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Re: Leaving Formwork in Place

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On 1/15/07, M. David Finley, P.E., P.A. <davidfinley(--nospam--at)bizsea.rr.com> wrote:
I have a situation that is structurally equivalent to a small grade beam system with a structural slab on top of the grade beams. 
 
Per ACI 318-05 section 7.7, the minimum cover for concrete cast against and permanently exposed to earth is 3".  Therefore, if the slab is cast directly on the ground, the beams & slab would have to have a minimum of 3" bottom cover and beams would have to have 3" side cover..
 
But what if a plywood form system is used for the beams and slabs and the formwork is simply left in place to deteriorate over time?  Can't the concrete then be considered just exposed to weather and for #5 bars (and smaller) the cover need only be 1.5" bottom & sides.  Is there any problem letting the wood formwork deteriorate? 
 
David Finley
M. David Finley, P.E., P.A.
2086 SW Main Boulevard - Suite 111
Lake City, FL  32025
386-752-6400


Generally formwork in contact with soil should be removed since it over time it will rot and not provide the kind of support that soil will.  Plus it will also be a source of food for termites which is a problem if oyu have a wooden structure on your grade beam /. slab system.

Concrete in the ground with plywood between the concrete & the earth is not the same as concrete merely exposed to weather.

Think of it as a concrete with a layer of mulch between the concrete & the earth;  because in a very sort time the plywood (unless treated for soil contact) will turn to mulch.

Once the plywood rots, you'll have the worst of both situations....a mulchy layer to trap moisture & ? against the concrete but without the more substantial strength of soil.

Just it is necessary to remove organics when prepping for concrete placement,  adding a complete layer of  "soon to be mulch"  isn't  very structurally sound.

cheers
Bob