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USE OF RECLAIMED ASPHALT PAVEMENT AND RECYCLED ASPHALT SHINGLES IN ASPHALT MIXTURES The practice of utilizing reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in new asphalt mixtures has increased in recent years because of its economic and environmental benefits. RAP has already become one of the most widely used materials in the United States, and RAS is emerging as a material of interest to the paving community. With increased demand and limited supplies of aggregate and binder, recycled materials with usable asphalt binders and aggregates can be valuable sources of these materials. Although the potential benefits associated with using these recycled materials are high, only a few state agencies currently use more than 25% RAP (designated as high RAP in this synthesis), RAS, or a combination of both in their roadway asphalt mixtures. The objective of this synthesis is to summarize current practices for the use of high RAP and RAS in the design, production, and construction of asphalt mixtures. The scope of this synthesis focuses on collecting information about the use, rather than just what is allowed, of high RAP, RAS, and/or a combination of RAP and RAS. A literature review, a survey of state agencies, and case examples were used to document current knowl- edge and practice. The literature review collected information about designing, producing and placing, testing, actual performance, and potential economic benefits when using high RAP, RAS, or a combination of both in asphalt mixtures. State materials engineers were surveyed to collect information about current practices for determining recycled material properties, developing mix designs, and using laboratory testing for assessing pavement performance. State construction engineers were surveyed about how to produce and place mixtures with recycled materials. Responses were received from 45 of the 51 agencies (50 states and the District of Columbia), an 88% response rate. Case examples were developed for five key topics. The first example shows how the Georgia Department of Transportation (DOT) developed and revised its specifications to encourage contractors to consistently submit mix designs using from 30% to 40% RAP in all pavement layers. The second example documents contractor practices and procedures used to produce and place high RAP mixtures for Georgia and five other surrounding states (Alabama, Florida, North Carolina, South Carolina, and Tennessee). The third case example provides guidance from a Missouri contractor for processing RAS for use in asphalt mixtures. The fourth case example shows how the Minnesota DOT collected performance data from non- state agency project roadway databases (i.e., county roadways) used in surface mixtures. The fifth example documents four recent research projects (three RAP, one RAS) designed to esti- mate the percentage of recycled asphalt binder that can be transferred to the virgin aggregate in the asphalt plant before the virgin asphalt is added (i.e., dry mixing). Information obtained from the literature and from the surveys show that recycled material asphalt influences the upper and lower critical performance grade (PG) temperatures with the upper critical temperatures changing about twice as fast as the lower critical temperatures. The asphalt in tear-off RAS, also referred to as postconsumer RAS, is stiffer than that from manufacturing waste (preconsumer) RAS. Either source of RAS has asphalt properties that are significantly stiffer than RAP asphalt. SUMMARY
2 Gradations of the recycled material aggregate are routinely directly measured. The aggregate specific gravity is estimated from measurements of the theoretical maximum specific gravity of the recycled material (prior to removing the asphalt), although a few agencies directly mea- sure the fine and coarse aggregate specific gravity after either ignition oven removal or solvent extraction of the asphalt. Laboratory practices for drying recycled materials, batching materials for sample prepara- tion material, preheating times and temperatures, and the order of the addition of materials to the mixing bowl vary considerably. Each state agency or group of researchers uses differ- ent test methods and criteria for the laboratory assessment of performance characteristics. At this time, there are no consistent practices for preparing, testing, and evaluating asphalt mixtures with recycled material content. RAP material can be obtained from the demolition of old pavement that produces chunks that have to be broken up or crushed, milling of existing pavement surfaces, and fresh mix- tures remaining from plant start-up, shutdown, or rejected out-of-specification mixtures. RAP aggregate gradations, dust content (i.e., percent passing 0.075-mm sieve), and asphalt content vary because of the types of equipment used to crush and/or mill the old pavement, processing practices, milling depths, and the types of mixtures in each layer milled. Fractionating the RAP into two, or at most three, sizes can help minimize material variability when higher percent- ages of RAP are used. Finer RAP fractions tend to have higher asphalt contents than coarser fractions, but can also have high percentages of minus 0.075-mm material that can limit the percentage of RAP that can be used (i.e., specification limits on dust-to-asphalt ratio). Several agencies use specific terms to designate RAP materials based on common aggre- gate characteristics, asphalt content and properties, how the RAP is processed (e.g., âextended RAPâ), and how the stockpile is built, tested, and maintained (e.g., âcaptiveâ and âcontinu- ousâ). However, this terminology is agency-specific; there is no consistency in terms. Separate stockpiles are required for manufacturer because the asphalt content and proper- ties are significantly different for manufacturer (pre-consumer) and tear-off (post-consumer) waster RAS. The aged tear-off shingle asphalt is significantly stiffer than the asphalt in manu- facturing waste RAS. Regardless of the type, RAS ground to a maximum particle size of 3/8 in. is more easily distributed throughout the asphalt mixture during production. The age, type, and equipment options (e.g., flighting, double drums, and separate drying drums for recycled materials) of the plant control the ability of the plant to remove any moisture in the recycled materials. The percentage and/or type of recycled material that can be added to the mixture is directly related to the ability of the plant to remove the moisture. Although covering the recycled material stockpiles help minimize moisture content, only a limited number of agencies indicated that this practice is either used by contractors in their state or is required by their agency. Contractor costs increase significantly because higher plant temperatures (i.e., increased energy consumption) are required to superheat the virgin aggregate for heat transfer to the recycled materials. Increased wear on plant equipment and baghouse damage resulting from the high heat and increased down time for maintenance also increase costs. High plant tem- peratures can also damage asphalt properties and increase the likelihood of penalties (dis- incentives) for out-of-specification mixture temperatures. These additional production costs can offset savings from lower material costs, which is one of the potential benefits attributed to the increased use of recycled materials. The pavement performance reported in the literature found that performance is related to construction difficulties, the percentage of virgin asphalt in the mixture, and changes in the upper virgin asphalt PG temperature. Early signs of pavement distress(es) in RAP mixtures
3 corresponded to documented construction difficulties such as visible deleterious materials (oversized RAP), dry looking mixtures (low asphalt contents), and mixture segregation. Reductions in load-related longitudinal cracking can be achieved by using a virgin asphalt with a reduced upper PG temperature. More than 5 years of service life is the minimum time needed for differences between virgin (control) mixtures and mixtures with recycled materials to emerge. Mixtures placed next to or over jointed or cracked portland cement concrete pavements show signs of reflective cracking, regardless of whether or not recycled materials are used in the mixtures. Suggestions for future research that may help increase the use of recycled materials in asphalt pavements included ⢠Improve laboratory procedures for drying, preparing, preheating, mixing, and compact- ing mixtures that more closely replicate what happens during production at the asphalt plant. ⢠Study of existing pavements with high RAP content (more than 25%), RAS, and combi- nations of RAP and RAS in surface mixtures for more direct correlation between the type and percentage of recycled materials and individual pavement distresses. ⢠Establish the expected service life of mixtures with recycled materials. This information is necessary for life-cycle cost calculations. ⢠Study recyclability of high RAP, RAS, and RAP/RAS combination mixtures. ⢠Investigate the impact of minimum and maximum silo storage times on recycled mate- rial asphalt mixtures.
