David ,you really dont need to thicken the slab for the posts....figure it out! Even a two post lift , take your car weight ,divide by two (for ea post), check your post base, most likely a 12 x 12 plate = 144 S.I.


use car weight sample 4000 Lbs, = 2000 per post 2000:144=13.9 PSI...what kind of concrete are you using? 3000 psi? lol most shops I`ve seen here in town have a 4" reinforced slab, and they are not prepared for special thickening for placings of lift posts...