Existing Concrete Slab on Grade with Cork Board - Overlay with new Concrete?

[CBSE]

I'm working with an Architect to turn a grade school gym into a fire station to house (4) trucks and (1) ambulance. The existing slab is 4" (probably un-reinforced) with 1/4" cork board overlay and then a gym floor on top. The glue holding the cork board down has asbestos in it and needs full abatement if it is removed...this is approximately $35,000. They want to add 4-6" of concrete height to the existing floor for drainage reasons.

Question, would it be feasible to come in with a 4" slab reinforced with WWF and pour it directly on top of the cork board? If not, why not? 1/4" cork board doesn't seam like much, and settlement would likely be negligible?

 

[CANPRO]

They want to add 4-6" of concrete height to the existing floor for drainage reasons

I think you may need that additional thickness for strength reasons as well. Wheel loads from a full size firetruck are no joke. I think you need more info on the existing sub-base and the particulars of the wheel loads before you can make an informed decision on the over-pour. Without knowing the quality of the sub-base I would be making some very conservative assumptions on the slab design...4" slab with WWF wouldn't be cutting it. I don't think pouring on the cork board is much of an issue.

 

[CBSE]

4" is probably light. The last fire-station I did I used 8" slab on grade with #4 bars @ 12" o.c. each way.

I was thinking 4" on top as that would give me around 8" total. The sub-grade is questionable though as I have no information on it. The building was built in 1961!!!

 

[CANPRO]

CBSE, the last slab on grade for a firestation I did was comparable to your previous design. In this case, I don't think two - 4" slabs = one - 8". Considering the questionable sub-base, I think that definitely warrants a closer look.

 

[Splitrings]

I would check out ACI 360, Design of Slabs on Grade. It references three methods of design; PCA, WRI & the COE Method. They are all based on Westergaard Theory. The methods all rely on the strength of the subgrade. Obviously your existing concrete will increase the support provided to the new slab but I am not sure you could quantify that. It may be best ignore it.

Emergency vehicles have axles that are rated higher than commercial vehicles. I have a single rear axle fire truck in my station that the rear axle is rated at 30,000#. Its not loaded quite that heavy but pretty close. Large ladder trucks can have dual rear axles rated at nearly 60,000#. Then there is the question will they support the ladder truck on the outriggers inside the station. These loads will far exceed the wheel loads.

I am assuming you are suggesting using WWF for the control of the crack widths and not strength? Personally, I wouldn't waste the time or money on rolled WWF. Either go with a plain slab with a close control joint spacing or heavier WWF sheets or even rebar along with a longer joint spacing.

 

[Splitrings]

Two 4" slabs don't equal an 8" slab as there is no way to transfer the shear between the slabs. I can't imagine the shear strength of cork is too high.

There are a lot of 6" and 7" plain concrete roads in this country that hold up. If you soils are decent, you might get away with a 6" slab.

 

[CBSE]

Splitrings, thanks for the info. I was thinking WWF for crack control. I think I can probably justify a 6" slab based on what trucks are being housed.

I was also thinking that the existing slab would act like a very well compacted sub-grade. I will keep looking into it.

 

[jike]

You need an 8" slab NOT two 4" slabs. Unless you can make them work as ONE, you would need to remove the cork and bottom slab.

Here is an idea (IF YOU CAN MAKE IT WORK......AND CAN PROVE IT).......

Epoxy vertical dowels thru the cork into the bottom slab at say 12" or 18" oc each way and let dowels project up into upper slab. If you can prove that you can create two 4" slabs that have equivalent bending or shear strength of 7" or 8" slab by testing or analysis........then you are good to go! Good Luck!

 

[haynewp]

I would have thought to remove the cork as the first step without any analysis due to local crushing under the wheels over time.

I have done 8 to 10" fire station slabs using PCASE. You have to know enough on the subgrade, subbase and the number of times the slab is expected to be loaded by the trucks. I would only try to make the existing slab work as composite (and not as a separate member sharing load) with a new pour by roughening the existing slab, epoxy doweling, and epoxying on top of the existing slab before the new pour. That's if I was to count on bending strength of the existing slab and not assume it is a subbase. 4" isn't a lot to dowel into though.

If that much would be spent getting rid of the cork, I would inquire into cost to remove the slab along with it and start new.

The idea of treating the slab as a subbase may work but I have never done this before.

I would want some type of geotech input on the soil below, at least allowable bearing and modulus, before going forward with any design.

 

[CBSE]

I will have to get a geotech involved because it will become an essential facility so I need to verify the existing structure can take the required loads...which I'm not to overly optimistic about.

My thinking with the 4" slab with the cork board now is that I will treat it as a base and not as part of the structural concrete. It seams to be a really fuzzy area and I haven't looked at doing something like this before and thought it would be worth exploring the possibility.

 

[KootK]

Would you still have abatement issues if you were to drill and epoxy dowels through the cork board? I would think so.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Splitrings]

I don't believe it is practical to plan on composite action between the slabs. A modulus of rupture for 3000 psi concrete is around 525 psi or so. That would equate to a shear force between the two 4" slabs of some thing like 5000# per square foot. That will require a pile of dowels and there is the question of whether you can develop the full shear strength with only 4" of embedment.

 

[CBSE]

The abatement requirements are interesting and I haven't looked into them too much (aside from walls). But my perception is that as long as you aren't disturbing more than 10sqft of area, you aren't required to have an abatement contractor in.

I think doweling into the existing slab is not a very good option. I'm not sure I really see any issues with over-laying an 8 or 6" slab (if it will work) and assuming the existing slab is base material. Will the cork board squish? I'm sure it will, but I'm not sure it will squish enough to notice. It's an interesting to talk about the possibility though!

 

[haynewp]

You should be able to get a stress strain compression curve from a cork mfr if you have an idea of the existing product, that curve can give you a general idea of the compression amount even if you don't know the exact product. The dip in the area where the wheels are driven daily may not be that noticeable but could affect the new drainage they are wanting to provide. You may can justify the dip smearing out over a wide enough area given the new slab stiffness. But that should be figured into the new slab design and not assume the soil modulus as the only spring parameter for the new slab.

 

[ztengguy]

Remember also the slab is going to be wet at least once a day when they wash the trucks. As it cracks, that water will get into the space between the slabs, what will that do for the cork?

tear it out and design a proper slab.