440 rhoades. CONCRETE AGGREGATE [Ch. 24 



NATURAL SAND AND GRAVEL AGGREGATES 



The properties of individual particles of sand and gravel (for ex- 

 ample, surface texture) or of assemblages of particles (for example, 

 grading), which determine suitability as aggregate, as well as the 

 characteristics of deposits as a whole which control feasibility and 

 economy of production, are all influenced by sedimentation and related 

 processes; indeed, the character of a natural sand and gravel usually 

 results almost wholly from (a) the kind of original or source material 

 from which it was derived and (6) the modifying influence of its sub- 

 sequent experiences as it was transported and deposited (Twenhofel, 

 1932). 



Processes of Sedimentation Influencing Sand and Gravel Aggre- 

 gates 



Sand and gravel, potentially useful as concrete aggregate, may be 

 transported and deposited by water, wind, ice, or gravity, or by any 

 combination of these agencies. Inasmuch as different mechanisms are 

 involved in transportation and deposition by these various agencies, 

 each will impart to the material involved certain characteristic and 

 recognizable properties. However, although the transportation and 

 deposition may differ both in mechanism and rigor, all these agencies 

 influence the material transported and deposited through some com- 

 bination of the following processes: (1) impact, abrasion, or crushing; 

 (2) sorting; (3) weathering, leaching, and chemical reaction. 



Weathering, leaching, and chemical reaction are not necessary ac- 

 companiments to sedimentation, but they frequently are inextricably 

 associated or occur concurrently with it, and in such cases their con- 

 sideration is essential to any complete understanding of the rock prop- 

 erties resulting as end products of the sedimentation process. 



These actions occur conjointly, and the final properties imparted to 

 a sand and gravel will be the summation of their combined effect ; but 

 certain of the resulting properties are most influenced by one or another 

 of these actions, as indicated in the following examples. 



Impact, abrasion, or crushing. This action, for example, is primarily 

 responsible for modifying the initial shape of the particles in the direc- 

 tion of increased roundness; for reduction in the size of all particles, 

 and sometimes in the elimination of all particles above a certain size; 

 for the elimination of soft constituents that may have been present in 

 the initial material ; or for the roughening of surface texture. 



