234 PLAIN CONCRETE 



on account of its lack of hardness and its liability to crumble, 

 and from the fact that its surface is likely to be unstable. 

 Slate and shale are hard, but they are of a laminated struc- 

 ture and break in such flaky shapes that they will not pack 

 closely. Cinders are frequently used as the aggregate for 

 concrete. Cinder concrete, however, does not possess suf- 

 ficient strength for structural purposes, and is generally used 

 for filling or for fireproofing. 



Based on percentages of efficiency, with trap rock taken 

 at 100, the following table gives a fair representation of the 

 comparative values of the different aggregates: 



COMPARATIVE VALUE OF DIFFERENT AGGREGATES 

 USED IN CONCRETE 



PROPORTIONING OF INGREDIENTS 



Effect on Strength and Imperviousness. The strength of 

 concrete depends on the strength of the cement and the 

 thoroughness with which the cement binds togethe. the 

 various pieces of aggregate. The more completely the voids 

 are filled, the more completely will the aggregate be held 

 together. Therefore, the more solid and condensed the con- 

 crete is, the less voids it will have, and the stronger it will 

 be. The same is true with regard to making concrete water- 

 proof: the more dense the concrete is, the more nearly water- 

 proof it is. 



A mixture of 1 part of cement, 1J parts of sand, and 3 parts 

 of stone, which would be considered extravagantly rich for 



