Adding a footing to the existing slab - dowels? vapor barrier?

[RabitPete]

When adding a thicker, reinforced footing in a middle of the existing PEMB slab, any pros and cons of tying new footing to the old slab with rebar dowels? There is a granular fill under existing slab, and it always makes me nervous, as excavation results in gaps under the old slab around the perimeter.

Also what are the thoughts on a vapor barrier? Add vapor barrier to the bottom and ignore sidewalls?

 

[SwinnyGG]

excavation results in gaps under the old slab around the perimeter

Assuming this footing is earth formed... properly placed (ie correctly consolidated) concrete will fill these voids as the footing is poured.

 

[dhengr]

RabitPete:
I’ve seen contractors salvage the existing vapor barrier by saw cutting for the new ftg., but not full depth of the existing slab. Start breaking up the slab at the middle, because you will ruin some of the vapor barrier (v.b.) as you open the hole. Then, working outward you can usually take care to protect the vapor barrier in the process, this is mostly hand and small tool work. Cut corner to corner slits in the v.b. and fold it back on top of the existing slab. Dig and line the new hole with v.b. and tie and tape the two v.b’s. together, down into the hole. Make the new ftg. large enough so you think you have managed any differential settlement btwn. the new ftg. and the slab.

 

[RabitPete]

Dowels will probably not help much. In case of a differential settlement they will simply break out of the existing slab, inspected the worksite today and the slab is only 4" to 5" thick