"The primary concern is to keep the heat in the mat long enough to get the compaction you need," he says. "If you don't do that and you have low density you have air voids in the mix, and that allows water to get in there, and it can freeze and cause the pavement to fail prematurely."
The first part of the process involves scheduling, which was made easier by the fact that Spokane Rock operates its own plant and by the fact that there was not a lot of other paving taking place at that time of year.
"They had their own haul truck and full control of their own plant, which helped with scheduling," Curtis says. "But because of the temperature we had to make sure the trucks were on site and ready to pave when the crew was ready for them. At the same time we didn't want so many trucks on site that they were stacked up with the hot mix just sitting there cooling, even though they were tarped."
Then, once the process started, everything had to be compressed.
"They couldn't let the paver get too far ahead," Curtis says. "They slowed it down so the breakdown and intermediate rollers could achieve even compaction." He says two rollers were used on the job with the breakdown roller hitting the new mix 30 to 45 seconds after it was placed by the paver. He says that on a 2-inch overlay at that temperature the roller operators have less than 10 minutes to achieve proper compaction, "So you need to get the breakdown and intermediate rollers on it right away."
Preventing a cold joint
"When you're paving up against a cold joint, the new asphalt you place is trying to interlock and mesh against a joint that's already cooled to 150°F or so," Curtis says. "It's very hard to get the new mix to intermingle and mesh with that cold joint and that's where shrinkage cracks occur."
On some jobs the longitudinal joint is a concern because if the paving run is long enough the edge of the pull has time to cool before the paver makes its next pass. Curtis says longitudinal joints were not a concern on this job, though they could be on other jobs in similar conditions.
"The paving runs in the parking lot were not so long that the edge of the paving run would cool off before the next pass was made," Curtis says. "They were short enough so they could make the run, then back up and pave the next lane right away before that joint was cool. It was just not that big or that long a parking lot."
But the end joints of the run were a different issue completely and CHEC and Spokane Rock came up with an innovative solution that worked very well.
As anyone paving a parking lot has experienced, paving passes are pulled in one direction, and paving stops 8 ft. or so from the edge of the lot so the paver can make an end run to tie all the edges together. "You always have an end run to tie all the cross runs into, and that end joint goes cold all the way across," Curtis says.
Where under good paving conditions the contractor could have left those ends open and simply paved across them on the last pass, that wasn't an option on this job. The joints would have been ice cold, and Curtis says shrinkage cracks would have been built into the pavement.
So to keep those ends hot to eliminate cold joints, Spokane Rock Products simply stockpiled material on top of the end of each run, then it tarped the stockpile. This was done at the end of each of nine pulls.
"To dump material they just raised up the screed and used the paver to dump material right at the end of the run," Curtis says. "They didn't use handwork at all because that would have taken too long and because that would cool down the mix. By placing that stockpile with the paver the exposure to the environment and the cooling of the mix was minimal."
When the crew was done with three or four pulls and ready to tie those ends together, they simply removed the tarps and shoveled off the stockpiled material, which was tossed into a truck and disposed of because you don't want any cold mix into the hopper where it would chunk up. The stockpiled material kept the ends of the 2-inch overlay warm, and the paver tied the ends together in passes perpendicular to the initial runs. Then they paved another three runs and repeated the process.
"Because of the way they had placed and tarped the hot mix the end joints were kept hot so they essentially were paving against a hot joint," Curtis says.