There are a lot of variations to pouring concrete and I haven't pour any in decades, but I was a Quality Control Inspector for a Prestress Concrete plant. As has been mentioned, alot, adding water to the mix isn't good for the strength. Also allowing a vibrator to be used to "move" concrete rather than a shovel is equally bad. Concrete mixes contain an air additive to allow the concrete to bend without breaking and when you vibrate it excessively, you loose that. (Don't mistake air entrainmnet for evaporated water).
I had a driveway poured to allow a loaded semi to drive on it, and it was spec'd out at 10 inches thick. The amount of wire and rebar in the concrete helps, but the thickness of the concrete has to be able to support the weight, or you get a cone shaped crack (wide at bottom, to a point at the top of the crack) if you look at a cross section of the concrete. The only way to change that is with thickness. I even asked the engineer to spec out the slab at 5,000 psi concrete and basically it didn't change it enough to warrant the additional cost of the batch. Thickness was the best answer.
The O/P mentioned light trucks with a semi annual propane truck rolling across it. You might get lucky, but in reality, you need to design the concrete slab for the worse case. Otherwise, you are going to have issues. If you get the propane truck across it without cracking, consider yourself lucky and got a decent job on the 6 inch slap. The fact that the propane truck is also turning while it is crossing the slab adds to the stress of the concrete. If it were me, I would consider adding an additional driveway, or widen the driveway enough (the driveway, not the slab!!) so that heavy trucks could use the driveway and not cross the slab. The little trucks are the ones rutting up your current driveway, which are causing the issues. With a wider driveway, the trucks could be directed down the unpaved section and not be an issue.
My driveway starts out at 40 feet wide (unpaved) and tapers to 10 foot wide. I have few issues with it, but if I or my neighbors have a delivery truck come down the road, my driveway is the only place to turn around a semi truck, and it becomes entertaining to watch newbies try to turn around on it. I have recently installed a 60 foot culvert on the other end of my property to add a larger driveway, but I haven't cut the trees to allow access yet. Letting the soil stabilize that I put over the culvert before I make it to where people can turn around on it. (you would be surprised at the number of people who would rather turn around on my driveway, then to use a street that turns off only another 50 feet further.
My suggestion, as far as the apron area, is to go with at least 6 inches, but if you want to drive a propane truck across it, go even thicker or don't be upset when the spring after he drives on it, that it shows cracks. You probably won't see them when the propane truck drives across it, but they will show up after a freezing winter.
David from jax
