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The strength and stability of aggregate such as gravel or crushed rock do not depend upon the crushing strength of the constituent particles but upon the shape and surface texture of the individual particles. The relative importance of these two factors was investigated experimentally in triaxial compression and in a new angle of repose apparatus. The shape and surface texture of selected aggregate were varied by abrasion and by immersion in acid. The size of particle was held constant at approximately ⅛ in.
A plot of strength against roughness indicated that strength increases with roughness to an optimum point beyond which an increase in roughness is accompanied by a decrease in strength. Specimens were tested at the maximum densities attained under standardized conditions of vibratory compaction. It is inferred that, other factors being held constant, roughness in excess of the critical value impedes the development of optimum structural arrangement of the constituent particles.
Test data for all materials tested, including pumice, crushed bricks, basalt, and river gravel, fell on the one curve, and all exhibited the same strength for the same degree of roughness, regardless of mineral composition. It was conclusively demonstrated that so-called “tough” and “hard” materials possess little if any strength advantage over relatively soft and friable materials, unless they differ in roughness.
Morris, Henry Clifton
Research Assistant, University of Washington, Seattle, Wash.