In the HMA facility, the RAP must be dried and heated prior to mixing into the HMA. Heat transfer to the RAP can be accomplished either conductively or convectively. Conductive heat transfer is used in almost all batch facility applications and when RAP is introduced in counterflow dryers while convective heat transfer is used in conventional batch plant aggregate dryers and parallel-flow drum mixes.
Since recycling began in the late 1970s, several approaches have been developed and are in use for drying and heating RAP in hot-mix facilities. Early in the evolution of this process, the industry’s primary concern was whether RAP could be processed practically and whether the resulting hot-mix product was sufficient to be used as structural pavement.
Once that was established, the industry shifted its attention to the environmental concerns of processing RAP in HMA facilities. Different approaches exhibited different levels of hydrocarbon and dust emissions. The concern over limiting the amount of hydrocarbon vapors, or “blue smoke” as it is commonly called, led to the development of new drying and processing technologies, such as the counterflow drum mixer, in an effort to process high percentages of RAP without negative impact on the emissions from a hot-mix facility.
No special techniques or equipment are required for laydown and compaction when using recycled mixes. However, recycled mixes are frequently placed at slightly cooler temperatures than virgin mixes in an effort to reduce the negative impact of super-heated temperatures on the facility equipment.
With slightly cooler mat temperatures, and depending on the RAP percentages used in the mix, paving personnel may find that rolling and compaction times are reduced. Conventional equipment, techniques and indicators of a completed mat apply.
Paving superintendents frequently claim that recycled mixes are “stiffer.” Translated, this means that roller and compactors need to “get on the mat” quickly and get their job done. Densities are typically achieved faster than with mixes made from all virgin materials. This is the only difference typically encountered when paving with recycled mixes.
The Federal Highway Administration’s (FHWA) Pavement Recycling Executive Summary and Report presents results of surveys of 17 states on the performance of RAP mixes. The report notes that many states have completed performance evaluations for internal use but have often not documented the findings in research reports.
Nine states were asked to gather performance information. The consensus of these states was the HMA with RAP performed as well as conventional HMA when the RAP mix was properly designed and produced. Failures that were noted could be traced to lack of proper consideration to good mix design practices. Verification of the RAP mix design during production is just as important as verification of a virgin material mix.
No significant differences between recycled and control pavement sections in pavement performance condition, structural numbers or recovered asphalt cement properties were reported.
Use of RAP is now a well-established, routine production practice in the HMA industry. With the growth of cold milling of HMA pavements as a rehabilitation strategy, production of RAP is estimated at 45 million tons per year. The FHWA estimates that one-third of all HMA removed is recycled into HMA production.
Industry experience in the last 20 years has proven that HMA is a resource that can be recycled time after time. Most agencies believe that mixes made with RAP have performance equal to or better than mixes made with all virgin materials, provided that appropriate mix design and process control during production are present. RAP mixes can be designed to meet all applicable HMA performance requirements.
Various processing technologies have been developed to permit HMA facilities to be retrofitted for various percentages of RAP. Visible and gaseous emission considerations have been addressed to maintain acceptable air quality.
Crushing, stockpiling and handling techniques have been developed to ensure consistency and quality in the RAP materials.
Recycling HMA is therefore a “win-win” scenario. The consumer wins with lower construction and rehabilitation costs without compromising quality, thereby stretching tax dollars and allowing more roads to be kept in better driving condition than if all virgin materials are used. The public also wins by reduced volume of construction rubble in landfills and dumping sites. The construction industry wins because higher volumes of production from the same fixed investment lowers business risk and keeps more people employed.
RAP reaps green for environment and business
Reclaimed asphalt pavement (RAP) not only is good for the environment, it also can be good for business. A year ago, Bond Paving Co., Gulfport, Miss., took delivery on a RAP plant and is crushing an average of 700 to 800 tons a day.
“It’s been by far the best investment I’ve made in a long time,” said James Bond Sr. The company, which has been in operation for 40 years, uses the plant on its contracted city and county projects.
About 25%of the crushed RAP is added to mixes in the company’s asphalt plant. “By using crushed RAP in your asphalt mix you get better stability,” said James Bond Jr.
The 100-tph-capacity Telsmith RP3036 RAP plant is moved every few months to a customer who needs to crush RAP. It takes the company one day to move the plant and six hours for it to be erected.
The plant consists of a 3036 HSI impact crusher, a 36-in. x 12-in. vibrating grizzly feeder and a 4-ft x 12-ft double-deck Valu-King screen. The plant produces a finished product of -1/2 in.