Next: Interfacial Zone Microstructure
Up: Main
Previous: CH percolation: Leaching
Mortar and concrete are random composite materials, with the fine and coarse aggregate acting as the inclusions and the cement paste acting as the matrix. The only real difference between mortar and concrete is in the size of the aggregates used. Typically, the maximum aggregate diameter in a mortar is 1- 3 millimeters, while the maximum aggregate diameter in a commercial concrete is around 30 millimeters. Concrete used for dams and other such structures, known as mass concrete, can have aggregate diameters even up to 150 millimeters or so [39].
To handle mortar and concrete theoretically, we average out the cement paste micrometer length scale to avoid having to deal with microstructure on too wide a range of length scales at the same time. The transport properties of the aggregate are measurable and usually constant in time, while the transport properties of the cement paste depend on the original water:cement ratio, kind and quantity of admixtures, hydration time, and to some degree on the initial particle size distribution. However, it will be shown below that the interfacial transition zone between the cement paste and aggregates may play a critical role in determining the bulk transport properties [40]. Therefore, when averaging over the cement paste microstructure, the micrometer scale interfacial transition zone, which is determined from cement paste microstructure, must not be omitted. This makes the concrete random composite problem difficult, since micrometer and millimeter length scales have to be simultaneously considered. Also, mortar and concrete have aggregate volume fractions of up to 70% or so, which implies that the spacing between particle surfaces is on the order of 100 micrometers, as seen by SEM in cross-section [41]. Therefore the micrometer cement paste scale can also clearly come in here, especially, as we will see below, as the interfacial zone is itself on the order of 20-50 micrometers. There has also been some work using x-ray microtomography to directly image the aggregate-matrix structure in mortars, and in particular the spacing between aggregate surfaces, in three dimensions [42].