This thesis presents the results of experimental and theoretical studies of the micromechanical behavior of concrete under different loading conditions. Cylindrical specimens of normal and high-strength concrete were subjected to testing under uniaxial and confined compression. An alloy with a low melting point was used as a pore fluid. At the stress or strain of interest, this alloy was solidified to preserve the stress-induced microcracks as they exist under load.

Scanning electron microscopy (SEM) was employed to capture images from the cross sections of the concrete specimens. These images were then used to study the generation, orientation, density, length, and branching of the compressive stress-induced microcracks and the effect of confinement on microcrack behavior. The microcracks were generated by a number of different mechanisms and had an orientation that was generally within 15 degrees of the direction of the maximum applied stress. The density, average length, and branching of the microcracks decreased as the confining stress increased. The confining stress showed a pronounced influence on interfacial cracks, also known as transition zone cracks, which occur at the interface of cement paste and aggregate. The amount of interfacial cracking decreased significantly as the confining stress was increased. Stereological analysis which interprets three-dimensional structures by means of two-dimensional sections, was used on the computerized images. Crack orientation, crack surface area, and crack length were determined stereologically. The resulting stereological measurements indicated that the crack orientation, surface area, and length decreased as the confining stress increased.

Three micromechanical models, the differential scheme, the Mori-Tanaka method, and a crack growth simulation model were used to examine the experimentally obtained data against the theoretically developed micromechanical models. The final modulus of elasticity for the concrete specimens was calculated using the first two models, based on the measured crack densities, which gave an approximation that was very close to the actual measured moduli. The crack growth model was used to generate and propagate microcracks for uniaxial and fully confined cases, and it also revealed behavior similar to that shown in the experimental results.

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