Pouring concrete over existing slab?

[GPintheMitten]

I have a building out back that was built on a slab. It is about 14x26. The old slab is cracked. I am considering moving my wood working tools out there and want to be able to move them around on rollers. There won't be any vehicles driving or parked in there.

What do you think?

 

[KYErik]

I wonder why the original slab broke? If there is settling underneath, the new slab might do the same. I would also be worried about water getting between the two layers and causing it to heave and break the top slab.

Is there any way to just patch the cracks in the original slab?

 

[Hollow]

Can you not get someone to cut the cracks out and pour in new concrete?

 

[NormL]

I have a building out back that was built on a slab. It is about 14x26. The old slab is cracked. I am considering moving my wood working tools out there and want to be able to move them around on rollers. There won't be any vehicles driving or parked in there.

What do you think?

That's precisely how we did our shop floor. The old concrete was cracked and settled but showed no signs of movement anymore so we did 'er right, I believe. We started by bracing the old building (20x24'), rolling our tandem axle utility trailer in, put some of the weight of the building on the trailer and then went all the way around, cutting the sill plates and about 18" of rot off. Then we drove the building right off the pad and set it down out of the way. The concrete contractor came and put up his forms, put down crushed rock to raise and level the pad so it would no longer flood, put in the plumbing for floor heat, poured the floor, removed the forms, collected his cheque and went fishing. We waited a couple weeks for the concrete to cure a little and then moved the building back, raised it till there was 4' of air under the new sill plates and then put 4' pony walls under that. Now we have 10' ceiling instead of the 7.5' we had before and a wood working shop that does not flood every time it rains. That's the short version of how we did our's. It took us all summer in 2008 and finishing it up with insulation and interior walls took a good portion of the winter. The floor has not cracked nor heaved for us and so building over the old floor was the right call. Since we heat all winter, heaving due to frost will not happen. Good luck with your project.

 

[GPintheMitten]

I suppose I could patch.

I moved a 14x21 garage once. Broke up the slab and driveway and hauled it out. Put up new forms, poured a new slab with rat walls then moved the garage back. I used rollers to move it. It was the talk of the neighborhood. They were all surprised at my success. Later I put a new concrete driveway in.

But I dont want to do that again.

I have replaced all the rotting sills with treated 4x4s on this building. It was built to low to the ground.

 

[Egon]

Go for it. Use a 5 to 6 inch layer of new concrete tied to the old concrete with some rebar in drilled holes.:thumbsup:

 

[EddieWalker]

I disagree with tying the two slabs together. I think you should treat the existing slab as a base and put down a membrane to make sure the new slab does not bond with the old slab. Then if you have any movement in the old slab, the new slab shouldn't be affected.

How big is the existing slab and why did it fail? How hard would it bet to remove the existing slab? Best would be to get rid of what's there and start fresh.

Eddie

 

[2458n]

They make a polymer that serves as a bonding agent when pouring new concrete over old. Need someone who does concrete for a living to weigh in on this as alot of money at stake for making a wrong choice.

 

[Cat_Driver]

The main thing is to lad up the new concrete with fibers. Do some research and you will see synthetic fibers, stainless steel fibers, etc. etc. No problem with the slab your covering cracking moving etc.

High-performance fibers made of PVA, which is short for polyvinyl alcohol, were developed some 20 years ago by Kuraray, a Japanese company. When added to concrete or mortar, the fibers develop a molecular and chemical bond with the cement during hydration and curing. The result: concrete with high tensile strength and amazing ductility whose makeup can significantly reduce a project's steel load.

Technology - PVA Fibers - Concrete Decor Magazine - Vol. 9 No. 7 | Decorative Concrete

I often lay concrete on soft beach sand and never put a piece of rebar in it and drive heavy equipment on it and never cracks. No rock base, no hard pack - nothing. Just fiber concrete.

I got started on fibers in concrete by talking to a professor who came up with concrete fibers to retrofit the bridges in Ca. against earthquakes.

He also make a canoe out concrete, made it light weight and floated it with people in it.

Clubs & Activities | Civil Engineering Technology, Environmental Management & Safety | CAST | RIT

Cal Poly Takes National Concrete Canoe Championship

GFRC - Stromberg Architectural Glass Fiber Reinforced Concrete

 

[GPintheMitten]

I have used concrete mix with fibre before.

I can't remove the old pad, at least not easily. The building was built on the perimeter of it. No foundation other than the pad. I think it was poured thicker on the perimeter to support the walls.

The pad cracks don't appear to have moved in the 6 years I have lived hear. It is a little uneven at the cracks but appears to be stable now. The building is pretty straight.