• You need to drill the old slab and "glue" rebar dowels into it, at least 4 inches deep. We go 6-12for slabs at work. The rebar should be 3/8 and the glue is a special epoxy from Simpson, Home depot has it for around $30. If you are not truly concerned with doing it the right way and just want the 2 to stay together, then i suggest you dowel every 24" at a skew 4" deep, alternating the directions of the holes. This way the slab wont pull out since dowels are not perpendicular to the original slab, but rather countering each other. If you want it done the right way, you will need a compressor to blow the dust out of each hole, a brush to clean the inside of the hole, the epoxy and the dispensing gun ($50 or so) and a really good hammer drill and bit in order to penetrate deep. GL.
  • If it was my slab, I would not dowel them together. Everything depends on the soil type and compaction of the sub soil and whether or not there is gravel and if so how thick the layer of gravel is. That said, I believe it will be very difficult to achieve the same compaction of the new slab that equals the existing slab and would expect settling to occur over the long haul and if the slabs are doweled together I would expect settling cracking to occur in either the new or existing slab. IMHO keep them separate and use a high quality urethane caulk/sealant in the joint between the two slabs.

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