I'm the opposite on this one. Give me a 35 degree morning every time, because once you're moving and pulling a screed you warm up quick, and the mud gives you way more open time before it kicks. Last July we poured 80 yards on a Walmart pad in 98 degree heat with the sun beating down, and I had two guys puking by 11 and the mix went stiff on us before we even got the power trowel out. Heat like that isn't just rough on the guys, it flat out works against you. Where I draw the line is below 30 with wind, that's when I tell the GC we're waiting.