SILOS OF REINFORCED CONCRETE 223 



out finally and quickly filled up, and the remaining part of the floor, over 

 the sump, laid down. 



Scaffolding. — In order to provide a means for lifting the forms, and 

 also to support a platform for the workmen to stand on, scaffolding is 

 required. 



It may consist of eight rough poles erected at intervals round the silo, 

 on its outer side, with their lower ends sunk into the ground. These 

 poles are braced together, and carry horizontal beams at a level con- 

 siderably higher than the top of the silo wall. From these beams four 

 blocks and tackle are suspended, by means of which the forms may be 

 lifted and adjusted as required. 



Building the Wall. — While the floor is being laid it is advisable to 

 form a groove in its surface, running right around it, in such a position 

 that it is directly below the middle of the thickness of the silo wall. 

 When the wall is built on top of this groove, the concrete of the 

 wall enters the groove and forms a key between wall and floor which 

 will prevent leakage between them. In fact the wall and floor are 

 tongued and grooved together. 



The building of the wall may be commenced by scribing two circles 

 on the floor (already laid), marking the positions of the outer and inner 

 faces of the wall respectively. The forms are then laid with their 

 upper surface perfectly level, and their cylindrical faces up to these 

 circles. Since the forms have been constructed with their cylindrical 

 surfaces exactly at right angles to their top face, when the latter is 

 set level with the aid of a spirit-level their cylindrical surfaces will be 

 truly vertical. 



After the forms have been filled, and the concrete has set hard 

 enough to resist indentation by the thumb nail, they may be moved 

 to a higher position. In this position the forms should overlap, and 

 grip that part of the wall already constructed, by about 6 inches. Each 

 time the forms are moved up, their top face must be set dead level by 

 means of the spirit-level. This will result in the production of a truly 

 vertical wall. 



Eacli time the forms are set in a new position, the upper surface 

 of the old ring, or tier of concrete, which was laid with the forms in 

 their previous position, should be cleaned and grouted over with 1 part 

 cement to 1 part sand, immediately before beginning to fill in the forms 

 in their new position. If this grouting be not done, leakage of silage 



