THE VIRTUAL CEMENT AND CONCRETE TESTING LABORATORY

The computational materials science of cement and concrete has been an active research area at NIST since the early 1980s, in close synergy with experimental materials science. A wide range of computational tools has been developed to model microstructure, simulate hydration, and calculate physical properties. This digital "tool-kiti" (Garboczi and others, 1999) includes programs for simulating cement hydration and building three-dimensional cement paste microstructures, assembling three-dimensional concrete microstructures using model aggregates, analyzing microstructures using percolation concepts, and computing physical (thermal, electrical, diffusional, and mechanical) properties using finite difference, finite element, and random walker algorithms. These are all described in an "Electronic Monograph," which is a web-based book of well over 2000 pages used by an average of 9000+ users per month from 80+ countries (Garboczi and others, 1997). However, it was recognized that there were important gaps between the assumptions (e.g., restricted curing conditions, constraints on starting materials and admixtures, and simplified aggregate shapes) of the models and current industry practice. Born out of the desire to advance the application of computational materials science by making the models able to address an increased number of real issues of concrete practice, the Virtual Cement and Concrete Testing Laboratory (VCCTL) consortium was established in January 2001. The ultimate objective of the VCCTL project is to develop a virtual testing system, using a suite of integrated software modules for designing and testing cement-based materials in a virtual environment, which can accurately predict durability and service life based on detailed knowledge of starting materials, curing conditions, and environmental factors. A pictorial representation of the current modules is provided in Figure 10.3.1. This consortium is a joint NIST/industry effort, and involves a number of leading cement, concrete, aggregates, and admixture companies and associations. The major advancements needed in the models have been identified, and a joint computational/experimental materials science research program laid out. The major current and continuing research topics of the consortium are:

1. enhancements to the cement hydration and microstructure development model to consider additional supplementary materials such as slag and limestone and the prediction of pore solution ionic concentrations;
2. computation of the elastic properties (elastic modulus, creep, shrinkage) of three-dimensional microstructures;
3. experimental measurement and computer modeling of the rheological properties (viscosity, yield stress) of cement-based materials;
4. systematic and comprehensive characterization of materials, including chemical composition, phase identification, and particle size distribution of cement;
5. detailed characterization of aggregate size and shape distributions; and
6. user-friendly scientific visualization of microstructures and dynamic phenomena.

This chapter describes the progress to date with an outline of future research needed. Following these descriptions, three case studies are presented briefly to illustrate the range of issues that have been addressed using the current VCCTL software. Additional details about the consortium itself are provided at the end of the chapter in an appendix.