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Examination of fracture toughness of concretes was made using Mode I (tension at bending) and Mode II (shearing) fracture. Subjected to examination were gravel and dolomite concretes in their natural states and the same concretes as made from paraffinated aggregates. Gravel and dolomite conretes with diverse water–cement (W–C) ratios were also examined. The values of the stress intensity factors, KIc and KIIc, and those of fracture energy, JIc and JIIc, were determined. In the case of concretes with variable W/C ratios, regression equations were also determined that described the dependence of the stress intensity factors and fracture energies on the W–C ratio. The paraffination of aggregates resulted in a considerable drop in the stress intensity factors studied as compared with those of concretes made from non-paraffinated aggregates. This drop was 34% for gravel as examined according to Mode I fracture, and 27% as examined according to Mode II fracture. For dolomite concrete drops were 19 and 28%, respectively. An increased W–C ratio caused a dramatic drop of both stress intensity factors. By addition of a super-plasticizer to the concrete mixture an evident improvement in the strength properties of both types of concrete occurred. Microstructural examinations performed have clearly confirmed the relationship between the type of aggregate used for concrete making and the microstructure of the concrete, particularly within the area of the contact layer between the aggregate and the cement paste.