Asphalt Separated

Oct. 1, 2005

Asphalt pickup has been a long-standing nuisance for paving contractors. When fine particles separate from the aggregate structure and adhere to the surface of a roller, openings are left in the asphalt mat and another step must be added to the paving process. A freshly paved surface that hasn’t even been opened to traffic must already be patched, which increases material and labor costs and slows down a project.

Asphalt pickup has been a long-standing nuisance for paving contractors. When fine particles separate from the aggregate structure and adhere to the surface of a roller, openings are left in the asphalt mat and another step must be added to the paving process. A freshly paved surface that hasn’t even been opened to traffic must already be patched, which increases material and labor costs and slows down a project.

As the usage of Superpave asphalt mixes grows, the popularity of pneumatic-tire rollers returns. And with the renewed popularity of pneumatics comes increased concern about asphalt pickup. While asphalt pickup may occur when using steel-wheel rollers, the issue can be nearly eliminated with the use of the roller’s water system and wheel scrapers. The solution isn’t as easy on a pneumatic-tire roller, thanks to chemistry.

Pneumatic tires and liquid asphalt have an affinity for each other because of their respective chemical structures. The rubber in the tires and the petroleum in the asphalt form a chemical bond that causes the 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-tire rollers, and the issue of asphalt pickup, is out of the question.

However, 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 to get a paving job done right the first time.

Releasing the problem

One of today’s most popular methods of combating asphalt pickup involves utilizing a chemical release agent. Release agents work by acting 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.

Release agents have long been utilized in the asphalt industry, first developed for use with asphalt-hauling trucks. When trucks released asphalt into the hopper of a paver or in front of a material transfer device, remnants of the load were left in the truck bed. If the remnants weren’t removed, they kept building up with each subsequent load. In addition, material that remained in the truck was waste that couldn’t be used on a site; this waste ultimately increased the cost of a project. Asphalt plants began to coat truck beds with a release agent to eliminate these problems.

When polymer-modified asphalts were introduced to the market and their high polymer content caused increased asphalt pickup issues, paving contractors started looking for a substance that could alleviate this problem. Contractors first used a jobsite 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 jobsite 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 issues diesel causes, many state DOTs have banned its use as a release agent.

Looking for a safer and more effective alternative, contractors applied the release agent concept asphalt plants used with trucks to pneumatic rollers and found success. The lubricating action of the release agent allows the pneumatic tires to move across the hot asphalt without inducing pickup.

Several kinds of release agents are on the market with bases including from silicon, vegetable oil and emulsified wax. These commercial release agents are highly effective lubricants and do not cause the asphalt mix to break down, unlike the diesel that was previously used on jobsites. 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. Review your state’s requirements before purchasing a release agent.

After selecting a suitable release agent, the substance must be applied properly to the pneumatic tires for it to do its job. The most efficient method of application is by mixing the release agent with water and spraying the liquid on the tires via the roller’s water tank. While this step may seem self explanatory, how the agent is mixed with the water can affect how well it will work.

The release agent must be evenly dispersed throughout the filled water tank to ensure a proper concentration is achieved. An effective mixing method involves three steps. 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.

Pneumatic know-how

While the advent of release agents has greatly reduced asphalt pickup, release agents work best when the roller is operated properly. In fact, when combating asphalt pickup, proper pneumatic roller operation is just as important as choosing the proper release agent.

Correct pneumatic-tire roller operation begins with checking the air pressure in the tires. This is a simple, yet often overlooked, operation procedure, but ensuring the air pressure is correct helps a roller operator reduce the possibility of material pickup. If one tire is underinflated, the lack of pressure will cause the 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 are near the temperature of the asphalt, the petroleum in the asphalt will stay hot enough to act as a lubricant for the rubber tires, much like a release agent. As soon as the tires cool to a temperature below that of the hot asphalt, aggregate will begin to adhere to the rubber.

An appropriate tire temperature can be achieved by first running the roller up and down a compacted surface to begin building heat in the tires. A roller can then make passes over a test area of hot asphalt to increase the tire temperature to as much as 250 °. Once this temperature is reached, it is important to keep the roller 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 mph. Pneumatic tires can cool quickly and begin to induce pickup, so it is imperative the operator keeps the roller moving to greatly diminish the chance the problem will occur.

Roller add-ons

Pneumatic-tire roller manufacturers have realized that even with help from release agents and proper roller operation, contractors can still feel the pain from asphalt pickup. In response, manufacturers have developed tools that, when added to the roller, help reduce the occurrence.

Heat retention shields are one of the tools available to alleviate asphalt pickup. These shields consist 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 because wind is 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 wind cannot reach the outside tires, keeping the heat from the asphalt mat around them.

Cocoa mats also are available to help eliminate the beginnings of asphalt pickup. These are attached to the roller’s frame and lie against each tire to loosen any aggregate that sticks to the rubber. Since even a small amount of asphalt on the tires will induce more pickup, the use of cocoa mats can help prevent minor buildup from turning into a major problem.

Asphalt pickup also continues to diminish thanks to engineering developments in the pneumatic tire industry. Some pneumatic tire manufacturers have taken steps to reduce pickup headaches by developing tires imbedded with a silicon-based substance that produces a “non-stick” effect and repels hot asphalt.

With the help of these new tools and technologies as well as old-fashioned roller know-how, contractors continue to get a better handle on the problem of asphalt pickup. By avoiding a sticky situation, contractors can dedicate more time to improving the quality of the finished product and continue paving their way to business success.

About The Author: Deahl is manager, national accounts and training, for Bomag Americas Inc., Kewanee, Ill.

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