Concrete slab above cold cellar

[chrisca91]

Hello everyone,

This is not very related to garage talk but I was hoping someone might be able to provide some input!

Last week, a 5-6" concrete slab was poured above a cold cellar in my home. The slab is suspended, the contractor installed wood forms underneath to enable them to pour the concrete. Above the wood forms there is a 2" styrofoam board. The existing foundation wall of the house along the perimeter is providing support to the slab. The slab is not very big, it is approximately 7'x8' and the suspended portion where there is only wood underneath covers about 35ft2. The contractor added rebar but it wasn't very much I felt.

I was doing sounding tests by tapping on the concrete with a hammer and all along the perimeter of the slab it was very solid. However, in the middle where it is suspended it was more hollow which is indicative of a void. Is this a problem or is this happening because the slab is suspended in the middle?

Thanks and I appreciate any input.

 

[ConCretin]

The difference in sound you hear might is probably just the resonance of the structure where it is supported at the edges vs unsupported in the middle. It's actually pretty unusual to get a true void in a slab. Gravity tends to be your friend.

I wouldn't worry too much about the amount of rebar. That is a pretty small slab and would probably be fine without any reinforcing especially considering there is no live load. There's a little more to it with a suspended slab but basically rebar is only there to hold cracks together once they occur and five inches of concrete spanning 8' isn't likely to crack.

 

[kbs2244]

I agree with the above

my grandfather dug out the dirt under a concrete floored front porch
he held the slab up with 2x4s until he got to the edges
then he laid block walls to hold the slab/floor/ceiling up
the room was apx 20lx8wx10h
no insulation, the cold dirt was the whole idea
no rebar in the apx 10 inch thick slab that he had poured on raw dirt 5 years beforet

the thermal mass of that floor was amazing
the porch was on the SW side of tfw house and got winter sun all day
my grandmother still had to put down rugs to prevent cold feet in the winter

that cold cellar is still in use after 150 years

 

[chrisca91]

Thank you guys for the replies, it makes sense and provided comfort.

Please see pictures below with the wooden forms for when the concrete was poured.

I am planning on the leaving the forms on. Some folks experienced mold issues in cold cellars but there are two 6" ventilation holes in my cold cellar that I could utilize for ventilation. I think that's plenty. I'd just have to add some covers on the outside to block animals and such from entering.

Would be interested in reading people's thoughts.

 

[kbs2244]

we never had mold problems under the porch
it seems to me that venting would introduce warm air and negate the whole idea of a cold room