By: Chuck Deahl
The last thing any paving contractor wants is to have a freshly paved surface ruined before it has even been opened to traffic. But that's exactly what can happen with asphalt pickup, a long-standing nuisance in the paving business. When fine particles separate from the aggregate structure and adhere to the surface of the roller, openings are left in the asphalt mat. As a result, the contractor must take the extra step of patching the surface, which both slows down the project and increases the costs of material and labor.
The growing usage of Superpave asphalt mixes has renewed the popularity of pneumatic-tired rollers. And with the return of pneumatics comes increased concern about asphalt pickup. While this could also occur when using steel-wheel rollers, the asphalt pickup issue can be virtually eliminated with the use of the roller's water system and wheel scrapers. But because of the principles of chemistry at work, solving the pickup problem is not as simple on a pneumatic-tired roller.
Pneumatic tires and liquid asphalt have an affinity for each other due to their respective chemical structures. The rubber in the tires and the petroleum in the asphalt form a chemical bond that causes asphalt to attach to the tire. Since pneumatics are more appropriate than steel-wheel rollers when compacting certain kinds of asphalt, eliminating the use of pneumatic-tired rollers, and with it the issue of asphalt pickup, is not a viable option.
But a paving contractor doesn't have to battle the problem of pickup alone. An arsenal of tools is available to reduce the occurrence of asphalt pickup on pneumatic roller tires. These tools help a contractor complete a high quality project in a timely manner, ensuring the job is done right the first time.
Keep Them Separated
Utilizing a chemical release agent is one of today's most popular methods of combating asphalt pickup. Release agents act as a lubricating barrier between the tires of a pneumatic roller and the asphalt mat. This barrier inhibits the rubber tires and petroleum-based asphalt from bonding.
First developed for use with asphalt-hauling trucks, release agents are not new to the industry. After trucks would empty asphalt into a paver's hopper or in front of a material transfer device, remnants of the load were left in the truck bed. If not removed, these remnants would continue to build up with each consequent load. Additionally, this remaining material was waste that couldn't be used on a site. Disposal of the waste ultimately increased the cost of a project. To eliminate this problem, asphalt plants began to coat truck beds with a release agent.
When polymer-modified asphalts were introduced to the market, their high polymer content caused increased asphalt pickup issues. When looking for a substance that could alleviate the problem, paving contractors first used a job-site staple, diesel fuel, to reduce pickup.
However, diesel was not a long-term solution for several reasons. First, the fuel raised concerns about possible contamination of job-site soil and nearby ground water. Second, the fumes from the fuel could adversely affect the health of the roller operator and other members of the paving crew. Still, the largest barrier to adopting diesel as a release agent was the simple fact that diesel can cut through an asphalt mat, thereby reducing its strength and quality. If asphalt pickup is a problem because it damages the mat surface, a substance that further weakens the mat is obviously not a solution. In fact, because of the many issues caused by diesel, many state DOTs have banned its use as a release agent.
Looking for a safer and more effective alternative, contractors took notice of the release agent approach used by asphalt plants with their hauling trucks. Applying the concept to pneumatic rollers proved successful, thanks to the release agent's lubricating action, which allows pneumatic tires to move across hot asphalt without inducing pickup.