Mickey_s, thanks for your explanation. Do you know of any public source where I can get the mix specifications for tracks that you mention?
Warning! It follows a long rant containing common information on asphalt for tracks... not suitable for all publics.
At the Autódromo de Rosario the structural team had to devise their own mixture, based on normal road specifications. Some tests were made with the pilots, actually resurfacing several times the same curve with test mixtures, and measuring the curve average speed attained. The record for the Autódromo was broken as soon as they raced on the new surface. To tell the truth, some curves lacking transitions were corrected.
I also worked on drainage design for El Dorado's second runway. Let me give this long explanation about it:
I suppose you know the work of Harry Cedergren
, back in the 70's: in his seminal paper, he measured infiltration of water amounting to 40% (on dense bitumen) to 60% (on concrete) of the rain that pours on the highway. When I read about this for the first time in my life, it seemed counterintuitive. After all, we are taught that dense asphalt is, in principle, impervious to water. We tend to forget the high cracking it develops over time.
We take very seriously this infiltration problem in Colombia, where you have a lot of rain (Bogotá, up to 2.500 mm/year; Chocó, up to 10.750 mm/year
). Besides, most soils in Colombia have a lot of clay: rocks degrade because of humidity into clay, not into sand by temperature cracking, like in non-equatorial countries.
Hailstorm in Bogotá (the white stuff is hail, not snow: no winter here).
Besides, as you surely knows, highway engineers make all possible efforts to compact the granular base, wich creates a layer of very low permeability layer under the asphalt. As Cedergreen pointed out and you also surely know, the water pressure, or lateral hydraulic gradient, in this layer is very low, which makes hard for the water to drain.
The end result is that, when you construct in an equatorial country a percolating runway or highway, maybe 100 meters across, on a cut made on clay, you are creating basically a water pool under the track. I have personally found liquid water on test ditches, under the asphalt, one month after the last rain, in places with low watertable.
We started back in the 80's to use Cedergren system for drain calculations. To make short a long story, we use transversal drains in almost any asphalt or concrete surface we construct, specially on wide roads or runways. If we don't, asphalt life is reduced in half or more.
In a hard tropical rain, like the ones we have in Bogotá, (with drops the size of cherrys...
) I've seen personally El Dorado's runway drainages spilling water with such pressure that it reaches one meter away from the exit of the pipe, after a fall of maybe one and a half meters. All the time we inspect drainages, airplanes are landing (we do not stand on the runway) but I do not find it scary, but exhilarating. Actually, when I was younger and made aircraft models, I went once in a while to the end of the runway just to see the aircrafts taking off (sort of aircraft-spotting)...
Now I have to wonder if the bumps on the entrance of curves the pilots complain so frequently are caused by this lack of transversal drainage under the surface
. After all, as you know well, the load of car on a track is negligible for structural purposes, so, in principle, tracks should not present any bumps or irregularities caused by traffic loads. I am not sure about shear stresses by braking, that I suppose are phenomenal.
Anyway, I'd bet my slide rule on the idea that most bumps are caused by temperature stress (freezing of soil or heating of surface) and soil humidity. At the entrance of curves, where the superelevation is higher, the drainage problems and braking stresses are higher...
Finally, did you look at MC photo
I mentioned? Take a good look at the quality of the patch to the left... it is really such an ordinary repair job as it seems to me, or it is just that I am not understanding something?