By Chuck Deahl, Manager, National Accounts And Training, BOMAG Americas, Inc.
October 1, 2006
Associated Construction Publications
Tips for keeping pneumatic-tired rollers free of asphalt pickup – The last thing any paving contractor wants is to have a freshly paved surface ruined before it has even been opened to traffic. 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. 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. 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.
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 – Using an asphalt release agent is one of today’s most popular methods of combating asphalt pickup. Asphalt 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, asphalt 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 asphalt release agents.
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 an asphalt 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 an asphalt release agent.
Looking for a safer and more effective alternative, contractors took notice of the asphalt release agent approach used by asphalt plants with their hauling trucks. Applying the concept to pneumatic rollers proved successful, thanks to the asphalt release agent’s lubricating action, which allows pneumatic tires to move across hot asphalt without inducing pickup.
Several kinds of asphalt release agents are available, with bases ranging from silicon to vegetable oil to emulsified wax. These commercial asphalt release agents are highly effective lubricants that do not cause asphalt mix to break down like diesel does. Most states have lists of asphalt 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 asphalt 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 asphalt release agent is mixed with the water can directly impact its effectiveness. To achieve a proper concentration, the asphalt 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 asphalt release agent as directed by its manufacturer. Finally, fill the rest of the tank with water. By following these three steps, the asphalt release agent will properly mix with the water to achieve a concentration capable of repelling asphalt from the roller’s tires.
Knowing Pneumatics – While the issue of asphalt pickup has been greatly reduced by the advent of asphalt 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.
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.