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We would like to back a truck over a garage floor recently poured. The slab is 6 inches thick with 1/2 inch rebar in a 2 foot grid throughout. The concrete is a 6.5 sack mix and has cured 60 days. The garage floor has no walls erected. Our intent is to pour a walk around three sides of the slab and they are only about 40 percent reachable to pour from the truck without using a wheel borrow to place the concrete. The concrete company estimates their loaded truck will weigh about 37,000 pounds. Two sets of dual wheels in rear and one set of single wheels in the front. Any suggestions will be appreciated. The concrete is poured over a soil surface compacted to 95 percent minimum compaction. No voids, no rock.

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    They do not have a pumper truck ?
    – Alaska Man
    Jul 7 '20 at 16:28
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    Have you check with the company that poured the garage for guidance?
    – JACK
    Jul 7 '20 at 16:56
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    As @JACK suggested, or provide these details to the company supplying the new mix to see what they say? I'm pretty sure they won't want to be responsible for repouring your existing floor, so I doubt they'd be willing to drive their trucks onto something they think they'd damage...
    – FreeMan
    Jul 7 '20 at 17:37
  • Thanks @JACK. Such a small job (4 cy±) the cost of a pumper brought in is prohibitive. Did talk to the concrete supplier. They think it will be ok with no more than 3 cy at a time. I was just looking for another opinion. I, personally, think it will be fine but I would rather not break the floor. If I have the slightest doubt, I will not have the truck driven on it. I am just asking the opinion of others and there is not, nor will there be any liability associated with those kind individuals seeking to help out. So, if any of you have any thoughts I would be pleased to hear from you. Thanks Jul 7 '20 at 23:20
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The slab you’re driving on is a slab on grade that does not rely on reinforcing to span voids.

If, as you say, the ground is compacted to 95%, (which is about the most aggregates can be compacted,) then the load will be transferred to the ground without the slab bending.

With 6 1/2 sack concrete, it’s compressive strength is probably between 4,000 psi and 5,000 psi depending on the amount of water, aggregate, etc. Therefore, one square foot of concrete will support between 576,000 lbs. (4,000 x 12 x 12) and 720,000 lbs. (5,000 x 12” x 12”).

Therefore, one square foot of concrete is substantially less than the area of 8 or 10 tires from the concrete truck.

Note: Don’t let the truck “bump up” onto the edge of the slab...it’ll crack the edge. If possible let it bump up on timbers and have timbers along the edge of the slab and on the slab for the first 12” or so. This will allow the weight to distribute out into the slab.

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This is a tough one I have seen full trucks break curbs. At 1/2 truck your slab should be thick enough and has cured long enough was definitely mixed strong enough at 6.5/ yard it should hold the weight but make sure the truck is not full as I have seen both curbs and driveways crumble under a full truck’s weight but those pours were lower strength pours 3 bag in most cases.

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Doing a proper check is too complicated for this site. The slab will attempt to distribute the wheel loads across an area of soil. If the soil is soft, the slab will have to bend more in order to distribute the load over a larger area, and it is more likely to crack. And you haven't given us enough information to even estimate the stiffness of strength of the soil. (The compaction ratio doesn't tell us anything about the type of soil or its quality.) If you want a reliable answer, you should hire an engineer to do the calculations.

An alternative approach could be to cover the slab with a thick layer of gravel to protect it from the local effects of the wheel loads.

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