Thread: concrete over concrete

Had a guy call me about house he was thinking of purchasing. He said that the 30 year old slab foundation house had originally had a "sunken" living room, approx 10-12 inches lower than the rest of the slab. The current owners had brought the sunken area up level to the rest of the slab by filling the sunken area with concrete. He also said he observed some buckling of the wood flooring over the newer pour but did not know if the floor damage was a result of the added concrete or from bad installation, moisture, etc. These are the only details I have.

I'm looking for input/discussion on the possible ramifications or issues with pouring over an existing slab, particularly 10-12 additional inches depth of concrete over an approx. 20ftx20ft area and whether the existing slab would support the new pour (or any other potential issues).

Thanks,

Eric

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The original slab should not be a problem as the new slab and the old slab together are creating a 20' x 20' x 16" deep footing for coffee table to set on.

If compacted properly to start with, that likely is not a problem, however, if they did not wait long enough for that concrete to fully cure and hydrate, the buckling could be from the moisture which was still in that large amount of concrete.

Think of that as the amount of concrete in 3 2-car garage floors stacked on top one another - that's a lot of concrete (by residential standards) and you've got to make sure the moisture is all hydrated before covering it with anything.

Did they say anything about how long the concrete had been left uncovered before installing the wood floor?

Also, what kind of wood floor (nail down on top of nailed down plywood, glue down, floating, etc.) and what type of buckling (entire floor buckling upward, individual boards cupping, etc.)?

Ae you sure they didn't build a false deck over the floor, then concrete it? Or maybe fill it with styrofoam or some other filler then pour the concrete?
I'm having trouble believing anyone would that much concrete in there, given the current cost per yard.

Originally Posted by Jerry Peck

The original slab should not be a problem as the new slab and the old slab together are creating a 20' x 20' x 16" deep footing for coffee table to set on.

If compacted properly to start with, that likely is not a problem, however, if they did not wait long enough for that concrete to fully cure and hydrate, the buckling could be from the moisture which was still in that large amount of concrete.

Think of that as the amount of concrete in 3 2-car garage floors stacked on top one another - that's a lot of concrete (by residential standards) and you've got to make sure the moisture is all hydrated before covering it with anything.

Did they say anything about how long the concrete had been left uncovered before installing the wood floor?

Also, what kind of wood floor (nail down on top of nailed down plywood, glue down, floating, etc.) and what type of buckling (entire floor buckling upward, individual boards cupping, etc.)?

JP: Maybe. This all depends on what sort of original slab and what sort of soil are in play. Uneven loading of a post-tensioned slab cannot be done without the approval of the design engineer, which is not likely to be forthcoming without the addition of piers.

Additionally, you have just formed a huge cold joint for termites to move through undetected so that they can eat that coffee table that sits there all alone.

Originally Posted by A.D. Miller

JP: Maybe. This all depends on what sort of original slab and what sort of soil are in play. Uneven loading of a post-tensioned slab cannot be done without the approval of the design engineer, which is not likely to be forthcoming without the addition of piers.

You are correct - I did not consider floating rafts in my equation, I am used to construction based on the building sits there, come heck or high water and high winds ... well, okay, maybe not high winds - but we keep trying to make advances in that direction.

I understand things HAVE to be done differently in areas where the earth shakes, then you have to rock and roll with the punches or it will all fall down.

But, come on, having the house float on soil by making it a raft?

Additionally, you have just formed a huge cold joint for termites to move through undetected so that they can eat that coffee table that sits there all alone.

What cold joint?

The original slab is still there.

The original slab had a recessed area (the house we had in South Florida had a 15 foot by 20 foot recessed area for the living room, down 6", and the family room was 11 feet by 21 feet recessed down 6". The problem there was the original builder, architect, AHJ, inspectors - everyone - forgot that if you have a recessed area, that the recessed area *IS STILL REQUIRED* to be above grade by the same 6" as the non-recessed area - thousands of homes were built that way in South Florida, then people wondered why the had leaks at grade level into the house at those recessed areas - fortunately, we did not have that problem) - but I digress, there is no cold joint exposed to termites, not unless the termites are already coming in.

What cold joint?

JP: Now I know, that down in Flahdah, the joints probably just get puffed right down to the very ash and never have a chance to get cold. But:

ACI 116R-00, “Cement and Concrete
Terminology,” defines a

cold joint as a joint or discontinuity
resulting from a delay in placement of sufficient duration to
preclude intermingling and bonding of the material in two

successive lifts of concrete, mortar, or the like.

The original slab is still there.

JP: Yes it is. And, that's the problem.

there is no cold joint exposed to termites, not unless the termites are already coming in

JP: Termites are a successful species, unlike us. The are always already coming in . . .

Aaron,

You are totally missing one concept there: There is a solid, monolithic, slab in contact with the soil.

There are no cold joints in contact with the soil.

There IS a cold joint between the original slab and the new in-fill concrete, but, as I stated, that would not be a termite problem UNLESS the termites were already there - which is what I said.

There are no cold joints in contact with the soil.

JP: I Never said there were.

There IS a cold joint between the original slab and the new in-fill concrete, but, as I stated, that would not be a termite problem UNLESS the termites were already there - which is what I said.

JP: Agreed. And, as I said, they are always there. "Solid" does not mean without cracks or fissures. I have yet to see concrete without cracks. Termites move through cracks. They also move through cold joints. When they are combined in the same slab, they eat your coffee table.

(south Florida) excessive recent rains have caused flooding in our 12 inch deep 20 x 18 sunken living room ...we were considering filling it with concrete -this site brought up the following issue that the south florida original (now deseased builder)foundation pourers of my 30 year old home could have neglected to considered pouring extra concrete fooing under it ...(my sister suggested just gunniteing the room and making a 12 inch deep pond for maybe catfish)... they poured main floors and and maybe as a secondary pour the living room...there appears to be a seam all around the edges...(in the past have used some rubbery water stop stuff to fill top of the crack)... yesterday we dug down looking for tree roots or big crack in the 10 feet by 15 feet planter box that is against house...the house wall appears to be concrete block with stucco on about one inch footing on nice white sand ....bottom of footing is three inches or so above top of living room concrete floor..so when water level is really high, the water runs under my entry through the white sand and comes out the interior side of the living room...I wondered why the water coming in was so crystal clear.. so I'm thinking first to dig down about three more inches and put concrete strip maybe 6 inches deep by four inches wide..at footing level.across the planter box area...sure would save some concrete...if the house was done right...how deep should that one inch footing have been ( in south florida)...I have a makeshift temporary sheet of block plastic and bricks filling the dug up area....????have any of you seen an alternate way to address our issue?????...

The best way to address that is to break out the concrete slab in that sunken area, bring in clean sand fill, compact it, termite treat it, install moisture barrier, WWM, dowels into existing slab where it turned down to the sunken slab, then pour it with concrete to the level of the rest of the house.

I used to live in South Florida and told many builder (while working for them), many clients (while doing my inspections) that not only is it stupid to building the house at the level and then dig DOWN into the water table for a sunken area, but that it is also against the code and FEMA requirements for lowest floor level (it was both of those, but many did not understand that).

The correct way would to have had that sunken area at the lowest floor elevation and above grade, THEN bring in fill and raise the rest of the house up - but the argument was that would cost a lot more.

Yeah, it does cost more to do it right most of the time.

What city are you in?

I always also wrote up those planters against the house wall as you WILL get leakage into the house through the planter, but they were VERY common for many builders to install them. The best way would be to leave an air space between the planter and the house, that way the planter could not leak into the house.

You really need to hire a contractor who can cut out the old slab and do it back the right way.

i agree with Jerry

hi JP...Thank you for your speedy professional and informative reply...in retrospective I was young, the house was cute..we live in the city of Sunrise...from day one we've had evil water spirits (just kidding)/interesting problems to solve...first was water coming out under baseboards under all the windows...someone just neglected to put any bottom screws in the windows....a plumbing leak that involved jack hammers ocurred because my copper pipes under the house are on sand...(you know that codes been changed)... the other planter box with no drain holes was high and on side of bath tub wall... when the tiles started falling off and some clay looking wall board got moldy...I chopped out the bottom and there was bare concrete block with poor mortar...a few of my chop outs are still that...we're retired...financially some solutions will have to take a back burner...chopping out that slab is a different solution that we will give consideration to.... pegg

Originally Posted by pegg webb

we live in the city of Sunrise...

Yep, I know what you mean about those houses - Sunrise thought they were the toughest for building and inspections, but it was all in their minds. Plantation thought the same thing, as did Parkland, as did ...

I lived in Pembroke Pines for 20 years and inspected all over South Florida, worked for developers in Cooper City, Sunrise, Plantation, Parkland, Coral Springs, etc., before I started inspecting.

Found that the largest developers made the biggest mistakes, and thought they knew everything.

As a youngster in NY state over by Buffalo we knew some Webbs - any family in that area?

Some get water on floor/slab of addition-back porch.....
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This HO was getting water on potch slab because there were open joints
and cracks/crevices on the outside,BELOW ground level....the left side.
Adding soil/raising the grade or longer downspout extensions did not
solve the problem because this stuff doesn`t seal the cracks/joints where
the water enters.
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