Slab on Grade Concrete Cover

[jbjones2001]

In the area where I practice we often are required to use structural (pile supported) one and two way slabs (7"-10" reinforced) on grade due to poor soils and liquefaction concerns. We have attempted to reason using less than 3" of cover many ways in an effort to cut down on the volume of concrete required. In the past we have used mud mats (working slabs)to create conistency of rebar placement and improve construction quality to justify reducing cover to 2". We recently have debate using biodegradable void forms so that the concrete will not be cast directly against grade. This leads me to a couple of questions:
1) How high off of grade would the slab have to be to justify only using minimal slab cover.
2) Have any of you used void forms in such a scenario?
3) Do any of you use working slabs to justify a reduced cover?

Any comments are greatly appreciated

 

[AJUK]

Have you thought about using a lean mix blinding to the excavated soil surface thus allowing a reduced cover? In the UK, we also have a rule that rebar in RC concrete cast against the ground should have 3" cover. This can reduce to as low as 1.25" depending on exposure and concrete grade.

Lean mix is much cheaper and also provides a clean, stable working platform for steel fixing.

 

[CTW]

I've pondered this same question and came to the conclusion that if you use a layer of stone you can reduce the cover. ACI uses the term "earth" which in my opinion means any material other than soil or the insitu material. So I would say (and have done this many times) that a cover of two inches would be sufficient as long as there is some material other than soil that the slab is cast on.

but to answer your questions:
(1) i would think that a vapor barrier with a layer of stone would allow you to use a minimum cover of say 3/4". this way the slab is not cast against "earth".

(2) can't help on the void forms.

(3) i think any material you can use under the slab (especially the lean mix mentioned by AJUK) other than soil will allow you to reduce the cover.

it's just going to be a judgement call. i would feel leary of using a cover less than 1.5" just because the actual cover after the bars are placed may vary by 1/2" or so.