Topping Slab for Mat Foundation

[smwa]

Hi All,

I'm designing a mat foundation with a 5" topping slab. The topping slab is added so the architect can depress the finish elevation for walk-off mats and restroom tiles, and also coloring the topping slab w/o coloring the entire mat.

My question is: Should the topping slab be poured directly on the mat foundation, or should there be a slip plane between the two, so the topping slab can shrink as it cures.

If the topping slab is poured directly on the mat, should I provide more contraction joints than the typical 15ft spacing I use for slab-on-grade?

Thanks in advance,

SMWA

 

[hokie66]

There should be a slip plane between the structural and non-structural slabs. Plastic sheets are often used. Is there a membrane under the mat? If so, be careful about too many vapour barriers.

 

[smwa]

Thanks for your response hokie66. Yes, there will be a 10mm vapor barrier under the mat foundation.

SMWA

 

[hokie66]

10mm vapor barrier? That's thick. What's it made of? Is it a membrane against hydrostatic pressure?

I'm not knowledgeable about vapor transmission, but would consult with someone who is before you sandwich the mat between two impervious surfaces.

 

[smwa]

hokie66,

Sorry, I meant 10 mil vapor barrier.
I guess the vapor barrier under the mat slab is unnecessary if there is a vapor barrier between the topping slab & the mat. Would there be any reason to have a vapor barrier under the mat?

Thanks,

SMWA

 

[hokie66]

I doubt there is a reason for a vapor barrier under the mat, but there may be need for a membrane. Big difference.

 

[structuresguy]

I would put the vapor retarder (the 10mil product, which should not be called barriers because they are not barriers) between the mat and the topping. Reason being is that it is more likely to be damaged during construction of the mat, due to heavy rebar stands, vibrators, etc... No reason I can see to have it below the mat, heck it might even be under water in that case, depending on your water table height. Here in florida that could definitely be a problem, and a standard 10mil vapor retarder with taped seems would in no way be "water proof". So i think it would be more effective and less prone to damage over the mat.