Footers under exsiting slab

[ErickD]

I have a project that calls for the installation of footers under an existing 4-5 in slab.

I have done this before (did a trench under the slab and the fill with concrete) but the problem then was that with shrinkage there was a small gab between the footer and the slab and I was wondering what good the footer is doing if it is not fully supporting the slab.

1. Am I too paranoid
2. Is there is a way to pour the footers so that it will really provide support to the slab
3. Anything else.

Thanks

Erick

[dave_k]

Re: Footers under exsiting slab

Am I missing something here? i just trench to the bottom of the footing depth then finish the to the top of the slab. The concrete does shrink a little bit however a good finisher knows this and orders a rich mix that can be finished at a low slump and can generally nail the grade flush with the existing slab.

[ErickD]

Re: Footers under exsiting slab

In both case the footers are completely under the slab (no increase in slab size)

[James Eggert]

Re: Footers under exsiting slab

As Dave said...what's wrong with removing the slab where someone has specified adding footings, excavate to the required depth, then fill completely back to top of slab?

Are you adding footings for carrying loads above the slab, or are you adding a footing to support the slab itself?

[ErickD]

Re: Footers under exsiting slab

The footings is to carry additional load. In this case the room is an aluminum sun room and they want to enclose it (frame) and add a truss roof. The load is not great but on a frame house you are require some footing.

I think what Dave said makes sense. Thanks for the input. Erick

[Allan Edwards]

Re: Footers under exsiting slab

Why not remove the 4" slab and replace it with a well reinforced new foundation with proper beams/footings and rebar. Removing a 4" slab is fairly inexpensive, trying to underpin a foundation is labor intensive, seems it wouldn't be that expensive to just start clean.

Just a thought.

[NW Architect]

Re: Footers under exsiting slab

What Allen said.
But back to the original question, yes, there will be a micro-space between top of footing and underside of slab. The objective id to keep the slab from deflecting any MORE than the dimension of that gap.

[dave_k]

Re: Footers under exsiting slab

yes, there will be a micro-space between top of footing and underside of slab. The objective id to keep the slab from deflecting any MORE than the dimension of that gap.

That's interesting. I've never heard of that being an issue. Different regions different problems. If an engineer wants to keep the slab from bonding to a footing they will spec a bond breaker.

Though it's not required it's common practice up here to dust the ftgs on the exterior basement walls with sand or a layer of stone then use flexcell around the perimeter. This acts as a screed but it also allows any condensate to drain into the drainage layer.

It's a nice thought but I don't know how effective it really is.

[robinnc1]

Re: Footers under exsiting slab

I'm I missing something here? Why not pour the footings and slab all in one?