New Conc. Topping Slab on Exist SOG 1

[LPPE]

I have retail store with an existing concrete slab on grade. The client wants to raise the perimeter of the store (about 50% of the entire floor area) 2.5 inches. Portions of the new floor will be tiled, while the rest will be finished smooth.

I searched the threads for concrete topping slabs with no definate concensus.

My concerns are cracking of the 2.5" topping. Some have mentioned using WWM, others mention using fibers. I'm leaning towards the WWM on chairs, but worried about accurate placement. Fibermesh is tough to finish, and from what I've seen, leaves a "hairy" finish.

Any thoughts, recommendations?

 

[jike]

Check with the tile manufacturer and installer to see if you can use fibermesh. That would be my first choice.

Are you using bonded or unbonded topping?

Take great care in providing sufficient control joints.

 

[LPPE]

Because it's an existing concrete slab on grade and the topping is not serving any special strutural purpose except transfering live loads to the existing slab below, I don't think there would be a need for a bonded topping. I will specify, though, that the existing slab shall be cleaned/power washed with H2O.

You're right on - provide plenty of control joints. I'll hear an ear-full if this topping cracks!

 

[hokie66]

The topping will certainly crack if it is bonded to the existing slab. It has to shrink, and the existing slab will restrain it, causing tension and inevitable cracking of the topping. The width of these cracks would depend on the amount of reinforcing provided. I doubt that fibers would help.

Small cracks in the tiled area should not be an issue if a flexible tile adhesive is used.

I would specify a bond breaker between the two slabs, either liquid applied or a plastic sheet.

 

[LPPE]

Great point, hokie66, about the topping slab shrinking while the existing will not. I will specify a bond breaker, reinforce the topping slab with WWM, and use a 3/8" max. aggregate mix.

Does anyone think a 4ksi mix over a 3ksi mix would be beneficial?

 

[jike]

4 ksi mix will cause more shrinkage.

 

[Rejido]

I think we don't have to worry on shrinkage crack 'cause we can just provide shrinkage reinforcements... What do you think? I think this will already be enough to prevent the topping from craking caused by shrinkage.

engr. rejido

 

[lkjh345]

FWIW, 'success' or 'failure' on this project will be as much about preparing the owner on realistic expectations as on technical issues.

Hopefully you have an owner who is aware that there will be some slight cracking visible in the exposured surfaces no matter what is done.

Providing WWF reinforcing, with a bond breaker and plenty of joints will certainly help keep cracking to a minimun, but I would make it clear to the owner that he should not expect to see a 100% crack free surface when its done. Its just the nature of the concrete to crack.

Unfortunately, over the years, I have found this is not the expectation of all owners when dealing with exposed concrete floor.

Just my 2 cents worth.

 

[LPPE]

lkjh345 - That reminds me of an owner who hated any cracks in his concrete slabs, and at one meeting with the concrete contractor, the concrete guy (with thick Italian accent) says "Concrete - is like a woman - you never know whatta she gunna do next."

 

[cvg]

provide a bond breaker between the tile and the topping slab to prevent the cracks from reflecting through the tile

 

[hokie66]

Rejido,

Shrinkage reinforcement does not prevent cracking, it controls the width of the cracks.

cvg,

You don't want a bond breaker if the tiles are thin set, but a flexible adhesive is necessary. A bond breaker could be used under thick set tiles, but I don't think the OP contemplated a thick set method.

 

[ali07]

I think providing bondbreaker in conrete topping would certainly resulting in a floor sound as topping is only 2.5" thk.

 

[csd72]

2.5" is very thin to put reinforcing in, but as long as it is inside then it is workable. A fine mesh with close spacings would be best.

The bond breaker and plenty of joints is a good idea.

Also:

Specify that there is to be a joint in the topping slab over every existing joint.

Specify that the tiled finishes are to have joints over your slab joints.

I would also specify that the slab be left for at least 2 weeks (or more) prior to tile placement to allow for a lot of the shrinkage to take place.