Aggregates, Concrete

Aggregates, Concrete 

natural or artificial bulk stone materials—the main component of concrete. The proper selection of aggregates, which account for up to 85 percent of the mass of the concrete, makes possible the regulation of the properties of concrete and a decrease in its cost. The strength and granulometric structure of aggregates, as well as the quantity of various impurities in the aggregates, have a significant influence on the quality of the concrete. A distinction is made between aggregates for the manufacture of ordinary (heavyweight) concrete (heavyweight aggregates) and aggregates for the manufacture of lightweight (porous) concrete. Depending on the dimensions of the granules, aggregates are classified as fine (sand) and coarse (gravel or crushed stone).

Aggregates for ordinary (heavyweight) concrete. Natural (mainly quartz) and granulated sand with granule size of 0.14–5.0 mm are used as a fine aggregate in heavyweight concrete. The content of harmful impurities in the sand is strictly limited, particularly argillaceous admixtures, which impede the adhesion of sand granules to the cement stone, thereby reducing the durability of the concrete. Gravel or crushed stone from rocks (less frequently crushed cinder and brick stone) with a granule size of 5–70 mm is used as a coarse aggregate. Gravel is usually rounded and has a smooth granule surface. The quantity of harmful impurities contained in gravel (dust, silt, clay, and organic substances) should not exceed 1 percent by weight. Crushed stone is produced by crushing rock or coarse gravel. The granule surface of crushed stone is coarser than that of gravel, which promotes better adhesion of the granules to cement stone. Aggregates for heat-resistant concrete are produced by breaking up ceramic brick or firebrick, blast-furnace slag, and fireclay. Heavy metal materials—magnetite, limonite, barite, and scrap cast iron—are used as aggregates in concrete designed to protect against radioactive effects (particularly heavy-weight concrete).

Aggregates for lightweight (porous) concrete. Aggregates for lightweight (porous) concrete consist of stone materials with granules from 5 mm (fine aggregates) to 40 mm (coarse aggregates). Both natural and artificial aggregates are used. Natural aggregates are produced by crushing porous volcanic or sedimentary rock (pumice, tuff, and porous limestone). Artificial aggregates (porous clay filler, expanded perlite and vermiculite, and agloporit) are produced by calcination of swelling rock or from industrial wastes (scoriaceous pumice, cinder gravel, fuel scoria, and ashes). The presence of chemically active harmful impurities, which cause reduced durability of concrete under service conditions, is inadmissible in porous aggregates. Porous aggregates have considerable potential for development, since designs based on them are contributing to the increased effectiveness of construction (improvement of thermotechnical and acoustical indicators of safety designs and considerable reduction of weight of buildings and structures).

REFERENCE

Stroitel’nye materialy. Edited by M. I. Khigerovich. Moscow, 1970.

G. I. GORCHAKOV