When you know you have the mix right and it’s delivered at the correct temperature all the way to the screed, all that’s left is to compact it into the perfect mat. A smooth finish that brings a bonus is as easy as rolling it out. Of course, there are few of us who think getting perfect density is easy. That’s why OEMs like Bomag, Caterpillar, Roadtec, Sakai and others prepare guides; industry paving consultants like John Ball, Tim Murphy, Chuck Deahl and others offer on-site training; and both onboard and aftermarket intelligent compaction (IC) devices are available to help veteran operators and new employees who influence the mat last do the best job possible. Let’s take a look at some basic tips and interesting twists that help roller operators finish the job well.
Stay on the Mat
The roller operator should know that a divot is a small hole or tear in the mat that occurs when a bit of asphalt material sticks to the roller’s steel drum and lifts off—or out of—the new pavement. This is referred to as “asphalt pick-up” and it is to be avoided. As the roller moves across the hot mat with this chunk of material stuck to the drum, she collects additional material with that chunk, adding to the asphalt pick-up and creating additional, increasingly larger divots in the mat.
A member of the team must recognize what’s happening and stop it. Have the roller come to a stop on the coolest portion of the mat at a 45 degree angle to the screed. A member of the team must scrape the pick-up off the steel drum, collect it so it doesn’t fall onto the mat where it will cause segregation, and check the roller’s spray nozzles on the spray bar, its lubrication overall and its drums’ temperatures.
If the drums are cool from high winds and ambient temperatures at the job site, you need to find a way to block the winds and hold heat against the drums. If a couple of spray nozzles are blocked on the spray bar, you must clean them so you get good lubrication of the drum. Be aware that dirty water in the water tanks can clog the nozzles and will cause you to perform maintenance steps more often. If you want to stay ahead of spray system problems, be sure to change the main spray system filters on the OEM’s recommended schedule. They test these things; it’s in their best interest and yours if their equipment performs well.
Staying on the mat keeps drums from picking up debris. Don’t let roller operators overhang longitudinal joints too far or pavement edges. If they pick up dirt, rocks or grass, you can bet you’ll find that transferring to your mat and ruining your day.
Staying on the mat keeps drums warm. Once you’ve heated the drums to the point they’ll work without worry of asphalt pick-up, you want to keep them heated. By staying in motion, the operator doesn’t create bumps and creases in the mat and he keeps the roller’s drums heated for optimum use. Even if the paver has to stop, the roller operator doesn’t.
Find the Middle Ground
Intermediate rolling—or compaction that takes place in the intermediate rolling zone—typically takes place with a pneumatic tired roller. Your roller operator will find asphalt pick-up happens with rubber tires on the pneumatic roller just as easily as, if not more easily than, it does on a steel drum roller. A number of OEMs give tips for reducing pick-up with these machines, including a couple you would use with steel drum rollers.
For instance, look at the tire scrapers/drum scrapers and distribution mats. You want these in the right position and well-maintained before you start a job. Also, use a bio-degradable release agent to clean tires. The folks at Rushing Enterprises have helped a number of crews get creative with the use of their release agent; don’t forget creativity in the rolling zone. In the event you do experience pick-up during roller, additional release agent can help in the cleaning. Remember to stop the machine on the coolest portion of the mat at a 45 degree angle to the screed, and then clean the material, capturing it in a manner that keeps it off the mat. While this may give tires time to cool back there, it prevents gouges in the mat where the heavy machine sits. When the tires are clean, run the roller in a warm area of the mat to heat the tires up again.
Smooth Out the Marks
For pneumatic tired rollers specifically, heating them means working them on a warm section of the mat before moving to the temperature zone that is the intermediate rolling zone. If you notice that the tire marks are deeper than you prefer, you can adjust the intermediate zone “back” from the screed or decrease the tire pressure—when specs allow—to reduce the tire contact pressure. Make sure an experienced roller operator or paving foreman handles the latter solution.
When a vibratory roller leaves marks in the mat, you have a number of settings you can fix to smooth out the problem. First of all, too much vibratory energy is going into the mat. If the intermediate and finish rollers can’t smooth the honeycomb within the asphalt layer into its appropriate state, they won’t achieve your best density. They won’t “take out” the marks.
First, check the working speed of the vibratory roller in breakdown position. Is it traveling so that it hits the mat with 10 to 12 impacts per foot? The team at Caterpillar Paving Products widens that margin to 8 to 14 impacts per foot. If not, you want to switch to a lower amplitude setting with a slightly higher frequency.
Changing the speed of the machine might not be an option when the paver is moving at a pace that keeps up with the plant and the haul trucks. You have a perishable product beneath your drums; changing the number of times you hit it as you roll it will be your best bet. But you don’t want to hit it with the same amount of force if you’re increasing the number of times you’re hitting it per foot. If the machine you have will allow it, try operating with one drum vibrating and the other in static mode, if state specs will allow it, and see if that eliminates the marks in the mat.
Remember, the marks in the mat are a sign that something isn’t right. You want to correct it the best you can so the pneumatic tire roller in the intermediate zone is working toward achieving density, not correcting mistakes.