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Pouring a concrete carport slab in NH Andrew Duane 08-10-2005
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Posted by Andrew Duane on August 10, 2005, 6:46 am


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.



PexSupply Full Banner
Posted by Joseph Meehan on August 10, 2005, 1:53 pm


Andrew Duane wrote:
> 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.

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.


--
Joseph Meehan

Dia duit




Posted by Fred on August 10, 2005, 2:37 pm



> 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.
>




Posted by Sacramento Dave on August 10, 2005, 3:46 pm



> 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.
>

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.




Posted by on August 10, 2005, 9:52 am


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.



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