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We need to get our cracked concrete driveway replaced. The damage seems to be a result of tree roots, and the tree has has been removed. Many people who we've spoken to say the concrete should be 6in thick. The contractor who we've chosen for the job insists that 4.5in of 4000 PSI fibermesh concrete is sufficient. Putting 6in will just increase costs.

Is 4.5in of concrete sufficient? My concern was whether 4.5in of fibermesh concrete would hold up if a mover/UPS truck backs into our driveway.

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Concrete is one of the more tricky trades. Ground prep is the #1 important thing. Proper base thickness, and proper compaction are key. #2 is a proper mix; too wet and too much water (typical) weaken the concrete. Instead of fiber mesh, use plasticizers to allow the concrete to run better but without adding too much water. And finally proper finishing can have a pronounced effect in many possible ways (cracking, spalling, blowouts, etc) All these things are much more important than thicker concrete.

On that note, going thicker with the concrete does add strength namely for two reasons: first you can add rebar in the bottom 1/3 of the pad to reliably strengthen it (4" +/- concrete this is very hard to do and does not have the same effect); second it makes the pad be able to handle more ground settling without cracking (given the other conditions are the same).

But an additional 33% more concrete will add significant material cost that would be better spent in other areas first (proper ground prep, proper mix and additives, proper concrete contractor)

As for the fibers, the small microfibers really don't do as much as one might want. Mostly they help with keeping the surface intact over time; they do not help with over all strength of the pad. Macro fibers can add strength to the pad, but mostly due to cracks from expansion and contraction, not from ground settling (from my understanding) and you will see them because the fibers are long.

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Ooooh, fiber mesh. Proprietary crap like that is how sellers pad their profit margins. It is like when car dealers sell you "undercoating".

The weight of the vehicle and thickness of the concrete does not matter. What matters is the subsurface. If the subsurface is solid 1" (inch) of concrete will support a cement truck. If it is not, it could break under its own weight no matter how thick it is.

This shows the cross section of a Roman road:

roman road cross section

As you can see the pavement on top is relatively thin. What is important is the 10 feet of drainage and support under the pavement that makes the road last for thousands of years. The same principle applies to your driveway.

  • What is drainage? – user2233706 Jun 19 '15 at 18:51
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    Drainage is a way for water to travel away by the force of gravity. – Tyler Durden Jun 19 '15 at 19:16
  • How does drainage apply to a concrete driveway? Are you referring to the pitch of the driveway so that water can run off? – user2233706 Jun 19 '15 at 19:53
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    @user2233706 If water gets under the driveway it will gradually destroy it. – Tyler Durden Jun 19 '15 at 20:01
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    I know little about concrete but I enjoyed the back and forth you guys are having. I think @TylerDurden is overestimating your knowledge about concrete. Let me help (hopefully) translate. He is saying that you are asking the wrong question. Since you have an existing driveway that has cracked, it is likely because your subsurface (whats on the ground under the concrete) wasn't quality before. If they pull up the concrete and don't replace what is under it with a quality base for concrete, the thickness of your slab isn't gonna matter much (if at all). – kinar Jun 19 '15 at 20:58

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