456 rhoades. CONCRETE AGGREGATE [Ch. 24 



Coral and algal reef materials, such as occur profusely in the South 

 Pacific, have had extensive local use in the production of concrete, but 

 the special considerations and techniques involved in this class of con- 

 crete construction are beyond the scope of this discussion (Rasmusson, 

 1946). 



Residual Deposits 



Residual materials remaining in an area as the end products of 

 weathering and erosion are rarely usable as concrete aggregate. Al- 

 though they owe their existence to their durability and therefore are 

 frequently hard and firm, they are normally ungraded in size and usu- 

 ally limited in quantity. Characteristically, they are admixed with 

 substances unsuitable for the production of concrete aggregate. 



Boulders of chalcedonic siltstone occurring over a considerable area 

 in southwestern North Dakota and northwestern South Dakota furnish 

 an example of the occasional usefulness of residual material. It was 

 proposed that concrete aggregate for construction in that area be pro- 

 duced through the collection, crushing, and screening of these materials. 

 Although laboratory tests indicated that satisfactory and durable con- 

 crete could be made with aggregates thus produced, excessive costs 

 would be involved. The use of such material would be considered only 

 in areas that lack material more suitable or more easily processed. 



QUARRIED SEDIMENTARY ROCK 



The suitability of a rock ledge for the production of concrete ag- 

 gregate through quarrying and crushing depends on three general 

 factors: (a) the intrinsic quality, physical and chemical, of the rock; 

 (b) the uniformity of the available working face; and (c) textural and 

 structural characteristics that influence the "crushing characteristics" 

 of the rock (ideally, a rock should crush to firm particles, roughly 

 equidimensional in shape, with a minimum of "powdering" or frag- 

 mentation into extremely fine sizes) (ASTM, 1948a) . 



The physical and chemical characteristics of quarried aggregate are 

 those of the "source material" before the modifications — beneficial or 

 harmful — which the processes of transportation and deposition would 

 impose if the same material were to be transformed naturally into 

 sand and gravel. Any beneficiation which a natural rock exposure may 

 require to render it suitable as concrete aggregate must be applied 

 artificially through crushing, screening, and washing. Different kinds 

 of rocks will respond differently to such processing. 



Ledge rocks are commonly variable in hardness because of localized 



