Chris
Hi Liam, Concrete is funny stuff. Add ‘extra’ or more water in the original mix than it was designed for and the ultimate strength will be reduced. But wetting or even flooding it with water after it has been placed and started to set is called ‘curing’ it, and is required in most concrete specs, as it increases ultimate strength and resistance to cracking. So may depend on when the rain started, and how heavy it was.
Not sure what you mean by ‘soft’. Of course if you mean after 5 days you can still stick your finger in it that sounds like a significant problem. That can be checked by drilling a core or two (maybe somewhere to be covered later) and compression testing it in a lab. To see whether it’s equivalent cube strength at 28 days meets the strength in the design (usually 20 or 30MPa). If it’s OK then maybe it’s just a surface thing, and if you’re grinding it off anyway not a major problem. If it’s slightly under an engineer may say it’s still adequate for the situation. But maybe you can negotiate a rebate from the builder, or maybe not if he’s been good in other areas. If it’s so low it’s not adequate to do the job, then I guess it has to come out, unless you can come up with some kind of alternative design like putting another slab over the top. To my mind if you test and it passes you pay, and if it fails the builder pays for testing. But that may depend on what your contract says.