446 



rhoades. CONCRETE AGGREGATE 



[Ch. 24 



manufacture of concrete was obtained by simple screening to effect the 

 final grading shown. More elaborate processing may be required in 

 other places: sands and gravels in the Colorado River at the "Wah- 

 weap" deposit near Glen Canyon Dam site, for example, contain the 

 fine sizes in such excess that some 40 percent more sand than is re- 



Screen size 

 •*200 *100 *50 *30 *16 *8 *4 



W 3 A" l l 4" 3" 6" 12* 



Fig. 



'(-e— — Sand— — ■>+* — ~ Gravel ->l 



1. Comparison of gradings of natural and processed aggregates with ASTM 



specification limits. (Aggregate from North Platte River investigated for Kortes 



Dam, Wyoming.) 



quired must be processed and discarded in order to produce the requi- 

 site quantity of coarser material. 



Attrition has had the effect in the Missouri Basin states, Kansas, 

 Nebraska, and South Dakota, of eliminating entirely the coarse-gravel 

 fraction of the river gravels (Scholer and Gibson). These gravels, 

 derived initially from the Rocky Mountain area to the west, contain 

 few, if any, particles over 1% inches in size at any location east of the 

 Colorado state line, although to the west coarser materials become in- 

 creasingly abundant, and near the front of the mountains the gravels 

 are fully graded. 



River transportation normally causes a concentration of hard and 

 firm particles through the selective elimination of softer materials by 

 attrition. However, the strength and hardness of the particles are 

 related closely to the source material from which the sediment was 



