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Leaving Formwork in Place

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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