Slab on ground repair

I have a client with an agricultural use (i.e. building codes do not apply) with forklifts who has a 4-inch slab on ground that is unreinforced (save for plastic fiber - ha) and is breaking up beneath him. (No - I didn't design or recommend this slab). To make matters worse, he has radient heat and drain lines beneath it which would probably have to be replaced if the slab were to be demoed and replaced. Add to that the heating equipment that has been installed on top of the slab, and he is in no mood to replace it.

The concrete was specced at 3000 psi and met the break testing during placement. No testing was made of the crushed stone or subgrade however. Most cracking appears to be as a result of poor depth and spacing of control joints (1/4" and 25') as well as little to no curing measures during very hot weather. It has begun cracking as well where the forklift traffic is heaviest. I don't see the need to get core samples because of the abysmal condition of the slab, which speaks for itself. I think it should all come up (all 24,000 sf).

The concrete contractor accepts blame for the cracking, but wants to put a 3-inch topping slab w/WWF over it after making repairs to the cracking, and the owner is listening. I see this as throwing good money after bad as I've never heard of useing a topping slab to repair a slab on grade, nor think that a 3-inch WWF slab would hold up under forklift traffic.

Any thoughts/opinions? Are there any references I can use here to support my case?
Thanks in advance for your input.

 

Okay, since it's in the back of my mind, I'll go ahead and tell you what I think has the best chance of working within the confines of a 3-inch topping (which I am NOT taking responsibility for).

3-inch slab with high strength concrete (say 5,500) and #4s @ 10" e.w. (.006Ag) on chairs and pumped, well bonded to the slab below via physical etching and cleansing, water cured for 7 days min, no control joints or discontinuous rebar.

Tear me apart.

(And while I'm at it, does anyone see going to a 4-inch topping as supremely better?)

 

[cvg]

4 inch - yes, and why not? the extra inch of concrete will cost practically nothing and will make it much easier to get minimum cover over your steel. Not to mention the fact that it will increase your section depth by a third.

 

[hokie66]

The owner and contractor will only get one chance at this. 4" overlay is a better approach, chair the bars so they are in the approximate centroid of the overlay.

 

[Ron]

If you use a bonded overlay, get ready for problems. I would isolate and reinforce...better chance of fewer complaints.

 

[Zambo]

We don't have photographs so it is impossible to guess the cause of the cracking. Are there any depressions that suggest the subbase is failing?

If the cracks are not due to soft spots below the slab but are due to the slab thickness and the lack of reinforcing I think the 4" topping could improve the condition to make the slab serviceable.

I don't like joints in slabs for forklift use. I'm also interested about the issue of bonding to the existing slab against isolation. All things considered I suggest sand blasting the existing slab surface (concrete planing better) and then bonding new to old.

The surface of the new concrete must be "closed", by that I mean not just tamped but trowelled (either manually or by power float/trowel). Of course curing is critical and needs to be supervised.

 

The slab does not exhibit signs of subbase failure and the contractor plans to repair all cracking due to the poor curing process.

To respond to Ron, I would like to know why bonding with reinforcement should be expected to give us any more than cosmetic cracks translating from the old slab. My concern about isolating panels is the forklift traffic. Everytime we create a seam between panels, the forklifts will start their damage here.