Striking or Removal of Centering

The centering used for construction of arches should be erased, only while the arch has developed enough strength. The centering must be eased two days before its removal so in which the voussoirs might close in and compress the mortar, but the centering must be completely erased before any masonry is constructed on the top of the arch. It is necessary because the removal of centering after the masonry construction might cause a small settlement of the arch, which in turn, might cause cracks in arch masonry and hence structural weakness. No load should be placed on the arch unless the centering has been erased.

For little spans, loosening the folding wedges does the removal of centering. While the span is more than 7.5 m, using sand boxes under the centering trusses, to prevent shocks, does lowering and removal of centering. A sand box, display in Figure 17, is placed below the prop. Sand is filled in box with a plugged hole at its bottom. Prop rests on the steel plate placed on the top of sand. Within order to lower the centering, plug is taken out because of which the sand flows out and lowers the prop gradually.