Cost of concrete driveway

[AndyS]

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

[Robert Allison]

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.

[RicodJour]

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

[Bob Morrison]

I concur with Rico's statement. A thicker unreinforced slab on a properly prepared base will give you better service in the long run.

[Bobk207]

I'd say it was a bid from someone who really wanted the job & was proabably going to skimp on the details.

I would suggest writing up (or obtaining) at least some sort of simple specification for the prep, base, rebar (size & placement) & concrete (w/c ratio, strength) . Also some words about the finishing & quality of the result.

IMO 4" is was too thin & I'm not a huge fan of unreinforced thicker slabs.

I would go with #5" min with #4 @ 18" both ways ......I hate jagged, uneven cracked slabs.

I like them to crack in the control cuts :)

I'll probably catch some flak from the guys who don't like rebar in thin slabs (they insist it does no good)

My experience is that it holds the pieces together

You're getting ready to spend a fair chunk of $'s.........how do you want the result to look in a few years?

cheers Bob

[RicodJour]

3" is the current accepted minimum for concrete cover over/under rebar, right? That's a 6.5" slab. You're really good at spending this guy's money! ;)

I'm curious why he needs a 14' wide driveway. Maybe so two cars can pass? I'd go with a narrower driveway for the bulk of it and a couple bump outs* to allow passing. It'd be a lot cheaper.

R

• I got a laugh out of that one. Firefox's spell checker flagged bumpouts as incorrect, and gave me the choice of bump outs or bum pouts. Bum pouts?! Has anyone in the history of the English language ever combined bum and pouts??? Methinks not.

[carolyn]

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

[DanG]

At 14' wide, the concrete needs a saw joint down the center and cross cuts every 7 to 10 feet. There will be a lot of lineal expansion in 310', I would use expansion every 60'. The rebar is OK, but 4" is minimal. There should be a minimum of 1 1/2" cover on rebar, making 1/2" bar almost too big to cover with 4". Money is better spent on subsoil preparation and drainage.

$3 would be low in central Oklahoma, but our stone is all imported. ___________________________ Keep the whole world singing. . . . DanG

[Bobk207]

I agree that 14' seemed a bit wide for a 300' driveway.....the house I sold last year had a 15' driveway but it was only 65' long....more of an extended parking area.

In this case maybe a narrower drive with "bum pouts" & an enlarged parking area....the material saved could go thicken the slab

I believe that cover requirements (& have had success with) are 3" for soil contact & 2" elsewhere. so that would be 5.5". Unless they've changed?

Yeah, I'll admit I'm good at spending money BUT it all depends on what the guy wants preformance wise. Is he going to be disappointed when the slab cracks (not in the cuts) AND shifts levels? And he's got cracked up mess

If not, go thin but otherwise the 5.5 to 6" ain't that much more material cost over the suggested 5" min. The extra concrete & rebar wiil add a few thousand $'s to the job & (IMO) greatly enhance the long term performance of the slab.

Soil condtions & weather could have major effects on the slab (I don't the local conditons in Eureka, TX) but in SoCal they're generally mild.

RIco did you get the post base photos, I sent last week?

cheers Bob

[Bob Morrison]

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.

[Glenn]

[Bob Morrison]

Bob:

A minor correction to your calculation:

3" clear for the soil + #4 each way (= 2*5/8 OD) + 2" clear from top = 6-1/4". I would use a minimum slab thickness of 6-1/2"

And this assumes that no salt for de-icing is used on the driveway. If salt is to be used, then the top cover should be increased to at least 3 inches and even that is going to rust eventually. So, if salt is to be used then add the cost of epoxy coating the rebar to the cost of the project.

Boy, talk about spending other people's money! We sure can add it up fast.

[HerHusband]

Wide loads maybe?

Another option to cut costs, though it's a bit of an older look, would be to pour "strips" on each side of the driveway. Maybe 2 feet wide on each side. Vehicle tires usually don't run down the middle of the driveway, so you could save money by not filling the center with concrete. Pour the strips, and place gravel or grass in the center and on each side. You could always upgrade later and fill the center areas with concrete as time and money allows.

Just a thought...

Anthony

[Bob Morrison]

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]

Anthony:

Good suggestion!

[Glenn]

I disagree. I've seen that done. The end result looks like a patched up mess. If you can't do it right, I suggest you just put down some gravel until you can do it right.

BTW, all driveways around here are 4" thick including mine and though it be 30 years old, is not broken up. #4 rebar 24" ew oc chaired on pour. Matter of fact, you won't find a crack in it. KC area.

[RicodJour]

No, Bob. I did delete my spam folder contents - I have my spam setting on Kill instead of Stun - but I reviewed it before deletion. Maybe I missed it. Please send it again. Thanks.

R

[RicodJour]

What the hell are you saying, Bob?! Anthony _sucks_ at spending other people's money. We're going to have to get the smoking jacket back and change the secret handshake again... ;)

When I lived in New Orleans about ten years ago, they upgraded Tchoupitoulas. Major truck route following the river. Obviously they have problems being on silty sand with an extremely high water table, so they couldn't prepare a BMOC (Bob Morrison Official Compacted) base. They designed the concrete road bed as a beam. The amount of concrete and rebar in that slab was astounding. I'd tease the guys working by pointing out that retaining walls were supposed to be _vertical_.

R

[Matt Whiting]

Bob, what is the design wheel load if there is no rebar in a slab of the same thickness?

Matt

[Bob Morrison]

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?

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.