ODOT Research Executive Summary Report State Job Number:

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1 Ohio Department of Transportation Office of Statewide Planning & Research Research Section 1980 West Broad Street, Mail Stop 3280 Columbus, OH FHWA Report Number: Influence of Warm Mix Asphalt on Aging of Asphalt Binders FHWA/OH-2014/13 Report Publication Date: November 2014 ODOT Project Duration: 28 months Start Date: August 16, 2012 Completion Date: December 16, 2014 Total Project Funding: $168,000 Research Agency: Researchers: ODOT Technical Liaisons: Project Background The University of Akron and Ohio University Ala R. Abbas, Munir Nazzal, Savas Kaya, Sunday Akinbowale, Bijay Subedi, Lana Abu Qtaish, and Mir Shahnewaz Arefin Mr. David Powers and Mr. Eric Biehl For copies of this final report go to Warm mix asphalt (WMA) has become more widely adopted in the United States due to its environmental benefits, energy savings, enhanced compaction, and increased haul distances. Over the last decade, different types of WMA technologies have been marketed and used in Ohio. However, foamed WMA produced by water injection has gained popularity among asphalt mix producers as it allows for the production of WMA with a standard grade asphalt binder through a one-time mechanical plant modification, eliminating the need for costly additives associated with other WMA technologies. In recent years, the amount of foamed WMA used in Ohio has increased from approximately 10,000 tons in 2008 to more than 10,000,000 tons in 2013, which represents nearly 60% of the total amount of asphalt mixtures produced in the state. To date, satisfactory performance has been obtained for pavements constructed using foamed WMA, with minimal issues arising from the reduction of the production temperature. However, one subject that has not been thoroughly studied that might affect the performance and durability of foamed WMA is binder aging. Since lower temperatures are used during the production of foamed WMA, it is generally expected that the asphalt binders in these mixtures will undergo less aging, leading to lower resistance to permanent deformation but better resistance to thermal and fatigue cracking than traditional hot mix asphalt (HMA). However, the difference in aging between foamed WMA and HMA may also be affected by other factors such as the binder type, aggregate type, aggregate gradation, and air void content within the mix. Therefore, there is a need to investigate the aging characteristics of foamed WMA mixtures to better understand their influence on pavement performance. Page 1 of 5

2 Study Objectives The primary objective of this study is to examine the short-term and long-term aging characteristics of foamed WMA as compared to traditional HMA. The specific objectives include: - Evaluate the rheological, chemical, and morphological properties of unaged and aged asphalt binders recovered from foamed WMA and HMA mixture at different stages of aging. - Study field aging in foamed WMA and traditional HMA mixtures. - Compare the standard laboratory aging procedures that are used for short-term and long-term aging of asphalt binders and asphalt mixtures. - Determine the ability of the standard laboratory aging procedures for asphalt binders and asphalt mixtures to simulate field aging of foamed WMA and HMA mixtures. Description of Work A laboratory testing plan was implemented in this study to examine the short-term and long-term aging characteristics of foamed WMA as compared to traditional HMA (Figure 1). As can be noticed from this figure, the laboratory testing plan included a binder aging study and a mixture aging study. The binder aging study evaluated the short-term and long-term aging characteristics of the selected asphalt binders. The mixture aging study was divided into three components. The first component investigated the effect of the extraction and recovery procedure on the rheological properties of the two asphalt binders that were used in the preparation of the laboratory-produced asphalt mixtures. The second component evaluated the short-term and long-term aging characteristics of binders recovered from laboratoryproduced asphalt mixtures. The third component focused on comparing the long-term aging in binders recovered from field-placed foamed WMA and HMA mixtures. Laboratory Testing Plan Binder Aging Mixture Aging Effect of Binder Extraction and Recovery Laboratory Mixture Aging Field Mixture Aging Figure 1: Laboratory Testing Plan. Laboratory Binder Aging Two types of asphalt binders (one polymer-modified PG asphalt binder and one neat PG asphalt binder) that are typically used in surface mixtures in Ohio were included in this study. The short-term aging of the two asphalt binders was simulated using a Despatch rolling thin film oven (RTFO) according to AASHTO T 240, and the long-term aging of the asphalt binders was simulated using a pressure aging vessel (PAV) from Applied Test System (Cheswick, Pennsylvania) according to AASHTO R 28. The dynamic shear rheometer (DSR) was used to characterize the viscoelastic behavior of the unaged, RTFO-aged, and PAV-aged asphalt binders at intermediate and high service temperatures. Temperature and frequency sweeps were conducted using a research grade DSR device from Rheometric Scientific (currently owned by TA Instruments). The dynamic shear modulus, G*, and phase angle, δ, were obtained at each loading frequency and testing temperature. Page 2 of 5

3 In addition, Fourier transform infrared (FTIR) spectroscopy was used to identify and quantify the amount of functional groups present in the asphalt binders, gel permeation chromatography (GPC) was used to determine the molecular size distribution within the unaged, RTFO-aged, and PAV-aged asphalt binders, and atomic force microscopy (AFM) was utilized to examine the effect of aging on the microstructure and morphology of the unaged and aged asphalt binders. Effect of Binder Extraction and Recovery In order to examine the extent of binder aging in laboratory-prepared and field-placed mixtures, it is necessary to extract and recover the asphalt binders from these mixtures. The asphalt binders are generally extracted in accordance with AASHTO T 164 (Quantitative Extraction of Asphalt Binder from Hot Mix Asphalt) and are recovered in accordance with AASHTO T 170 (Recovery of Asphalt Binder from Solution by Abson Method). Because these procedures introduce a solvent during the extraction and heat during the recovery, they are expected to have some effect on the physical and chemical characteristics of the recovered asphalt binders. A laboratory testing plan was designed to quantify the effect of the extraction and recovery procedures on the two asphalt binders (PG and PG 64-22) that were used in the laboratoryproduced asphalt mixtures. To determine the sensitivity of these asphalt binders to extraction and recovery, controlled amounts of trichloroethylene (TCE), the solvent used in AASHTO T 164, and dust were added to the unaged, RTFO-aged, and PAV-aged binders of both PG grades. AASHTO T 164 and AASHTO T 170 were then used to recover the asphalt binders from the resulting solutions. The DSR test was used to characterize the viscoelastic behavior of the original and recovered unaged, RTFO-aged, and PAV-aged asphalt binders, and x-ray diffraction (XRD) was used to identify the presence of any limestone dust remaining in the recovered asphalt binder. Laboratory Mixture Aging Asphalt mixture aging was evaluated by examining the physical, chemical and morphological properties of asphalt binders recovered from laboratory-produced foamed WMA and HMA mixtures at different stages of aging (immediately after mixing, short-term aging, and long-term aging) and by comparing the dynamic modulus, E*, of short-term and long-term aged foamed WMA and HMA asphalt mixtures. In this study, two asphalt binders (PG and PG 64-22) and one aggregate (limestone) were used in the preparation of the foamed WMA and HMA asphalt mixtures. The aggregate gradation met the Ohio Department of Transportation (ODOT) Construction and Material Specifications (C&MS) requirements for Item 442 (Superpave Asphalt Concrete) Type A with a nominal maximum aggregate size (NMAS) of 12.5 mm. The aggregate blend was prepared by mixing 55% #8 limestone aggregate, 30% limestone sand, and 15% natural sand. An optimum asphalt binder content of 5.7% was used in the preparation of the asphalt mixtures. None of the asphalt mixtures contained reclaimed asphalt pavement (RAP). It is noted that ODOT requires using PG for Superpave surface mixtures. However, PG was included in this study to allow for determining the effect of the asphalt binder type on mixture aging. A Wirtgen WLB10 laboratory-scale asphalt binder foaming device was utilized to foam the asphalt binder by injecting cold water into the heated asphalt binder. This device employs a process similar to that used by large-scale foaming systems that are incorporated into commercial asphalt plants. A foaming water content of 1.8% by weight of the asphalt binder was used in the production of the foamed asphalt binder. This quantity represents the maximum water content permitted by ODOT for foamed WMA mixtures. In addition, the foamed WMA mixtures were produced at 30 o F (16.7 o C) lower mixing and compaction temperatures than the traditional HMA mixtures. This temperature reduction is consistent with current ODOT specifications for foamed WMA mixtures that allow using a compaction temperature 30 o F (16.7 o C) lower than that of the HMA. ODOT, however, does not control the mixing temperature of the foamed WMA. It is up to the contractor to determine the appropriate mixing temperature for this material. Page 3 of 5

4 Field Mixture Aging This study also involved investigating the effect of aging on foamed WMA and HMA mixtures placed in the field. Field cores were collected from four roadway sections in Ohio (US Route 224 in Portage County, State Route 303 in Summit County, US Route 62 in Pickaway County, and State Route 49 in Miami County) that were constructed using both foamed WMA and HMA mixtures prepared using the same materials (asphalt binder and aggregates), aggregate gradation, and asphalt binder content. All pavement sections were constructed in 2008 as part of ODOT s initial field implementation of foamed WMA in Ohio. The asphalt binder was extracted from the field cores using AASHTO T 164 and recovered using AASHTO T 170. The recovered binders were examined for the same physical, chemical, and morphological properties using the same test procedures as the laboratory-produced foamed WMA and HMA mixtures. Research Findings & Conclusions Based on the experimental test results, the following observations and conclusions were made: Laboratory aging of foamed WMA and HMA mixtures: - In general, comparable or slightly higher G*/sinδ and G*sinδ values were obtained using the DSR test for asphalt binders recovered from laboratory-prepared HMA mixtures than those recovered from laboratory-prepared foamed WMA mixtures. This was the case for both short-term and longterm aged mixtures. This indicates that laboratory-prepared foamed WMA mixtures undergo comparable or slightly lower levels of aging than traditional HMA mixtures. - The conventional DSR test results were consistent with the FTIR, GPC, and AFM test results in that the carbonyl and sulfoxide indices from the FTIR, the percentage of large molecular size (LMS) from the GPC, and the reduced modulus (E reduced ) from the AFM indicated a slightly higher level of aging for the laboratory-prepared HMA mixtures than the laboratory-prepared foamed WMA mixtures. - The laboratory-prepared foamed WMA mixtures also exhibited a slightly lower dynamic modulus than the traditional HMA mixtures. This was the case for both short-term and long-term oven aged asphalt mixtures. - Aging had a pronounced effect on the dynamic modulus of foamed WMA and HMA mixtures prepared using PG and little effect on the dynamic modulus of foamed WMA and HMA mixtures prepared using PG This indicates that the effect of aging on the dynamic modulus is highly influenced by the type of asphalt binder used in the asphalt mixture. Effect of asphalt binder extraction and recovery: - Little effect was observed for the extraction and recovery procedures on the rheological properties of PG especially for the unaged and RTFO-aged asphalt binders. However, significantly lower G*/sinδ and G*sinδ values were obtained for the recovered PG asphalt binder. This implies that the PG asphalt binder is more sensitive to the extraction and recovery procedures using TCE than PG Little difference in G*/sinδ and G*sinδ was observed for asphalt binders recovered from binder/tce solutions with and without dust. This suggests that the undertaken extraction procedure was able to remove most of the dust that was introduced into the binder/tce solutions. - The same dominant peaks for limestone dust were observed in the XRD test results for recovered asphalt binders obtained from binder/tce solutions containing dust. This indicates that some traces of dust remained in the recovered asphalt binders even though the effect was minimal on the DSR test results. Comparison of laboratory binder and laboratory mixture aging: - In general, the G*/sinδ and G*sinδ values obtained for asphalt binders recovered from short-term oven aged foamed WMA and HMA mixtures were slightly higher than those obtained for the corresponding RTFO-aged binders, while the G*/sinδ and G*sinδ values obtained for asphalt binders recovered from long-term oven aged foamed WMA and HMA mixtures were not Page 4 of 5

5 consistently higher or lower than those obtained for the corresponding PAV-aged binders. This indicates that the RTFO test procedure results in less aging than the short-term oven aging procedure specified in AASHTO R30, while the PAV test procedure results in comparable aging to the long-term oven aging procedure specified in AASHTO R30. Similar results were also obtained from the FTIR, GPC, and AFM tests. Field aging of foamed WMA and HMA mixtures: - No consistent differences were observed in the DSR, FTIR, GPC, and AFM tests for asphalt binders recovered from field-placed foamed WMA and HMA mixtures. This was the case for both PG and PG asphalt binders. This implies that both mix types had comparable levels of aging with no mix type showing significantly higher levels of aging than the other. - In general, the DSR test results obtained for the asphalt binders recovered from the foamed WMA and HMA field cores fell within the range obtained for the RTFO-aged and PAV-aged binders recovered from binder/tce solutions containing dust, with the DSR test results for the asphalt binders recovered from the field cores being closer to those obtained for the RTFO-aged binders. This was not unexpected since the PAV test was designed to simulate asphalt binder aging after 7 to 10 years of service, while the field cores were obtained 6 years after placement of the surface course. Recommendations for Implementation of Research Findings This study investigated the short-term and long-term aging characteristics of foamed WMA mixtures in comparison to traditional HMA mixtures. The experimental test results showed a slightly lower level of aging for laboratory-prepared foamed WMA mixtures than for laboratory-prepared traditional HMA mixtures. However, no consistent differences in the level of aging were observed for foamed WMA and HMA mixtures placed in the field in Consequently, there is no need to modify the asphalt binder and/or asphalt mixture laboratory aging procedures to simulate the short-term and long-term aging of foamed WMA mixtures. The extraction and recovery procedures were observed to have a significant influence on the rheological properties of the recovered PG asphalt binder and little influence on the rheological properties of the recovered PG asphalt binder. It is recommended to expand this study to evaluate the effect of the extraction and recovery procedures using trichloroethylene (TCE) on additional asphalt binders. Further research can also be conducted to determine if alternative solvents can be used instead of TCE for asphalt binder extraction. The effect of aging on foamed WMA and HMA mixtures was highly influenced by the type of asphalt binder used in the asphalt mixture more so than the mix type. In this study, foamed WMA and HMA mixtures prepared using PG were found to be more susceptible to aging than foamed WMA and HMA mixtures prepared using PG Because field placed asphalt mixtures may eventually be used as reclaimed asphalt pavement (RAP) in future construction projects, the difference in binder aging shall be taken into consideration in the mix design of new asphalt mixtures. Page 5 of 5