Slab on Grade over Existing Slab on Grade 1

[Jc67roch]

I have a project (a large warehouse, pre-engineered metal building to be erected over existing footings and slab) where we need to put a new concrete floor slab over an existing, but rough condition, concrete slab. The existing slab is 6 to 7 inches thick, and the new one needs to support forklift traffic and large storage racks (point loads). I was proposing to use a 2-4 inch thick sand or gravel separation layer, and pour a new heavy duty 8” concrete slab over the existing. The thought being that the sand layer would prevent existing slab joints and cracks from reflecting up through the new slab (essentially make the new slab structurally independent). But the 8 inch thickness is causing some financial heartburn for the client due to the large size of this warehouse.

I have attached some photos of the existing slab conditions and the rough detail for the sand layer concept.

Does anyone know of a concrete separation membrane product that we could up down directly on the existing slab, and then install the new slab right on top? My thinking is it would keep the slabs independent/prevent reflection of joints and cracks, but maybe allow me to take advantage of the combined thickness for the loading as well as anchorages.

Does anyone have thoughts or experience with bonding an overlay (2" maybe) over an existing slab and simply cutting in joints to match the existing slab below?

Have you ever done something like any of these options? Suggestions? Any input you can provide would be appreciated! Looking for a way to significantly reduce the thickness of that new top slab.
Thanks!

 

[AskTooMuch]

We use cement stabilized sand on top of existing slab. Haven't heard any complains.

 

[canwesteng]

Why do you need 8" of concrete on top of gravel that is on top of existing concrete? I might also check just adding bar to say a 5" slab (maybe dense mesh to a 4" slab) and getting rid of the gravel, might be cheaper and do a better job of controlling cracks.

 

[SlideRuleEra]

1) I was proposing to use a 2-4 inch thick sand or gravel separation layer, and pour a new heavy duty 8” concrete slab over the existing.

2) Does anyone know of a concrete separation membrane product that we could up down directly on the existing slab, and then install the new slab right on top?

3) Does anyone have... experience with bonding an overlay (2" maybe) over an existing slab and simply cutting in joints to match the existing slab below?

4) But the 8 inch thickness is causing some financial heartburn for the client...

1) An "unbonded overlay", good choice based on the photos.

2) Wishful thinking.

3) Yes, a "bonded overlay, works if the existing concrete is in very good condition and the surface has been properly prepared. IMHO, at least some and probably most of the existing slab in the photos is not suitable for a bonded overlay... too many existing cracks.

4) If there was no cracking (not true per the photos), the Client would likely faint when shown the cost to prepare the surface shown for a bonded overlay. Your unbonded overlay proposal will seem "cheap".

See "Guide to Concrete Overlays, Fourth Edition"
... overlays are not just for highways and bridges, the principle can be used (successfully) on just about any concrete suface.

http://www.SlideRuleEra.net

 

[dik]

Any existing cracking will likely 'telegraph' through the new slab, unless there is a gran buffer between.

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

-Dik

 

[Muraks]

2"-4" filling material looks has not height enough to compact well.

In my opinion first step should be calculating of cost of removing the existing layer:
[ul]
Calculate the existing rebar quantity in the existing concrete[ul]
Take 3 proposals from wreckling companies and give them the existing rebar[ul]
If any proposal is OK for budget disposal all site concrete and [ul]
Fill the area with selected material and compact well[/li]
[/ul]

If there is no way to remove existing because of budget, you should use gravel for that layer. And shouldn't use any sheet between gravel and new concrete layer.