A good paving job will have a smooth, dense appearance that looks flat and tight, without any gaps between the aggregates at the surface. Yet, that appearance doesn't just happen. There are a lot of factors that affect mat quality.
The paver operator, the paving mix, even the distance from the asphalt plant to the jobsite all play a role in achieving optimum mat density. But the last person to touch the pavement is the roller operator. He/she is the one who has the greatest influence on smoothness.
"Over the years, operator performance has become more and more critical to the bottom-line success of most any paving job," says Todd Mansell, technical marketing manager, Sakai America. "With job profits based in part on bonuses and penalties in many jurisdictions, the operator must be able to adapt to different jobsite conditions by using all of the features offered on an asphalt roller."
Train for optimal performance
Most roller manufacturers, in conjunction with their dealers, offer training for end users of their equipment. That training is available in a variety of forms, including group seminars, hands-on courses or one-on-one training on the jobsite or at the manufacturer's training grounds.
Training gives operators the knowledge to overcome some of the hurdles that can crop up on a project, such as difficult weather conditions, clumpy mixes, extremely thick or thin lifts, joining of hot and cold lanes, etc. "The ability for an operator to successfully adapt his/her rolling pattern to successfully compact mixes such as Superpave, FAA and the ever-increasing variety of mix designs is arguably the greatest training challenge that must be met in order to maintain quality and productivity," says Mansell.
According to Wayne Tomlinson, training specialist, Volvo Road Institute, operators often retake courses offered through the Institute, in part to get hands-on training, as well as to get the latest information about what's happening in the industry. "During the three-day training, attendees will actually lay a mat and practice rolling patterns," he points out. "We have a lot of guys who keep coming back year after year."
"While the basics of paving haven't really changed, there can be some changes when new technology and machines are introduced," says Bill Nittinger, Northeast regional manager, Dynapac.
"Operators can learn how to use the machine the right way to give them the best results. It's good to keep the information in front of operators. A lot of it may seem basic, but the rules aren't always followed."
Compaction technology continues to evolve as manufacturers introduce new features to simplify operation. For example, Dynapac units offer soft stop and start features. And Sakai's ExactCompact monitors impact spacing, while Intelligent Compaction plots roller laps.
"[Such features] are a great help to the busy roller operator, allowing him to focus on the job at hand and make changes if necessary by using real-time information," says Mansell.
But operators need to be trained on how to use such features in order to achieve maximum benefit. "A lot of people may not understand that the operator is just as important as the machine," says Nittinger. "You can have the best machine in the world, but if the operator isn't running it at the right settings, the right speed, etc., it can affect the quality of the mat, and that quality is important to the life of the road. The better the operator, the better the machine, the better the performance and the longer the life of the road."
Operator training helps to minimize some of the most common mistakes when operating a roller - mistakes that can result in corrugated or "washboard" roads with dips, divots and indentations that compromise mat quality and road life.
"A good roller operator understands the process and what his/her job is within that process," says Bruce Monical, marketing manager, Hamm Compaction Division. "He/she can make or break the contractor's ability to achieve final payout. A good operator can help you; a poorly trained operator can hurt you."
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."