Effect of water/cement ratio and silica fume addition on the fracture toughness and morphology of fractured surfaces of gravel concretes

The results of the fracture toughness investigations fbr 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, 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. (C) 2000 Elsevier Science Ltd. All rights reserved.