Question raised during current widespread SoCal wildfires.
Question raised during current widespread SoCal wildfires.
Off the cuff guesses.
How about the portion of asphalt that is actually combustible is a small percentage of the total, and the other aggregate material makes it hard to get enough oxygen to anything more than the surface.
It really doesn't get that hot because it's part of the ground.
I would guess that if you get it hot enough it would burn. According to this MSDS for asphalt, the autoignition temperature is 900F. But they list the boiling point as greater than 600F, so it would probably melt and vaporize before it burned.
Asphalts are very high molecular weight hydrocarbons, with virtually no volatile fraction. This article has more info.
Have you ever seen the area of asphault dirrectly underneith a bad car fire? It indeed melts. I don't know that I've ever seen it burn, but probably for the reasons that swift mentioned.
Yes, Swift's references are quite thorough---thanks!..."Almost all asphalt used today is derived from the bottom of the barrel--that is, the last cut in the petroleum refinery after naphtha, gasoline, kerosene, and other fractions have been removed from crude oil"...
Believe it or not, I once had a research project on what the petroleum industry calls "bottoms" (more literally the bottom of the fractionating column, rather than the bottom of the barrel). The only current way to "crack" them is thermal cracking, which is very energy intensive, and yields a lot of undesirable stuff, like methane, and not a lot of C8-C10 cuts that go into gasoline. So, if the price is right, they don't bother and just turn them into asphalt. My project ( a summer internship ) was trying to find a cracking catalyst for them (we hadn't by the end of the summer).
I just wanted to add that by the time asphalt has been in a road surface for several years, the miniscule amount of voc/svocs will have volatilized even further than what was present in the fresh asphalt, likely driving the autoignition temperature even higher.
I thought I had recalled pictures of asphalt burning when lava flowed over it, but the more images I googled up, the less convinced I was that the asphalt was burning even then. And images of cooled lava on top of roads, look almost like you could scrap the lava off, and the road would still be there.
That asphalt is tough stuff, until it rains and freezes, rains and freezes. can you say pothole the size of your car.
Interesting photo-essay on "de-asphalting" what were originally brick roads:...The main piece of equipment that has allowed a successful restoration program is the “Road Master” which produces over 12 million BTU’s of 100% radiant heat energy to melt the asphalt surface...special teeth, that will not damage the brick surface below, loosen the heated asphalt to allow it to be scraped off with a road grader...
I've gotten asphalt to burn when using an acetylene torch. It was a small and somewhat unstable flame that could barely hold it's own. There could have been other impurities in it that were helping, such as leaked engine oil.
I know that when there was a fire at the condo complex where I used to live, there was a 6-8 inch deep and wide pit in the asphalt where police suspected that the initial incindiary was tossed. But a carport full of gasoline and oil burning directly on asphalt did not damage it visibly.
STARGAZING: All I see are the lights of a billion places I'll never go. --Howard Tayler, Schlock Mercenary
This state experimented a few years ago with asphalt containing ground up rubber from recycled tires. It caught fire from spontaneous combustion. Just smoldered, though.
Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.
Asphalt pavement contains about 5% to 5.5% of non-cutback AC (Asphalt Cement). This percent may vary in other regions depending on the type of rock and the fines ratio. AC must be > 250 deg. F to be thin enough to pump. It can be heated to over 400 F without problems. Seal coat surfaces, where oil is sprayed and rocks are sprinkled on top, must be shot at temperatures over 300F and, when crumb rubber is an addative, around 400F or more.
These temperatures are problematic for handling so cutbacks and emulsions (~1/3 water) have come along and helped. Diesel and naptha were the two best cutbacks for effectiveness. These reduced application temperatures to below 150F and were effective. Unfortunately, the flash point of naptha is 85F, so there are many stories of equipment, power lines, and other things burning down when operators were careless. [We had a 2000 gal. distributor truck blow up in our shop by a mere spark on the ground 10 ft. away.]
Also, besides low concentration and high flash point, the reluctance for a road to burn might also be found given the fact it is always on the bottom of a fire and not in a fire.
Thank you George!
Just to amplify that see: http://www.afscme.org/issues/1363.cfm
And to add a personal experience, I restored a car which the PO had 'water-proofed' by pouring asphalt into all the nooks and crannies where sensible car owners pour Waxoyl. I suppose he was successfull, as it not only kept water out, it allowed him to fill in all the rust holes without a trace !
Welding patches on to fill the holes was ..... entertaining, shall we say? Especialy for onlookers. Burning asphalt would drip out of the body work. If it landed on bare skin you knew why they poured boiling pitch from castle battlements.
But even large drips that fell to the floor, still burning would soon go out as the cold floor cooled the mass and stopped it vapourising. Solid earth/ground has enormous thermal capacity. Even though under the embers of a camp fire, the ground will be hot for a long time, the ground around it will stay cool.
Thanks. Nice story, , and nice link. I knew if I hung around this site long enough, y'all would find my only strong suit; asphalt equipment R me.
I don't know about setting it on fire, but i'm fairly certain that the layer of dirt that imbeds itself into the surface (causing it to lighten up in color over time) also helps to suppress fires.
However, for fun, pour some LOX onto asphalt in a good location and wait for a car to drive over it. Just be prepared to be arrested afterward for assult, if not depraved homicide.
What's the ignition source?
I believe he means the embrittlement from freezing.
The color of the Sun when viewed through an AM29.35 atmosphere (without heavy particulate concentrations from St. Helens or California wildfires). That's assuming you can see the flames peakin' through the man-made micro Bok nebula caused by the smoke.