198 SAND AND CEMENT 



properties of sand. The greater these voids, the more 

 cement paste will be required to fill them in order to give a 

 dense mortar. The percentage of voids may be determined 

 by observing the quantity of water that can be introduced 

 into a vessel filled with sand, but it is best computed as 

 follows: 



100 X weight per cubic foot 



percentage of voids =100 



62.5 X specific gravity 



EXAMPLE. What is the percentage of voids in a sand 

 having a specific gravity of 2.65 and weighing 105 Ib. per 

 cu. ft.? 



SOLUTION. Substituting in the formula, the percentage 

 of voids is 



100 = 100-63.4 = 36.6 



62.5X2.65 



The percentage of voids depends principally on the size 

 and shape of the sand grains and the gradation of its fine- 

 ness, and hence will vary from 25 to 50%. Sand contain- 

 ing over 45% of voids should not be used to make mortars. 



The shape of the grains of sand is of chief importance in 

 the influence that the sand exerts on the percentage of voids. 

 Obviously, a sand with rounded grains will compact into a 

 more dense mass than one whose grains are angular or flat 

 like particles of mica. Therefore the more nearly the grains 

 approach the spherical in shape, the more dense and strong 

 will be the mortar. This fact is contrary to the common 

 opinion on the subject. 



The fineness of sand is determined by passing a dried 

 sample through a series of sieves having 10, 20, 30, 40, 50, 

 74, 100, and 200 meshes, respectively, to the linear inch; 

 The result of this test, expressed in the amount of sand 

 passing each sieve, is known as the granulometric composition 

 of the sand. Material that does not pass a i-in. screen is 

 not considered to be sand, and should be separated by 

 screening. Sand that is practically all retained on a No-. 30 

 sieve is called coarse, while 80 or 90% of sand known as 

 fine will pass through this sieve. Fine sand produces a 

 weaker mortar than coarse sand, but a mixture of fine and 

 coarse sand will surpass either one. 



