I MULTI-SCALE DIGITAL-IMAGE-BASED MODELLING OF CEMENT-BASED MATERIALS ~ D.P, BENTZ*, E.J. GARBOCZI*, H.M. JENNINGS+, AND D.A. QUENARD”* * Building and Fke Research Laboratory, Building 226, Room B-350, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA + l)ep~ment ofMaterials Science and Engineering, Northwestern University, Evanston, IL 60208 USA ““ (JentreScientifique et Technique du Batiment, Saint-Martin d’Heres, FRANCE ABSTRACT Computer modelling of the properties and performance of cement-based materials is complicated by the large range of relevant size scales. Processes occurring in the nanometer- sized pores ultimately tiect the performance of these materials at the structural level of meters and larger. One approach to alleviating thk complication is the development of a suite of models, consisting of individual digjtal-image-hazed structural models for the calcium silicate hydrate gel at the nanometer level, the hydrated cement paste at the micrometer level, and a mortar or concrete at the millimeter level. Computations performed at one level provide input properties to be used in simulations of performance at the next higher level. This methodology is demonstrated for the property of ionic diffusivity in saturated concrete. The more complicated problem of drying shrinkage is also addressed. INTRODUCTION Predicting the performance of concrete structures is made difficult by the compkity of the material, which has a complex microstructure [1] and exhibits composite behavior at a seriesof length scales. At the scale of millimeters, one can view the concrete as a composite of aggregates and air voids embedded in a continuous cement paste matrix. Even here, one may need to account for the difference in cement paste microstructure and properties in the interracialzones surrounding each inclusion [2, 3, 4, 5, 6]. At the scale of micrometers, the cement paste is a composite of unhydrated cement particles, hydration products (crystalline and amorphous), and capillary porosity. Fbmlly, the major hydration product of calcium silicate cements, calcium silicate hydrate (C-S-H) gel, is itself a composite of nanometer-sized “particles* and pores. A better understanding of the physical processes occurring at each of these scales and their interactions is necessary to increase the predictability of the performance of concrete. For example, it is the capillary forces which develop in the capillary and gel pores that are mainly responsible for the drying shrinkage of concrete. Thus, to reliably predict the drying shrinkageof 8 field concrete, one must understand the matex’i~ at the sc~e of ficr?rneters and even nanometers [7]. While this large scale rmge cannot be eazily incorporated into a single model for concrete microstructure, it is possible to develop an integrated model in 33 ~ate.ial. Re.earc!k ?ocle~y. ~~crostructure O? R~~nt-B=ed Systems/ Mat. es. OC.~ym . Proc. 0,37001995 Materials Resee h Bonding and Interfaces in Cement itious Materials, Symposia, November 28-December 1 1994 Boston MA Diamond, S. Mindqss, S. Glasser F P., Roberts, t.W., ~kalny, ~.P., Wakeley, L.fi., Editors, *S VO1. 370