92 SILOS: CONSTRUCTION AND SERVICE 



tative sample of the moist aggregate should be measured and 

 the quantity of water determined which is required to fill to 

 the surface of the aggregate. The proportion that this 

 quantity of water, plus 10 per cent, is to the quantity of 

 aggregate, is the proportion of cement to the aggregate which 

 is required to make a dense concrete. 



Tamping. From this discussion it is easy to see why 

 concrete should be tamped, or, more properly speaking, 

 vibrated, as any motion of the mass will cause the finer parti- 

 cles to settle more closely into the spaces between the larger 

 particles. In all silo work a very dense concrete should be 

 used. In order to remove forms promptly it is also very 

 desirable to have a mixture rich enough to set quickly. This 

 is especially true of the concrete used in the roof, 



The presence of clay or dirt prevents the union of the 

 cement and the particles of sand and gravel. Small quanti- 

 ties of dirt are usually present in bank gravel, but should not 

 be more than 10 per cent. It is a simple matter to determine 

 the quantity of dirt in gravel by simply washing a measured 

 portion of the material. The wash water should be set aside 

 to settle, and the quantity of dirt settling out should be meas- 

 ured. The proportion of this to the quantity of gravel is 

 determined. 



Storing Cement. It seems almost needless to say that 

 cement should be very carefully stored in a dry place. Any 

 cement having hard lumps distributed through it should not 

 be used. Aside from what has been given, little or no testing 

 seems advisable on the average job. 



If there is any question as to the quality of the cement, a 

 trial sample may be mixed in the proper proportions with the 

 gravel and allowed to stand a few days before the main 



