458 ehoades. CONCRETE AGGREGATE [Ch. 24 



measure, for instance, their specific gravity, absorption, abrasive re- 

 sistance, soundness under freezing and thawing (or under the more 

 rigorous and accelerated test with sodium or magnesium sulphate). 

 Such tests have been correlated with field service over many years, and 

 on the basis of this long experience they serve as guide lines for estab- 

 lishing specification limits and for the selection and rejection of pro- 

 posed aggregate materials. 



Today there is an increasing recognition by concrete technologists 

 that these empirical tests, however useful they may have been for the 

 practical purposes of the past, do not measure specifically the funda- 

 mental properties that define the "concrete-making" potentialities of 

 rock materials (Blanks, 1949) . 



Surface texture, for example, is generally admitted to be highly sig- 

 nificant in the fabrication of concrete because of its profound effect on 

 the bond that will develop between an aggregate and the enclosing 

 cement. But that property is never measured except in research in- 

 vestigations; there exists no test amenable to routine application by 

 which roughness or smoothness of aggregate particles may be defined ; 

 there is no specification in which surface texture is used as a criterion 

 for acceptance or rejection; and, indeed, there exists only a general 

 and qualitative understanding of the effects of different kinds and 

 degrees of surface texture. Many other properties of aggregates are 

 understood in the same imperfect way: they are recognized as im- 

 portant; their effect on concrete can be gaged qualitatively; but their 

 significance lacks, and badly needs, quantitative definition. Concrete 

 technology requires that research of the future be directed toward the 

 determination and elucidation of the properties of rocks and minerals, 

 in terms of their quantitative significance to concrete. 



An intimate knowledge of the inner character of rocks and minerals 

 will be a first requisite to this research. This knowledge will be con- 

 tributed chiefly by geologists and petrographers, with their special 

 knowledge of the origins, histories, compositions, and textures on which 

 the properties of rocks and minerals depend. But, if these contribu- 

 tions are to be directly useful, geologists and petrographers must also 

 have insight into the concepts and technology of concrete. Research 

 will progress only haltingly in this direction until the resources of 

 the geological sciences are better mobilized for its prosecution — until 

 more geologists acquire some competence in concrete technology, 

 through academic training or practical experience, and become preoc- 

 cupied with its problems. 



