Ch. 24] PROPERTIES OF CONCRETE AGGREGATE 439 



be obtained. But additional research will be required to define entirely 

 the properties that are admissible and inadmissible, and to develop 

 simple tests which will discriminate conclusively between acceptable 

 and unacceptable aggregate materials. 



PROPERTIES OF CONCRETE AGGREGATE 



One important set of properties that influence the quality or suit- 

 ability of concrete aggregate includes: strength, hardness, compres- 

 sibility, durability, elasticity, particle shape, surface texture, specific 

 gravity, porosity, volume change with varying thermal or moisture 

 conditions, presence or absence of deleterious impurities or coatings, 

 mineralogic composition, and a variety of chemical properties related 

 to the stability of the aggregates in the aggressively alkaline environ- 

 ment of concrete or their resistance to the agencies of natural weather- 

 ing. These various properties apply to individual particles of aggre- 

 gate. They may not always be equally important, their importance 

 depending on the use, purpose, and environment for which the concrete 

 is designed, but each influences the serviceability, durability, or cost 

 of concrete for general or special purposes; and together they con- 

 tribute fundamentally to the behavior of a concrete mass under dif- 

 ferent service conditions. For the benefit of readers unfamiliar with 

 concrete technology these properties are summarized briefly in the ap- 

 pendix of this chapter from the standpoint of their significance to 

 concrete. More complete discussion and extensive bibliographies may 

 be found in American Society for Testing Materials (1948a, b, c), U. S. 

 Bureau of Reclamation (1949), Rhoades and Mielenz (1946), Blanks 

 (1949). 



Another set of properties, related not to individual particles but to 

 the overall assemblage of particles, includes minimum size, maximum 

 size, size gradation, and degree of mineralogic or lithologic diversity. 

 (See appendix of this chapter and ASTM, 1948b; U. S. Bureau of 

 Reclamation, 1949; Twenhofel, 1932.) 



A third set of properties, applying to whole deposits of aggregate, 

 that influence the quantity of material available, the optimum method 

 of excavation, the kind of beneficiation required, and like considera- 

 tions, defines the adequacy of any aggregate source and the cost of ag- 

 gregate production. This category of factors includes the area, depth, 

 variability or stratification, overburden, topography, and depth to 

 ground water (ASTM, 1948c; U. S. Bureau of Reclamation, 1949). 



