PRACTICAL SILO CONSTRUCTION 49 



broken in each course laid, and the work of laying 

 greatly simplified. This can also be arranged so as to 

 bring the hollows or holes in blocks made by the gas- 

 pipe exactly over those in the block below, thus per- 

 mitting vertical reinforcement, if so desired. 



CONCRETE FOB SILO WORE 



For the manufacture of silo blocks or monolithic 

 silo walls the usual mixture is 1:2:4, using clean, 

 sharp sand that is well graded, and crushed stone or 

 coarse gravel for the larger aggregate. This should 

 not be over J " in diameter. The materials call for 

 thorough mixing and enough moisture to completely 

 wet the mix. This makes it impossible to raise the 

 forms until the concrete placed has had time to 

 harden or set. While this may seem a hardship to 

 the contractor in a hurry, it is the one and only safe 

 policy, even when a semi-dry mixture is employed. 

 Many jobs are injured by haste in construction ; for, 

 if the concrete has not had from 24 to 36 hours to 

 harden, even under the most favorable conditions it 

 is a menace to the workmen employed on any job 

 where the concrete must support the staging or scaf- 

 fold. Where the forms can be filled in a few hours 

 or one-half a day, the concrete can easily be given 

 24 hours to harden without delaying the work. 



Where the mixture is made semi-dry it must be 

 tamped very solidly ; but in the event of a wet mix- 

 ture the tamping can be replaced by placing the 

 mixture in thin layers or courses and pressing it out 



