I suspect local construction practices play a pretty big role too...The contractor/architect with the winning proposal picked the surface. As others have said, either one functions. It should be noted that "asphalt" runways are actually composite, as true asphalt (aka bitumen in other countries- yes, classically a coal product but now available from other sources) is not very strong and subject to distortion in high temperatures. Some thick asphalt roads actually sink 2-3 inches under the weight of bus traffic on a hot summer day, causing lots of problems for smaller vehicles. Then they just resurface it in September.... But layered composite asphalt is much stronger, and doesn't need expansion joints like concrete, and actually (in my opinion) makes a better road/runway surface. (but the incessant thump-thump of the expansion joints in the concrete interstate highways in the Southwest US brings back fond childhood memories...)
The differences in initial cost vs. maintenance have been noted. As to other differences, concrete has a higher compressive strength but is more brittle, asphalt is more flexible. Concrete expands laterally, whereas asphalt is more prone to volumetric expansion (the deck will rise and fall a few inches to absorb the expansion, but not crack like concrete)
|Quoting HAWK21M (Reply 13):|
How much would the spacing for expansion between Concrete slabs be approx.
That depends a lot on the mixture (lots of different things can go into "concrete", and the proportion makes a big difference in expansion ratios), and a variety of other things including climate, scale, and substrate. In vertical construction (which I am more familiar with), it's not uncommon to see expansion joints every 35-50 feet. (Some buildings use other methods to control expansion and only have joints every 150 feet, but in my experience those buildings usually have water leaks at the joints...)
Flat decks are a whole different ball game, and I see up to about 200 foot spans. I believe the joints at IAD
are just slightly more frequent than the runway lights, how are those spaced? 150 feet or so I think. But smaller width runways will require more frequent joints. The lateral expansion means a basically square slab is the best shape to compensate, so figure not more than 1-1/3 the width of the runway if no joints run the length. But I sometimes see smaller airports and municipal roads with a patchwork of square pours not more than 25 feet square with expansion joints. I expect this has to do with the construction method...enough concrete has to be poured at the same time to fill a section, because if you pour a new patch against a section that has already cured without an expansion joint, you risk a stress fracture at that point because new concrete doesn't bond well with existing (cured) material. So smaller projects with limited concrete access (only 5 trucks a day, for example) will pour smaller patches with more (but narrower) expansion joints. This also makes repair and maintenance easier, which is important if corrosives are used during freezing weather. A small patch of concrete is easier to break up and re-pour than a much larger one- although, as has been noted, this means closing that part of the runway until the concrete cures. It is also used for special purpose areas- so you see concrete "islands" around mx and refueling stations in a sea of asphalt. (concrete is a must where mx is performed, as asphalt doesn't have the compressive strength to hold up the heavy jacks.) But you aren't likely to find an asphalt island anywhere in an otherwise concrete facility.