Slab Repair 1

[PRE EIT]

Hi I am working on a building that was destroyed by a fire. The building was a garage that had a ton of oil drums in it. Anyways the slab is in very bad shape and i'm assuming it isn't all pre-existing. What is the best way to go about leveling out / repairing the slab?

 

[JAE]

Are you talking about the interior slab-on-grade or the exterior pavement?

If pavement - you are in the wrong forum.

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[JAE]

Or is the "pavement" we are seeing here in this photo the former interior slab-on-grade?

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[PRE EIT]

@JAE Yes the photo is looking at the former interior slab. Notice the former exterior masonry that has been cut to the ground

 

[JAE]

I'd be inclined to remove all the former slab-on-grade, dig down and inspect/repair the remaining foundation masonry and build up from there if you are simply replacing the lost building with a new one.

However, sometimes the hassle of dealing with an old foundation and trying to inspect, fix, repair it to serve a new building is more than the work involved in just tearing it all up and starting anew - free from the constraints of the old.

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[MotorCity]

At first glance, I don't see anything wrong with the slab. I would leave it as is. It may come down to cost, whether or not the insurance company is going to pay to replace the slab.

 

[JoshPlumSE]

I'd get a contractor out there to "pothole" the slab at a few locations (like where you want to see the foundation for the CMU). That way, you get an idea about about what kind of shape the slab is in. This one doesn't look too bad. So, I expect it can be saved.

However, I was working on a project where we thought about spot repairing an existing slab. But, when we opened it up, we realized it was garbage.... literally. They had demo'ed some previous TI work at some point and the debris formed the base of the slab.

 

[JAE]

If they poured a lot of water on it there could be mush for sub grade.

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[BUGGAR]

"The building was a garage that had a ton of oil drums in it." Could this mean a ground pollution problem? This could dictate what you do.

 

[gte447f]

I am with MotorCity... it doesn't look bad to me. I would specify remove any loose or deteriorated concrete and patch with a repair mortar. You could seal and inject any cracks.

 

[BridgeSmith]

I would recommend following JoshPlum's suggestion and take some cores to have tested. We had a bridge where a truck crashed under it. The extreme heat from the diesel-fueled fire dehydrated the cement in the concrete. It looked ok, but it had very little strength. We had to remove about 2" of the surfaces of the columns and concrete girders and shotcrete everything.

 

[oldestguy]

Sometimes a way to detect weakened concrete is with the chain drag method. You go over the whole thing rattling a chain around on the surface and listen for a nice "hard" reaction sound. A dull sound means bad stuff there. The chain should be at least 1/4" rod chain link size. About a yard or more is needed. At the weak questionable areas do your coring.

 

[BridgeSmith]

A chain drag works well to detect delaminated concrete; not so sure it will detect weak concrete.

 

[gte447f]

I like HotRod10's suggestion to test some core samples. It would be interesting to see if there were any correlation between test results and the visual appearance of different areas of the slab after the fire. Not sure if insurance will pay for this testing or not, but it would be worth suggesting.

 

[Teguci]

If the bones are good and you just want a new top surface you could look into hydro demolition. High mobilization cost so it probably isn't quite right for this situation but a good tool in a repair engineer's belt.

 

[BridgeSmith]

Even out here 'in the sticks', hydrodemolition has become our go-to bridge deck repair strategy. With removal of 1/4" plus whatever is loose or weak in the top 3.5", we've been seeing bids around $50/square yard. That assumes there is source for lots of clean water readily available. We generally estimate 100 gallons per square yard.

Edit: For a pad the size showing in the picture, which is the size of one of our smaller bridge decks, you might be looking at closer to $100/SY, accounting for that high mobilization cost mentioned by Teguci.

 

[charliealphabravo]

Hopefully you have some sense of what areas were exposed to fire based on debris or the distribution of fire in the superstructure. Many times the fire/heat doesn't make it down to the slab or the slab is protected by contents and fallen ceiling finishes. If you had spilled oil that was burning on the slab then it is probably a writeoff.

You can do manually sounding (chain drag or hammer) to check for dead concrete. Concrete affected by heat will often be discolored pink or yellow. You may need to have the slab cleaned to see this. Both of these diagnostic methods are described in ACI.

Edit: Just as an armchair opinion, I don't see evidence of heat damage in the photograph.

 

[Ron]

Core the slab. Have a petrographic examination done. Check particularly for carbonation (this can be done without petrography) from the surface, down. If you have carbonation to a significant depth (1/4" or more), you might want to consider scabbling the concrete to a depth of about 3/4" and providing an overlay of polymer modified topping (SIKA 123 or similar). If your petrography shows microcracking to greater depths than the carbonation, it would probably be easier to replace the slab. If you have neither, scabble the surface and provide a competent polymer modified topping (again...SIKA 123 or similar).

This is a large slab area. It would be cost effective to have a proper evaluation of the concrete and would also meet your standard of care for evaluation for future issues if they develop.