Replies

1. 
   sungod | Oct 09, 2003 06:47pm | #1

   Mesh alone will not stop the driveway from cracking.  The driveway will crack from too much weight on it, especially if the sub-base is not compacted.  After grading and before setting form, drive your vehicles on it for a while.  If the tire tracks aren't deeper than the center of the driveway, its compacted very well.

   All cracks are from how the concrete is formed and cured.  It will crack where one truck poured load ended and the where the next starts.  It will  crack where one side is in sun and the other in shade.  If rebar is heated by the sun and too close to the surface, it will crack there.  On the inside corner of a "L" shaped pour it will crack.  If a portion is on plastic and the other is dirt.it will crack.

   Its all a mater of how the crystals form in the concrete, if the crystals start at different times or grows at different rates, there will be a crack.

   Also, sand under a slab gives tree roots an easy path to grow under and pop up it up.

2. 
   kaorisdad | Oct 09, 2003 07:54pm | #2

   For a driveway you can use 6 x 6 8/8 or 6 x 6 6/6.  However, I'd recommend using rebar - #4's at 12 or 18-inches OC each way.  Use dobies (little concrete blocks with tie wire - you can get them at Home Depot) to block them up.  Mesh tends to get trampled as concrete is placed and if it is on the bottom, it does no good at all.  I think the other post mentioned about cracks at 'L' shaped transitions.  Cracks can be avoided by placing a 4-ft long rebar piece at a diagonal at the apex of the 'L'.

   Don't forget to add control joints for shrinkage cracks.  These should spaced at about 10-feet.

   Slab cracking can be minimized by proper curing and the amount of water in the mix.  Cement content also helps - watch the Water-Cement ratio.  Use at least a 5-sack mix, 6 is even better.  Specify a 4 or 5-inch slump.  Usually cracking is due to shrinkage caused by rapid hydration or drying out.  That's why curing is important.

3. 
   brownbagg | Oct 09, 2003 07:56pm | #3

   O.K. once again. First, five inches of concrete is great, that is more than you need but still good. wire mesh has no effect on load, that is the job of rebar, but the average car does not produce enought weight to need rebar. So on a driveway you really do not need rebar. If you want to,you can. Now for wire is a joke, just use fiber mesh. wire only controls cracking and does a poor job of that. Fiber is cheaper in the long run since no labor or waste of material,and does a better job.

4. 
   VaTom | Oct 10, 2003 02:05am | #4

   I poured a 4" driveway that later had a lot of concete truck traffic in the course of building our concrete house.  6x6 was completely adequate, carefully placed.  Only cracking (minimal) was at the control joints, 15'.  The foundation under the concrete was undisturbed, no topsoil.

   Rebar is totally unnecessary unless you're putting the concrete in tension, which should never be allowed in a slab on grade.

   Since, I've poured a number of slabs and can tell no difference between 6x6 and fibers, other than convenience.  Only problem that ever occurred was when I let a professional finisher do one slab.  He insisted on too much water and it cracked.  I work with 3-4" slump.  More effort, but no problems.  Covered cure time.

   You've already paid more attention to what's under the concrete than most.  Money very well spent.  You'll do fine.  I wasn't offered a choice in 6x6 and used whatever was available. 

   PAHS Designer/Builder- Bury it!

5. 
   PandaW | Jun 02, 2023 02:44am | #5

   You're correct that mesh reinforcement alone may not prevent cracking in a driveway. The factors you mentioned, such as the weight on the driveway and the compaction of the sub-base, are crucial in determining the durability of the concrete.

   Compacting the sub-base is essential because it ensures a stable foundation for the driveway. As you mentioned, driving vehicles on the sub-base can help compact them effectively by assessing whether the tire tracks are more profound than the center of the driveway.

   In terms of concrete cracking, you've provided some accurate insights. Concrete is prone to cracking due to various factors, including how it is formed, poured, and cured. Temperature differentials, differential drying shrinkage, and uneven curing can all contribute to cracking.

   https://www.breaconcrete.com/home

6. 
   obeeswax | Jun 03, 2023 11:16am | #6

   We had a concrete drive poured many years ago and the cement contractor said if I wanted rebar, I’d have to tie them myself. Since I just had major foot surgery, that wasn’t possible, so I insisted he use fiber which was still relatively new at the time and he scoffed. I learned later he was an alcoholic and most probably under the influence at the time. I also insisted on ten foot control joints. The driveway has lasted even with the weight of the tree trimmer’s trucks and the roofer’s trucks. The drive was framed with 2x6” which the contractor also thought overkill and was surprised when I suggested he use Crisco on the boards he looked at me like I’d lost my mind but it was a trick a cement contractor taught my family back in the 1960’s so the boards could be reused. A 2 x 6 will give you a 5 ½” slab. With the advent of using deliveries to the house and some quite heavy I’m happy we went with the thicker slab. So when you’re figuring out the thickness of the gravel, count on heavier loads coming over it like the roofer’s dump truck.

   We now live at a new place with a 3 ½” cement drive. Trees have uplifted it and the weight over those areas have cracked it. Instead of tearing those areas out, we will do a careful cut out of the whole section and replace with pavers as we don’t want to lose the trees. Hopefully it will look good…as we are extending the small turn out so vehicles can actually turn out without driving in the grass. Tree roots, especially the feeders can extend far beyond the canopy and lift the cement. Places where they had the control joints 20-30’ apart have cracks. Cement contractors are hard to come by around our parts, our brick mason is having problems just getting cement delivered. It’s also a good time to think about putting utilities underground, if they have to go under the driveway. Check with local building code, I believe ours went from white to gray some years back.