PROPORTIONS AND MIXTURES 121 



the use to which the concrete is to be put. The three proper- 

 ties which are most often required are: (1) Strength, as in 

 bridges, buildings, etc. ; (2) resistance to wear, as in concrete 

 sidewalks and roads; (3) water-tightness, as in water tanks, 

 silos, etc. The practical mixtures that are ordinarily used 

 for different kinds of concrete work are as follows : 



114. Standard Mixtures. Rich mixture of 1 part ce- 

 ment, 1-1/2 parts sand, and 3 parts broken stone, or gravel, 

 commonly called a 1 : 1-1/2 : 3 mixture, is used for columns 

 of reinforced concrete buildings, for thin water-tight walls 

 where very dense, strong concrete is required, and under all 

 similar conditions. 



A good, standard mixture of 1 part cement, 2 parts sand, 

 and 4 parts broken stone, commonly called 1:2:4 mixture, 

 is used for reinforced concrete work of all kinds, for water 

 tanks, thin walls, etc. 



Medium mixture of 1 part cement, 2-1/2 pares sand, and 5 

 parts broken stone, commonly called 1 : 2-1/2 : 5 mixture, is 

 used for all plain concrete, that is, concrete without rein- 

 forcing for foundations, walls, floors, etc. When the walls 

 are to be water-tight, a 1 : 2 : 4 mixture should be used in- 

 stead. 



Lean mixture of 1 part cement, 3 parts ot sand, and 6 parts 

 broken stone, commonly called 1:3:6 mixture, is used for 

 very heavy mass concrete where the loads are wholly com- 

 pressive. Still leaner mixtures are sometimes used for very 

 heavy foundations and abutments, but are not recommended 

 for general use. 



115. Common Errors in Proportioning Concrete. A 

 rather common error that is made by the inexperienced con- 



