JOURNAL OF MATERIALS SCIENCE 33 (1998) 1819 — 1825 The examination of the fracture toughness of concretes with diverse structure G. PROKOPSKI Rzeszo ´ w University of Technology, 35-959 Rzeszo ´ w, Powstan ´ co ´ w Warszawy 6, Poland J. HALBINIAK AND B. LANGIER Technical University of Cz e ‘ stochowa, 42-200 Cz e ‘ stochowa, Armii Krajowej 3, Poland 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, K I c and K II c , and those of fracture energy, J I c and J II c , 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.  1998 Chapman & Hall 1. Introduction The strength of concrete, as traditionally understood and analysed, is compressive strength; which, though easy to evaluate, is not a perfect mechanical quantity because it evaluates only the maximum force that is carried by a concrete specimen of a specific form. Observations of failure of concrete specimens under a compressive load indicate that as increase in force often still occurs when the specimen already shows cracking, i.e. when it is already beyond its failure stage. This imperfection does not occur in the testing of concrete for fracture toughness. In this examination, the values of critical force and critical stresses are determined at the moment of dramatic propagation of the crack existing in the specimen. In recent years, intensive research work has been undertaken aimed at determining the influence of the composition of the concrete mixture, including the type and amount of coarse aggregate, the W—C ratio and the associated character of the aggregate—cement paste interface layer, on the properties of hardened concrete. Particularly intensive investigations are fo- cused on the properties of the interface layer, which extends by approximately 50 m from the coarse ag- gregate—cement paste contact layer, and on the deter- mination of its effect on the course of the failure process and the strength of the concrete. The existence of an interfacial layer of different microstructure from that of the cement paste beyond this layer has been confirmed by several researchers [1— 4]. Studies on the effect of the interfacial layer on the mechanical proper- ties of concrete have been conducted in recent years mainly applying the methods of fracture mechanics [5—8]. An argument still exits among researchers concern- ing the influence of specimen size on the obtained values of fracture mechanics quantities, such as the stress intensity factors, K  and K  , or fracture en- ergy, J  and J  . It has been proposed that the height of test specimens subject to Mode I fracture, i.e. ten- sion at bending, should be a minimum of 229 mm [9] or at least 50 diameters of the maximum grains of the coarse aggregate [10]. The subsequent results of works performed by different researchers [11, 12] have shown that the size of the concrete specimens to be examined does not have to be so large. It has been stated in the RILEM draft recommendation [13] that the minimum height of a bent specimen should be minimum 150 mm when using a coarse aggregate with a grain size of 25 mm and a minimum of 250 mm when 0022—2461  1998 Chapman & Hall 1819