Ch. 24] CONCRETE AGGREGATE RESEARCH 457 



leaching or cementation. Dolomites frequently are fractured, lime- 

 stones frequently are leached and rendered porous along channelways 

 penetrated by ground water. Concretionary structures representing 

 local concentrations of mineral material deposited interstitially in 

 sandstones, siltstones, or shales, or by replacement in limestone, occur 

 sporadically in sedimentary formations. Limestones may contain in- 

 terstitial clay, which renders the material susceptible to volume 

 changes, or may contain chert lenses that are physically unsound or 

 chemically reactive with the alkalies in cement, as in many localities 

 in the southeastern United States. 



The rock at a given quarry site may be massive and uniform in com- 

 position, or it may be stratified and composed of more than one rock 

 type. Individual strata in a quarry may be hard or soft, porous or 

 dense, jointed or unfractured. Limestones may be interbedded with 

 shales, sandstones with siltstones, and quartzites with dolomites. Strat- 

 ification may be vague or pronounced, the layers thick or thin. The 

 thickness of layers may control the size and shape of the aggregate pro- 

 duced: rocks with thin stratification or lamellar structure will crush 

 into planar or elongated pieces. Limestone may possess good crushing 

 characteristics if dense and massive, but may produce particles of poor 

 shape or an excess of fines if it has a lamellar "grain" or is closely 

 fractured or highly clayey. Similarly sandstone may be hard or soft 

 and thus crush well or poorly depending on the kind and amount of 

 cement that binds the grains together. 



These factors of petrography, texture, and structure are inherent in 

 the sedimentary origins of rocks; the sedimentary processes by which 

 a rock has formed will determine its intrinsic physical or chemical 

 quality, the uniformity of the product which may be produced, and 

 the practicability and ease with which it may be crushed and other- 

 wise processed into aggregate suitable for concrete. 



CONCRETE AGGREGATE RESEARCH 



Past and current research on concrete aggregate has been a joint 

 venture by engineers, chemists, physicists, and, recently, geologists. 

 In the future, even more than in the past, research in concrete aggregate 

 will call for the joint preoccupation of scientists and engineers of 

 various specializations, with geologists playing an increasingly im- 

 portant role through analysis and interpretation of rock and mineral 

 properties. 



Concrete aggregates have been appraised and selected in the past 

 primarily through the application of a series of empirical tests which 



