Concrete Pour Over Existing Slab

[ml504]

First off I would like to say how addicting this site is and that the amount of knowledge I've gained is amazing. I have been lurking around for several months but I'm finally about to begin my project. It will be a 50'x50'x14' shop to house my restoration projects.

My question is this, there is an existing 20'x24' 4-inch thick slab in the center of where my new shop is going. Can I level up the remaining ground to the existing slab height and then pour over that? The plans call for a 4" thick slab with a 12"x12" footing around the perimeter of the building.

Any thoughts, suggestions, or ideas would be greatly appreciated.

Thanks,
Matt

 

[rsanter]

I belive you can as long as the surrounding soil is properly compacted

bob

 

[1320stang]

Do you plan on putting a lift in? You might take and rough up the surface of the existing concrete, drill some holes in it to epoxy rebar into it and tie it into your new slab. Prior to pouring the slab, pour some bonding agent on it as well.

But I agree, proper compaction of the dirt around it is crucial.

 

[BooUrns!]

Can you guarantee that the substrate was effectively compacted under the existing slab? You don't want a crack in the old slab tranfering up into the new floor. You're looking at around 10-12K in concrete alone for your project, are you willing to take the risk?

 

[Kevin54]

You can if the slab is in good shape, but personally I would not do it. If you get any settlement at all around the slab, the weight of the new concrete will let it crack where the new and the old is. You would have to make doubly sure that your base material would be tamped down really well, along with using rebar from the top of the old slab into the new to try and prevent it.

 

[walrus]

if you have to have an excavator to prepare the site for the slab, he rip that old one out in 30 minutes or so. Or if its in good shape I see no reason why you can't pour over it assuming you prepare the surronding area correctly

 

[Ezzie]

I'd smash it up into small pieces with the excavator, backhoe or jack hammer and just leave it in place as a base. Then when you are doing your aggregate layer in preparation for the pour, compact the whole business. This way you don't have to dispose of the old concrete and the small voids in and around of the old concrete will help underfloor drainage just as a coarse aggregate base would.

 

[mpraddict]

I would expect to see a crack in the new slab where to old slab ends. It would be very difficult to ensure the same compaction at both the newly prepped area and under the old slab. I agree with others, bust it up.

 

[ml504]

Thanks for the replies everyone. I am reviewing my options at this point as it looks as though the slab may be coming up.

 

[Torque1st]

Anywhere the old slab is discontinuous, at edges or any existing crack, will increase the stress on the new slab at those locations. Rip out the existing slab.

 

[woodbutch]

From my experience finishing concrete over an existing slab at the same time you are finishing slab on grade is real pita, I'd rip it out. And 50' x 50' not big enough,go 50' x 100'.

 

[mmg440]

Leave the old slab for the entrance / parking area of your new building and move the location of the structure so it won't be over it at all.

 

[Torque1st]

Leave the old slab for the entrance / parking area of your new building and move the location of the structure so it won't be over it at all.

Excellent idea!

 

[WoodzyFF]

Hello,

I just joined this forum and am in need of some advice. I recently purchased a home with a detatched garage built in the early 1970's. The garage was built on a concrete block foundation that ranges from 4 to one blocks tall. (Uneven bedrock terrain) It looks like the garage was built on top of that and then a slab poured inside as there is a 1/2" to 3/4" gap between the slab and concrete block. The walls sit on the concrete blocks.

I plan on tearing the garage down in the spring as it is to far gone to salvage. Can I repour on top of the existing slab and tie in the foundation walls? I have never seen this construction before and I'm curious if it is okay to build a new garage on. (I'm thinking of putting a second level apartment on top if the foundation is sound.)

Sorry there are no good detail pics right now but I will post some in the next couple days.

Any help/ideas/questions would be greatly apprieciated.

- WoodzyFF
Fort Frances, ON

 

[Stargeezer]

RE: Pour over. I had this done recently. My concrete mason would not do this unless the new pour was at a minimum 4 inches thick atop the older pour. For me this worked and he did it 18 months ago and it is still perfect and uncracked. So I guess you could if you take out the extra thickness insurance.

 

[timewarp]

Unless the new slab is the same size or smaller than the old slab you will have cracks at the edges of the old slab, and any cracks in the existing slab will transmit directly to the new slab if poured directly over it. I have been told that you can avoid the transfer of cracks to the new slab by a layer of sand at least 2 inches thick on top of the old slab.

 

[Charles (in GA)]

If the slabs will be exactly the same size, I would do it if the old slab is in good condition (no uneven cracking or shifting, just hairline cracks with no vertical shifting of the slab) I would use a cement bonding agent to assure adhesion to the old slab.

Slabs are expensive to remove and dispose of, this will raise the building by 4 inches and you won't have that removal expense.

Charles