“It’s hard to get density for rubber, because it’s so ‘spongy’,” Caltrans’ Chang said. “That’s why we use three rollers. We’ve found that if we keep the breakdown roller going with the vibrator on, and the secondary roller off and hanging back a ways, we get better compaction, with the smaller roller doing finish. I’m trying to get up to 95 percent compaction if we can get it; that’s why we’re trying different methods of compaction.”
The rubber used was not a “terminal blend” in which the crumb rubber is added by the liquid asphalt distributor. Instead, crumb rubber was added to the PG 64-16 liquid asphalt at a nearby Granite plant via “super sacks” at a temperature of 380 to 400 degrees F, and with the rubberized binder then brought via tanker to the Ukiah North plant. Nominal rubber dosage was 18 percent by weight.
“Rubberized binder does not retain the same performance spec,” Plouff said. “You can’t apply a lot of the same tests because the rubberized asphalt becomes too elastic. But its elasticity is one of the benefits, leading to noise reduction and enhanced durability.”