PROPORTIONS AND MIXTURES 123 



for all standard proportions is to take the sum of the propor- 

 tions and divide into the number 11; the quotient will be the 

 number of barrels of cement required to make one cubic yard 

 of concrete of the particular proportion. For example, ** 



= 1-4/7 barrels of cement, or 6-2/7 sacks (4 sacks to a barrel; 

 for one cubic yard of concrete. Since 54 cubic feet, or 2 cubic 

 yards, of concrete is required in the above job, it will take 

 2 x 6-2/7, or 12-4/7 sacks of cement, 25-1/7 cubic feet of sand, 

 and 50-2/7 cubic feet of coarse aggregate. For a small job, 

 the first method may be used, but with the larger job, the 

 latter method, which is more accurate, should be adopted. 



117. Requirements of Good Mixing. The require- 

 ments of good mixing are: (1) That every particle of sand and 

 stone is coated with cement paste, (2) that the sand and stone 

 are evenly distributed through the mass, and (3) that the 

 whole mixture is of a uniform consistency. A poorly-mixed 

 concrete may be known by its lack of uniformity in color and 

 the separation of fine and coarse material. It is just as im- 

 portant to have materials thoroly and carefully mixed as to 

 have them properly proportioned. It is considered so impor- 

 tant by well-informed concrete contractors, that they require 

 the materials to be mixed for a definite period of time, if 

 mixed by machine method, or turned a definite number of 

 times if mixed by hand. Up to a certain limit, it has been 

 found that the strength of the concrete is directly propor- 

 tional to the length of time it has been kept in the mixer. 

 (In the specifications for the construction of some concrete 

 work, the time of mixing is definitely stated.) 



118. Hand-Mixing. A water-tight platform is the first 

 requirement for successful hand-mixing. In mixing by hand, 



