Anyone lay their own concrete?

My shop is 36x48 with a 4" slab. #2 rebar, 12" on center with two beams and a rail. When I estimated, it came out to around what your saying, yours with just a flat slab should be almost 24 yds ((40x48)/81) will give the yardage. So, for mine, I didn't take into account the extra for the beams etc and the contractor, old dude working with his son, looks at it all for a few minutes and says 34 yds... ouch... 34 put me over 4k which required a cash payment before delivery so I got 33 and hoped for the best. We ended up with about 1/4 yd or less overage. Total cost, all in was 8400 which was about materials x2. make sure you ask for a 4.5" "slump" and don't let them add water to random spots to make it easier on themselves. You'll get a low spot. I had it done when it's was colder so I didn't worry about covering it, but if you have to cover it to keep it wet, use the black plastic. The clear will act like a lense and dry to top too fast.

Moral of the story, get a pro.