I recently laid a concrete base for a large shed, I decided to delay the job due to the 33deg weather. The concrete company were extremely helpful. Re-arranged it to a better day 18 to 21 deg. Slab wes 26ft x10 ft and I still struggled by the time I got to the end . How much are we talking about when you say your builder didn’t level out the slab -a few mm or is it a lot more ? Is it generally uneven all over? Photos would help . However as with other posts with similar issues to yours you need to sweep the entire slab to remove loose dust and particles. Seal the floor with SBR and let it dry . From this point you have a few options- levelling compound standard or laytex, there are several other types of floor treatments but most are expensive and can be tricky to apply. My choice for this issue is a 1.1/2 or 2 “ layer of floor screed ( concrete sand / cement mix ) as it’s a large area split it into manageable sections and apply the screed . As the screed is very fine bits of grit and not large aggregate you can achieve a very flat surface, it can be then floated with a trowel to a finer surface. I think your builder could of taken steps to prevent your situation in the conditions he was working in . Was it his decision to lay the floor - did he explain that the concrete was likely to set before he finished the floor - did he advise to lay concrete on a cooler day - or was it your decision to go ahead with the job. I ask all this because your builder could be held responsible for your uneven floor and as a result should put it right if he took the decision to go ahead given the weather conditions. Hope this is helpful.
