CONCRETE 



*33 



mean grains that are angular with sharp corners, for many 

 good sands are composed of spherical grains. In the exami- 

 nation of a sand bank an idea of the quality of the sand to 

 be used as fine aggregate may be had by means of samples 

 of the sand rubbed between the palms. Good sand will 

 feel sharp, as described above, and will not leave a great 



quantity of dirt on the hands. 

 If a more complete test is desired, 

 it may be secured by means of 

 filling a straight-sided glass vessel 

 from one-third to one-half full of 

 sand; adding enough water to 

 cover the sand from 2 to 4 inches ; 

 shaking thoroughly, and then 

 letting the vessel stand until the 

 sand has settled. The clean sand 

 will be on the bottom and the dirt 

 on top. By measurement of the 

 depth of the sand and of the 

 worthless material, the percentage 

 of dirt may be obtained. Sand 

 containing 3 or 4 per cent of dirt 

 is very clean; 10 to 12 per cent 

 dirt may be used without affect- 

 ing the strength of the concrete 

 seriously. 



If screenings are used for fine 

 aggregate, they should be from good hard stone and clean. 

 If sand is used in cement mortar, it must be screened to be 

 separated from the coarser material, as mortar requires a 

 fairly fine aggregate. 



For obtaining the best results in concrete, the percentage 

 of voids should be as small as possible. This is secured by 

 use of an aggregate ranging from coarse to very fine grains, 

 so that the finer grains fill in between the larger ones, and 

 the still finer fill in between the medium grains. This gives 



/•'£> 



> (. * c ct ■> " O ... 



\-.q : :0,-of.o-o\°. 



T 



k 



Fig. 



127. Method of testing 

 dirty sand 



D 



X 100 =per cent of dirt 



D = thickness of dirt layer 

 T = total height of dirt and clean 

 aggregate 



