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Subject Author Date
Cost of concrete driveway AndyS 11-02-2006
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Posted by Matt Whiting on November 4, 2006, 1:39 pm
Bob Morrison wrote:

> In a previous post Matt Whiting wrote...
>
>>Bob, what is the design wheel load if there is no rebar in a slab of the
>>same thickness?
>>
>
>
> Using a modulus of rupture (bending stress) for concrete of 2*SQRT(f'c)
> and f'c=3000 psi, a span of 12" and an slab thickness of 5"
>
> M = PL/4 = Fb * Sx = 2*SQRT(3000)*[(12)(5^2)/6] = 5500 #-in
>
> For L = 12" P = 1835# or about one light pickup truck wheel load
>
> If you increase the slab to 6-1/2" then P = 3100#
>
> Compare the unreinforced slab to the 6-1/2" slab with #4 @ 18" o/c, which
> gave a P = 4000#
>
> If you increase the concrete strength to 4000 psi, then
> P = 3100# * SQRT(4000/3000) = 3600#
>
> Pretty simple calculation isn't it?

Yes, but I'm still surprised the rebar adds so little additional
strength. I guess being nearly on the neutral axis is probably the killer.

Matt

Posted by RicodJour on November 4, 2006, 1:55 pm
Matt Whiting wrote:
>
> Yes, but I'm still surprised the rebar adds so little additional
> strength. I guess being nearly on the neutral axis is probably the killer.

That's the whole point about rebar in driveway and other slabs. People
view rebar as a panacea - that it's a simple way to make a slab far
stronger, and it's not. At least not the way it's usually done, and
not the way it can be done in thin slabs on grade.

R


Posted by HerHusband on November 5, 2006, 12:12 pm
>> I'm still surprised the rebar adds so little additional strength.

> That's the whole point about rebar in driveway and other slabs.
> People view rebar as a panacea - that it's a simple way to make a slab
> far stronger, and it's not.

I'm no engineer, but wouldn't the rebar help control cracking and just hold
things together when it does crack?

I'm probably wrong, but I would think the slab could settle unevenly if it
cracked without the rebar, whereas it might stay a little more stable with
the rebar?

In any case, rebar is cheap, so I've always added a couple of bars in the
sidewalks and stuff we've poured. It usually only added an extra $10 or so
to the total cost, so why not?

Anthony

Posted by Bobk207 on November 5, 2006, 10:27 pm

HerHusband wrote:
> >> I'm still surprised the rebar adds so little additional strength.
>
> > That's the whole point about rebar in driveway and other slabs.
> > People view rebar as a panacea - that it's a simple way to make a slab
> > far stronger, and it's not.
>
> I'm no engineer, but wouldn't the rebar help control cracking and just hold
> things together when it does crack?
>
> I'm probably wrong, but I would think the slab could settle unevenly if it
> cracked without the rebar, whereas it might stay a little more stable with
> the rebar?
>
> In any case, rebar is cheap, so I've always added a couple of bars in the
> sidewalks and stuff we've poured. It usually only added an extra $10 or so
> to the total cost, so why not?
>
> Anthony

I agree with you about rebar holding things together when it does
crack but this drivway is pretty big

If it has rebar @ 18" oc (both ways) we're talking about 6000 ft of
rebar....... so buying & placing the stuff would take some $'s.

IMO worth it, but others disagree

cheers
Bob


Posted by butwhat on November 6, 2006, 3:41 am
I put rebar in everything. Did anybody recommend a mix?
I'm also curious as to what effect a driveway mat under the concrete
would have.
A friend of mine uses them to drive semis over.
He swears by the road mats.
I usually try to put down 12" of clean rock covered with visqueen for a
drainage base.


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