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Page 94
Suggested Citation:"Chapter Six - Conclusions ." National Academies of Sciences, Engineering, and Medicine. 2016. Use of Reclaimed Asphalt Pavement and Recycled Asphalt Shingles in Asphalt Mixtures. Washington, DC: The National Academies Press. doi: 10.17226/23641.
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Page 94
Page 95
Suggested Citation:"Chapter Six - Conclusions ." National Academies of Sciences, Engineering, and Medicine. 2016. Use of Reclaimed Asphalt Pavement and Recycled Asphalt Shingles in Asphalt Mixtures. Washington, DC: The National Academies Press. doi: 10.17226/23641.
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Page 95
Page 96
Suggested Citation:"Chapter Six - Conclusions ." National Academies of Sciences, Engineering, and Medicine. 2016. Use of Reclaimed Asphalt Pavement and Recycled Asphalt Shingles in Asphalt Mixtures. Washington, DC: The National Academies Press. doi: 10.17226/23641.
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Page 96
Page 97
Suggested Citation:"Chapter Six - Conclusions ." National Academies of Sciences, Engineering, and Medicine. 2016. Use of Reclaimed Asphalt Pavement and Recycled Asphalt Shingles in Asphalt Mixtures. Washington, DC: The National Academies Press. doi: 10.17226/23641.
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Page 97

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95 • Recycled material aggregate gradations are the most frequently determined aggregate property. – Other consensus and/or source properties are infre- quently measured during mix design. – Quality control/quality assurance consensus and/or source property testing during production, if done at all, is usually conducted using the total asphalt mix- ture sample. • Recycled material asphalt is recovered with either the Abson or Rotavapor recovery method and the properties are determined for the high temperature shear modulus, G*, using the dynamic shear rheometer, and low tem- perature stiffness and m-value with the bending beam rheometer are determined. – However, agencies may determine these properties for any one of the following: n As-recovered asphalt; n After recovery and rolling thin film oven (RTFO); and n After recovery, RTFO, and pressure aging vessel aging. – Recycled material asphalt increases both the upper and lower critical performance grade (PG) temperatures; however, the upper critical temperature increases about twice as much as the lower critical temperature. n A strong linear correlation exists between the change in the upper critical temperature and the change in the lower critical temperature. • Specific gravities of the recycled material asphalt and aggregates are typically: – Assumed for the asphalt to be between 1.01 and 1.035, or the virgin asphalt-specific gravity is used for the recycled material asphalt. – Calculated for the aggregate using measured theoreti- cal maximum specific gravity of the recycled material. SELECTING VIRGIN ASPHALT GRADE • Selecting the virgin asphalt grade is accomplished by: – Extracting, recovering, and testing the recycled material asphalt so that properties for blending charts can be measured. – Limiting the percentage of recycled material so that blending charts are not needed. – Development of agency-specific virgin asphalt selec- tion tables that define the required virgin asphalt for a full range of recycled material percentages. – Using agency-defined asphalt grade “bumping.” Information reported in the literature review and from each of the state agency surveys is summarized in this chapter. The information is organized by the key topics that include general availability of reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS) materials, recycled material properties and testing, selection of the virgin asphalt grade, recycled material mix design practices and proce- dures, perceived and reported pavement performance of recycled material asphalt mixtures, asphalt plant practices including RAP and RAS stockpiling, asphalt plant opera- tions and equipment, transfer and placement of recycled material mixtures, and information for field inspectors. Suggestions for future work are included at the end of this chapter. AVAILABILITY OF RECYCLED MATERIALS • Recycled materials are generally available: – Statewide for RAP, with some excess of RAP in cities or urban areas, or in one or more districts within a state. n Some agencies note that they would prefer to have more access to RAP supplies. – Only in limited areas for RAS. n Supplies, usually in excess of demand, are available in urban or limited locations within a state. RECYCLED MATERIAL PROPERTIES AND TESTING • Recycled material asphalt content is most frequently determined using the ignition oven method, although the majority of these agencies also use either centri- fuge and/or reflux solvent extraction. Trichloroethylene solvent is typically used, although some agencies use n-propyl bromide. – Eight agencies specifically noted that they no longer use any solvent extraction in their laboratories. – Ignition oven correction factors are essential for mix- tures with aggregates that degrade during testing or nonasphalt material components that burn off (i.e., RAS backing) and can be determined by: n Comparing results from the ignition oven to those from solvent extraction. n Preparing mixtures with known material contents and material properties and evaluating changes after ignition oven testing. chapter six CONCLUSIONS

96 RECYCLED MATERIAL MIX DESIGNS • For adjusting calculations of the total asphalt content. – Review appropriate asphalt availability factors for both RAP and RAS. – Consider limiting the contribution of the recycled material asphalt to the total asphalt content by using a: n Minimum asphalt binder ratio for ensuring that a minimum percentage of virgin asphalt is included in the mixture. n Maximum recycled binder ratio to limit the amount of recycled material asphalt that is included in the mixture. • Review the batch weight (masses) used to prepare materials for the mix design sample preparation. – The mass of the asphalt content in the recycled material that does not contribute to the asphalt con- tent [i.e., 1 - (asphalt availability factor)] is consid- ered with the mass of the recycled material aggregate. • Consider checking compaction levels and increasing asphalt content: – Overcompaction of mix design samples can lead to the selection of an asphalt content that is too low. Higher RAP and/or RAS mixtures tend to look dry. n Some agencies found that dry looking mixtures have difficulties during construction and show early signs of pavement distress(es). ○ These agencies reduced compaction levels and/or increased the percentage of virgin asphalt to improve constructability and performance. • Sizing (sieving) recycled materials for batching may or may not be done. The recycled materials can be batched: – As-received without any additional fractionating into individual sieve sizes. n Definitions of coarse and fine fractions depend on how the contractor separates the recycled materials for stockpiling. – Sieved into individual sieve sizes for better control of gradations for mix design purposes for: n Only the coarse fraction, n Both the coarse and fine fraction, or n Only the fine fraction (typically for dust control). • Drying recycled materials for batching is agency-specific. – Materials may be dried: Not at all; n Under a fan, in a conventional oven, or in a micro- wave oven; n From 1 h to overnight; n At temperatures from room temperature to 300°F (149°C); and n There is no standardized definition for “dry.” – If defined as “dried to a constant mass,” the times between subsequent weighing vary or are not well- defined, and the acceptable change in mass ranges from 0.05% to 0.5% of mass. • Preheating temperatures for virgin aggregate and recycled materials before mixing are variable and agency-specific, however: – Most agencies use higher preheating temperatures for virgin aggregates. – Agencies may or may not preheat recycled materials. n If preheated, the temperature used is generally lower than that used for virgin aggregate. – Some agencies combine the virgin aggregate and recycled materials and heat together (i.e., use the same temperature for virgin and recycled materials). • Order of addition of materials to the mixing bowl is agency-specific; however, there are general trends that indicate the: – Virgin aggregates are added first, followed by the recycled materials (if not batched with the virgin aggregate), and may or may not be briefly dry mixed, before the addition of the virgin asphalt. – Mixing is complete based on a visually uniform coat- ing of materials, although some agencies use set mix- ing times that typically range from 1 to 10 minutes depending on the type of laboratory mixer. • Short-term aging of the mixture most frequently uses 2 hours; however, other times used for short-term aging included 1.5 h, 4 h, and 15 h ± 3 h. – Short-term aging temperatures of 140°F to 335°F (60°C to 168°C). • NDesign for compacting mix design samples range from a single value of 65 for almost all of the agency’s dense mixtures to multiple numbers of gyrations based on traf- fic levels or positions in the pavement structure. – Marshall mix designs are still used with: n 70 blows per side for base course mixtures. n 35 blows per side for stone matrix asphalt mixtures. n 6-in. (150-mm) sized molds for large stone mixtures by one agency. • Mixture volumetric calculations require accurate infor- mation about the asphalt content, asphalt specific gravity, and aggregate specific gravities and asphalt absorption capacities for each source of aggregates in the mixture. • Air void, voids in mineral aggregates, and dust–to-asphalt ratio specification requirements can be more difficult to meet when: – The percentage of RAP increases above 25%. n These volumetric properties can be difficult to achieve, although less frequently, even when the percentage of RAP is less than 25%. – The mixture contains RAS (any percentage). – The mixture contains a combination of RAP and RAS (any percentage). • Performance-based mixture property testing by agencies: – Most frequently evaluate the rutting resistance using either the Asphalt Pavement Analyzer or Hamburg loaded wheel devices. – Less frequently evaluates the mixture stiffness using either resilient modulus or dynamic modulus testing and usually for research purposes rather than as a part of the mix design process. – Occasionally use either the disc-shaped compact tension or semi-circular bend to evaluate low tem-

97 perature, reflective, and/or top-down fatigue crack- ing potential primarily for research purposes. n One agency allows the contractor to use a higher percentage of RAP based on acceptable semi- circular bend test results. PAVEMENT PERFORMANCE OF RECYCLED MATERIAL MIXTURES • Agency perceptions of pavement performance for mix- tures with increases in the RAP percentage or when RAS and/or a combination of RAP and RAS is used are that: – Rutting decreases with the use of either RAP or RAS, or increasing percentages of RAP, because these materials increase the mixture stiffness. – Cracking potential, either traffic-related or reflective, may increase, depending on where the mixture is located in the pavement structure. – Thermal cracking potential increases because of the increasing mixture stiffness. – Moisture sensitivity may, or may not, increase. • Literature review pavement performance for recycled material asphalt mixtures found that: – Pavement performance can be related to the percent- age of virgin asphalt in the mixtures. – Decreasing the upper performance-grade tempera- ture reduced the traffic-related cracking without sig- nificantly influencing the rutting resistance. – After between 5 and 10 years of performance, mixture with up to 30% RAP had similar performance com- pared with control sections almost half of the time (Long Term Pavement Performance SPS-5 sections). n When there was a difference in the pavement per- formance, the control sections (no RAP) performed better than the RAP sections approximately 30% of the time. n RAP sections performed better than the control sections approximately 20% of the time. – Projects that documented construction difficulties also showed early signs of pavement distresses. – At least 3 to 5 years of performance data are required to see a significant difference between mixtures with and without recycled materials. – Pavements constructed over, or next to portland cement jointed or cracked concrete pavements are typically prone to reflective cracking. – Additional sources of pavement performance data may be obtained by retrieving data for nonagency projects. ASPHALT PLANT PRACTICES AND OPERATIONS Recycled Materials Stockpiling and Processing • RAP stockpiling and processing use: – Terminology, when used by agencies, to identify and differentiate between different types of RAP stock- piles is agency-specific. There are no standardized terms at this time. – RAP scalping screen sizes that are typically: n 19 mm (3⁄4 in.) n 12.5 mm (½ in.) n 9⁄16 in. – RAP screen sizes for fractionating into coarse RAP (retained on) and fine RAP (100% passing) include: n 4.75-mm (No. 4) n 9.5-mm (3⁄8 in.) n 2.36-mm (No. 8). – Separate stockpiles are used for RAP obtained with micro-milling machines because of the high fines (dust) content. – Separate stockpiles can be continuously built, main- tained, and tested (quality control) n Variability is minimized when RAP is separated by mixture types with similar mixture characteristics (i.e., gradations, asphalt content). – Additional quality control testing is important to man- age RAP asphalt and RAP gradation variability when using a higher percentage of RAP. – Contaminates in the RAP stockpiles are to be evaluated. • Examples of contaminates in RAP include crack filling materials, geotextile fabrics, vegetation growing on stock- piles, or trash. • RAS stockpiling and processing: – Remove contaminates from the RAS supply prior to processing. n Any contaminates left in the RAS are ground up along with the RAS. – Keep separate stockpiles for manufacturer waste RAS and tear-off RAS. – Maximum size no larger than 100% passing the 3⁄8-in. (9.5-mm) sieve. n Some agencies use smaller sieve sizes for 100% passing. – Add sand to RAS to help prevent clumping. – Avoid grinding in hot, wet weather. • RAP and RAS quality control/quality assurance testing at the asphalt plant and during production and placement: – Asphalt content is frequently measured using the igni- tion oven. – Washed aggregate gradations are usually evaluated using aggregates retained after ignition oven testing. – Aggregate specific gravities are most frequently cal- culated using theoretical maximum specific gravity measurements for a RAP stockpile sample. – Aggregate consensus and source properties, when measured, typically use samples of asphalt mixtures from haul truck or behind the paver for testing. Asphalt Plant Operations and Equipment • Feeding recycled materials into asphalt plant can be: – More difficult because of crusting on the stockpile sur- face, clumping, and bridging of materials over weigh belt scales.

98 n In-line crushing and/or sizing systems breaks up, or scalps off, oversized materials. – Easier when using additional cold feed bins. n Bins with nonstick surfaces, steeper side slopes, or a self-relieving bottom help material that tends to stick and clump in the bin flow out more uniformly. – Metered into the plant more uniformly when: n Separate weigh belt scales are provided for each type of recycled material. • At times, asphalt plants slow production rates and increase plant temperatures for better drying when using more than 25% RAP. • Asphalt plant modifications that help use or increase the percentage of recycled materials include: – Addition of a separate system to any type of plant for drying and preheating recycled materials. – Batch plants: n Consider screw conveyor or belt scale to move recycled materials into pugmill. n Add venting capability to remove steam produced by moisture in the recycled materials. n Bypass main vibrating screen and add recycled materials directly into the No. 1 bin. n Add a separate unit for drying, proportioning, and feeding recycled materials directly into pugmill. – Parallel flow drum mix plant: n Review flighting inside drum to improve heat trans- fer, mixing, and retention time in drum. n Adjust location of RAP collar on drum. – Counterflow drum mix plant: n Change flighting inside drum to improve heat trans- fer, mixing, and retention time in drum. Recycled Material Placement • Recycled material transfer to paver and placement: – Stiffer recycled material mixtures: n Tend to move from haul truck into the paver hop- per in large clumps rather than flowing like virgin asphalt mixtures. n Kicker paddles to move stiffer material under gear box at center of screed. n Inclined to crust because of cooling in the paver wings or on the top of windrows. n Can make it more difficult to achieve the required density at joints. n May be more temperature sensitive. INFORMATION FOR FIELD INSPECTOR • Field inspectors check the following for: – RAP mixtures: n Ensure contractor starts with clean pavement sur- face before milling. n Watch milling operations. n Monitor quality (i.e., variability) of RAP stockpiles. n Ensure consistency of mixture during production. n Monitor temperature segregate, clumping, mixture temperature. n Evaluate mat behind paver for evidence of foreign materials (e.g., crack filling material), clumps of asphalt and fines, texture (e.g., segregation, pulling, tearing, and streaking). n Closely monitor joint density and mat density (i.e., air voids). – RAS mixtures: n Ensure that the uniform amount of RAS, at the correct percentage, is fed into the plant. n Evaluate mat behind the paver for evidence of foreign materials, visible RAS, clumps, and dust balls. n Check and/or adjust asphalt content if RAS mixture looks dry. n Closely monitor mat density (i.e., air voids). SUGGESTIONS FOR FUTURE RESEARCH • Evaluate the recycled material asphalt transfer to virgin aggregate under actual plant conditions and develop lab- oratory mixing methods that replicate the transfer that occurs during production. • Improve laboratory procedures for drying, preparing, preheating, mixing, and compacting mixtures to more closely replicate what happens during production at the asphalt plant. • Investigate the potential for high RAP, RAS, and RAP/ RAS combination mixtures to be effectively recycled dur- ing future maintenance and reconstruction projects. Infor- mation is essential as to how these mixtures can influence milling operations, affect the choice(s) of pavement pres- ervation surface treatments, and address any issues with in-place recycling methods (hot in-place, cold in-place, and full-depth reclamation). • Develop information about the expected service life of high RAP, RAS, and RAP/RAS combination mixtures that can be used for estimating life-cycle costs. The development of expected service life could be aided by a significantly large performance database that represents performance for a wide range of environmental and traffic conditions. • Investigate the impact of minimum and maximum silo storage times on recycled material asphalt mixtures. • Investigate the impact of reduced temperatures when using warm mix asphalt technologies on the percentage of recycled material asphalt that contributes to the total asphalt content.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 495: Use of Reclaimed Asphalt Pavement and Recycled Asphalt Shingles in Asphalt Mixtures summarizes current practices for the use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in the design, production, and construction of asphalt mixtures. It 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.

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