(5) Concreting
Once the reinforcements, cables laying and anchorages are
done, the other side of the steel form-work as well as the
end form-works are closed up, ready to receive fresh fluid
concrete. External vibrators are installed on the steel form-
work at certain positions to assist in the compaction besides
the internal vibrators. A concrete pump is used to pump
the concrete from the trucks into the 2m high form-work.
Preparation and mixing of concrete is another important skillful work and is done in a batching plant. The concrete has to achieve certain strength, in this case 45N/mm2 in 28 days, 40N/mm2 at transfer (the strength for pre-stressing), be workable for pumping and flow into all corners inside of form-work with the assistance of vibrators, and will not set too fast to allow concreting to be completed. Admixture is added to improve the above properties and accelerate the strength so that the girder beam can be completed and removed from the casting yard. This time is set for 3-7days so that next beam can be started. The concrete volume required in this case is 30 cubic meters and required 2 hours to complete the work.
Many problems can arise in this stage. Interruption of concrete supply, stoppage of electricity supply, poor mixing, malfunction of concrete pump, high temperature, large air bubbles trapped in the form-work, displacements of steel and cables, puncture of cable ducts, safety issues, etc.,the workers have to be repeatedly cautioned and reminded.
No comments:
Post a Comment