Some of the topics training sessions focus upon include:
- Maintaining proper speed - Roller speed has one of the greatest influences on mat quality. Driving too slow can over compact the surface; driving too fast can leave gaps and compromise safety if the operator gets too close to the paver. Using the correct speed will also help to maintain the appropriate impact spacing to prevent washboarding. For most jobs, correct spacing is between 10 and 14 impacts per foot. "Even if the washboarding isn't readily seen at the time the road is paved, over time, it increases its affect because of the way the asphalt continues to compact... as cars continuously drive over it," says Monical.
- Monitoring mat temperature - Typically, asphalt comes out of the plant at about 300° F. At that temperature, it is relatively viscous and liquid and is readily compacted. As it cools, it firms up and compaction becomes increasingly difficult. At about 100° to 150° F, it has cooled enough that the aggregates no longer move. "A good operator understands the temperature envelope in which he has to work," says Monical.
- Determining when to use vibratory and static modes - Vibratory compaction creates shock waves that compact from the bottom up, while static compaction works from the top down. "For most jobs, you will use a combination of compaction modes, starting with vibratory compaction and finishing with static," Nittinger comments. "We recommend that you roll in vibratory mode out of the gate. With vibration, you're getting the desired density, and with static you're finalizing the compaction."
- Selecting the right amplitude - Generally, amplitude settings are determined by the depth of the lift. Machines with variable amplitudes allow the operator to fine tune the setting to the mix. Lower amplitude settings are recommended for lifts of 2 in. or less; higher amplitude settings should be used for deeper lifts.
- "If you're on a thin lift and the operator is rolling in high amplitude, it will create roller rebound, resulting in a corrugated/washboard surface," says Nittinger.
- Avoiding sharp turns or sudden speed changes - Sharp turns can tear the mat, and decelerating/accelerating quickly can rip/tear the mat or leave indentations.
"When you tear the mat, no matter what you do, you can't get rid of it," Nittinger says. "You have to completely re-lay the asphalt. Dents and shove marks are also hard to cover up."
So change directions gradually. "Stopping at a slight angle (about 20°) will also minimize dips in the asphalt," says Tomlinson.
Keep lines of communication open
Much of what happens on the jobsite is linked to communication between workers. This includes communication between the paver and roller operators, as well as the individual in charge of checking asphalt temperature.
The paver operator and compactor operator need to communicate to ensure both are moving at a pace that provides for optimum density and smoothness. Yet, paving crews are often paid based on how much they lay, says Tomlinson. That means they may be tempted to speed through the process, which can create a couple of problems.
For one, there is a certain amount of pre-compaction generated by the paver. These density levels should be in the mid 70s to low 80s. "If the paver is going too fast, density may only be in the mid 60s," says Tomlinson. "Also, if the compactor operator has to speed up to keep pace with the paver, that changes the impact spacing. Either scenario is asking a lot of the compactor operator. The paver will either have to slow down, or you will have to add additional compactors to maintain a quality mat."
In addition, if the paver is going too fast, it may run out of material and the operator will have to stop and wait. "When that happens, the paver screed settles and makes a dip," says Monical. "A good paving crew operates at a reasonable speed so the roller operator can keep pace, and the paver never has to stop. Once the paver starts, it needs to keep going."