If you have noticed ugly brown sports or streaks on your parking lot or within your parking lines, you are not alone. This unsightly occurrence is happening more and more. It is not only found on asphalt parking lots, but driveways, tennis or basketball courts or any asphalt surface. Many times you do not even notice it until you put a fresh coat of paint on your lines or sealcoat your lot. These brown spots sometimes bleed right through bright new coating and really stand out.
If you see these brown spots on your line paint take a closer look at your parking lot. You will most likely see them there as well. The color difference of white or yellow line paint makes them really stand out, more so than the black or gray color of the pavement.
These brown spots are actually metallic metals called Pyrites (Iron). They are sometimes found in the stone or aggregate used to make the asphalt. Asphalt allows water to penetrate through it. The moisture accumulating on your parking lot goes into the pavement. If your pavement has pyrites in it, it becomes wet, the pyrites (iron) then begin to rust. The rust stain eventually comes to the surface causing the ugly brown spots or streaks in the asphalt.
In the early stages these rust spots are just cosmetically undesirable. Eventually the rusting aggregate begins to swell causing a small bump in the pavement. You would not even notice the bump unless you rubbed your hand over it. These small bumps will eventually erupt or pop, creating a small hole in your asphalt. They are typically the size of a small marble or less.
These eruptions allow an easier access for salt and water to penetrate into the asphalt surface. This causes more damage to the pavement. Sealcoating the pavement will help slow down the increased number of rusting pyrites by preventing some of the water from entering the pavement. Eventually the old rust spots will reappear along with the new ones. There are specialty sealers such as “rust arrest” that claim to hide and significantly retard the continued rusting of the pyrites in the pavement. We have tried other products with similar claims but have had very little success.
It is not the fault of the contractor that installed the pavement, they just purchase the asphalt from the asphalt plant, who has little to no say about the stone being used in the production of the asphalt. Asphalt plants source of aggregate varies from time to time. It is impossible for the producer of asphalt to guarantee that the asphalt will not contain pyrites. At this time it is impossible for a line stripper or sealcoating company to prevent them from bleeding through a newly coated asphalt area.
Below is a photo of rust spots:
