Slab on ground repair
Slab on ground repair
(OP)
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.
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.





RE: Slab on ground repair
It sounds like the slab wasn't designed at all...just placed by the contractor.
How large is the forklift? What is the tire size and payload?
A 4-inch slab is bare minimum for NO traffic. When you put forklift traffic on it you are bound to have issues, particularly with 3000 psi. Chances are that it isn't 4" consistently and if you have a gravel subbase, you are probably getting a lot of restraint to shrinkage...all bad combinations.
I would start over with the slab...tear it out and place a proper 6" thick slab (assuming the forklift loadings work out), put proper joints in at 12 to 15 feet each way, and finish/cure the slab properly for the application.
RE: Slab on ground repair
RE: Slab on ground repair
Thanks,
RE: Slab on ground repair
Don't think a 3-in topping will do the job. You'd need control joints with dowels at 7 ft spacing and I'm not sure you'd have sufficient concrete for the dowels to work (i.e., you'd just have over an inch above and below the dowel).
You need to reconstruct the slab and have it properly designed.
f-d
¡papá gordo ain't no madre flaca!
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
I say this in somewhat opposition to "hokie66" who's implying that reinforcement is not the main thing. I think it is the main thing 'cause even if you originally had a 6-in thick slab, there'd be cracking. Even if you had control joints at 15 ft, you'd develop problems if reinforcement was not provided at the control joints.
Forklifts are rough on joints and cracks!
f-d
¡papá gordo ain't no madre flaca!
RE: Slab on ground repair
RE: Slab on ground repair
It turns out the indivudual wheel (which I believe are pneumatic) loads are 4K. The contractor's engineer designed the topping assuming it would act as a single slab, dispersing the load to the soil base. Demonstrating that the total thickness, which includes the #57 stone base, creates a presure prism which allows a stress less than the allowable load; he's done.
RE: Slab on ground repair
When that happens, the slab will bend more at the joints/cracks as load goes across them and more cracks will develop.
RE: Slab on ground repair
this is an interesting topic. I've worked with some experts on this, but am no expert myself. Just a man with thoughts and such. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
What if we heavily reinforced the slab as stated and poured it over a continous mat of suspended rebar, then left out all joints. Another option would be to cut joints over the rebar within 12 hours just to control cracking somewhat over the tops of the bars.
Again, this would have the effect of providing continous strength over the entire mat. Any feedback on the effects of volumetric shrinkage would be appreciated.
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
f-d
¡papá gordo ain't no madre flaca!
RE: Slab on ground repair
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?)
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
RE: Slab on ground repair
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.
RE: Slab on ground repair
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.