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.
Several kinds of release agents are available, with bases ranging from silicon to vegetable oil to emulsified wax. These commercial release agents are highly effective lubricants and do not cause asphalt mix to break down like diesel does. Most states have lists of release agents that have been approved for use, as well as guidelines for what kind of agent is appropriate for a particular asphalt mix. An individual state's requirements should be reviewed before purchasing a release agent.
Once a suitable release agent is selected, the substance must be properly applied to the pneumatic tires for it to do its job. The most efficient method of application is to mix the agent with water before spraying the liquid on the tires via the roller's water tank. This step may seem self-explanatory, but how the agent is mixed with the water can directly impact its effectiveness.
To achieve a proper concentration, the release agent must be evenly dispersed throughout the filled water tank. Effective mixing is a three-step process. First, add a few gallons of water to the bottom of the tank. Next, add the amount of release agent as directed by its manufacturer. Finally, fill the rest of the tank with water. By following these three steps, the release agent will properly mix with the water to achieve a concentration capable of repelling asphalt from the roller's tires.
While the issue of asphalt pickup has been greatly reduced by the advent of release agents, these agents work best when the roller is operated properly. In fact, proper pneumatic roller operation is just as important as choosing the proper release agent when it comes to combating asphalt pickup.
Proper operation begins with checking the tires to ensure the air pressure is correct. This simple, yet often overlooked, operation procedure helps a roller operator reduce the possibility of material pickup. The lack of pressure from an under-inflated tire will cause asphalt to bind to the tire instead of the mat. Having correct air pressure in all the roller's tires will help reduce pickup on the job.
Another crucial step in proper operation involves warming up the tires to a temperature near that of the hot asphalt before applying the roller to the asphalt. If the tires kept at a hot temperature, the petroleum in the asphalt will stay hot enough to act as a lubricant for the rubber tires, much like a release agent. However, as soon as the tires cool to a temperature below that of the hot asphalt, aggregate will begin to adhere to the rubber.
The appropriate tire temperature can be achieved by first running the roller up and down a compacted surface to build heat in the tires. A roller can then make passes over a test area of hot asphalt to raise the temperature to as much as 250 degrees. Once this temperature is reached, the roller must be kept moving to maintain the heat in the tires.
Even if the rest of the equipment in the paving train stops, the pneumatic roller should continue running over the asphalt at 2.5 to 3 miles per hour. Pneumatic tires can cool quickly, so it is imperative to keep the roller moving to reduce the chances that a pickup problem will occur.
Some Added Help
Even with the assistance of release agents and proper roller operation, paving contractors can still encounter difficulties with asphalt pickup. Realizing this, pneumatic-tired roller manufacturers have developed additional tools to help reduce the occurrence of a problem.
One of the tools available to alleviate asphalt pickup is a heat retention shield, which consists of a rubberized material that acts as an insulator to keep heat around the tires. Asphalt pickup usually begins on the outside tires of pneumatics, as these tires cool faster due to wind constantly blowing on them. When the tires cool, the hot asphalt is more apt to stick to the rubber. By using heat retention shields, the outside tires are unaffected by the wind and retain the heat from the asphalt mat around them.
The beginning phase of asphalt pickup can also be prevented through the use of cocoa mats. These are attached to the roller's frame and lay against each tire to loosen any aggregate that might stick to the rubber. Since even just a small amount of asphalt on the tires will induce more pickup, the use of cocoa mats can help prevent minor buildup from becoming a major problem.
Engineering developments in the pneumatic tire industry continue to diminish the occurrence of asphalt pickup. Some pneumatic tire manufacturers have even developed tires embedded with a silicon-based substance that produces a "non-stick" effect and repels hot asphalt.
Thanks to new tools and technologies and a little old-fashioned roller know-how, paving contractors are getting a better handle on the problem of asphalt pickup. This allows contractors to focus their energy on improving the quality of the finished product, rather than wasting valuable time in a sticky situation.
Chuck Deahl is manager of national accounts and training for BOMAG Americas, Inc.
Bomag rollers and pavers