Effect of coarse aggregate size and matrix quality on ITZ and failure behavior of concrete under uniaxial compression

Effects of coarse aggregate size and water/cement (w/c) ratio of the matrix on the formation of interfacial transition zone (ITZ) and subsequently on 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 of different sizes. The ITZ properties and the failure process of concrete were investigated through tensile strength tests both before and after compressive preloading, stress–axial strain, stress–volumetric strain and stress–lateral strain measurements. It was observed that ITZ becomes critical for larger aggregates and lower w/c ratio mortar matrices. The negative effect of smooth surface texture of the aggregate and the large difference between aggregate and matrix moduli of elasticity on the properties of ITZ is of paramount importance for low w/c ratio composites. The effect of reduced bond properties of ITZ relative to its matrix was reflected in the lower critical stress levels for the low w/c ratio composites with larger aggregates.