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Aggregates for use in concrete, and to a large extent the individual constituent particles, must possess a reasonably high degree of inherent strength, tenacity, and stability to resist, without detrimental degradation, the static and dynamic stresses, impacts, and wearing actions to which it may be exposed both in concrete production operations and, ultimately, in concrete in service. In many uses of concrete the roughest treatment to which an aggregate may be subjected in terms of mechanical forces and attrition is in the concrete production process. Actions involved in production are fairly predictable; those in service may be less predictable. In end uses such as beams, columns, covered slabs, walls, footings, and other mostly structural or architectural elements the only real strength property needed of the aggregate after the concrete is in place is that necessary to give the concrete enough strength to resist the distributed service loads. Other uses may expose aggregate near or at the surface to a variety of localized impact and abrasive stresses which will be of overriding importance in aggregate evaluation and selection. Examples include pavements and slabs exposed to heavy traffic and hydraulic structures subject to eroding forces of moving water and sediment material.
Director of engineering research, National Sand and Gravel Association, National Ready Mixed Concrete Association, Silver Spring, Md.