122 CEMENT AND CONCRETE 



crete worker is to assume that when mixing one cubic foot of 

 cement, two cubic feet of sand, and four cubic feet of gravel, 

 he will secure seven cubic feet of concrete. This is an entirely 

 erroneous idea, as the sand would simply fill the voids in the 

 coarse material, and the cement would fill the voids in the 

 sand and coat the particles of sand and gravel or stone. Since 

 the amount of cement and sand used is more than enough to 

 fill the voids in the gravel, the resulting concrete will be 

 slightly more than four cubic feet, about 4.25 under average 

 conditions. The same error is often made when unscreened, 

 bank-run materials are used. In attempting to secure the 

 equivalent of a 1 : 2 : 4 mixture, the contractor will use one 

 part of cement to six parts of bank-run material, when, in 

 reality, he should use only about 4-1/4 cubic feet of bank-run 

 material to one cubic foot of cement to get the equivalent of a 

 1:2:4 mixture. This is assuming that the bank-run mate- 

 rial is of the correct proportion of one part of fine aggregate to 

 two parts of coarse aggregate, which should be accurately de- 

 termined by testing. The only safe method of using bank- 

 run materials is to screen them before using. Then when the 

 materials are used, the proportions can be definitely secured. 



116. Determining Quantities for a Job. In deter- 

 mining the quantities of material for a job, one must remem- 

 ber that the volume of concrete is only a little greater than 

 the volume of coarse aggregate ; in fact, this is often taken as a 

 basis for estimate of materials needed. For example, sup- 

 pose it is. required to make 54 cubic feet of concrete of a 1 : 2 : 

 4 mixture. It is assumed that 54 cubic feet of coarse aggregate, 

 27 cubic feet of sand, and 13-1/2 cubic feet, or 13-1/2 sacks 

 of cement are required. Another rule which may be used 



