In business, as in daily life, few things are certain when planning for the future. In the world of asphalt, one thing that is certain in the years to come is the growing demand for Reclaimed Asphalt Pavement.
But, planning to increase RAP content at the plant is a multi-faceted effort and one that requires special planning across the entire operation, from collection, transportation and stockpiling of the material, through to the plant’s handling, mixing and processing of the material.
Nearly 50 years ago the EPA and FHWA, in a study and report to Congress, identified asphalt pavement as the nation’s foremost recycled product and today, according to the National Asphalt Pavement Association (NAPA), “It continues to be reclaimed and reused at a greater rate than any other product in the U.S.”
Already, some asphalt plants are providing mixes containing 65% and higher levels of RAP content for select applications and, while pavement mixes in the U.S generally are said to be plus or minus 20% RAP, companies continue to work on methods to conveniently and economically go well beyond that level.
In fact, the 2018 annual NAPA survey of asphalt mix producers showed that new mixes contained more than 82.2 million tons of RAP, a nearly 8% increase over the prior year’s construction season. The nationwide average percentage in mixtures rose to 21.1%, a new high for the annual survey, which is conducted under contract with the FHWA.
The Economic Value of RAP
The number one benefit of using RAP is cost. The aggregate itself hasn’t really aged. It has the same physical properties as it did the day it was originally crushed and used to make a road.
“The return on investment is high,” said Michael Varner, Astec vice president, engineering. “If you look at the evaluation of recycle, it adds anywhere from 34 to 44 cents per percent per mixed ton. That number comes from the value that is inherent in the materials that make up RAP—the liquid that is in the RAP, the asphalt cement or the binder, and the rock.
Recycle is worth the material it’s replacing. Some areas have more asphalt pavement to be reclaimed than other areas. Using recycled aggregate saves money that would’ve been spent on virgin aggregate.”
As interest continues to increase across the country for more RAP in new pavement, asphalt plant technology also has improved for a smoother transition toward higher percentages in the mix.
The benefits and savings of mixtures with high RAP content requiring less reliance on costly virgin aggregates are well documented and asphalt plants, especially older operations, still need to plan ahead and make important adjustments and process changes in order to avoid problems when aiming higher than 40% RAP content.
“In order to use RAP content above 40%, heating of the RAP is essential to ensure enough workability,” according to a research team working with the Worcester Polytechnic Institute. “In a conventional drum mix plant, 60-70% RAP content can be processed but it is restricted to 50%, because of the short-term ageing and gaseous emission which occurs due to the direct exposure of RAP to the burner flame,” researchers Martins Zaumanis, Rajib B. Mallick and Robert Frank noted in a paper presented at the E&E Congress 2016 meeting held in Prague, Czech Republic.
Take Advantage of New Technology
As for individual plants, the process used in drum plants was long considered to be the way to go when looking to increase RAP content. Feeding the RAP into the system continuously releases moisture as the RAP is heated releasing smaller amounts of steam for exit through the plant’s exhaust system.
Batch plants traditionally had been much more challenged in the ability to increase RAP content into mixes since the process called for dropping the reclaimed material into the mix as a batch, emitting larger amounts of steam and potentially blowing out high quantities of dust and causing emissions problems for the plant.
But, as with many technological advancements in the recent years, engineers have begun finding ways to equip all plants with cost-effective measures to raise RAP use in mixtures beyond previous process limits and keeping emissions at acceptable levels.
A Double Barrel mixer by Chattanooga, Tennessee-based Astec Inc., when set up in combination with a batch tower, can allow for the production of a variety of batch mixes and high-volume continuous mixes using RAP.
Key cost saving features of the Double Barrel mixer include a unique design that utilizes the entire drum length for more efficient drying, a technique that allows the material in the mixing chamber to capture heat radiated from the process, and a thick layer of insulation surrounded by the shell holds heat in to save energy.
The company also offers a Double Barrel XHR high RAP aggregate dryer with external mixer that is capable of producing 65% RAP in mixtures based on 5% RAP moisture. A stainless steel drum and combustion flighting are made to withstand the higher temperatures associated with running high RAP. Sequential mixing ensures ingredients are added to the hot mixture in an order that allows better temperature equalization and even distribution of all particles throughout the mix.
The included V-Pac stack temperature control system can maintain low exhaust temperature when running high RAP content. The control system saves energy by adjusting the drum to the optimal rotation speed for maximum heat exchange to the aggregate and minimum heat lost out of the stack. Thus, operators get optimum fuel usage, which in turn means they do not burn excessive fuel.
The XHR system separates the drying process from the mixing process allowing for RAP to be preconditioned through the external part of the Double Barrel dryer before being discharged to the external mixer where the liquid binder is added. Sequential mixing ensures ingredients are added to the hot mixture in an order that allows better temperature equalization and even distribution of all particles throughout the mix.
Overcoming Obstacles and Challenges
While plant manufacturers have made progress toward increasing RAP content at both drum and batch operations, even long-time and established plants still face a variety of obstacles and challenges in order to successfully meet the growing demand for reclaimed material use in their asphalt mixtures.
Older plants might have outdated RAP collars that limit mixtures to between 10% and 20% to avoid becoming clogged by moisture and dust. As RAP content increases, it may be wise to confer with plant manufacturers in order to widen some drum openings to prevent clogging when feeding RAP into the system and to assess the maximum RAP capability of the system as a whole. Using both drying and mixing drums in the process can make increasing the mix less challenging since that approach can eliminate the need for RAP collars.
Other factors to consider when taking steps to increase RAP content involve careful stockpile management and consideration of the size and pitch of the conveyor systems to make certain that they are capable of handling the larger amount of recycled materials without reaching capacity limits, causing problems, delays and downtime.
“Poor management of RAP stockpiles is commonly cited as a reason agencies are reluctant to increase allowable RAP contents in asphalt mixtures,” according to NAPA. “For production of quality mixes with high RAP contents, excellent materials management practices are essential.”
The pavement association suggests taking an initial four-step analytical approach in order to establish realistic goals for how much RAP can be used at an individual plant:
- An inventory of RAP on hand and RAP generated per year;
- A summary of mixes produced per year by mix types and customers;
- Determining the maximum amount of RAP that can be used;
- A comparison of the quantity of RAP available to the amount of RAP needed.
“The management of RAP is important because it holds many resources with high value to the end user, such as oil and sometimes scarce aggregate sizes,” Patrick Reaver, product development manager for Astec Mobile Screens, told For Construction Pros a while back. “If RAP is not managed properly, you can create contamination or lose control of what mixture of these resources were used, thereby decreasing the value of the RAP.”
Some agencies may permit only “classified RAP,” i.e. reclaimed asphalt from their projects, to be used since the origin of the materials is known and the aggregate and binder quality were satisfactory in the original pavement. Space permitting, plants may consider placing incoming RAP from different sources in separate stockpiles.
In addition, moisture content of aggregates and RAP is a primary factor affecting an asphalt plant’s production rate and drying costs, according to NAPA, which points out that the best practice to minimize moisture accumulation is to cover the stockpile with a shelter or building to prevent precipitation from getting to the RAP. An alternate technique would be to use conical stockpiles that naturally shed rain or snow and to place the stockpile on a paved and sloped surface to help drain the water. In addition, managing the stockpiles so that moisture content remains consistent in the feed material is important.
Partner for Success
While there are many variables for asphalt plants to consider along the way, a good strategy would also include a cooperative approach in the research, sampling, testing and analysis of RAP across many agencies and environments to stay ahead of the curve as RAP percentages continue to increase. And, don’t overlook the importance of staying up-to-date on plant equipment advancements along the way.
RAP processing is very different from that of traditional aggregate crushing, Astec’s Reaver has pointed out. It often is one of the last things plants may think about, but can have negative implications for maintenance, production and efficiency if producers attempt to use equipment not designed for the materials and tasks at hand.
Established asphalt plants can benefit greatly by working closely with a plant manufacturer to determine the best and most cost-effective systems for their particular operations.