Dear Straight Dope:
I was driving into work today and passed a sign that said "fresh oil." The sign was on a street that had been freshly stripped of its blacktop and was down to what looked like concrete. The question occurred to me: Why do we put oil on streets like this? Does it dry out otherwise? Does it help a fresh layer of asphalt adhere somehow? I am sure that this will be a chip shot for you to answer, hopefully just the question for a lazy summer day.
From your description, it sounds like the street in question was concrete with an asphalt overlay, which was being outfitted with something like Petromat. Petromat is a synthetic polypropylene fabric (the trade name is owned by BP/Amoco), which is placed between the road base and the asphalt-concrete upper surface to provide a water barrier, strengthen the road, and prevent cracking of the overlay. When used during resurfacing, it’s almost always soaked with an oily-looking asphalt sealer (sometimes called an asphalt emulsion or a bitumen sealer). Sometimes, depending on the application, sand is spread on top of the sealer and mat to help bind excess sealer to the surface. The coating of sealer is called the "tack coat." The term sealer notwithstanding, it’s fair to say one purpose of the stuff is to help the various road layers adhere. In case you’re wondering how much sealer you’d need, Amoco recommends 0.2 to 0.3 gallons per square yard.
Petromat and similar products also come in a tape-like form for spot sealing of large cracks and joints. A Petromat-fiberglass composite with the trade name Petrogrid is used where higher-strength reinforcement is needed.
Other uses of asphalt emulsions and bitumen road sealers (see note 1) don’t involve fabric matting. One common resurfacing technique is what’s known as "chip sealing," which involves laying down a thick layer of bitumen, then covering it with an aggregate made of rock, gravel, or even bottom ash and slag from coal power plants. The other common application is direct spraying of bitumen sealer onto gravel roads to bind and strengthen the road surface and reduce road dust.
"Fresh oil" is a misnomer, undoubtedly used because "fresh bitumen sealer" would baffle the average motorist. In many locations, the use of actual crude oil or motor oil on gravel road surfaces is banned for environmental reasons. My civil engineering friends tell me even thick oil won’t bind dust and gravel as well as an asphalt or bitumen-based product. That doesn’t stop individuals in rural areas from sometimes pouring waste motor oil onto their driveways for dust control–which is unfortunate, because the oil often contains hazardous metals and other toxic chemicals that are washed into the environment by rain. While road sealer isn’t something you want to dump into the environment either, it’s more likely to stay where you put it and typically contains fewer contaminants.
Note 1. Although "bitumen" and "asphalt" are often used interchangeably, it’s probably more accurate to say that asphalt is a type of bitumen, with bitumens including asphalts, tars, and similar viscous carbon-based substances derived or evolved from crude oil, coal tar, wood tar, and other base stock.
Petromat Installation Guide, Amoco Fabrics and Fibers Company.
Material Safety Data Sheet: Petromat, Amoco Fabrics & Fibers Company, 1999
"Chip Seals," in Caltrans Maintenance Technical Advisory Guide, Caltrans Division of Maintenance, 2003
Civil Engineering Handbook, Chen, W.F., editor, 1995.
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