2 post lifts

Big difference in strength between acstd. four bag mix and a six bag. That's the number of bags if cement in one yard of concrete. The size of the aggregate in the concrete is also a factor.

I actually had my concrete guy pour a 12" thick area when I was building my shop. But when I bought my lift the base wasn't anywhere near where it needed to be that's one reason four the shallower pour under my lift. That and what was recommended to me by the concrete guy and the refi mix company.

I worked on a concrete crew doing big subdivision work in 1969.... One summer was enough. We would pour 4-6 driveways and a couple basements in our mid summer days....14 hours. Wow...