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C.M. Neubauer and H.M. Jennings
Department of Materials Science and Engineering
Department of Civil Engineering
Northwestern University
Evanston, IL 60208
E.J. Garboczi
National Institute of Standards and Technology
Building Materials Division, 226/B348
Gaithersburg, MD 20899
Abstract
Mortar and concrete are composite materials with overall properties that are influenced by the arrangement and characteristics of each constituent in the microstructure. Elastic shrinkage, like other properties of mortar and concrete, must be described by a three-phase model: aggregate, bulk cement paste, and interfacial zone cement paste. A simple two-dimensional digital-image- based model of a mortar is developed based on the hard core/soft shell percolation model. Specific attention is given to the properties of the interfacial transition zone between cement paste and aggregate. The elastic shrinkage properties of this model are computed numerically, using a specialized finite-element technique. The effects of varying the elastic moduli and shrinkage properties of the interfacial zone and bulk cement paste are examined in parameter studies, both analytically, for low aggregate content, and numerically, for arbitrary aggregate content. Special attention is given to the effect that the topology of the interfacial zone has on overall shrinkage. A comparison is made between model predictions and the limited available experimental results (0.35 w/c, 65% hydration), assuming an interfacial zone width of 20 micrometers. Based on this comparison, this paper introduces the prediction that the Young's modulus of the interfacial zone is 1/3 to 1/2 of the modulus for bulk cement paste, and that the unrestrained shrinkage of the interfacial zone material is close to the shrinkage of bulk cement paste when both are averaged over a 20- micrometer thick interfacial zone. This finding is supported by further analysis of a 3- D microstructural model of cement paste.