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Formwork in dead space

Formwork in dead space

Formwork in dead space

(OP)
An existing reinforced concrete exterior stair (4 steps) and landing at the top of the stairs has to be demolished to gain access to the waterprooofing membrane on the structural slab below, and then the stair and landing must be repoured. The formwork for the new stair and landing cannot be removed because it is trapped all around by concrete walls.

What is the best way to form this?
Is it with wood forms, perhaps using pressure treated wood to protect against rot(although perhaps it does not matter if the wood forms rot)?
Or does steel deck form (with rebar in the flutes of the form so that the steel deck provides no permanent role) make any sense? I am somewhat reluctant to use steeol deck form because I think it promotes cracking of the concrete at the high point of the flutes and requires building dwarf walls to support it, but also because probably wood forms would still be needed for the sloped soffit of the stair flight.

If wood forms are left in place, is there any danger of attracting termites, and can the wood be treated for this?
I had thought of placing a urethane membrane on top of the landing and stairs to lessen the possibility of moisture getting to the wood, but that would add cost and would be harder to keep clean than exposed concrete.

Any thoughts, recommendations or comments on:

- wood form vs. steel deck?
- use of pressure treated wood to protect against rot?
- need for treating wood to repel termites, and of so, what would the treatment be?

RE: Formwork in dead space

If the formwork has no effect on your repair, leave it to the contractor. I've done some transit station work and this sort of thing comes up regularly. We just call out "lost form" on the drawings and that's it.

RE: Formwork in dead space

This sounds like a good place for polystyrene blocks under the stair and landing.

RE: Formwork in dead space

(OP)
hokie66 - yes, good idea. I should have thought of that. Thanks.

RE: Formwork in dead space

Ajk1:
There are also small precasting outfits around who precast 3,4,5 riser stairs, with landings and of various widths. They truck them to the site and set them in place.

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