Raised Concrete Slab

10 Replies

I have found a potential investment property in Charleston, SC and came across this during the walk-through (see attached picture).  Has anyone ever seen a concrete slab that has been raised?  The house is in a flood plain, so I can see a benefit to using concrete over a traditional wood supported floor when moisture/rot is an issue.  However in the picture, there is rebar exposure in the bottom of the slab between the supports.  I know the house isn't going to fall in tomorrow, but at some point this issue will need to be addressed.  How would you go about repairing or replacing something like this?  I know I could patch the concrete, but I would assume that is a very temporary fix.  Has anyone else dealt with this?

Thanks BP Nation

Wow, great picture for discussion! I have seen this for 2nd story and above but for the ground floor WITH a crawl space, nope. I would be concerned for a number of reasons but situations like this always mean an opportunity. This situation calls for a structural engineer and not a home inspector. If time allows, look up a local engineer and ask him/her for their help. a few hundred dollars may yield information that would allow you to go back to the current owner with structural remedies. Include this report with your offer. Good luck!  

This does not look good. It looks like someone got the jacking wrong and then mislaid the rebar. Or it could be a failed post tension system but this highly unlikely as Post Tension tends to be in commercial construction. 

For reading on how this is supposed to be done check here

I suggest calling Kelly Powell at Kelly Engineering to see if he can take a look. But there is no guarantee that this is savable. 

That doesn't look like rebar. That's wire mesh instead. When they poured it they didn't bring it into the center of the slab but left it on the bottom.

@Mike Reynolds nice catch! Just curious on your thoughts on this. I usually thought elevated slabs were built with block and a draining gravel supporting the slab. Have you seen something like this where someone pours a slab over a pier system and then use a jack system to allow it to set? 

Could be kind of an interesting application...What do you think? 

@Jeffrey Stasz

 No, I havent seen anything like that before. Keep in mind that I have set forms as far as thirty feet below ground and as high as 300 feet in the air. I was never a finisher because a commercial carpenter usually didnt do that but on some occasions. Back in the old days we used plywood between the I beams. Shored them up and poured on top of that. That doesnt look like what happened here. It almost looks like they poured a regular slab and raised it up. 

I would have sure quit my job if they had asked me to put those piers in. 



Originally posted by @Jeffrey Stasz :

@Mike Reynolds nice catch! Just curious on your thoughts on this. I usually thought elevated slabs were built with block and a draining gravel supporting the slab. Have you seen something like this where someone pours a slab over a pier system and then use a jack system to allow it to set? 

Could be kind of an interesting application...What do you think? 

That reminded me of a remodel I did twenty or so years ago. I went under the house and the piers were Cypress stumps. Nothing else. I asked how old the house was and they said it was built in 1865. I remember the date because I was thinking it was built right after the war. This house was so unique. The windows were only 8 inches wide and floor to ceiling tall. They pivoted just so that you couldnt get a straight shot with an arrow aimed at the bed when they were asleep. And you couldnt crawl inside either but would let a breeze blow through. I wish I had took pictures of that house but the stumps are still there and not rotted at all. 

That is odd, no poured beams over the piers. Looks like rebar, about 1’ spacing. I assume it has two mats of rebar, one top, one bottom of the slab. The deteriorating rebar is an issue, as that bottom mat is under tension.

I know these piers are not proper in CA. When there is a lateral movement (CA) the slab above falls. I would think one need to build more complaint posts all around the existing posts strap together and add more support on flare up for added support.  The pier in front most likely is tilted. This is definitely a foundation contractor challenge. Most GC will not want to  be involved. A structure PE approval should be on the permit application.

@Tj Estes , this is only my opinion so you'll have to seek out the advice of a structural PE in your state. The slab on grade concrete foundation is designed for compression and uses minimum rebar to control shrinkage and temperature cracks. From the photo, the exposed rebar appears to be welded wire fiber (WWF) which is typical in slab foundation. Raising the slab and placing it on top of piers introduces bending and one-way and two-way (punching) shear into the equation, which the slab was not designed for. Furthermore, I doubt that the pier is physically tied via post-installed anchors so there's no lateral resistance or moment transfer. I beg that you don't buy this POS since the only fix is to either bring the slab back to grade or throw crap tons of steel members underneath the concrete and create a diaphragm to transfer the loads correctly.  If you have any engineering related question, feel free to ask. 

Update:

I decided to pass on the house. @Victor Evans , I had the same line of thought as you on this one.  The numbers on the house itself worked out for my team, but we likely would have blown our whole rehab budget experimenting on a proper fix for the floor.  I have recently found out that someone is buying the house, and I was told they haven't even checked under the house.  They may have their hands full on this one.  Anyways, thanks to all of you for your responses!

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