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.

 

[Ron]

A topping slab can be used; however, if there is any faulting of the existing cracks, they will reflect through, even with WWF.

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.

 

[hokie66]

Topping it could work...but then maybe not. It will be a gamble...just make sure the risk isn't yours. Depends a lot on the type forklift, how big, what type wheels, etc.

 

Will check on forklift and get back to you.

Thanks,

 

[fattdad]

You need dowels at the control joints. WWF won't help. 25 ft is too great of spacing for a 4-in thick concrete slab (30xthickness is typical, no greater than 15 ft).

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!

 

[beton1]

This sounds like a project with little or no engineering input, and possibly no general contractor. Why would the concrete contractor just 'accept' fault for the cracking, and suggest a topping slab, especially if there was no base/subgrade testing. I agree with Ron and others about the basics, joint spacing etc., but the whole thing is obviously inadequate for the intended purpose. A topping is a 50-50 proposition. So the question is, who came up with the 4" slab, the owner, who does not want it removed because of the radiant heat etc., or the concrete contractor? And was forklift traffic known by all the parties to be part of the equation.

 

[hokie66]

The main thing is the slab thickness, not reinforcement or jointing. If the risk is taken and a topping is placed, dowels and jointing would do no good in such a thin topping. You would just let shrinkage cracking occur where it wants to, controlled of course to the extent possible by mix design and curing. I agree that a properly designed slab is best, but can understand the owner's reluctance to do that. It's his and the contractor's money, so as long as they take the risk, who's to tell them otherwise.

 

[fattdad]

forklift traffic and cracks don't mix at all. Further distress will occur as there won't be sufficient aggregate interlock over time. Dowels are needed at the control joints. A topping slab won't allow sufficient thickness to provide this detail. A new slab is needed or the owner needs to recognize that whatever is done is temporary.

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!

 

[hokie66]

Actually, fork lifts are worse on joints than they are on cracks. Even with dowelled joints designed in accordance with current recommendations, the joints are usually the first thing to fail. The current trend where I am is to use jointless floors with heavy crack control reinforcement throughout.

 

I appreciate all your input. I would like to ask fattdad what he means by "there won't be sufficient aggregate interlock over time". The "over time" comment doesn't make sense to me. Also, what would be the risks if you ran continuous WWF everywhere, including under the saw joints? Dowels are sure to be problematic in a 3-inch slab. Would this potentially cause the entire slab to shrink?

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.

 

[Ron]

What f-d is referring to is the load transfer capability across a crack or a joint. When you have a "random" crack or a sawcut joint, the load is transferred (in this case wheel loads from a forklift) across the crack or joint by the friction of the coarse aggregate particles exposed within the joint or crack face. As time goes on, the concrete shrinks (drying shrinkage) and the joint or crack gets wider. As the crack or joint widens, the friction interlock becomes less and less, eventually losing all contact and load transfer capability.


When that happens, the slab will bend more at the joints/cracks as load goes across them and more cracks will develop.

 

[fattdad]

what Ron said.

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!

 

[hokie66]

Just a bit more on your aggregate interlock query. Ron is correct, but with enough reinforcement, the cracks can be kept narrow enough so that aggregate interlock is maintained. Not the case with the usual WWF, but with about 0.6Ag reinforcement, then the cracks stay narrow.

 

[hokie66]

That should be 0.6%Ag.

 

I'd like to go back to the concept of a jointless floor for this topping. (This is usually done for a suspended slab right? It is apparent that the joints are of major concern. I realize that doweling the joints may be ideal, but for such a thin top slab (3-in), I fear allowing any movement will only add to the troubles.

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.

 

[cvg]

I'm not sure there has been any recommendation for a thin topping slab. Recommend either complete remove and replacement or a thicker topping slab, probably about 6 inches thick. Note that it would be cheaper to increase the slab thickness than it would be to put in "heavy reinforcement" and likely would be more successful.

 

I mentioned the 3-inch topping slab from the beginning. Is this thickness utterly out of the question? (This is what I've essentially already told my client).

 

[cvg]

you've got 5 thumbs down for the thin topping slab. Again, increase the slab thickness, eliminate the heavy reinforcement, maybe or maybe not put in dowels, sawcut and finish properly and you might be able to make it work.

 

[hokie66]

I'm the only one who has suggested the 3" topping may be worth the risk in this case, provided the risk is not yours. And yes, if I were the owner and decided to go this way, I would heavily reinforce the topping and provide no joints. Clean the bottom slab, probably by a whip blast, chair the bars, place the overlay, wet cure the slab for a week, and hope. Again, make sure the risk is not yours. As to the price of doing it, that would have to be compared, but I think the reinforced topping would be cheaper in an existing facility.

 

[fattdad]

. . . and pump the concrete (well, don't tailgate). You'll need a good mix design.

f-d

¡papá gordo ain’t no madre flaca!