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Concrete Topping Slab Over Concrete Slab On Grade

Concrete Topping Slab Over Concrete Slab On Grade

Concrete Topping Slab Over Concrete Slab On Grade

I am working on a project that involves adding office space to an existing 14' tall single story building.  The building is about 60 years old constructed of c.m.u. block walls.  The plans indicate demolishing an existing 6" exterior slab on grade (approximately 40' x 12') and replacing it with a new 4" concrete slab as this area will be part of the new interior office space at job completion.  The contractor wishes to leave the slab in place and pour over it up to the new interior top of slab elevation.  From a structural standpoint, is this a reasonable request as the new slab will be poured over two different substrates (the existing slab and soil)?  Is there any surface preparation that should be done to the existing slab?

RE: Concrete Topping Slab Over Concrete Slab On Grade

It can be done,and structurally it is not an issue.

Assuming you have the appropriate grade matching,etc. worked out, then you can specify a topping slab.

You have two choices for the topping...an unbonded topping or a bonded topping.  

An unbonded topping should be completed unbonded...put a bond breaking membrane between them.  Cut control joints on a very pattern that does not exceed 10 feet in either direction and make the pattern square or nearly so.

For a bonded topping, make sure you use a good bonding agent, prepare the existing slab by scabbling the surface and make your control joints in the new slab match the old. If you have cracks in the existing slab, they will likely reflect through the new slab.

If you are going to have forklift or other traffic on the slab, make sure you design for the stress interface.

RE: Concrete Topping Slab Over Concrete Slab On Grade

I think there shall be a joint, or sill plate, seperating the int & ext slabs. Then follow the advices from Ron.  

RE: Concrete Topping Slab Over Concrete Slab On Grade

One other issue is that of vapor pressure.  The old slab may or may not have a vapor retarder (barrier?) under it.  If the new space is to be an office, with tile or carpet on the floor, then any moisture migrating up through the "on-soil" portion of the slab might be a problem....depends on the dryness or wetness of the underlying soils.  Many times, the moisture content under a buildng grows over time.

If you go unbonded, do what Ron suggests - perhaps consider as your bond breaker a vapor barrier that continues on over the soil supported area.

Where the new slab changes from over concrete to over soil, you might want to detail the slab at this point to take into consideration the different support conditions and accommodate any differential movements that might occur - a control joint or expansion joint perhaps....even consider a thickened slab there.


RE: Concrete Topping Slab Over Concrete Slab On Grade

I think JAE made some important points.  Years ago, I specified a slab on grade for a pharmacy.  Ordinarily, I had been using vapor barrier under grade slabs but the latest fad of the day was that a vapor barrier was unnecessary, maybe even detrimental to the performance of the floor, so I decided to omit it.   

The floor was finished with asphalt tile and about a year later, there were literally thousands of dime sized popouts all over the floor.  These were caused by minute coal fragments in the aggregate which produced methane gas as a result of water migration from below.  The testing company reported that the coal content was within acceptable limits and that the reason for the popouts was the lack of a vapor barrier below the slab.

That was the last time I omitted the vapor barrier.


RE: Concrete Topping Slab Over Concrete Slab On Grade


RE: Concrete Topping Slab Over Concrete Slab On Grade

What about footings and frost heave??

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