A wide range of concretes was evaluated to explain the reasons for large strain values of high strength concretes (HSCs) at peak stresses under different loading conditions, such as uniaxial compression, uniaxial tension, bending and torsion. To determine the influences of constituents on the stress distributions at the matrix-aggregate interface, around the aggregate and in the matrix close to the aggregate, concrete was considered as a three-phase composite material consisting of a continuous mortar matrix, model aggregate and the interfacial zone between cement and aggregate. The results obtained show that in normal strength concretes (NSCs), i.e. the hard inclusion case, the elastic mismatch of aggregate and matrix is significant and large tangential, radial and shear stresses occur at the interface. However; in both HSCs and lightweight concretes (LCs), the elastic modulus of the aggregate is closer to that of the matrix, and lower tangential, radial and shear stress distributions occur at the aggregate-matrix interface, resulting in these concretes having a much more uniform stress distribution at the interfaces than NSCs. In both HSCs and LCs, tensile stresses occur at the tips of the aggregate (at the poles in the model) perpendicular to the applied stress, and tangential stresses in the matrix close to the interface or at the aggregate surface are larger than those in NSC ones. These imply that the crack will be forced to go through the aggregate and lower strains will develop in ascending branches of these concretes. Based on the model proposed and on additional microstructural studies, it can be concluded that the levels of strains observed at peak stresses under the different loading conditions are as expected. (C) 1998 Elsevier Science Ltd. All rights reserved.