1012 N'ew York State Dairymen's Association 



costly mistakes by not informing themselves properly concerning 

 the principles and correct methods of making good concrete. 



For materials, Portland cement is the only kind we need to con- 

 sider as practical. Portland cement is an artificial mixtnre of 

 lime carbonate (-5 parts), silica, alnmina and iron oxide, or, in 

 general terms, lime and clay. The brand selected is immaterial 

 so long as it conforms to the specifications of the American 

 Society for Testing ^iaterials. The main differences are in tlie 

 color and the rate of setting up. 



The greatest care should be used in selecting the sand and gravel 

 which constitutes the bulk of raw material. Sand is that portion 

 of grains and pebbles which will pass through a oue-fourth inch 

 ring or four-mesh sieve, while gravel is that portion which is 

 retained on the quarter and passes through a one and one-half inch 

 ring. Both should be free from loam or vegetable matter and for 

 best and most economical purposes, should l)e well graded from 

 fine 10 coarse, always having the larger proportion coarse. It is 

 often economical to screen fine sand over a fortv-mesh sieve and 

 discard the screenings. Sharp sand is preferable to smooth ; tht- 

 rough surface helps the adherence of the cement particles and is 

 heavier, therefore, with less voids and requires less cement. 



In finished concrete we want every surface covered with a 

 film of cement, so the coarser the material the less surface ex- 

 posed per given volume, and the greater the variation in the size 

 of the sand grains the less is required to fill the voids. 



The chief use of aggregate is to increase the volume of con- 

 crete without decreasing its strength. By knowing exactly the 

 proportions of sand and gravel, the required amount of cement 

 can be determined. As a rule the volume of .correctly propor- 

 tioned concrete is about one-eighth greater than the aggregate it 

 contains. 



The water used to crystalize the cement should be free from 

 alkalies or acids, and added until for general work the mixture 

 holds the water but spreads out on the mixing floor; or, if a so- 

 called slush, to a quaking consistency. 



Comparative volumes are used in proportioning materials. The 

 standard or first-class mixtnre contains 1 part cement, 2 parts 

 sand and 4 parts gravel or broken stone. However, we should not 



