Using old slab as base for new slab?

[gearheads78]

I am about to build a new building 40 x 50 over where a 24 x 36 is now. There is not anything wrong with the slab I have and it would be a good thing to get the new building up 4" higher due to drainage. Any drawbacks to doing this way?

 

[IDASHO]

The portion NOT being poured over the existing slab has a much higher chance of settling than the section poured over the old slab.

 

[Gary S]

If you don't disturb the soil around the old slab, you shouldn't have any settling to worry about.
I've done 2 garages in the past 18 years. On both occasions I poured new concrete over the old slab and enlarged it. Neither one has has any settling or cracking problems...........yet.

 

[NUTTSGT]

Seeing you're in TX, I doubt you have any freezing/thawing problems like us farther up north. On the house garage, I utilized the old grage floor and poured right over it. I compacted and wet the stone for the new floor with a vibrating compactor. It did crack, right in the relief cut like it should, that was poured 6 years ago.

 

[gearheads78]

thanks guys...you are right I don't have to worry about the cold. We usually only have a few days a year in the 20's or colder

 

[willymakeit]

We are doing this on a large comm. project I am the super on. There are no problems if your org. slab is good. Use plenty of rebar and a vapor barrier that is placed below the concrete. This also has epoxy coated walls and floors.