CHAPTER XIII 

 PROPORTIONS AND MIXTURES; HANDLING OF CONCRETE 



112. Proportions. The theory of proper proportions is 

 to use just enough sand to fill the air spaces or voids in the 

 coarse aggregate, and enough cement to fill the air spaces in 

 the sand, and also to coat each particle and thus serve as a 

 binder. The small contractor in actual practice rarely at- 

 tempts to carry this out; in fact, he seldom accurately meas- 

 ures the materials that go into the job. He uses a little ce- 

 ment, some sand and gravel, and, under average conditions, 

 may get fair results. It is no wonder, however, that we find 

 sidewalks going to pieces, foundations of buildings cracking 

 and disintegrating when the work is done in such a haphazard 

 fashion. 



To make a concrete that is strong as well as economical, it 

 is essential that the materials be well graded from the larger 

 to the smaller-sized particles so that the voids around the 

 particles are reduced to a minimum. The absolute elimina- 

 tion of voids is an ideal condition which we should strive to 

 obtain. However, the densest concrete is not always the 

 strongest. In some cases, a rather porous mixture with a 

 small amount of fine aggregate is stronger than another piece 

 of concrete with a great deal of fine aggregate and a small 

 amount of coarse material, although the latter mixture would 

 be the denser of the two. 



113. Requirements of Good Concrete. The proper 

 proportions to use, under practical conditions, will depend on 



120 



