CONCRETE CONSTRUCTION 245 



Proportions of Materials. Theoretically, 

 the ideal concrete is a mixture in which all 

 the spaces between the stones or gravel are 



Fig. 127. Proportions of cement, sand and stone re- 

 quired to form concrete 



filled with sand, and all the spaces between 

 the grains of sand are filled with cement. 

 From this it will be seen that the total bulk 

 of concrete would not be greatly in excess 

 of the bulk of stone or gravel, as the sand and 

 cement would go to fill the vacant spaces 

 (voids). This is really true except that, as 

 usually proportioned, a slight excess of ce- 

 ment is allowed. This is wise in order to 

 insure that there shall be a film of cement 

 about each stone and grain of sand, so they 

 may be all bound together in a solid mass. 

 The common formula for most concrete 

 work is known as the i :2 :4 mixture. In this 

 there are: i part by measure of cement, 2 

 parts of sand, and 4 parts of stone or gravel. 



