Thread: How To Fill In Sunken Area in Slab?

I have a (former) kitchen on the ground floor that was "sunken" into the slab about 6". Area is about 8' x 8', by 6" deep. Looks like it was all poured that way originally. The slab under the kitchen looks like it's in good shape, no cracks, no moisture. The rest of the larger slab is in similar sound condition.

I want to fill in the sunken area so that entire ground floor is one level slab. How best to do this?

I assume fill with some broken concrete and pour over it. Do I need reinforcing bar or mesh? Do I need to treat the existing concrete in any fashion to help the new concrete adhere? Or do I want it to NOT adhere for any reason? Is there any other acceptable fill that is easier to bring in than wheelbarrows full of broken concrete?

Thanks!

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Make sure the slab has finished sinking first.
Check the drainage around the house.
Forget the broken concrete part and use a wheel barrow or two full of readymix.
Check out etching the old floor with a chemical cleaner before the pour.
But I advise # 1 to talk to an expert concrete worker before you start.

Originally Posted by John Kogel

Make sure the slab has finished sinking first.

John,

The OP said the sunken area was poured that way.

Originally Posted by Charles Zito

Looks like it was all poured that way originally.

Originally Posted by John Kogel

1)Make sure the slab has finished sinking first.
2)Check the drainage around the house.
3)Forget the broken concrete part and use a wheel barrow or two full of readymix.
4)Check out etching the old floor with a chemical cleaner before the pour.
5)But I advise # 1 to talk to an expert concrete worker before you start.

Originally Posted by Bruce Breedlove

John,

The OP said the sunken area was poured that way.

Charles,
Skip steps 1 and 2 above, start at step #3.

At 6", you can fill with wood framing at a much lower cost. However, it will feel different when you walk on it, so if that is important, then you are looking at concrete, gypcrete or something similar.

Originally Posted by Bruce Breedlove

John,

The OP said the sunken area was poured that way.

Yep, I meant formed and framed that way, rather than cut out of the slab later. It's definitely a square, stepped area 6" below the main slab -- not an area that has dropped or sunken due to deterioration.

Call a tile flooring installation company, a good one. They use a "self-leveling concrete" for that purpose, and I can testify that it works extremely well. Just don't know what it's called.

Floor leveling concrete without stone are not made to be poured 6 inches thick. They are not strong enough. At least that I'm aware of... Just use regular concrete and add an expansion joint around the perimeter.

It sounds like slab on slab, wood framed would be the least expensive, but if it's concrete you want 6x6 mesh is necessary.

BTW that's 1.2 yards of concrete, it can be done by hand but preferably with a mixer.

Charles,

As some have already mentioned, framing in the area with dimensional, treated framing lumber (secured properly to the existing slab) and decking the area with glued and screwed down plywood will be the most cost effective. It can be made very sound if installed properly and leveled to the surrounding slab. It really depends on the floor covering that will be going into the area. If carpet, wood or laminate, I would just do the wood framed method.

If going with a hard good like tile or marble (or even vinyl) I would put in a 2" pad of clean sand, expansion joint around the perimeter, then just pour a stiff mix (as stiff as possible- stiffer means harder when cured). It would be easier just to call a concrete mix company out, pay their min. cost (usually a 2-3 yard minimum- but you only get what you need- less than 2 yards). The easiest way to get it placed would be to contract with a pumping company (usually a 1 hour minimum- $75-$100) and have the mix company deliver a pump mix (3/8" rock instead of 3/4") and have the pump guy place it for you. Wheel barrowing 1.5+ yards is allot of work. As one poster has already mentioned, adding a 6"x6"x#10 wire mesh can help in minimizing cracking (make sure it is pulled up into the placed concrte and not tramped down). You can also order you concrete with fiber-mesh instead of wire (material mixed into the concrete at the plant)

If you don't have concrete experience you should hire someone who knows how to rod off, tamp, float and finish your floor. That will ensure the flatest, best finish for the floor. You can come back later and fill the expansion joint area with a joint mastic or just level/ fill with a concrete floor patching compound. That can be left to the floor layers before they install you floor.

Good luck.

I would simply add and compact two inches of fill sand to create a break between the two slabs and reduce the amount of concrete needed. After all.. the original floor is probably 4" thick anyway and has stood the test of time from the sound of it. I agree that commercial concrete is probably the best and fastest. A One guy wheeling and one guy finishing would make short work of it. I would probably not use expansion around the perimeter since this is a controlled environment. Movement of the floor is probably non-existent. A treated framed floor would work nicely, but wouldn't be as solid as Mitchel pointed out if you plan on a high end tile floor. Mesh and fiber in the pour would be fine, but probably overkill.

Consider slabjacking . Nobody pours a concrete pad sunken 6".

Sunken kitchens and other "sunken rooms" were a design fad back in the 50's and 60's. Just ankle breakers really. I still see them once in a great while.

I would go with the wood framing idea. It is less costly and could be done in a day and ready for use in another day, or a weekend project.

Will you need to move any electrical?

Originally Posted by Mark M Johnson

I would simply add and compact two inches of fill sand to create a break between the two slabs and reduce the amount of concrete needed. After all.. the original floor is probably 4" thick anyway and has stood the test of time from the sound of it. I agree that commercial concrete is probably the best and fastest. A One guy wheeling and one guy finishing would make short work of it. I would probably not use expansion around the perimeter since this is a controlled environment. Movement of the floor is probably non-existent. A treated framed floor would work nicely, but wouldn't be as solid as Mitchel pointed out if you plan on a high end tile floor. Mesh and fiber in the pour would be fine, but probably overkill.

Considering the low cost of expansion material, eliminating it from this project would just be stupid. The concrete you pour is sure to expand and contract seasonally and put pressure on existing concrete causing cracking. Put it in if you pour concrete. Also, inclusion of fiber-mesh is minimal cost (maybe 3-5 dollars per yard. Well worth the investment to minimize cracking.

Do they use special expanding concrete in California? Concrete does not change dimensionally with the seasons. It moves with frost and soil conditions but this is an interior pour so there will be no movement. Expansion joint is not needed.

Originally Posted by Steve Duchene

Do they use special expanding concrete in California? Concrete does not change dimensionally with the seasons. It moves with frost and soil conditions but this is an interior pour so there will be no movement. Expansion joint is not needed.

Holy expanding concrete Batman!!! Where did you get that information? I guess expansion joint material is sold only for totally bogus reasons. Guess I'll stop using it cuz we don't sell concrete that expands, or contracts, in California.

I think it would be interesting for Steve D. to explain why in the Spring and middle of Summer here "in California" (no threat of freeze/ thaw in my area and poured on a sandy loam with no expansive soils) when I pour concrete (flatwork, foundation, regardless of size) it is subject to cracking, if I don't put deep joints in or saw-cut it at proper directions and intervals (which only control cracking). Or why it might be customary to place expansion joint material at concrete to concrete joints. Is this only for during the curing process, or does it come into play even years down the line and all time spans in between. Why?

Please explain all concrete knowing one.

My only concern with the wood framing could be moisture issues unless it had ventilation. We have framed wood floors over existing concrete slabs (garage converted to living space) architects and ahj's were very rigid about meeting the designed ventilation requirements.

The up side of wood framing is it would make a great hiding place for those valuables.

Originally Posted by Mitchell Toelle

I think it would be interesting for Steve D. to explain why in the Spring and middle of Summer here "in California" (no threat of freeze/ thaw in my area and poured on a sandy loam with no expansive soils) when I pour concrete (flatwork, foundation, regardless of size) it is subject to cracking, if I don't put deep joints in or saw-cut it at proper directions and intervals (which only control cracking). Or why it might be customary to place expansion joint material at concrete to concrete joints. Is this only for during the curing process, or does it come into play even years down the line and all time spans in between. Why?

Please explain all concrete knowing one.

Concrete is at it largest point when it is poured. As it cures it shrinks. The expansion joints are really more of a control joint.

Originally Posted by Scott Patterson

Concrete is at it largest point when it is poured. As it cures it shrinks. The expansion joints are really more of a control joint.

Case in point.

From the Concrete Network .com:
Contraction Joints

Now let's talk about what are arguably the most important type of joints—certainly the most likely to cause problems. Both isolation and construction joints are formed before the concrete is poured; contraction joints (or control joints) are "placed" in the fresh concrete before it has a chance to create its own joints—also known as cracks. What a contraction joint really is in the end is a crack in the slab that we force to follow a line of our own choosing. We create a weakened line across the slab and let nature take its course. When the slab does crack, that's called "joint activation."

Slabs crack directly beneath saw cut contraction joints and transfer load between panels by aggregate interlock.

Here are a few things to consider about contraction joints:

• After concrete is placed it is going to shrink. We can reduce shrinkage with good mixes, but it is always going to shrink and we need to accept that fact and know how to accommodate that shrinkage. A smooth, unrutted subbase and a moisture barrier directly under the slab reduce friction between the slab and the subbase and reduce internal restraint.
• Contraction joints are formed by saw cutting, by tooling a joint with a grooving tool, or by inserting a plastic strip into the concrete during finishing (zip-strip). Proper timing and depth of cut are essential. If you wait too long, the slab will crack where it wants to rather than where you want it to. And if the joint is not cut deep enough it will not create the plane of weakness needed to guide the crack.

Take in consideration if there is any supply or drain pipes under the slab. Would they bare the weight, probably, would they be hard to get to after the pour if need be.........probably. Need to add any outlets be a good time to lay some conduit.

RUSSEL,
IT IS CALLED "ARDEX".
IT'S TO LEVEL AREAS THAT ARE 1/2 INCH LOW. NOT PRACTICAL FOR 6 INCH SLABS.
IT'S EXPENSIVE AND SETS UP QUICK, BEST TO HAVE A FLOOR INSTALER DO THE WORK.
yeah the caps were locked and i'm not screaming, now or before.

Had a very similiar situation at my own home and I chose to lay down pex piping (for radiant floor heating) and then poured Gypcrete product over it and I will tell you, I dont regret it for a minute. The radiant floor heating works like a charm and the Gypcrete product is a, almost perfect, self leveling product. Also Gypcrete is made for radiant floor heating systems. Good Luck

Concrete is not poured it is placed. Using the correct terminology will sometimes bolster peoples faith that we are knowledgeable on the subject. It is just a trick, I know, but we all use tricks to pad our image.
Please do not take offense. I am trying to bolster everyone's image with their clients.

Leigh,

So far no one has called it "cement". We ARE making progress!

Originally Posted by Leigh Goodman

Concrete is not poured it is placed. Using the correct terminology will sometimes bolster peoples faith that we are knowledgeable on the subject. It is just a trick, I know, but we all use tricks to pad our image.
Please do not take offense. I am trying to bolster everyone's image with their clients.

Leigh,
I don't do flat work but I have poured many footings. We back the truck in and pour the concrete in it's place.

Trying to convince people that they do not pour concrete, when in fact that is exactly what you do, makes convincing people there are no sub panels sound like an easy sell.

Leigh you keep telling your clients you place concrete and I will tell mine pour. Who do you think will get the first strange look..........

I have poured it at my place but never placed it at my pour.

One thing I haven't seen addressed is the question of what is the construction of the exterior walls. I would be leery of filling the area with concrete if the exterior walls were wood frame due to moisture issues. If the walls are concrete block or the slab was 6 inches above the current finished floor, it should be easy. I personally would consider placing something (visqueen for instance) between the existing floor and the new concrete if there is any chance that you would want to remove it and go back the the recessed slab in the future. It makes it much easier to remove.

I would put a layer of sand about 2 to 3 inches thick to save on concrete and then fill the remainder with concrete. Also take into consideration what type of flooring you plan to install on top of it. If you pump the concrete, they tend to use a very high slump which leads to more cracking (you can help minimize this by going with a 4000# mix). The ideal would be to use concrete with a 3 to 4 inch slump which does not shrink and crack as much but it is harder to get it into place unless you can get the chute from the truck to the location you are trying to fill.