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AGGREGATE, a term used in building and construction to designate the material with which cement, lime, gypsum or other adhesive material, such as bitumen, is mixed to form a concrete or mortar (q.v.). The most common aggregates are sand, crushed or broken stone, gravel or pebbles, broken blast-furnace slag, boiler ashes or clinker, burned shale and burned clay. The pur pose of the aggregate is to provide volume, freedom from change in volume, and resistance to wear or erosion and other desired physical properties, in the finished product.

Aggregates are divided into two general classes commonly known as fine aggregate and coarse aggregate. The fine aggregate is generally sand, crushed stone or crushed slag screenings; the coarse aggregate is gravel (pebbles), fragments of broken stone, slag and the other substances above mentioned. The dividing line between fine and coarse aggregate is usually a fin. screen opening, sometimes a fin. opening. Aggregates are mixed with the cement or binding material in proportions varying from one part of cementing material to one part of fine aggregate for a "rich" mortar to one part of cement to nine or ten parts of mixed fine and coarse aggregates for a "lean" concrete. The theory upon which such mixtures are proportioned is that there shall be suf ficient cementing material to coat all of the particles in the fine ag gregate, and that there shall be sufficient mortar so produced to fill all the voids in the coarse aggregate. The ideal concrete would consist of closely packed fragments of crushed stone, slag or pebbles in which all the fragments are coated with mortar, and all the voids between the coarser fragments filled with mortar. Under actual conditions in construction work there is practically always an excess of mortar. Nearly all natural sands require wash ing or rinsing to remove loam, clay, organic material, etc. A grad ual variation in size from about 5o mesh (5o openings per linear inch) to fin. mesh is desirable for the strongest mortar or concrete. The preparation of commercial fine aggregate therefore often re quires artificial grading or mixing of ,the various sizes. This is usually accomplished by hydraulic separation of the undesirable fine sizes, which usually are in excess. For coarse aggregate any hard, tough, durable rock may be crushed, screened and graded. The common commercial sizes vary from fin. to 31in. in largest dimension. The most desirable rocks for general construction pur poses are basalt, commonly called trap, and fairly pure limestone. Such rocks are not only durable but fire-resisting. Specification re quirements include cleanliness (maximum limits of extraneous impurities), limiting percentages of various sizes, soundness (freezing and thawing tests, for example), sometimes mineralogi cal composition (designed to insure soundness).

In this age of concrete construction and road building the busi ness of supplying aggregates for construction work has grown to large proportions. In the U.S. alone production amounts to ap proximately 300,000,00o tons per annum, valued roughly at $300, 000,000, requiring an investment of about $5oo,000,000 in wash ing, screening and crushing equipment and accessory machinery for mining and conveying the raw and finished materials.

BIBLIOGRAPHY. Industrial Minerals and Rocks (1937). (N. C. R.)Bibliography. Industrial Minerals and Rocks (1937). (N. C. R.)

fine, mortar, material, coarse and aggregates