448 



rhoades. CONCRETE AGGREGATE 



[Ch. 24 



surface and are most prevalent in the upper few feet. Concentrated 

 deposition in the upper layers of a deposit may cause the formation of 

 cemented layers that hinder excavation. 



The most common coatings are calcium carbonate and, inasmuch as 

 this material is chemically innocuous, they may in no wise impair 

 the quality of aggregate if they are firm and tightly adherent to the 



Fig. 2. Deposits of natural sand and gravel on the Missouri River near Canyon 

 Ferry Dam site, Montana. 



particles. Siliceous coatings — opaline or chalcedonic — are chemically 

 reactive with alkalies in cement, and, no matter how firm and adherent, 

 they will be detrimental to concrete unless used with cements low in 

 alkalies (ASTM, 1948a; McConnell et al., 1948). Coatings composed 

 of iron oxide, manganese oxide, or gypsum are not uncommon. Gyp- 

 sum coatings, being soluble and pulverulent, are detrimental, but they 

 are often removed by normal washing and screening. In many places, 

 as at Davis Dam (Colorado River, Arizona-Nevada) the upper several 

 feet of a terrace deposit must be removed and discarded because of del- 

 eterious coatings or cemented layers. 



Terraces frequently exhibit postdepositional weathering. Extreme 

 weathering may render a terrace deposit entirely unsuitable for use 



