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Old 09-12-2009, 10:34 AM   #1
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Should concrete driveway slab be thinner on the edge by the forms?


We hired a concrete contractor to put in a concrete apron on our driveway. In the specs he put that it would be 4.5 inches thick.

Now that he's done, when I measure around the edges it's only about 3-4 inches thick, and no where do the edges measure 4.5 inches thick. And come to think of it, he was using 2x4s for the forms on the edges, so that is probably why the sides avg. about 3.5 inches thick.

When I call him, I'm guessing he'll say it's thicker once you move in from the edges and in the middle, but I have no way of knowing.

Is this common for contractors to use 2x4s for forms for a 4.5 inch thick concrete slab, and then they just taper the sides by the forms so its thinner by the sides where the forms were? Just want to make sure we didn't get scammed!

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Old 09-12-2009, 05:46 PM   #2
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Should concrete driveway slab be thinner on the edge by the forms?


2x4's are commonly used to form slab edges. However, they should be staked up, which will elevate them to gain the necessary thickness. A small amount of concrete slumps underneath them, and is then covered with grade.

Under no circumstances should a driveway slab be 3.5" thin. Even a basement floor slab is a full 4" thick just about everywhere, and nobody drives cars on those. Of course he'll say that it gets deeper in the middle, but I'd suggest that he get a 1/4" hammer drill and you guys drill the middle of the slab 4.5" deep. If you get that deep and haven't popped through the bottom you'll at least know it was thicker. If not, problem. (the hole can be filled with hydraulic cement and will be too small to show up)

I would have serious reservations about making payment on the work, given the contract specs were not met.

I don't think you got scammed necessarily, but he cut a corner regardless. Might have been intentional, might not.

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Old 09-12-2009, 06:24 PM   #3
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Should concrete driveway slab be thinner on the edge by the forms?


it should conform to specs everywhere. And it should be the same thickness everywhere. Demand the specs are met.
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Old 09-12-2009, 07:26 PM   #4
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Should concrete driveway slab be thinner on the edge by the forms?


I'll even go one farther and say that a driveway should always be thicker at the edges. Any vehicle getting right on that edge (or rolling off it) will introduce forces much more concentrated at that relatively unsupported area than it is subjected to when a full slab supports the wheel.
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Old 09-13-2009, 01:32 AM   #5
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Should concrete driveway slab be thinner on the edge by the forms?


I am not a concrete expert, but have done several out building slabs, sidewalks, driveways, and patios as a DIY'er that have held up well over time. I make the edges of a driveway thicker by a shovel (~6-8" additional under the slab 4" depth, and about 8-10" wide).
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Old 09-13-2009, 09:23 AM   #6
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Should concrete driveway slab be thinner on the edge by the forms?


Quote:
Originally Posted by Studly View Post
We hired a concrete contractor to put in a concrete apron on our driveway. In the specs he put that it would be 4.5 inches thick.

Now that he's done, when I measure around the edges it's only about 3-4 inches thick, and no where do the edges measure 4.5 inches thick. And come to think of it, he was using 2x4s for the forms on the edges, so that is probably why the sides avg. about 3.5 inches thick.

When I call him, I'm guessing he'll say it's thicker once you move in from the edges and in the middle, but I have no way of knowing.

Is this common for contractors to use 2x4s for forms for a 4.5 inch thick concrete slab, and then they just taper the sides by the forms so its thinner by the sides where the forms were? Just want to make sure we didn't get scammed!
This is definately a common practice, at least for anything under 5"-6". What very well could be the case is that the plate compactor has a curled edge (as all newer compactors do), & when running next to the form, the last inch or so of gravel remains a little high. Ocaissionally, we will go along the edges with a shovel or hand float just to pull this little bit of gravel down. Does this make a better slab? No, it just gives a perception that the slab is thick than it is in reality. This tiny bit of gravel left high from the compactor probably only equals a few shovel-fulls of concrete over an entire driveway.

At the same time, there is a chance that the contractor didn't pour as thick as his contract stated. This is always a possibility, as ready-mix concrete is no longer a cheap commodity. I would never be willing to scimp on my materials, as it can cost a contractor far more in the long run to save a "few" dollars. I'm not saying other contractors have the same thought process, though. I'm meerly saying that using the formed edge to judge actual thickness of the slab is like "judging a book by it's cover."

Drilling the slab in a control joint with a 1/4" or smaller drill bit is a fairly accurite way to tell the thickness with minimal disturbance. If you could find out how much concrete was ordered vs. the actual square footage of driveway installed would also be an accurate way to figure the overall thickness.
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Old 09-14-2009, 09:24 AM   #7
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Should concrete driveway slab be thinner on the edge by the forms?


Thanks for the advice, everyone. Upon further measurement, I noticed that there is only one spot that's 3 inches deep on the edge, and that is because when I probed further, a piece of concrete must have came off with the forms or chipped off afterwards, on the bottom edge, because when I dug down and probed underneath a half inch in, it got thicker. In the other spots on the outside edge it's generally 3.5-3.75 inches thick.

We did talk to the contractor and he said it should be at least 3.5 inches around the outside edges of the apron because indeed, as Jomama said above, he used 2x4s for forms and kind of tapers the dirt under the outside edges with the compactor. But he said a couple inches in from the left and right edges, it's all 4.5 inches or above. He said he has the paperwork from the concrete order to prove it and that he actually ordered enough ready mix concrete to go 4.75 inches deep to have a cushion.

From everything I could find out, this contractor is reputable and has been in business for many decades and has no complaints on record from the BBB, so I'm not worried enough to drill into it and verify the thickness of everything. And it's reassuring as Jomama said that many contractors do it this way, and that I'm not working with an oddball one. But that is a good idea the KCtermite mentioned above, to drill into it, if you ever need to verify the thickness of a slab.

KCtermite, when you mentioned that most people just raise the 2x4 forms to get the desired height (in my case for a 4.5 inch slab, you'd have to raise them 1 inch on the stakes) you mentioned the concrete seeps out a bit on the bottom. But if it's an inch gap under the form, wouldn't a lot of concrete come out?

I know obviously it would be better to have the slab 4.5 inches thick throughout the slab and on the edges, but if it's 3.5" for a couple of the outermost inches on the left and right outside edges, using 5,000 PSI concrete, do you think it would hold up when it is only occassionally driven on? I say occassionally because you don't typically drive on the outside couple inches on the edges of the driveway and also this concrete slab is for an apron, so its only on the top four feet of the driveway, in front of the garage.

Thanks everyone for your advice and opinions!


Last edited by Studly; 09-14-2009 at 09:38 AM.
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