concrete formwork question

[bigengineer]

I have a situtation where perimeter masonry walls support an outdoor patio. The owner wants to use a structural reinforced slab instead of slab on grade. I know there are ways around having to shore the slab (light gauge framing supporting slab, composite deck, steel joists, etc.), so this is just a general question. Have you ever been in this type situation where the formwork was required but would be enclosed by the new concrete structure and have to be... abandoned for lack of a better word? Thanks.

 

[dik]

Typically use a 6" void form... this can be a cardboard type or my preference, a foamed plastic type. This is placed on moderately compacted granular fill (depending on the soils, sometimes no compaction) and the concrete is cast over it. You can still place a PEVB between the void form and the concrete slab.

Dik

 

[JAE]

We've used the cardboard forms, foam, and metal form deck. All left in place. I'd be wary of leaving wood formwork abandoned near a house if you are in termite country.

 

[DST148]

@bigengineer: We have done what we call structural slabs on grade for interior applications where for design purposes the slab is considered as a suspended structural slab. This is possible where the underlying soil/base is capable of supporting the weight of the structural slab and construction loads during the construction phase.
Your post does not mention about your region. If you are in a region where there are no frost protection requirements, then it is possible to place the slab directly on soil/base without using any sacrificial forms or frost protection measures.

 

[bigengineer]

Thanks for the replies. Actually, this is an elevated patio, about 5-6' above grade with perimeter masonry walls. The owner doesn't want to use fill inside the masonry walls and wants a free spanning r.c. slab spanning between the walls. There are no access doors, etc to remove form work later. And again, owner is requesting no fill material.

I can use a composite steel deck to achieve the spans he wants, but would be cheaper to do some metal decking (non-composite) as form work that is shored a couple/few times between permanent supports. Like JAE said, this would cause concerns with termites though with abandoned forms. Maybe form work from treated lumber?

 

[dik]

You can support metal deck on wood knee walls to cast the concrete and remove the temporary framing after the concrete is sufficiently hard. Metal deck forming can be less costly than plywood... and you may be able to obtain roof deck (not composite) from a local salvage supplier.

Dik

 

[JAE]

Don't use composite metal decking. The decking would serve as the structural reinforcement but because it is an exterior deck, it will rust and eventually you would have a slab with no reinforcement.

Use form deck and reinforce with rebar or do one of the other ideas above.

 

[hokie66]

Agree with JAE. Use whatever deck will carry the load during construction, but reinforced the slab as if the deck did not exist.

 

[dik]

Thanks, guys... I should have been a little more emphatic...

Dik

 

[asixth]

I have seen metal decking used as lost formwork on a footbridge project. The slab was reinforced with conventional reinforcement and the metal decking was lest to rust away over time.

Becareful with cardboard void formers. I used them once and they collapsed when the concrete was placed. Very costly.

 

[dik]

bigenginnerz:
You should have some openings for ventilation... Can have a louvred opening, or simply a steel grate/wire opening to keep birds and rodents out...

asixth:
Cardboard ones work OK, as long as they're kept dry prior to placing and contractors often wrap them in poly and provide a 1/8" or 1/4" masonite board on top to keep the rebar chairs from poking through.

Dik

 

[RHTPE]

bigenginner - I definitely agree with JAE & Hokie, do not depend on the metal deck for any composite action. Dik makes a good point about providing a couple of screened openings to ventilate the enclosed void. You don't mention the plan dimensions, but you may be able to lighten up the deck gauge by fabricating a simple truss from angle stock to hang off the walls and provide additional support at the middle of the deck span.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

You could also build another central wall support or piers to cut down on the slab span.