Using existing concrete paving as seal slab and putting 15'×15' (L×W) foundation on it

[AskTooMuch]

If I have an existing 6" thick reinforced paving. Let's say I have an equipment on skid that will required a 15'x 15' length×width of foundation, havent decided the thickness of foundation yet. What are the list of issues if I put this new foundation on top of paving (as seal slab) instead of breaking the paving and putting new foundation soil.

 

[r13]

Check shear capacity.

 

[RabitPete]

I would be concerned about subgrade quality under the existing slab, especially if equipment is heavy. Seen too many floors which cracked and sunk under the weight of forklifts and machinery, all due to poorly compacted subgrade.

 

[AskTooMuch]

Paving is outside and decades old. Lots of traffic has passed on this. I think it's sufficiently compacted. If I break the paving, the new foundation will likely just sit on the subgrade and I will just be adding isolation joint. Hence, do I even need to break the paving which will cost more. It seems to me, soil bearing wise, it's better to put on existing paving than on subgrade. Though perhaps I'm not seeing all the issues here. Particularly if this will cause paving to crack.

 

[oldestguy]

Your reasoning makes sense. Make use of that slab.

 

[spieng89]

Why not look at the loading of the item you are proposing to place on the slab? If the total load plus the dead load of the skid is less than vehicle traffic load i.e. AASHTO you'll probably be fine. Pavement design is a bit different than slab design as pavement are typically design as floating instead of rigid

 

[AskTooMuch]

Existing pavement has joints and the location of equipment is somewhere in that area.

 

[dik]

Can the equipment be relocated to fit in a sawcut segment? Can a new sawcut pattern be made to reflect the footprint so cracking, if it occurs, falls along the sawcuts? How heavy, and how is the load applied?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

The subgrade may have densified due to traffic load, but this type of compaction is uncontrolled, that resulting in subgrade with uneven strength, thus differential settlement can occur under heavy uneven load; depends on project location, frost heave can be a problem too.

As suggest by dik, you shall at least provide separation joint, otherwise unsightly cracks my occur around the skid.

 

[dik]

Depending on the load... I'd be least concerned about the subgrade loading unless the loading is high... then a footing would be in order.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EcoGen]

I'm not sure what kind of loading we're talking about here but like others have said, if the new loading is less than the vehicular loading (actual loading more so than design load) then it wouldn't be as much of a concern. But for any significantly higher loads, I think it's a different story. Then you would be assuming sufficient soil bearing capacity and also either 1)there are no pockets of hollow areas with settled soil below the slab or 2)the reinforced slab can span over these hollow areas of soil without cracking.

Either way, you'd be depending on the subgrade which you've never seen or tested. At that point I would do some test holes in the existing slab on grade to verify subgrade is still in full contact with the slab or even to test the soil to verify load bearing capacity if you haven't already at adjacent areas. As opposed to demo-ing the entire slab.

 

[AskTooMuch]

About 700psf, using 15'x15' foundation.

Edit: that's including wind load

 

[dik]

If uniformly loaded 700 psf is not a heavy loading generally... and the subgrade should accommodate it. You are looking at a total load of 160K which is a bunch and I'd be looking at putting this on a real foundation.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

If the 700 psf was derived by the weight diving the entire 15'x15' support area, depending on the type of skid, which is usually a steel frame, you may find the actual stress is much higher under the frame beams, which is a line load in nature. It would be worse, if the equipment has support legs, the load is non-uniform on the support frame.