Chapter 24 



INFLUENCE OF SEDIMENTATION ON CONCRETE 

 AGGREGATE 



Roger Rhoades 



Chief Geologist, U. S. Bureau of Reclamation 

 Denver, Colorado 



Concrete aggregate, that is, the rock and sand that are mixed with 

 cement to make concrete, may be natural sand and gravel, or quarried 

 rock crushed to sand and gravel sizes, or any combination of crushed 

 and natural material. Sand and gravel comprise slightly less than 

 two thirds of the concrete aggregate produced annually in the United 

 States. Quarried, sedimentary rock, predominantly limestone, will 

 constitute about one fifth of the total production. The processes of 

 sedimentation always influence and frequently control the develop- 

 ment of the various properties that combine to determine the use- 

 fulness of either sand and gravel or sedimentary ledge rock as aggregate 

 for concrete. 



The remaining annual production is represented by non-sedimentary 

 types, including crushed igneous and metamorphic rock. The latter 

 category of aggregate is in no way influenced by processes of sedi- 

 mentation and is therefore ignored in this presentation. 



Concrete in service is subjected to various external conditions which 

 may impose stress, strain, saturation, desiccation, chemical attack by 

 aggressive solutions, abrasion, impact, and temperature changes con- 

 ducive to expansion and contraction. Internal conditions inherent in 

 the chemistry of the cement itself subject the constituents of concrete 

 to additional rigors. In the first place, cement is an aggressively al- 

 kaline material capable of greater or lesser chemical interaction with 

 any aggregate material. In the second place, the hydration involved 

 in the setting of cement is accompanied by heat generation which, in 

 large masses of concrete from which heat is dissipated very slowly, 

 will cause significant expansion with attendant stresses and strains, 

 followed ultimately by a contraction, with its own complement of 

 stresses and strains, when cooling finally occurs. 



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