[Porsche-O-Phile]

Ouch, do you actually have to cut the slab to get at the piping? If so, that's a terrible design. Piping conduits are your friend for under-slab applications. If the monkeys that built the place actually laid down the pipe and poured the slab on top of it, there isn't much you can do short of abandoning the pipe and re-routing the lines or saw-cutting. If you decide to saw-cut the slab (dicey proposition), you could always add conduits for any new piping before re-pouring the portion of the slab, but at that point you're into the job for thousands - not to mention that cutting the reinforcing on an existing slab is never a great idea and should be avoided if at all possible. It's unlikely your slab is acting as a structural diapragm or anything, but it's still never a great idea and should involve consultation with an engineer or architect.

Flexible piping like PVC or thermoplastic is a good idea in an area like this because of seismic considerations (rigid metal piping will tend to spring leaks easier in a quake). PVC is a decent choice (if permitted) as it tends to have a bit more "give" to it - it's more flexible. If you use PVC, make sure they use a "tracer wire" on the piping so you (or the next guy) can find it 15 years from now. It's kind of hard to find PVC pipe with a metal detector.

I'm not sure how good the spray-on type "fix-it" stuff holds up over time. It might be good or it might just be a "band aid" type fix - I'm not terribly familiar with it (I can do some checking). I suppose it can't hurt to try as a short-term fix, right? If it holds, it holds. What kind of piping do you have now anyway? Copper? Steel? It's hard to make any sort of recommendation without knowing exactly what you've got.