[TheForge] Concrete forms OT, but important... for me anyhow.

[Andrew Vida]

The excavation under the house is now more or less complete.  I removed 
about 400 yards of clay from under and just outside the house.

The house is now standing on 18 high strength stilts that I fabricated 
from drill string.  It is a weird sight, what with all that daylight 
coming in from under the old footings.

My question is this: would 5/8 plywood suffice for concrete forms? The 
tallest section to be poured is about 8 feet, about 4 feet wide.  The 
rest is averaging about 5 feet.  I will be using 12" snap ties, on 24" 
centers with two layers of 2x4 criss crossing per the usual practice.

I was going to use 3/4 OSB, but am now having some second thoughts 
because of the tongue and grooved edges.  3/4 ply is painfully 
expensive, 5/8 being notably less costly.  My concern here is whether 
the 5/8 has the strength.

The damned lumber for the forms is almost as $$ as the concrete itself 
and the wife is having a heart attack at the costs.  I'm trying to be as 
efficient with the money as possible, but do not want to make the 
mistake of using insufficient materials.  A blow-out would be very bad, 
as you might surmise.

One other question: because I am forming to the irregular bottom of an 
existing footing I was going to rough-trace the profile onto wood, cut 
with a scroll saw, and fit it that way.  Has anyone here done anything 
like this before and if so, how did you build to the footing?  Since I 
have couple hundred tons of clay handy, I was thinking I might use it as 
a sealant to close up the little crannies at the top.

Thanks much,

     -Andy