Back propping of RC and post tensioned concrete floor 1

[oranda]

Where can I get some information on the above topic? For RC floor system, what would be the procedures for removing or stripping of formwork? For post tensioned floor system, can all the formwork be stripped after the floor is full stressed? Would back propping be required?

Thanks

 

[DTGT2002]

Your question is in regard to shoring and reshoring Reinforced Concrete Structure. Shores are the elements used to support the active level that is being formed and cast, and reshores are the elements used to support and distribute load either to the ground, or to other floors with residual capacity.

The best answer for the above question is to seek the assistance and input of a local formwork/falsework engineer. Next best is to further explain the problem.

Is you structure a multistory Reinforced Concrete Structure, or are you referring to a single elevated level? There are many parameters to consider.

The methods ao stripping are fairly straight forward. Once the level is shored, formed and cast, the concrete is left to cure until the slab has sufficient strength to support itself. The determination of sufficient strength must come from the slabs design engineer, and should be based on analysis of cylinder breaks, additionally supported by knowledge of the mix used. I generally prefer a slab to reach about 80-90 % of it's capacity before stripping, however, the job schedule may not reflect this. As such the concrete mix may be adjusted to give a higher early strength. Once the slab has reached it's required strength, it can be stripped in order to allow it to deflect, and carry it's on self weight, or if it is post-tensioned, it may then be stressed if the strength required for stress is also met. If their is only one level, the formwork may be removed by loosening the shoring system, removing the shores and pulling the plyform or forming surface from the slab. If the building is a multistory building, the slab will be stripped and allowed to deflect, and reshores will be placed snugly below the slab, allowing the construction load of subsequent levels to be passed first to the ground and later shared between the residual capacities of several elevated slabs.

As far as references, ACI 347 is a document which provides guidelines for Shoring operations, and ACI SP-4, Formwork for Concrete is a practical guide to many aspects of formwork and shoring. ACI comittee 347 is in the process of finalizing a document to address reshoring as we speak which will hopefully be published in the near future.

let me know if I have helped at all or if I can answer any more questions.

Daniel