concrete mixtures. For better understanding of the crack initiation and propagation in concretes with different coarse aggregates, a macroscopic failure surfaces examination of the tested beams is also presented. Both of the analyzed fracture mechanics parameters, i.e. K-Ic(S) and CTODc increased significantly in the case of concretes which were manufactured based on crushed aggregates. They amounted in comparison to concrete based on gravel aggregate at levels ranging from 20% for concrete with limestone aggregate, to over 30% for concrete with a granite aggregate, and to as much as over 70% for concrete with basalt aggregate. The fracture process in each series of concrete was: quasi-plastic in the case of gravel concrete, semi-brittle in the case of limestone concrete, and clearly brittle in the case of the concretes based on granite and basalt aggregate. The results obtained help to explain how the coarse aggregate type affects the strength parameters and fracture toughness at bending.