The VCCTL models start at the level of the cement and mineral admixture particles. At this level, the most important needs include the cement PSD and where the various chemical phases of the cement are distributed within the particles. Accurate bulk measures of a cement's chemical phases using the Rietveld analysis of x-ray diffraction data, combined with particle-size distribution information, can capture most of the information necessary for experimental characterization of a cement's properties of hydration. Because cement hydration takes place at the individual particle level, correct particle level information is needed as input into the VCCTL virtual hydration model.
Use of a combination of back-scattered electron scanning electron microscopy (SEM) and x-ray microprobe analysis will identify the chemical phase belonging to each subregion (pixel) of each cement particle examined. Figure 1(a) displays, in false coloration, an image of cement particles after all of the major chemical phases have been identified. Cement databases containing this kind of information are being systematically built up and incorporated into the VCCTL software package. Figure 1(b), on the other hand, shows in a three-dimensional display how virtual particles, with realistic chemistry and shape, are arranged in a model cement-paste microstructure just prior to the start of hydration.
Fig. 1(a): This "false color" image, produced by scanning electron microscopy and x-ray microprobe analysis, illustrates the various chemical phases in individual cement particles. The image has been enlarged from its real size of 256 x 200 µm (10 x 8 mil)
Fig. 1(b): This false color, three-dimensional image shows virtual cement particles mixed in water just prior to the start of hydration. The image is a cube, 100 µm (4 mil) on each side
Aggregate in concrete, both fine and coarse, can be of many different mineralogical types, and is sieved or crushed to the desired size distribution. The aggregate's internal porosity, water absorption, mineralogy, and mechanical properties can vary greatly depending on its type. While these properties can be measured, to make a virtual concrete one must generally use aggregate having realistic shapes. For predicting some properties, such as chloride diffusivity, the shape of model aggregate does not seem to matter much. For other properties, however, like fresh concrete rheology and mechanical properties—especially at early ages—aggregate shape does mean a lot. Few standard tests address aggregate shape; none attempt to characterize the full three-dimensional aspects of aggregate shape necessary for understanding and predicting its effect on concrete properties.
The shape of real aggregates can be characterized by a combination of x-ray computed tomography and mathematical analysis. Figure 2 depicts a virtual reality modeling language (VRML) picture of a fine and a coarse limestone aggregate obtained from one of the proficiency samples of the American Materials Reference Laboratory (AMRL). Association of State Highway and Traffic Officials (AASHTO). Databases are currently being augmented for various aggregates and incorporated into the VCCTL software. Similar techniques can be employed to characterize cement particle shape. Cement particles (usually about 10 to 20 µm [0.4 to 0.8 mil] in size) require x-ray microtomography to reach the diminutive length scales of approximately 1 µm (0.04 mil) per voxel necessary to capture their shape (a voxel is the term for "volume pixel," the smallest, identifiable box-shaped part of a three-dimensional digital image). Still, the three-dimensional shape of the very finest cement and admixture particles, 1 µm (0.04 mil) or less, cannot be so captured at the present time.
Fig. 2: Reconstructed VRML images of aggregates taken from AMRL proficiency samples. The large coarse aggregate (limestone), at left, is about 13 mm (1/2 in.) in size, and the fine aggregate (river sand), lower right, is about 1.5 mm (0.06 in.) in size