Dry products require less energy to heat. At a minimum, Jackson recommends stockpiling your materials on a sloped area so that the water can drain out of the stockpiles.Other considerations for stockpiling:
- Paving your storage area helps reduce stockpile loss, avoids contamination from mud, and allows for better drainage
- Cover your RAP stockpiles to keep them dry if you can
- First in - first out: If only one end is open, you are constantly adding wet material to the front of your stockpile and the dry material is not getting used.
“After a rain event you will lose production if you do not have a well-drained storage area,” says Jackson. “RAP will not drain as fast as clean, virgin aggregate.”
Drying the RAP & WMA
Trying to super heat the virgin aggregates to dry out the RAP can cause some very high stresses in the plant leading to increased maintenance, loss of production and a shorter life on your critical wear parts, such as the drum shell, tires and trunnions.
“I’ve had colleagues tell me that they have recorded shell temperatures of 1,200° F,” says Jackson. “We have been running warm mix using Evotherm 3G for the last four years and have noticed a decrease in temperature-induced wear in our plants.”
The Evotherm 3G also allows West Contracting’s road crews to achieve density easier so they are not telling the plant operators to turn up the heat because they cannot get compaction, explains Jackson.
“At the plant, we save burner fuel and wear on the plant,” he says. “Using warm mix is essential for producing the most economical high RAP mixes. One thing that we have noticed when using a surfactant warm mix additive is that some residual moisture increases the efficiency of the warm mix additive. Since the product is an adhesion promoter, the residual moisture does not promote stripping.”
In addition to reducing heat-related damage to your plant, using warm mix with a high RAP content helps combat stiff binders, says Jackson. “You’re not heating the virgin or reclaimed binder as much,” he says.
Plant component considerations
In addition to high heat issues, there are other components on your asphalt plant that need some considerations during production of high RAP mixes.
For example, RAP collars on older plants were probably designed to utilize 10-20% RAP. “As a result when you try to make the move to higher RAP utilization then the collars can plug, especially with wet recycled materials,” says Jackson. “We have found that by enlarging the openings on the inside of the drum and using a larger opening to feed the RAP into the drum helps solve this problem.”
Moisture and dust are contributing factor to clogged RAP collars. “Try to avoid having your dust return feed into the drum close to where the recycled materials enter the drum,” suggests Jackson. “The dust will bind to the moisture in the RAP and cause your plant to plug up.”
If you have both a drying drum and a mixing drum, then you don’t have to have a RAP collar. “You can feed the RAP directly into the back end of the mixing drum,” says Jackson. “It’s easier to increase the amount of RAP in the mix and the large opening to feed RAP into the drum can be inspected easily by your ground person.”
Flight adjustments in the drum are another consideration for high RAP mixes. “We have adjusted our flights in the drums over the years,” explains Jackson. “A lot of what we have learned is through trial and error.
“We have found that working with the right people at the plant manufacturer is the best way to learn how to make adjustments to the flight,” he continues. “I always recommend training, and the best training that I have had is at the Astec Advanced Customer Schools. We took what we learned there and modified our flights to our needs. It helps to have some great plant operators that can take what they have learned and figure out what is best for their plant.”
Another component that can make a difference during production of high RAP mixes is the conveyor belt and the proper incline. “In order to add additional incline belts to the plant, where we are already constrained on space, we tend to make the incline belts steeper,” says Jackson. “If the belt is too steep, then the large RAP aggregates tend to roll back down the belt toward the scalping screen.”