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Primary considerations in the selection of materials for coarse aggregate are those pertaining to freeze-thaw durability and the development of D-cracking in highway and airfield pavements. Two aspects of the problem are of particular importance: moisture movements and critical saturation of the aggregate, and the response of the aggregate to cyclic freezing and thawing in concrete.
Laboratory studies of coarse aggregates have indicated that nondurable materials are generally of sedimentary origin and may reach critical saturation when the concrete is in direct contact with either free water or capillary-held water. Absorption-adsorption and mercury intrusion studies have revealed differences in the pore structure of durable and nondurable materials, while laboratory tests of aggregates in concrete differentiate performance during freezing and thawing in line with field service records.
The most feasible method of utilizing potentially nondurable coarse aggregates is to reduce maximum particle sizes. The needed reduction can be determined from laboratory freeze-thaw tests and is found to vary with source. Gravel sources have the option of crushing oversize material for using naturally finer material to improve durability, while crushed stone sources may use alternatively selective quarrying to produce higher quality aggregates. Proper evaluation of aggregate materials is contingent on establishing laboratory procedures directed expressly to the problem of D-cracking.
aggregates, critical saturation, freeze-thaw, particle size, pavements, pore structure, sorption
Senior research petrographer, Portland Cement Association, Research and Development, Construction Technology Laboratories, Skokie, Ill.