70 Concrete Silos 



wall of 6-inch thickness the size should not exceed l J /2 

 inches. A mixture of particles of various sizes from y% 

 up to 11/2 inches makes the strongest wall. 



The water used for mixing concrete should be clean 

 and free from alkalis and acids. It is especially desir- 

 able to caution builders of farm structures on this 

 point, because drainage water from the barnyard or 

 water from a muddy stream may sometimes find its 

 way into the supply. 



The usual proportions for a 6-inch concrete wall are 

 1 part of cement to 2 parts sand and 4 parts stone. 



In filling the forms, only a few inches in depth 

 should be filled in at one place at a time. Depositing a 

 great quantity of concrete at one place puts a heavy 

 strain on the forms and has a tendency to force them 

 out of plumb. As the concrete is put into the form it 

 should be spaded with a piece of 1 by 3 inch board, 

 sharpened to a bevel edge. The purpose of the spading 

 is to remove all air bubbles and avoid the formation of 

 cavities. On the other hand, in a wet mixture as used 

 in silo building the spading must not be overdone, or 

 the heavier rock will sink to the bottom and the cement 

 and water will rise to the top. 



The exterior surface can be kept smooth by greasing 

 the outside form with soap or some cheap oil or grease. 

 To be effective this grease coat must be renewed at 

 each raising of the forms. No grease should be used on 

 the inside form, as this surface is to receive a brush 

 co'at of pure cement wash. Small particles of cement 

 will adhere to this form each time it is raised, and 

 before it is used again these should be removed with a 

 broom or a wooden trowel. If these are not removed 

 an undue amount of concrete will adhere, and this will 

 result in an unnecessarily rough wall. 



