CONCRETE 241 



cement be as good as any, hence care should be taken to 

 determine the cause of the lumping before discarding the 

 cement. 



190. Sand. Sand constitutes a large part of concrete. 

 It is extremely important to secure the proper kind of sand, 

 which should be coarse, clean, hard, and free from other 

 materials. The screening of the sand should be done at 

 the source of supply. This is accomplished by screening 

 what passes a j-inch sieve against a sieve containing forty 

 meshes to the linear inch and set at an angle of 45, using 

 the portion retained by the sieve. 



The following test will show the proportions of sand, clay 

 and loam in the source of supply of sand: Fill a pint pre- 

 serving jar to the height of 4 inches with the sand and 

 add water to within 1 inch of the top. The lid is then 

 fastened and the jar is shaken for ten minutes, after which 

 the contents of the jar are allowed to settle. The sand settles 

 to the bottom and the clay and other material gather at 

 the top. If more than one-half of clay and loam is present, 

 the sand should be rejected. If other sand is not con- 

 venient, the sand in question may be washed. When 

 washing is required, a simple way is to build a board plat- 

 form 10 to 15 feet long with a 12-inch fall. On the sides 

 and lower end, 2 by 8-inch pieces should be nailed to hold 

 the sand. The sand is spread on this platform to a depth 

 of 3 or 4 inches and is washed by means of a hose, the water 

 being applied at the elevated end of the platform and run 

 through the sand and over the lower end. The impuri- 

 ties in the sand should not amount to more than 10 per 

 cent of the whole. 



191. Gravel. The gravel or crushed stone which con- 

 stitutes a large part of the concrete should vary from that 

 retained on a j-inch screen to those that pass a 1^-inch 

 ring. This material should be as free as possible from 

 dirt and, if necessary, may be washed in the way suggested 

 for sand. 



