Short-Term Laboratory Conditioning of Asphalt Mixtures (2015)

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5 Background Asphalt mixtures may be produced in either batch or drum mix plants and then compacted at temperatures ranging from 220°F (104°C) to 325°F (163°C) (Kuennen 2004; Newcomb 2005a). The goal of asphalt mixture production is to ensure complete drying of the aggregate, proper coating and bond- ing of the aggregate with the binder, and adequate workability for handling and compaction. These processes are important to the mixture’s durability, resistance to permanent deforma- tion, and cracking. Recent advances in asphalt technology— including the use of polymer-modified binders, use of more angular aggregates, and increased compaction requirements— have resulted in increased mixing and compaction tempera- tures up to a limit of approximately 350°F (177°C), where polymer breakdown in the binder can occur. The use of warm mix asphalt (WMA) technology can lead to reduced produc- tion and paving temperatures without sacrificing the quality of the final product. This has led to a wider range of available production temperatures that may be employed by the con- tractor. The conditions under which hot mix asphalt (HMA) or WMA is produced and placed may have a profound impact on its performance properties. Traditionally, asphalt mixtures have been designed on the basis of volumetric parameters (Asphalt Institute 1984, 1995) in which the optimum asphalt content for a given aggregate gradation was dependent upon the compaction effort, air void (AV) content, voids in mineral aggregate, and absorption of the asphalt into aggregate voids. Laboratory mixing and com- pacting temperatures were dependent upon the stiffness or viscosity of the asphalt binder. This system was refined over decades of experience, first for the Marshall and Hveem pro- cedures, and then for Superpave. The volumetric procedures worked well for agencies and contractors in an era when the components for asphalt mixtures were relatively constant. In the last two decades, changes have occurred in these compo- nents, and these changes are beneficial but leave the asphalt community in need of changing its practices on how mixtures are designed and evaluated. Increased use of polymer modi- fiers, increased use of reclaimed asphalt pavement (RAP) and recycled asphalt shingle (RAS), and the advent of WMA are departures from the norm under which volumetric mixture design was developed. Compounding this complexity has been an evolution in asphalt plant design. The 1970s saw a rapid and persistent increase in the number of continuous plants (or drum mix plants [DMPs]) replacing batch mix plants (BMPs). In a BMP, the aggregates are dried prior to being loaded into hot bins in a batching tower. Gates on the bins are opened, allowing the proper proportion of aggregates to be weighed in a weigh bin prior to dropping into the pugmill. Asphalt binder is fed into the pugmill, usually through a weigh bucket. The aggregate and binder are then mixed and discharged either into a truck or onto a slat conveyor for loading into a silo. The material is then delivered to the job site via dump truck for deposition and paving. A variation in BMP design is for aggregates to be dried and then fed into a separate continuous pugmill, and then placed into a silo. In general, a continuous plant differs from a batch plant in that cold aggregates are fed onto a weight belt in the proper proportions prior to entering the elevated end of the drum for drying. The aggregates are dried as they tumble through the drum toward the lower end, where they are mixed with binder before exiting to a slat conveyor for loading into a silo where the mixture is kept until it is deposited into a truck for transport to a paving site. The drum may either have the burner directed in parallel with the downward flow of the aggregate or directed upward against the flow of the aggre- gate. These designs are referred to as parallel-flow drum and counter-flow drum (CFD), respectively. There are two other variations for continuous plants. One is a dryer mixing drum, in which the aggregate is dried in a drum before it is loaded into a pugmill at the end of the drum for mixing with asphalt. The other variation is the unitized drum mixer design, which is a variation of the CFD in which the aggregate is dried as it goes through an inner drum. At the end of the inner drum, the C H A P T E R 1

6material flow is reversed, feeding the aggregate into an outer drum where it is mixed with asphalt before being discharged. As discussed, the changes in mixture components, mixture processing, and plant design have left many paving technolo- gists questioning the validity of current mix design methods in adequately assessing the volumetric needs of asphalt mix- tures and the physical characteristics required to meet perfor- mance expectations. While a variety of other NCHRP projects (9-43, 9-47A, 9-48, 9-49, and 9-49A) either attempted or are attempting to address many of these issues, this project con- sidered the impact of climate, aggregate type (to reflect differ- ences in asphalt absorption), asphalt type and source, recycled material (RAP and RAS) inclusion, WMA technology, plant type, and production temperature on the volumetric and per- formance characteristics of asphalt mixtures during construc- tion and over an initial period of performance. Project Objectives and Scope The objectives of NCHRP Project 9-52 were to (1) develop a laboratory short-term aging protocol to simulate the aging and asphalt absorption of an asphalt mixture as it is produced in a plant and then loaded into a truck for transport and (2) develop a laboratory aging protocol to simulate the aging of the asphalt mixture through its initial period of performance. Accordingly, the project was divided in two phases: Phase I evaluated short-term aging protocols for HMA and WMA to simulate plant aging, and Phase II evaluated long-term aging protocols to simulate the aging of the asphalt mixture 1 to 2 years after construction. This chapter presents the background and literature review summary on short- and long-term laboratory conditioning of asphalt mixtures, including related work on conditioning completed in NCHRP Project 9-49 (Epps Martin et al. 2014). Chapter 2 focuses on the research approach and details of the field sites used in this project. Findings for Phases I and II of the project are presented in Chapter 3, with detailed test results and analyses provided in the appendices. Conclusions and rec- ommendations of this project are in Chapter 4. Previous Research on Short-Term Conditioning The aging of asphalt binders in mixtures has long been a concern to those in the pavement field. Several studies dating back to the 1960s and further evaluated the short-term aging of asphalt mixtures in the field, and a consistent conclusion was obtained that most short-term aging of asphalt mixtures occurred during production and construction due to the high temperatures involved (Heithaus and Johnson 1958; Traxler 1961; Chipperfield and Welch 1967). More recent studies have identified several factors of asphalt mixtures and production parameters that could significantly affect the short-term aging characteristics of asphalt mixtures; these factors include binder source, binder type, aggregate absorp- tion, plant type, production temperature, and silo storage time (Traxler 1961; Chipperfield and Welch 1967; Lund and Wilson 1984, 1986; Topal and Sengoz 2008; Zhao et al. 2009; Morian et al. 2011; Rashwan and Williams 2011; Mogawer et al. 2012; Daniel et al. 2014). A brief summary of these studies on the short-term aging of asphalt mixtures is pro- vided in Table 1-1. The standard practice for laboratory asphalt mix design is to simulate the binder absorption and aging that occurs dur- ing production and construction by short-term oven aging (STOA) or conditioning the loose mix prior to compaction for a specified time and temperature. For HMA, the recom- mended procedure in AASHTO R 30 for preparing specimens for volumetric mixture design is 2 hours at the compaction temperature (Tc), while it is 4 hours at 275°F (135°C) for preparing specimens for performance testing. However, the implementation of WMA raised the question of the impact of lower plant temperatures on the aging characteristics and absorption of asphalt by aggregates in asphalt mixtures and how to adequately simulate any differences in the laboratory. Preparation of WMA specimens for quality assurance is also challenging since the question arises about how to account realistically for the compaction of WMA mixtures when reheat- ing may be necessary to make them workable. Reference Short-Term Aging Major Finding Heithaus and Johnson 1958 Field Aging Most aging during production and construction through compaction Traxler 1961 Chipperfield and Welch 1967 Aschenbrener and Far 1994 Table 1-1. Previous research on short-term aging.

7 Traxler 1961 Factor on Field Aging Binder chemistry and aggregate absorption major effects Chipperfield and Welch 1967 Aggregate gradation no effect Terrel and Holen 1976 Plant type significant effect; DMP < BMP due to lower temperature and less moisture Lund and Wilson 1984 & 1986 Binder type and binder source significant effects Chollar et al.1989 Slightly more aging from DMP than BMP Mogawer et al. 2012 • Production temperature, silo storage, inclusion of recycled materials, and reheating significant effects • Softer binder with RAP = harder binder without RAP Daniel et al. 2014 • Production temperature and silo storage significant effects • Reduced difference in virgin vs. RAP mixtures after reheating Aschenbrener and Far 1994 Factor on Lab Aging Aggregate absorption important effect Topal and Sengoz 2008 • Binder type and binder source significant effects • Reduced aging with polymers Zhao et al. 2009 Morian et al. 2011 • Binder type and binder source significant effects • Reduced aging with polymers • Aggregate absorption and gradation important effects Aschenbrener and Far 1994 2 h at Tc • Reheating significant effect on Hamburg wheel-tracking test (HWTT) results • Recommend 2 h @ Tc Estakhri et al. 2010 4 h at 275°F (135°C) • WMA 4 h @ 275°F (135°C) comparable to HMA 4 h @ 250°F (121°C) • Recommend 4 h @ 275°F (135°C) for WMA Rashwan and Williams 2011 2 h at 302°F (150°C) (HMA) 2 and 4 h at 230°F (110°C) (WMA) E* and flow number higher for HMA with different temperature and for mixtures with RAP Reference Short-Term Aging Major Finding Jones et al. 2011 4 h at Tc • Equivalent HWTT results and heavy-vehicle-simulator rutting for HMA and WMA • More HWTT rutting in WMA without short-term aging Bonaquist 2011a 2 h at Tc4 h at Tc • Gmm (aggregate absorption) and indirect tensile strength comparable to cores at construction • Recommend 2 h @ Tc for WMA and suggested additional longer aging period for evaluating rutting and moisture susceptibility Hajj et al. 2011 4–15 h at 250°F (121°C) • Recommend compaction of WMA foaming within 4 h • Foaming effects lost @ 4–15 h @ 250°F (121°C) Clements et al. 2012 0.5, 2, 4, and 8 h at 275°F (135°C) (HMA) 0.5, 2, 4, and 8 h at 237°F (114°C) (WMA) • Equivalent disc-shaped compact tension results for WMA vs. HMA • Reduced E* and flow number and increased rutting for WMA vs. HMA Estakhri 2012 2 h at 275°F (135°C)4 h at 275°F (135°C) • Equivalent HWTT for WMA vs. HMA • Aging time and temperature effect on HWTT and overlay tester results Sharp and Malone 2013 1 h at 302°F (150°C) Recommend 1 h @ 302°F (150°C) for WMA Epps Martin et al. 2014 2 and 4 h at Tc 2 and 4 h at 275°F (135°C) 2 h at Tc + 16 h at 140°F (60°C) + 2 h at Tc • Effect on aging: STOA temperature > STOA time • Recommend 2 h @ 275°F (135°C) for HMA and 2 h @ 240°F (116°C) for WMA Table 1-1. (Continued).

8Over the last few decades, a number of studies were per- formed (1) to evaluate the effect of various laboratory STOA protocols for HMA and WMA in order to achieve equivalent binder aging and absorption that occur during production and construction in the field and (2) to evaluate the perfor- mance of short-term aged WMA compared to HMA. These studies are also summarized in Table 1-1. A study by Aschenbrener and Far (1994) was conducted to evaluate the short-term aging of HMA. Nine projects were selected throughout Colorado and four different laboratory STOA protocols including 0, 2, 4, and 8 hours at field com- paction temperatures were used to fabricate laboratory speci- mens. The specimens were tested to determine their theoretical maximum specific gravity (Gmm) values and Hamburg wheel- tracking test (HWTT) results in accordance with AASHTO T 209 and AASHTO T 324, and the results were compared against results obtained on counterpart mixtures produced in the field. During construction, the silo storage time and tem- perature were monitored in order to better quantify the short- term aging that occurred during plant production, and it was indicated that HMA stayed at elevated temperatures in the silo for approximately 1 to 2 hours at an average temperature of 263°F (128°C) prior to being loaded out and transported to the paving site. The comparison in Gmm values between laboratory mixtures with various STOA protocols versus field mixtures illustrated that for the majority of the mixtures, the laboratory STOA protocol of 2 to 4 hours at the field compac- tion temperatures produced asphalt mixtures that matched the asphalt absorption that occurred in the field specimens. The performance of the laboratory specimens in the HWTT was highly dependent upon the STOA time. In addition, labo- ratory specimens that were short-term aged for 1 to 3 hours at the field compaction temperature exhibited equivalent HWTT performances compared to field specimens. Thus, according to the Gmm and HWTT results, researchers recommended con- ditioning the laboratory-produced samples for 2 hours at the field compaction temperature in order to simulate asphalt aging and absorption during plant production. A more recent study by Estakhri et al. (2010) evaluated the effect of three laboratory STOA protocols on a WMA mixture prepared with Evotherm DAT™: 2 hours at 220°F (104°C), 2 hours at 275°F (135°C), and 4 hours at 275°F (135°C). WMA performance was evaluated using the HWTT per AASHTO T 324 and compared against that of HMA conditioned for 2 and 4 hours at 250°F (121°C). The target AV content of the mixtures was 7 ± 0.5 percent. In addition, WMA mixtures prepared with Advera® and Sasobit® conditioned for 2 hours at 220°F (104°C) and 4 hours at 275°F (135°C) were also tested and compared against the results of HMA conditioned for 2 hours at 250°F (121°C). The results for WMA Evo- therm DAT™ showed that the number of passes to generate a 0.5-in. (12.5-mm) rut depth increased with higher aging tem- perature and longer aging time. Improved rutting resistance due to increased laboratory STOA temperatures was also observed for the WMA mixtures prepared with the other two WMA technologies. In addition, the WMA Evotherm DAT™ mixture conditioned for 4 hours at 275°F (135°C) showed equivalent performance compared to the control HMA con- ditioned at 250°F (121°C). Based on these observations, a recommendation for laboratory STOA protocol of 4 hours at 275°F (135°C) for WMA was made and was incorporated in the mix design methods for the Texas Department of Transportation. The recently completed NCHRP Project 9-43 on mix design practices for WMA (Bonaquist 2011a) recommended a con- ditioning protocol for WMA of 2 hours at Tc for both volu- metric mixture design and performance testing, as stated in the draft appendix to AASHTO R 35. This conditioning pro- tocol was selected based on comparisons of maximum specific gravity (AASHTO T 209) and indirect tensile (IDT) strength (AASHTO T 283) of laboratory-mixed, laboratory-compacted (LMLC) specimens subjected to the conditioning protocol versus the results obtained for plant-mixed, field-compacted (PMFC) cores. The specific gravity comparison showed equiv- alent maximum theoretical density for LMLC specimens and PMFC cores, indicating the same binder absorption level. The difference in IDT strength between LMLC specimens and PMFC cores was also insignificant based on a paired t-test comparison with a 95-percent confidence interval. In addi- tion, further research was recommended to develop a two-step WMA conditioning procedure for the evaluation of moisture susceptibility and rutting resistance, similar to the conditioning protocol applied to HMA. The first step would be condition- ing for 2 hours at Tc to simulate binder absorption and aging during construction, and the second step would consist of an extended conditioning time at a representative high in-service temperature but no longer than 16 hours. NCHRP Project 9-49 (Epps Martin et al. 2014) performed a laboratory conditioning experiment focused on evaluation of the moisture susceptibility of WMA technologies and rec- ommended a laboratory STOA protocol of 2 hours at 275°F (135°C) for HMA and 2 hours at 240°F (116°C) for WMA in order to simulate the short-term aging of the asphalt mixture occurring during production and construction. In the study, different laboratory STOA protocols were selected based on the available literature and used for fabricating HMA and WMA LMLC specimens. These specimens were tested to deter- mine the effect of each laboratory STOA protocol on mixture resilient modulus (MR) stiffness. Cores at construction were incorporated as part of the experimental design to represent short-term aged HMA and WMA specimens produced in the plant. Laboratory STOA protocols were able to produce asphalt mixtures with significantly increased stiffness. Additionally, the effect from the short-term aging temperature was more

9 pronounced than the short-term aging time. Among the five selected laboratory STOA protocols, 2 hours at 275°F (135°C) and 2 hours at Tc yielded mixture-stiffness values similar to those of HMA and WMA cores at construction, respectively. Considering the difficulty in accurately defining Tc for WMA in the field and the common range of Tc for WMA, 2 hours at 240°F (116°C) instead of 2 hours at Tc was recommended as the standard laboratory conditioning protocol for WMA LMLC specimens. The final recommendation of NCHRP Project 9-49 for HMA LMLC specimens was 2 hours at 275°F (135°C). In general, the main findings of the previous studies on short-term aging of asphalt mixtures can be summarized as follows: • Substantial aging in the field occurs during production through placement and compaction. • Binder type, binder source, aggregate absorption, WMA technology, recycled materials (i.e., RAP and/or RAS), pro- duction temperature, and silo storage are factors that have a significant effect on mixture short-term aging characteristics. • An increase in laboratory STOA temperature, time, or both increases asphalt mixture stiffness. Short-term aging is more sensitive to STOA temperature than STOA time. • Several laboratory STOA protocols have been proposed to simulate the short-term aging occurring in WMA during production and construction. Despite the previous research efforts, there are still some aspects of the short-term aging of asphalt mixtures that have not been fully resolved, which include the following: • A standard laboratory STOA protocol for WMA. • A comprehensive study to establish short-term aging pro- tocols that encompass the effects of aggregate absorption, asphalt type and source, recycled material inclusion, WMA technology, plant type, and production temperature. Previous Research on Long-Term Aging Aging of asphalt pavements continues throughout their in- service life, though at a lower rate compared to that during production and construction. Therefore, it is important to account for the changes in asphalt mixture properties due to field aging when preparing laboratory samples for longer- term performance testing. The standard practice for labora- tory mix design of asphalt mixtures is to simulate the field aging by storing the compacted specimens for 5 days at 185°F (85°C) in accordance with AASHTO R 30. In the past few years, studies have evaluated the effect of field and laboratory long-term aging on asphalt mixture properties and identified reasonable correlations between field aging and laboratory long-term oven aging (LTOA) protocols. A brief summary of these studies is provided in Table 1-2. A study by Rolt (2000) evaluated the effect of field aging on asphalt mixture properties. Thirty-two full-scale test sec- tions were constructed with various factors including expo- sure temperature, pavement thickness, pavement density, and binder content. Cores were obtained from the pavements at various in-service times. Binders were extracted from these cores and then tested for binder viscosity. The viscosity results of the extracted binders were used to evaluate the effect of each variable on mixture aging characteristics. Based on the test results, it was concluded that the factors of exposure time and exposure temperature had a significant effect on mixture aging characteristics, while the effect from pavement thick- ness, pavement density, and binder contents was insignificant. A more recent study by Rondon et al. (2012) was performed to evaluate the evolution of asphalt mixture properties with environmental exposure. A typical HMA was subjected to 42 months of environmental exposure in Bogota, Colom- bia, prior to being tested in the laboratory to determine the changes in its mechanical properties. Test results indicated that the increase in mixture stiffness, rutting resistance, and fatigue resistance was observed for the first 29 months of environmen- tal exposure and could be attributed to the aging of the asphalt binder due to temperature and ultra-violet radiation. However, an opposite trend was shown between 30 and 42 months. Another study on laboratory aging by Safaei et al. (2014) evaluated the effect of long-term aging on HMA and WMA stiffness and fatigue resistance. In the study, laboratory HMA and WMA specimens were fabricated following the STOA protocol of 4 h at 275°F (135°C) for HMA and 2 h at 243°F (117°C) for WMA plus LTOA protocol of 2 days or 8 days at 185°F (85°C). Then, at different aging stages, these speci- mens were tested with dynamic modulus (E*) and direct ten- sion tests to quantify mixture performance–related property evolution with aging. It was indicated that laboratory LTOA was able to produce both HMA and WMA mixtures with a significant increase in mixture stiffness over the short-term aged mixtures. Additionally, a reduction of the difference in mixture stiffness between HMA and WMA was observed after laboratory LTOA. A study performed at the University of New Hampshire (Tarbox and Daniel 2012) evaluated the effects of laboratory LTOA on RAP mixtures. Plant-produced mixtures with 0, 20, 30, and 40 percent RAP were used to fabricate specimens in the laboratory and were then long-term aged for 2, 4, and 8 days at 185°F (85°C) prior to being tested with the E* test. The Global Aging System model was also used in the study to predict the changes in E* values at various long-term aging levels. These predictions were then compared against the measured values. Test results indicated that the laboratory LTOA was able to produce mixtures with a significant increase in stiffness

10 compared to the short-term aged mixtures, and that the stiff- ening effect was more pronounced for virgin mixtures (i.e., mixtures with 0 percent RAP) than the RAP mixtures. The reduced susceptibility of RAP mixtures to laboratory aging was due to the inclusion of already-aged binders in the RAP. The comparison in predicted versus measured E* values illus- trated that the Global Aging System model over-predicted the mixture aging compared to the laboratory LTOA protocols. Bell et al. (1994) evaluated the correlation between mix- ture aging in the field versus laboratory aging in terms of STOA and LTOA protocols. In the study, field cores with a wide range of in-service times were acquired and tested to determine their triaxial and diametral MR stiffness. Labora- tory specimens were fabricated using materials from the field sites and conditioned with the selected laboratory STOA and LTOA protocols prior to being tested. MR stiffness results indi- Reference Long-Term Aging Major Findings Kemp and Predoehl 1981 Field Aging Air temperature, voids, and aggregate porosity have significant effects Kari 1982 Pavement permeability and asphalt content have significant effects Rolt 2000 • Exposure time and ambient temperature have significant effects • Binder content, mixture AV, and filler content have no effect Rondon et al. 2012 • Increased mixture stiffness, rutting resistance, and fatigue resistance for first 29 months of environmental exposure • Opposite trend observed between 30 and 42 months Farrar et al. 2013 • Field aging not limited to the top 25 mm of the pavement • Field aging gradient observed West et al. 2014 • WMA has less aging than HMA during production • Reduced difference between WMA vs. HMA with field aging • Equivalent binder true grade and binder absorption for WMA vs. HMA after 2 years of field aging Morian et al. 2011 Lab Aging (3, 6, and 9 months at 60°C) • Increased mixture E* and binder carbonyl area (CA) with LTOA • Binder source has significant effect while aggregate source has no effect Azari and Mohseni 2013 Lab Aging (2 days at 85°C 5 days at 85°C) • Increased mixture resistance to permanent deformation with LTOA • Interdependence observed between STOA and LTOA Tarbox and Daniel 2012 Lab Aging (2 days at 85°C 4 days at 85°C 8 days at 85°C) • Increased stiffness with LTOA • Stiffening effect from LTOA: virgin mixture > RAP mixture • Global Aging System model > LTOA Safaei et al. 2014 Lab Aging (2 days at 85°C 8 days at 85°C) • Increased stiffness with LTOA • Reduced difference in stiffness for HMA vs. WMA with LTOA Bell et al. 1994 Field vs. Lab Aging (4 days at 100°C 8 days at 85°C) • STOA of 4 h at 135°C = field aging during the construction process • Effect on mixture aging: LTOA temperature > LTOA time • STOA plus LTOA of 4 days at 100°C and 8 days at 85°C = 9 years of field aging in Washington State Brown and Scholz 2000 Field vs. Lab Aging(4 days at 85°C) • Stiffness: LTOA of 4 days at 85°C = 15 years of field aging in the United States Harrigan 2007 Houston et al. 2005 Field vs. Lab Aging (5 days at 80°C 5 days at 85°C 5 days at 90°C) • Significant field and laboratory aging • AV content effect on field aging • AASHTO R 35 LTOA (5 days @ 85°C) vs. 7–10 years of field aging: lab > field when AV < 8%; lab < field when AV > 8% Epps Martin et al. 2014 Field vs. Lab Aging (1 to 16 weeks at 60°C) • Increased stiffness with field aging and laboratory LTOA • Pavement in-service temperature effect on field aging • Stiffness: WMA = HMA, after 6–8 months of field aging • Stiffness: STOA of 2 h at 135°C for HMA and 2 h at 116°C for WMA plus LTOA of 4–8 weeks at 60°C = first summer of field aging Table 1-2. Previous research on long-term aging.

11 cated that a laboratory STOA protocol of 4 h at 275°F (135°C) was representative of the short-term aging occurring during production and construction. In addition, a more significant hardening effect of asphalt mixtures was achieved by LTOA at 185°F (85°C) and LTOA at a higher temperature of 212°F (100°C) for a shorter time. According to the study, LTOA of 4 days at 212°F (100°C) or 8 days at 185°F (85°C) was repre- sentative of about 9 years of field aging for the climate condi- tions in Washington State. A study by Arizona State University (Harrigan 2007; Houston et al. 2005) evaluated the effects of asphalt mix- ture aging for performance testing and pavement structural design and identified the potential correlation between field aging and laboratory aging. In the study, field cores were obtained from three test sites after 7 to 10 years in service in Arizona, Minnesota (MnRoad), and Nevada (WesTrack). Plant mix was also obtained from the three sites and was compacted in the laboratory followed by LTOA of 5 days at 176°F (80°C), 185°F (85°C), and 194°F (90°C). Asphalt binders were extracted and recovered from the field cores and laboratory-aged samples and then tested with the dynamic shear rheometer (DSR) for determining viscosity in order to evaluate mixture aging in the field and laboratory. Test results indicated that pavement in-service temperature and pavement AV contents had significant effects on mixture aging in the field; warmer in-service temperatures and higher AV contents in the mixture were generally associated with increased mix- ture aging. The comparison in viscosity of extracted binders from field cores versus laboratory-aged specimens indicated that the laboratory LTOA protocol outlined by AASHTO R 30 produced more mixture aging than 7 to 10 years of aging in the field when the AV content of the field cores were less than 8 percent and that the reverse was observed when AV content were greater than 8 percent. Therefore, it was recommended that the laboratory aging protocol account for the AV content of the asphalt pavement. Performance evolution of HMA and WMA with field and laboratory aging was studied as part of NCHRP Project 9-49 (Epps Martin et al. 2014). This study provided prelimi- nary results toward understanding the correlation between intermediate- or long-term field aging and laboratory LTOA protocols, in addition to their effects on mixture properties. In the study, the laboratory STOA protocol of 2 hours at 275°F (135°C) for HMA and 2 hours at 240°F (116°C) for WMA on loose mixtures followed by LTOA protocol of 1 to 16 weeks at 140°F (60°C) on compacted specimens was selected for fabricating laboratory long-term aged HMA and WMA speci- mens from two field sites in Iowa and Texas based on available literature. Cores at construction and one or two sets of post- construction cores with certain in-service times were included in the experimental design to represent mixtures experiencing intermediate to long-term aging in the field. These long-term aged field and laboratory specimens were tested for MR stiff- ness to evaluate the evolution of mixture stiffness with field and laboratory aging, and to explore the potential correlation between field aging and laboratory LTOA protocols. A signifi- cant effect on increasing mixture stiffness was observed from intermediate- or long-term aging in the field and laboratory. Additionally, the effect of field aging on mixture stiffness was more pronounced for aging in the summer than aging in the winter, which was likely due to the higher in-service summer temperatures. The difference in mixture stiffness between HMA and WMA was reduced with intermediate- or long- term aging; equivalent mixture stiffness was even achieved after the first summer of field aging or laboratory LTOA of 2 weeks at 140°F (60°C). The correlation between field aging and laboratory LTOA protocols was also explored based on the MR stiffness. The laboratory STOA protocol of 2 hours at 275°F (135°C) for HMA and 2 hours at 240°F (116°C) for WMA on loose mixtures plus LTOA protocols of 4 to 8 weeks at 140°F (60°C) on compacted specimens were found to be representative of the first summer of field aging. In NCHRP Project 9-47A, West et al. (2014) established relationships between the engineering properties of WMA binders and mixtures and their field performance. Six exist- ing field sites and eight new field sites were sampled. Each of the field sites included at least an HMA control section and one WMA section. Six of the field sites included mul- tiple WMA technologies, such that a total of 26 WMA and 14 HMA mixtures were sampled. Loose mix was collected, and samples for volumetric analyses and performance tests were compacted on-site, without reheating. Mixture verifica- tions were performed according to the NCHRP 9-43 proto- cols. When performing the mixture verifications, blends were produced to match the field-produced gradation determined from extraction tests rather than the reported job-mix for- mula. Samples were prepared for moisture susceptibility, flow number, E*, and in some cases fatigue testing. Results from the testing of recovered binders taken at the time of construc- tion and the testing of laboratory-produced short-term aged specimens showed that WMA mixtures had slightly less aging than HMA, as indicated by lower binder true grade and lower mixture stiffness. After 2 years, recovered binder properties revealed that the true grades of the WMA and HMA binders were not substantially different. Furthermore, there had been little or no aging of binders tested at construction and up to 2 years later. Similarly, it was noted that although the initial absorption of the binder into the aggregate was slightly lower for WMA, after 2 years there was little, if any, difference in absorption. Ongoing NCHRP Projects 9-49A and 9-54 are directed toward investigating the long-term field performance of WMA technologies and evaluating long-term aging of asphalt mixtures for performance testing and prediction. Although

12 these two projects have already started, their conclusions were not available prior to the completion of NCHRP Project 9-52. To summarize, the main findings of the previous studies on long-term aging of asphalt mixtures are: • Field aging is primarily quantified based on pavement in-service time. • Pavement in-service temperature and time, pavement depth, AV content, and effective binder content in the mixture have significant effects on field long-term aging characteristics of asphalt mixtures. • Field aging gradient with depth in asphalt pavement has been observed. • Laboratory LTOA protocols produce asphalt mixtures with a significant increase in mixture stiffness and rutting resis- tance over unaged mixtures. • The laboratory long-term aging of asphalt mixtures is more sensitive to LTOA temperature than LTOA time. • The difference in mixture properties, including stiffness, strength, and rutting resistance, between HMA and WMA is reduced with field aging and laboratory LTOA. • Reasonable correlations with field aging and laboratory LTOA protocols have been proposed. Despite the previous research efforts on long-term aging of asphalt mixtures, the following issues still need to be fully addressed: • How to account for the differences in construction dates and climates of various field sites. • How to determine when WMA and HMA reach an equiva- lent stage of aging. • Which laboratory LTOA protocols are required to achieve equivalent mixture performance–related properties between WMA and HMA. • The need for a comprehensive study to explore the corre- lation between field aging and laboratory LTOA protocols that encompass the effects of aggregate absorption, asphalt type and source, recycled material inclusion, WMA tech- nology, plant type, and production temperature. • The LTOA protocols need to be further calibrated against field aging using the existing NCHRP 9-52 test sites.