Pervious/Asphalt Transition

Among the common questions that I encounter are those concerning the transition from asphalt to pervious concrete. Most of the questions of this transition revolve around concerns of degrading the asphalt base with water under the pervious or providing some sort of edge support and when something is designed, it is typically some sort of ribbon or flush curb. Sometimes this transition is a big concern and sometimes it is not even addressed.

It has always made sense to me that a ribbon or flush curb would help these issues, but reflecting on millions of sq ft of pervious concrete placement, I now feel that these are not really issues to be overly concerned with. In fact, the vast majority of projects of which I am involved has pervious concrete abutting asphalt with nothing separating the asphalt and pervious. None have suffered from failed edges from lack of edge support and none had degradation of the asphalt base from ground water (asphalt is commonly surrounded from porous surfaces, e.g. stone, grass, etc).

However, there is a transitional concern that I have seen that is not typically addressed, that of fines entering the pervious from the bordering pavement.This can happen with all pavements draining to pervious, but it becomes a true concern when asphalt is sloped toward the pervious. The asphalt industry will tell you that their pavement does not leach but I can tell you that we are getting and capturing their crap. This is why an asphalt pavement gets lighter with age, the binder is washing off and exposing agg and when that happens, agg washes away too. This becomes a maintenance issue for the owner and yes it can be maintained, but getting back to the ribbon or flush curb, here is how the designers at Athens/Clark County, GA addressed the issue.

And yes, I’m told it works great!