Sticky Situations

While asphalt pavement may be lighter than concrete and give crusher operators faster throughput rates, there are still many problems that can creep up. Crushing asphalt pavement can be done effectively by choosing the right equipment to do the job and making the right adjustments to account for its abrasive and sticky material makeup.

Production can be faster in cooler weather.

Final product size determines production rate.

Gary Smith, president of Construction Equipment Co. (CEC), Tualatin, Ore., says there are three things a contractor needs to consider in choosing the appropriate crusher for an asphalt project: the volumes of material you have to process, the size of the material you have to process, and how much of that material is free run–material in your raw product or feed that is already to the proper size.

“What that drives you to do is choose the proper size of crusher and more importantly the proper size of screen,” says Smith. “

D.J. Cavaliere, operations manager for COR Equipment Sales Stamford, Conn., says choosing the right crusher for asphalt pavement depends on climate, temperature, desired finished product size and the amount of asphalt you want to crush per hour.

“An impact rotor style (impactor) is by far the best type of crusher for asphalt crushing due to the high rpm spinning rotor,” he says.

While Stu Gamble of Sandvik agrees that an impactor is the kind of crusher you would use on clean asphalt, he points out that many times at job sites, there is contamination like concrete under asphalt or rebar. In those cases, it may make sense to use a jaw crusher before the impactor, he says. “There are a lot of considerations. It’s the incoming material, the contract, and what the rules and regulations are.”

Climate, including both moisture and temperature, can make a difference in how asphalt responds in a crusher, according to Gamble. In some hotter climates asphalt crushing is done at night. In a colder climate, production will be faster, he adds.

Another factor that makes a difference in production is the final product size. For example, if the final product size is a regulation ¾ inch, it will take longer to achieve that than if it is 2 inches, says Gamble. “Your production ends up being controlled by the final product size you are trying to get to,” he explains.

While asphalt is often crushed and reused onsite, there are also many instances where asphalt is not recycled onsite. According to Cavaliere, in highway milling projects, it is common to haul the milled material back to the asphalt plant where it is crushed, sized, then added to new asphalt mixes.

In Los Angeles and Portland, Ore., CEC has crushers in place that break up asphalt that is generated from the cities’ maintenance and repair work. The asphalt pavement is taken to central locations in those cities and stockpiled for processing.

Asphalt pavement is often crushed offsite.

Prescreening can make crushing easier.

ADAPTING TO MATERIAL
A fixed operator who processes strictly concrete may not have a large enough screen or a large enough prescreening or scalping capacity to remove the abrasive material in asphalt from the feed, according to Smith. In diesel fired operations, operators may not have proper air ventilation to the cooling system of the engine to keep it free from the asphalt particulate dust.

“Asphalt particulate dust can clog up the cooling systems of engines readily,” warns Smith. He says to consider the dust that blows around a job site. “If it is asphalt, it is going to be sticky and gooey so if it hits a hot radiator or a hot engine, it will melt itself to it.”

Asphalt will give operators a much faster throughput than concrete, says Smith. The finished product size will also differ from concrete.

Smith also says there is a trend in asphalt crushing toward producing refractionated RAP (recycled asphalt pavement), or different sizes.

“It dictates that you are probably going to need a larger screen in your processing plant than you would have if you just did an 1-¼-inch-minus concrete,” says Smith. “If you are going to do a large volume of [two sizes of] RAP, you may need a larger screen.”
 

ADDITIONAL ASSISTANCE
You may choose to prescreen a large volume of recycled asphalt before you ever put it into your processing plant. CEC has a product called Screen-It. Running the asphalt through the Screen-It pulls out the pieces that are already to spec so only the oversized material is processed.

“It is much easier on the crushing system if you can get all of that out ahead of time,” Smith remarks. “Screening is far less expensive than crushing, so by screening the finished product out of it, they’re able to cut the size of the crusher, the screen and processing plant down to a manageable size.”

Although asphalt is abrasive, it is much softer than concrete. Breaking the oil apart that binds the stones together is easier than breaking rock or concrete apart. According to Smith, the downside to crushing asphalt pavement is that “the high compressive strength and the abrasive nature of the raw material causes it to be harder on the crusher as far as wear costs go.”

CEC uses an addition called a ball deck, which is a cleaning system for the fine materials. “Because asphalt can get sticky, it can try to glue itself to the fine wire used in the screening. Using a cleaning system can help avoid that problem,” Smith explains.
 

WAYS IT PAYS
People are trending more toward reusing oil in order to save on the cost of hot asphalt, notes Smith. Often times through proper blending of material and proper heating methods in the asphalt plant, asphalt can be reused. In some cases, he says, asphalt can be reheated without using any virgin material at all.

“So by cutting down the need for virgin asphalt in the mix, they’re able to save on their oil costs, and depending on their ability to blend the material into the asphalt mix and what their specifications allow, they are able to cut their finished product cost because of the ability to reuse that oil,” says Smith.

The green movement has led to an increasing use of asphalt in alternative heating methods, some with regenerative agents and some with alternative heating methods such as microwaving, according to Smith. “This is kind of a blossoming new method,” he says. “It is a push across the country.”