Concrete Silos 67 



concrete mixer on the job, while the farmer himself 

 would frequently be satisfied to mix the concrete by 

 hand. On a structure such as a silo, which is built with 

 a comparatively thin wall, and which at the same time 

 has to withstand considerable strain, it is of the utmost 

 importance that the concrete should be well mixed, so 

 that it may attain its greatest strength. 



For the same reason, too, considerable care should 

 be given to the materials, making sure that they are 

 clean and properly graded to make a concrete of the 

 greatest density. Sand and gravel for silos are as 

 often as possible secured from deposits in the locality, 

 rather than being bought from commercial plants, and 

 hence need most careful inspection. Gravel taken 

 from a creek is often coated with clay loam, which 

 prevents the cement from making a good union, and 

 very often it contains particles that are too large or of 

 a crumbling character. Such gravel should be run 

 over a screen and w r ashed before using. Soft granite, 

 shale, slate rock or dusty cinders are not desirable. 

 The material should not easily crush and disintegrate 

 and should be suitable to give a good, strong union 

 with the cement. Above all it should be repeated that 

 it should be absolutely clean. 



In some localities there are natural deposits of 

 gravel containing varying proportions of sand. If 

 clean and not too coarse, such gravel is well suited for 

 silo building; but in using this material it is never safe 

 to assume that the proportion of sand to gravel is cor- 

 rect until a quantity has been run over a ^-inch screen 

 and the exact proportions determined. Usually such 

 gravel contains too much sand. 



For the foundation the stone may be as large as 

 will pass through a 2%-inch ring, while for the main 



