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Aggregates in stockpiles are collections of various sizes of irregularly shaped particles often with rough or convoluted surfaces, sometimes with pores and wrinkles or crevices that hold water. Because the particles do not fit together nicely, spaces called voids are left between them and the whole pile contains water in the pores and on the surface of each particle, even when located on the edge of the African desert. In order to use these aggregates as the largest part of a concrete mixture, five factors must be determined: (1) the volume of voids in order to fill them with paste and to float the particles apart with an excess of paste, (2) the volume that solids will contribute to the concrete mix, (3) the specific gravity in order to convert volume to weight, since most concrete is batched by weight, (4) the absorption because it affects the specific gravity that is used, and (5) the surface moisture because mixing water must be adjusted to compensate for it.
Professor, North Carolina State University, Raleigh, N.C.