Cost of concrete driveway

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Andy writes:
I just got a bid on a 310 long driveway, 14 feet wide.
It is to be 4" thick with rebar 18" OC.
The bid is for $2.65 per sq foot.
Is this a competitive bid ???
The area is North Texas
Andy in Eureka
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AndyS wrote:

That is way cheap. And way too thin. I would go with at least 5" of concrete for a driveway. Sidewalks are 4" thick.
Does that include base and prep work? To give you an example, for flatwork like that, the going rate around here is about 5-6.00 a square foot with base and prep. I would be careful about the prep especially. Without it, you are going to be unhappy.
--
Robert Allison
Rimshot, Inc.
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Robert Allison wrote:

I don't like rebar in such a thin slab, or in driveways at all for that matter. A thicker slab and no rebar is better than a thinner slab with rebar. As RA opined, the prep work is where corners would be cut and what would cause the driveway to fail.
I'd be nervous with a price that low. You need some additional bids.
R
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In a previous post RicodJour wrote...

I concur with Rico's statement. A thicker unreinforced slab on a properly prepared base will give you better service in the long run.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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RicodJour wrote:

Can you tell us why you don't like rebar in a thin slab? My understanding is that concrete has great compression strength, but snaps easily when pulled apart. The weight of a car on the top of a slab pushes down on the top of the slab, applying compression strength to the top, while the bottom of the pad is being pulled apart. Enough weight, and the cracks form in the bottom of the pad and tear through the top. Put the rebar in the lower half of the slab, and it will hold the bottom together and prevent that tear through.
I don't know how think it needs to be for what weights, and 5" may be enough for cars, but what about the 18 wheeler delivery truck, dropping off a new table saw? (Yeah, my saw and drill press were delivered in an 18 wheeler.
5" and rebar may be over minimum code for a driveway - but the worst that will happen is that it will be too strong and last much longer than it otherwise would.
Carolyn

Additional bids, and detailed bids, including the subbed. (I think that is the term.)
Carolyn
--
Carolyn Marenger


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In a previous post carolyn wrote...

In theory this is true if your subgrade is so poor that it won't support the weight of the vehicle above it. I can tell you that NOTHING will prevent shrinkage cracks in a slab. If you get one truck a year then a 5-inch slab on a good base will be fine. If you get one truck per month then up the thickness to 6 inches. If you expect frequent heavy traffic then make the slab thicker, but you can still leave the rebar out. Put the money into proper subgrade preparation and into proper crack control joints.

I regularly specify unreinforced slabs on grade. The American Concrete Institute has a whole book on this subject. Most city streets and much of the interstate highway system are unreinforced slabs on grade and are usually between 8 and 10 inches thick. My favorite example is an 18-inch unreinforced slab I specified for a log handling yard. It sits on a 24- inch deep compacted gravel base, which makes the total pavement thickness 42 inches. That pavement is subject to 130,000 pound axle loads and except for some surface scarring has held up very well.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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I don't disagree with no steel but if it is for me, I still want #4 rebar, 24" oc, 100% tied and on chairs

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In a previous post Glenn wrote...

Max code bar spacing is 18" o/c.
BTW, if you assume 3000 psi concrete, grade 40 bars and a span of 12", this slab will support a wheel design load of 4000 pounds. Doesn't sound to me like the rebar is worth the effort for such a thin slab unless you want to simply pour the concrete on unprepared rough ground. And, as I mentioned in another post you need at least 6-1/2" of concrete to meet minimum cover requirements.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Bob, what is the design wheel load if there is no rebar in a slab of the same thickness?
Matt
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In a previous post Matt Whiting wrote...

Using a modulus of rupture (bending stress) for concrete of 2*SQRT(f'c) and f'c000 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?
Of course it can be made more complex if you factor in the footprint of the tire. Using this sort of uniform load (32-35 psi depending on tire pressure) will either increase the design span or the allowable wheel load. This sort of calculation is not solvable by normal algebraic manipulation because there are too many unknowns and only a limited number of equations.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison 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.
Matt
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Matt Whiting wrote:

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
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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
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HerHusband wrote:

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
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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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In a previous post butwhat wrote...

12" of rock is a pretty substantial base. Unless you are on very soft ground I think you could eliminate any visqueen or geotextile fabric and still get excellent slab performance.
Remember that no matter what you do the slab will crack. so, put in crack control joints. At least then it will crack in a straight line.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Bob-
For the 14' driveway under discussion, if you spe'd 5" unreinforced.... what spacing would you spec for crack control joints?
Would you go the full 14' width with no joint? A spacing of 7' (across th4e width) is kinda short but 14' seems a little far.,
cheers Bob
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In a previous post Bobk207 wrote...

My spec for crack control joints reads:
Crack control at max 150 sq ft and max aspect ratio of 1.5:1
So, a joint pattern of 10x14 (140 SF) with an aspect ratio of 1.4 fits within the parameters. You could probably get away with 12x14, but that may be pushing it a bit. This is one of those things where more is better.
The joint is a 1" min deep sawcut or tooled joint. Some people like to use vinyl strips, but it is hard to get them straight. Sawing of joints should be done as soon as the slab is hard enough to walk on with out leaving a mark. The new lightweight joint saws are waterless and look something like a gas powered lawn edger.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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In a previous post HerHusband wrote...

Anthony:
Yes the rebar does hold things together when they crack (and it will crack). However, if there is no where for the slab to deflect then the cracked sections will stay together just fine.
However, the point of proper subgrade preparation is to prevent differential settlement by making the base consistent so the slab does not have to span any distance.
As I said before I regularly specify unreinforced slabs: no rebar, no welded wire fabric, no fibermesh, just plain concrete. These slabs perform just fine if the contractor takes the time to properly prepare the gravel base and subgrade. And as for gravel base I specify 3/4" minus compacted crushed rock, not pea gravel.
Elevated slabs are another matter and are not what is being discussed here.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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In a previous post Matt Whiting wrote...

You got it. The effective "d" for a 6-1/2 inch slab on grade is only 3-1/4". Not to mention that #4 @ 18" o/c is a pretty small amount of steel.
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Bob Morrison, PE, SE
R L Morrison Engineering Co
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