Pouring a concrete carport slab in NH

[Andrew Duane]

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. 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.

[trader4]

And to all of the above, I would add this. Forget about the contractor that didn't specify the thickness of the concrete. It's hard to see how someone in that business would fail to specify one of the most important things in a contract, unless it's deliberate.

[Joseph Meehan]

With 4000 test concrete and rebar plus mesh it sounds over engineered to to me (I like over engineered) Have you checked to see what is required by local code. I would be surprised if the code did not include specifications for a below frost footer and minimum specs for the slab.

[Fred]

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

[Sacramento Dave]

You might call some of the concrete suppliers , they could explain the deferent mixes 5 sack, 6 sack that will help you understand what the contractors are telling. I would definitely use #4 rebar 12" centers. The concrete will get some cracks in it rebar will hold it together. I have always felt wire mesh is poor quality. While there pouring and working the concrete it's getting pushed down ends up sitting the ground, then in time it rust. Some finishers will try to pull it up but still rebar much better.