THIRTY-FIFTH ANNUAL CONVENTION. 285 



ough tamping is necessary, as it brings all the particles into 

 intimate contact. In adding to the work of the previous day, it 

 should be first thoroughly wet, cement sprinkled on the hard- 

 ened concrete, and then two inches of freshly mixed concrete 

 put in place and thoroughly stirred to bring the finer material 

 into close contact with the hardened concrete. This care is 

 taken to prevent a poor union between the old and new work 

 and the subsequent air leakage or crack which might form. 

 The coarse material should be worked away from the sides of 

 the forms by a common spade, or what is better, a special con- 

 crete spade with holes in it to permit the finer material to pass 

 through, while the larger pieces are pushed back. For several 

 days the concrete while hardening, must be kept moist by sprink- 

 ling. 



Reinforcement. 



The height of the forms are 30 inches, which is just equal 

 to the height of the windows inside and the distance between the 

 windows. By placing the circular reinforcement two inches 

 above and below the doors it will come near the middle of the 

 forms each time that it is inserted. Reinforcement placed in 

 the center of the wall in this manner will be very satisfactory. 

 Bars, rods, or wire may be used for reinforcement, provided the 

 same cross section of steel is obtained. At each side of the doors 

 a medium sized bar of vertical reinforcement should be used, 

 placed somewhat nearer the outside of the wall than the inside. 



Common Mistakes in Concrete Silo Construction. 



In the construction of many silos an attempt has been made 

 to use sheet iron for the inner form, which springs out of shape 

 when the pressure of the concrete comes against it, making an 

 ill-shaped silo. The inner form should be so constructed as to 

 be rigid enough to retain its shape under the pressure of the fresh 

 concrete. Another serious mistake eliminated in the new design 

 is the door without a wooden frame. A wooden frame not only 

 rots away in time, but often causes serious air leaks into the silo. 



