Have you considered asphalt millings? If you are unfamiliar with them they are asphalt ground off city streets or highways. This is done prior to repaving.
Locally I can get them for $25 per yard delivered for a 20’x20’ pad 4" thick you are looking at 5-6 yards per pad materials ($150). I would triple that number if you are hiring outside labor and doing less than one or two pads at a time.
Depending on your soil type site prep work can be more important than the actual finish surface and can lead to additional costs. For areas that already being used at parking areas I normally would level out the ruts that form using a loader blade, then compact the area. Top soil usually is not a issue.
After digging out top soil and then compacting the subsoil you place 4-5" of loose milling and then compacting them into a relatively hard and durable surface. Ensure parking pad has positive drainage with no ponding. If this does happen add more milling till solved. An easy way to QC a job is with a garden hose. If pools form the job is not done.
It will then take one warm summer till the asphalt in the milling solidifies in an asphalt like material. If the area has people turning slowing over the new milling you will have problem areas. This is due to the car tires turning on the road surface and tearing up the millings.
If/when pot holes do form you can use cold patch that you can buy at any home Depot or use asphalt milling mixed with a little drive way sealer. The second solution is by far cheaper. You can rent a compactor for roughly $100 per day.
Depending on the quality of the milling you can get a surface indistinguishable from asphalt. Quality depends on how old and dried out the source asphalt the milling came from. This in my experience is a crap shoot. I just put a coat if driveway sealer on milling I find sub par.
As for life of the product… This all depends on too many factors to give a honest answer. Quality of milling, subsoil, thickness of millings, quallity of workmanship, climate, weight of vehicles driving on the roadway, etc. That being said, with yearly maintenance and upkeep you should be able to get 20 year before any major issues. Just fill in any pot holes in each spring and put a sealer coat on tired sections as needed.
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