Crumb rubber modified asphalt comes down the road every 10 years or so. This seems to be about the cycle where the greenies that promote it forget what a dismal failure it was last time it was in vogue and about the time all of the politicos turn over so there's a fresh batch to influence with ********. Arizona hasn't done the rest of us any favors with some of the baloney they have been spouting about the benefits of rubberized asphalt. As an example, it is true that it appears to be quieter than normal asphalt. However, what we've found out is that this effect is short lived. As the new surface gets some wear on it, the noise characteristics revert to what a normal ashalt surface would be in fairly short order. This degradation in sound attenuation is accellerated in places north of Arizona by studded tires. Note, any asphalt surface will be quieter than concrete. You don't hear these tidbits coming from the folks in Arizona.
Performance is really what gets lost every ten years. When we specify/design this stuff, we count on properties of the asphalt oil we select to help with performance. Specifiers are always trying to balance two things: ductility and stability; or, said another way, cracking vs rutting. The difficulty is balancing one vs. the other, because the things that you can do to help one, tends to hurt the other. When we add an item like crumb rubber to the asphalt it modifies the properties in a way that is not particularly conducive to getting the performance we desire. Rubber modifies the asphalt oil in such a way that we don't get the stiffness we would like to have. This is great for ductility, not so great for stablity. In a place like Arizona that has huge problems with thermal cracking due to the wicked weather, this probably works out for them and winds up giving them a net increase over their typically dismal performance. In other places where we actually expect performance, the material tends to rut and is decidedly not good with studded tires. Someone asked about the stretch of I-5 between Everett and Lynnwood in WA. The last time I looked, it was rolling over and dying a spectacular early death. In all fairness, there were a lot of things going on there (it is a test section) and that premature failure may not be entirely due to rubberized asphalt - although it is almost certainly a contributor.
If it were up to me, tires make pretty good fuel for a cement kiln.
