The objective of the proposed investigation is to introduce a new approach to the study of the micromechanical behavior of concrete by combining its materials science and structural engineering aspects, based on the assessment of the qualitative and quantitative significance and role of cement paste-sand particle interfaces in stress-strain behavior of concrete that is subjected to pure uniaxial compressive force. In the experimental part, mortar cylinders having sand particles of different packing characteristics are tested for their stress- strain behavior and their micromechanical properties. Concrete cylinders will also be tested to compare the significance of the coarse aggregate with the sand particles. These are then correlated with the microstructural properties using SEM with backscatter electron analysis and image analysis capabilities, mercury intrusion porosimetry, and electrical conductivity measurement techniques. In the analytical part, a model is developed which describes the micromechanical behavior of concrete and defines quantitatively the role and significance of interfacial parameters. In the model development a nonlinear fracture mechanics approach is taken using the finite element method as a tool to analyze crack propagation and localized deformation. The relationship between crack propagation and sand particles packing characteristics will be studied. In this approach, interfaces are treated as weak planes or initial flaws. Results of this investigation can be extended to other cases including durability, creep, and shrinkage considerations, and optimum mixture proportioning of conventional and advanced cement-based concrete composites.