Pour new concrete garage floor over existing concrete pad

Question:

Is it ok to pour new concrete slab over existing concrete pad? I have an existing 20ft x 24ft slab that I would like to turn into floor slab for a new detached garage 30ftx40ft. The existing slab is 4 inches lower than the top of slab for the new garage.

Answer:
Hi,
There are a lot of opinions regarding this matter. I don't agree with the idea that it is best to breakup the existing slab, haul away, and pour a new slab. What is most important for concrete slab's life is a solid base, there isn't any more solid base than an existing slab beneath. If the height or elevation of the new slab going to be an issue, than it's an entirely different matter.

I would pour a 4 inch thick slab over your existing slab and that should be ok. The areas at the perimeter of the existing slabs will have the tendency to settle first. A good compaction will be required. Also, add some dowls at the perimeter of the existing slab about 18 inches on center #4 bars about 24 inches long. These will not only help tie the two slabs together, they will also address settlement issues.

hope this help
Tonga