Effect of water/cement ratio and silica fume addition on the fracture toughness and morphology of fractured surfaces of gravel concretes G. Prokopski a, *, B. Langier b a Rzeszo Âw University of Technology, Powstan Âco Âw Warszawy 6, 35 - 959 Rzeszo Âw, Poland b Technical University of Cze Îstochowa, Akademicka 3, 42 - 200 Cze Îstochowa, Poland Received 12 November 1999; accepted 5 June 2000 Abstract The results of the fracture toughness investigations for concretes made from natural gravel aggregate, with diverse water / cement ratio (W / C = 0.33, 0.43, 0.53 and 0.63), without silica fume and with a silica fume addition are discussed. The critical values of the stress intensity factor, K Ic S , as well as, the critical values of crack tip opening displacement, CTOD c were determined. Also, the examination results for profile roughness parameter, R L , and fractal dimension, D, of concrete specimen fractures obtained in fracture toughness tests were performed. The largest values of the stress intensity factor, K Ic S , were showed by concretes with the lowest water / cement ratio, W / C = 0.33 (both with and without silica fume addition). This was caused by considerably lower porosity of the aggregate ± cement paste transition zone as observed in microstructural examinations, which had in this case a compact structure with a small number of structural defects. Cracks, upon reaching the critical force P Q , ran through the coarse aggregate grains, and the obtained fractures were flat in character. The examined parameters of fracture morphology, i.e., the profile line development degree, R L , and the fractal dimension, D, reached the smallest values for those fractures. As the water / cement ratio increased, an increase in the structural porosity of the aggregate ± cement paste transition zone occurred, which caused a promoted propagation of cracks and resulted in the obtaining of lower values of stress intensity factor, K Ic S . Cracks in this case propagated avoiding coarse gravel grains (an overgrain fracture formed), which resulted in increased fracture surface roughness and in a rise of the values of both examined parameters of fracture surface morphology, R L and D. D 2000 Elsevier Science Ltd. All rights reserved. Keywords: Interfacial transition zone; Fracture toughness; Silica fume; Concrete 1. Introduction Concrete is a multi-phase material, which makes its properties impossible to be determined without an analysis of the effect of its individual components on the service properties and designation for various applications. Being the most common material in building construction, con- crete is continuously subject to intensive and comprehen- sive investigations. Of the factors that influence the properties of concrete, particular importance is attributed to the aggregate ± cement paste transition layer that is regarded as the most sensitive area within the structure of concrete. The structure of the transition zone, and thus its properties, are influenced by several factors, such as the type of components used (coarse aggregate, cement, additions and admixtures) and the water ± cement ratio. A particularly significant role is attrib- uted to the latter of the above factors, which has been confirmed by numerous studies in which an increased porosity has been found to occur in the area of the aggregate±cement paste interface caused by the higher water/cement ratio in this region with a simultaneous decrease in water/cement in the bulk of the matrix [1,2]. The examinations of the matrix structure clearly indi- cate the occurrence of porosity gradient with the maximum at the surface of the aggregate grains [3±5]. The cause of the increased porosity and the related properties of the transition zone are ascribed by many researchers to water / cement ratio. According to Ref. [6], the structure of porosity of the transition zone, its thickness and properties, are all closely * Corresponding author. Tel.: + 48 - 17 - 865 - 1439; fax: + 48 - 17 - 854 - 3365. E-mail address: grzeprok@ewa.prz.rzeszow.pl (G. Prokopski). 0008-8846/00/$ ± see front matter D 2000 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(00)00332-X Cement and Concrete Research 30 (2000) 1427 ± 1433