CONCRETE 



135 



a lower percentage of voids than an aggregate which con- 



sists of grains of uniform size, 

 preferable to one of fine grains 

 because, as a rule, the coarse 

 sand contains some fine grains 

 which mix in between the 

 coarser ones. This makes a 

 more compact mass, and it 

 takes less cement to make a 

 mortar of the same strength. 

 With the same percentage of 

 voids a coarser sand requires 

 less cement for a given strength 

 for the reason that there is less 

 total surface of grains to be 

 covered with cement. 



Standard sand. A clean, 

 white, spherical-grained sand 

 found in the vicinity of Ottawa, 



A sand of coarse grains is 



Fig. 133. Concretion that is frequently 



found in gravel too weak for 



use in concrete 



Fig. 132. Standard Ottawa sand 



Illinois, is used through- 

 out the United States as 

 the standard sand for 

 making comparative tests 

 in concrete work. (Fig. 



132.) 



Bank-run gravel. Grav- 

 el, as ordinarily found in 

 the bank, does not con- 

 tain the proper propor- 

 tions for making the best 

 concrete. However, in 

 practice the gravel is gen- 

 erally used as found, 

 though much more satis- 

 factory results can be 

 secured if the materials 



