"Andrew Duane" wrote in message
oups.com...
I've had a 20x23' excavation done next to my house for an exterior
carport. The hole has been filled with 6" of compacted 3/4" road-fill,
ready to have a concrete slab poured on it.

The concrete contractors that have bothered to call me back (2 out of
9!) have given me wildly different estimates, and slightly different
specs for the job. The estimates I'm going to settle by trying to get
one or two more, but I wanted to ask about the specs.

The slab is butted against the house foundation, 23' deep, 20' wide.
The expensive contractor spec'd a 6" thick slab, #4 rebar plus wire
mesh. The cheaper estimate said "rebar plus mesh" but didn't spec a
thickness, just 4000 PSI. Given how many yards he said I'd need
(6-6.5), he seems to be pouring a 4" thick slab.

Looks like it. 5.68 yards to be exact for 4" slab and 8.52 yards for 6" or
150% more concrete. Like another poster said #4 rebar with 12" on center
both ways is pretty good, make sure the rebar is at the center of the
concrete and not at the bottom. Plus mesh? Don't know what for but make sure
he has the rebars 12" OC both ways. Make sure your soil is compacted to
something like 98% or whatever requirements in your area which could be much
more important than the thickness of your concrete.

In my area, Bay Area, many times they don't even use steel but we don't have
ice either. Part of my driveway is only 2" thick! Damn no cracks so far even
with all the earthquakes over the years. 6" concrete in my area you could
put a semi on it.

Is that sufficient in
northern New England? He will put an expansion joint in the middle.

I don't know a lot about concrete structures, and want it to last. What
kind of questions should I be asking?

I also got some quotes for asphalt paving, just as a backup. Any
particular reason to like one over the other? I tend to prefer concrete
slightly for the durability.