The effects of aggregate type and size are important parameters in the formation of interfacial transition zone (ITZ) structure and subsequently in the failure process of concrete. The influence of surface, rigidity and size of aggregates and water/cement (w/c) ratio of the matrix on bond strength at the ITZ and the interrelationship between the bond and the matrix in the failure process of concrete under uniaxial compression were studied. For this purpose a series of experiments were designed and carried out on mortars (with two different w/c ratios) containing single spherical steel aggregates. The ITZ properties and the failure process of concrete were investigated by means of tensile and compressive strength, and stress-volumetric strain measurements. It was observed that the effect of aggregate properties (high modulus of elasticity, smooth surface texture and size) on the weakness of ITZ and the failure process of concrete are of paramount importance for low w/c ratio composites. The effect of reduced bond properties was reflected in lowered critical stress levels for the low w/c ratio composites with larger aggregates. (C) 2002 Elsevier Science B.V. All rights reserved.