QUIKRETE 50-lbs Fast-Setting Concrete Mix Item #: 10437 | Model #: 100450

While I believe this product would work fine for what you want to do, I don't see any reason to go to the added expense of using the red-bag fast setting Quikrete. Why not just use the regular product, since I cannot believe that getting a fast curing time is critical for you project.

QUIKRETE Fast Setting Concrete Mix, like all other concretes, should be used in an application no less than 2” thick – if applying over gravel as you describe, a thickness of 4”, which is the thickness of a typical slab, is more appropriate. Please note that QUIKRETE Fast Setting Concrete Mix MUST be mixed with water before being placed for all applications other than for setting posts.

A thin layer of concrete over your gravel will not work - it doesn't matter if you use the 50#, 80# or high-strength Quikrete products. The thin layer will simply crack and fragment when a load (car, truck, boat, etc) is parked on it. A concrete driveway should be at least 4" in thickness and reinforced with re-bar or heavy steel mesh. Properly done: your extra parking space should be framed, graded, compacted, re-barred, your concrete product placed, screeded, floated, and finish troweled or brush-finished. After the concrete has cured, I suggest applying a waterproofer, such as, ChemStop WB Heavy Duty to help prevent water and road chemical damage.

No, this product is designed to pour dry into a pre-dug hole around a post which you wish to use to support something like a mailbox, fence post or deck post. Your project requires a concrete product which sets slowly and should be at least 5 inches thick when finished and smoothed out to match the surface height of the existing driveway. You should consider using rebar to reinforce the new concrete. Someone at Lowes should be able to explain how to use rebar or they have how to books on projects like this explaining all aspects of this type project.