Need a bit of guidance here,

I have tough yard and my contractors are going to pour concrete over some large boulders and filled in the remaining area with parts of other boulders (6in to 1 foot). They are planning on using rebar but not much else. I have read this is not optimal but they are pretty deep into the project now - thoughts? Should I ask them to use extra rebar? Adjust the concrete mix so it is more liquid? Wet the rocks jut in advance of the pour so the concrete binds better. Any help you can provide is genuinely appreciated, a bit outside my comfort zone here.

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  • I would want to fill the deeper areas because thickness variations will cause cracks no matter how much rebar is in there.
    – Ed Beal
    Jun 21 '20 at 7:41
  • Watch blancolirio's videos on the Oroville Dam reconstruction, for a subject lesson in how to lay concrete on irregular bedrock. Short answer: 6" at a time with an eye on the weather. Jun 21 '20 at 13:50
  • @EdBeal Ed, how do you explain thickened slabs for footings then? Not all slabs are a uniform thickness. When we add a footing in a slab we thicken the slab and add rebar.
    – Lee Sam
    Jun 21 '20 at 14:23
  • With a slab that varies from nothing to a foot thick rebar and welded wire may help but the ones I have seen were cracked and I was removing them.
    – Ed Beal
    Jun 21 '20 at 14:58
  • 1
    What is the slab being used for? If it's just an outside patio, some cracks might be annoying, but not a problem. If it'll be inside space, that's a totally different issue
    – FreeMan
    Jun 25 '20 at 13:28

A vibrator will get excellent penetration of regular concrete into the broken rock. Adding more water to concrete mix reduces the strength. I poured a car park addition to a drive way using broken concrete from a sidewalk . The concrete minimum thickness was 2 " or less . It was in fine shape 10 years later when I moved . Of course I doubt it would be strong enough for moving traffic.


Concrete slabs crack when the thickness varies without rebar.

There is a recommended ratio of area of concrete to area of rebar. This is NOT structural steel, but rather a ratio of reinforcing steel to concrete and is often called “temperature steel” and is represented by the symbol Pe.

If the slab can vary from 6” deep to 12” deep, as you say, the deeper slab should have twice the reinforcement in BOTH directions.

Looking at the form work, it appears the slab could be as thin as 1”. If so, this is obviously a problem and will crack. There should be a minimum of 4” thick concrete and it should have rebar extending through this portion of the slab. (4” is minimum because you need 1 1/2” cover over and under the rebar.)

  • I agree that the thickness will be an issue, I would want to add fiberglass to the pour
    – Ed Beal
    Jun 21 '20 at 7:19
  • Thank Lee Sam, great eyes, yes that upper corner is too thin, I have that on mu list to bring up and will ensure it is 4in thick throughout. What I am really hopping for is some guidance on what to do to ensure a better outcome from what I have above - what should I use to filling the gaps between the big rocks they are using now as filler? Or should try negotiating they pull out all the big rocks and use another material altogether as the base (if so what would you suggest?), I feel like my contractor is giving me a bit of a run around so I am trying to be informed when I push back. Thanks Jun 21 '20 at 18:43
  • Big rocks are not a problem as long as there is uniform concrete coverage and adequate rebar for temperature fluctuations. If the big rocks protrude up above grade and if you have a few in a row, it will weaken the slab. We like things uniform, including the grade, thickness of the concrete, etc. If things vary we add reinforcement. Notify your contractor that the base needs to be uniform, thickness of concrete needs to be uniform or it will crack without additional rebar. Let them know you’re going to take pictures of the pour and measurements of everything.
    – Lee Sam
    Jun 21 '20 at 19:28

I've been removing some concrete this weekend that I poured years ago over rocks (but without the large boulders). I made the mix a bit more liquid and used a rebar to push the mix into all cracks/spaces by hand, rocks become aggregate. It was as strong as anything for domestic use. Don't build a nuclear bunker if it is only for a car park. BTW, what is this slab for?


For what its worth, my honest opinion is that it would have been better to add more rock in low spot mid left and jack hammer or chip/saw on the right and do the same for upper left corner. Potentially build up forms to higher grade if needed--looks like a porch or look out decking of sorts.

  • You should say why it's better. Bald assertion doesn't provide much to the community.
    – isherwood
    Dec 22 '20 at 19:12

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