438 rhoades. CONCRETE AGGREGATE [Ch. 24 



The different constituents of concrete are chemically and physically 

 heterogeneous. Cements differ widely in composition and in physical 

 character and behavior, and the rock and sand comprising the ag- 

 gregate — excluding certain monomineralic types, such as limestone or 

 quartz — will generally contain a variety of minerals, each with its own 

 physical and chemical characteristics and with its own unique re- 

 sponse to environmental conditions. 



These separate and diverse constituents of concrete react individu- 

 ally to the internal and external influences to which the concrete is 

 subjected, and they interact with each other and thus determine its 

 durability and serviceability and its appropriateness and effectiveness 

 for its intended use. Inasmuch as aggregates normally comprise about 

 75 percent of concrete, their physical and chemical properties pro- 

 foundly influence the overall character and behavior of any concrete 

 mass. 



Ideally, the selection of concrete aggregates should be based on a 

 detailed appraisal of their chemical and physical properties, so that 

 materials may be chosen that will react and interact harmoniously and 

 compatibly and impart to the concrete properties consistent with its 

 purpose. Deleterious reactions or incompatible interactions result in- 

 evitably in premature deterioration or ineffectual service. 



Practically, it is not now possible in the manufacture of concrete to 

 pay full deference to these considerations. In the first place, the 

 variables involved are numerous and intricately interrelated, and the 

 isolation of any separate variable for individual study, although prac- 

 tical on a research basis, is impractical as a routine, and suitable 

 laboratory tests have not been standardized for general application. 

 In the second place, in the present state of concrete technology there 

 are many factors of uncertain and obscure significance, and various 

 current hypotheses lack certain confirmation. Furthermore, the use- 

 fulness of concrete as a construction material depends in part on its 

 relatively low cost. Economic considerations, therefore, prohibit costly 

 beneficiation or the arbitrary rejection of a cheap local material and 

 the costly importation of aggregate from a more distant source unless 

 the advantages to be gained are assured and commensurate with the 

 added expense. 



Current research is rapidly resolving the remaining technical un- 

 certainties in aggregate selection; and the most enlightened current 

 practice involves the formulation and progressive revision of specifica- 

 tions and the development of acceptance tests that will exclude in- 

 appropriate materials unequivocally, no matter how cheaply they may 



