Does your asphalt have rust stains? Asphalt, also known as asphalt concrete, is typically a mixture of 95% aggregates (stone, sand and gravel) and 5% asphalt cement. Asphalt cement has a highly viscous nature so it must be heated in order to mix with the aggregate.
The aggregates used in the mixture contain metallic metal (pyrite). The mineral pyrite, or iron pyrite, also known as fool’s gold, is an iron sulfide and is the most common sulfide mineral. Iron in the pyrite changes into Iron Oxide (rust) when the aggregate is exposed to air and moisture.
The rust bearing aggregate continues to grow, and finally explodes, creating rust streaks. There are specialty primers on the market designed to deter the rust from bleeding through the sealer. These primers are expensive and only marginally effective. The rust stains will always bleed back through the sealer, it’s just a matter of how soon.
This year the record amount of rain has caused the stains to reappear sooner than usual. When the ground is saturated with water, it leaches up through the base of your driveway causing the metallic mineral to oxidize. The result is brown or brick-red colored spots that will leave stains or streaks in your driveway. Oftentimes a homeowner will not notice the spots until after a fresh coat of sealcoating has been applied. The spots were always there but the jet-black glossy finish seems to make them more noticeable as they bleed through the asphalt sealer. These stains are
Jet-Seal’s number one source of customer complaints.
Who is to blame? That’s a difficult question to answer.
Most state DOT’s, (Department of Transportation) allow a certain amount of pyrite into the asphalt mixture. The contractor doesn’t know what the percentage is in any batch and it’s impossible to know by looking. Asphalt with a high pyrite content doesn’t look any different from a batch of asphalt with a low content or no pyrite at all. The mixture is black, and the aging of asphalt may take months until the aggregates are exposed to the elements of weather that cause oxidation.
The sealcoating contractor or the product used is not to blame. These stains will bleed back through even the highest quality pavement sealers.
Fortunately, the pyrite or the stains it leaves does not compromise the integrity of the driveway. When the pyrite explodes it leaves a pea sized divot and a potential entry for water. A well-planned maintenance schedule that includes crack sealing and sealcoating will protect your driveway from water entry and the effects of the central Ohio freeze / thaw cycle.