The key here is of course observation of real asphalt surfaces. Most any place you go a weathered,cracked and pot holed parking lot should be readily accessible. Check out abandoned or seldom used strip malls that have existed for a number of years.Be sure to take some photos for later reference. Cracks in the surface often have the look of lightning bolt patterns.For the actual asphalt itself I have used those thin sheets of paper backed Styrofoam sheet that are available just about everywhere for creating posters and signs. Be sure to peal the paper off of both sides of it to start because if you don''t it WILL warp. I use a small wire bristle brush to create wear and texture to the surface. The cracks can be produced by using a tooth pick.
For color I use acrylic flat colors and apply with a bristle brush. Paved surfaces are NOT black,. They are shades of dark gray.I often mix some black with the dark gray to darken the color,but I personally suggest starting with a dark gray as a base.Good luck with your surface project.Be sure to post photos here when it's finished.