Keywords: micromechanics, ITZ, microcracks, debonding, elastic limit, interface, concrete, mortar, cement paste..

Crack initiation in concrete, represents the elastic limit of the material, typically starting in the 15 microns thick interfacial transition zones (ITZs) around aggregates. Recently, two possible modes for crack initiation have been identified from inspection of post-failure concrete fragments: clean separation of aggregates from ITZs and ITZ cracking resulting in a thin layer of cement paste remaining attached to the aggregate surface. As for micromechanics modeling, concrete is idealized as a perfectly bonded three-phase composite consisting of spherical aggregates, covered by ITZs, and embedded in a mortar matrix. Mortar, in turn, is also represented as a matrix inclusion composite consisting of spherical sand grains, covered by ITZs, and embedded in a cement paste matrix. ITZs are considered to be perfectly bonded two-dimensional interface phases at concrete and mortar scales. At the micrometer-scale, however, the ITZs are resolved as three-dimensional spherical shells. In this paper this multiscale continuum micromechanics model is used to study stress concentration and amplification characteristics throughout the scales. Microscopic tractions and stresses, respectively, are larger around coarse aggregates than the ones around sand grains. Thus, the model predicts that microcracking starts around the coarse aggregates, and this is in agreement with independent experimental observations.

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