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12 Overview of Research Approach This section briefly explains the research approach. Figure 2 presents a flow chart to describe the research plan. The subsequent sections explain the components within the flow chart. The suggested future research topics shown in the figure are described in Chapter 4. The individual tasks included in the overall research approach are as follows: 1. AIP selection to track the oxidation levels of laboratory-aged mixtures and field cores. 2. Sensitivity study to estimate the sensitivity of the mechanical properties of asphalt concrete to asphalt binder oxidation. 3. Selection of long-term aging method. 4. Determination of project-specific aging durations by matching the AIPs measured from laboratory-aged loose mixtures with those from field cores. 5. Climate-based determination of predefined aging durations to determine the required labo- ratory aging duration to match the field aging at any United States location of interest and depth of interest using EICM hourly pavement temperature data. 6. Development of pavement aging model by calibrating the rheology-based kinetics model against the AIPs measured from different depths of field cores using temperature profiles obtained from the EICM. AIP Selection The development of the long-term aging procedure and kinetics model hinged on the com- parison of key AIPs of laboratory-aged binders and binders extracted and recovered from field cores. Therefore, an evaluation of candidate chemical and rheological AIPs was conducted in order to select AIPs to track the oxidation levels of field- and laboratory-aged materials when developing the long-term aging procedure and associated kinetics model. A wide range of laboratory- and field-aged materials were used to evaluate and subsequently select the AIPs. The chemical AIPs evaluated include the carbonyl absorbance area, C + S absorbance area, and C + S absorbance peaks determined using attenuated total reflectance (ATR) FTIR. The chemi- cal AIPs were evaluated based on their correlation to the duration of the laboratory aging. The rheological AIPs evaluated included the dynamic shear modulus, zero shear viscosity, Glover- Rowe (G-R) parameter, and crossover modulus. The rheological AIPs were evaluated based on the strength of their relationship to the chemical changes that were induced by oxidation. Sensitivity Study The goal of the sensitivity study was to estimate the sensitivity of the mechanical properties of asphalt concrete to asphalt binder oxidation. The sensitivity study provided thresholds by which C H A P T E R 2 Research Approach
Research Approach 13 to evaluate the significance of observed differences in asphalt binder AIPs in terms of asphalt mixture performance. To accomplish this goal, experimental characterization was performed at multiple length scales, i.e., asphalt binder, asphalt mastic, and asphalt fine aggregate matrix (FAM). Here, asphalt mastic refers only to the portion of the asphalt mixture that includes the asphalt binder and the filler (i.e., particles finer than 75 mm). FAM includes portions of asphalt mastic, fine aggregate (< 0.6 mm), and some of the air voids. Testing was performed using (a) asphalt binder to establish baseline properties and evaluate the degree of oxidation, (b) asphalt mastic to consider physicochemical aspects, and (c) FAM to consider air voids and aggregate inter action effects. For all the tests, the asphalt binder was first oxidized using standard laboratory aging proce- dures [rolling thin film oven (RTFO) and PAV]. The aging process was undertaken to create test materials that replicated, to the degree possible, asphalt binder that exists in asphalt pave- ments of relatively young, medium, and old ages. For this purpose, the relationships among PAV aging time, MAAT, and in-service aging time that was developed during NCHRP Proj- ect 9-23 were utilized (Houston et al. 2005). Correlations between the MAAT and asphalt binder grade were established to utilize this function, and the fact that these aging conditions affected the rheology of the asphalt was verified by determining the performance grade (PG) of the aged asphalt. Figure 2. Research approach flow chart.
14 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction After oxidizing the asphalt, it was either tested directly, blended with filler particles to create mastic, which was then tested, or blended with filler and fine aggregate to create FAM, which was then tested. The testing consisted of temperature and frequency sweep tests to establish the dynamic modulus values of the materials and time sweep tests to establish the fatigue properties of the materials. Subsequently, these experiments were analyzed to determine the sensitivity of the material responses to binder oxidation. Selection of the Proposed Aging Method An experimental program was executed to select the proposed aging method, in which the following factors were evaluated: (a) state of the material during aging (compacted speci- men versus loose mix), (b) pressure level (oven aging versus pressurized aging), and (c) aging temperature (95Â°C versus 135Â°C). The integrity of the specimens following aging, the rate of oxidation quantified using the AIPs of the extracted binder, versatility, ability to mimic field oxidation reactions, and the cost of the various procedures were compared in order to select the most promising aging procedure. The aforementioned analysis was conducted using hot mix asphalt (HMA) mixtures. However, a complementary analysis of the long-term aging of WMA mixtures also was conducted. To assess the aging level achieved during the aging trials, comparisons were made between the AIPs of the binder extracted and recovered from long- term laboratory-aged mixtures, binder aged using the standard RTFO and PAV, and binder extracted and recovered from field cores acquired from in-service pavements. Loose Mix Versus Compacted Specimen Aging Both compacted and loose mixture aging trials were conducted to determine the optimal state of the material to use for long-term aging. The specimen integrity following laboratory aging and the efficiency of oxidation were used to evaluate the state of the material. For the compacted specimens, two geometries were considered: standard 100-mm diameter specimens and small-specimen geometry 38-mm diameter specimens. The motivation behind the use of small specimens was to reduce the diffusion path. The primary concerns associated with the laboratory aging of compacted specimens are slump, changes in air void content, and the existence of an oxidation gradient. Therefore, the dimensions and air void contents of compacted specimens were compared before and after aging to determine if the specimens were damaged during the laboratory aging process. In addition, differences in the rheology and chemical compositions, along with the distance from the specimen periphery, were used to detect the presence (if any) of an aging gradient. For the loose mixtures, the primary specimen integrity concern was compactability. There- fore, the number of gyrations required to meet the target air void content was compared between the short- and long-term aged mixtures in order to assess compactability. In addition, air void content measurements were used to verify that the desired compaction level had been met. Dynamic modulus and cyclic fatigue tests were conducted using short- and long-term aged specimens to further assess if specimen integrity had been compromised as a result of the long- term aging procedures applied to both the loose mixtures and compacted specimens. The AIPs of the binder extracted and recovered from the long-term aged materials were used to assess the relative efficiency of the loose and compacted specimen aging procedures. Oven Versus Pressure Aging The application of pressure to expedite the long-term aging of the loose mixtures and com- pacted specimens was evaluated by conducting aging trials in both an oven and a binder PAV. The ability of the PAV to improve the efficiency of laboratory aging was assessed through
Research Approach 15 comparisons of extracted and recovered asphalt binder AIPs following long-term aging. Damage induced by the application and release of pressure to the compacted specimens also was assessed. Aging Temperature, 95Ã§C Versus 135Ã§C Aging Several researchers have proposed the oven aging of loose (uncompacted) asphalt mix- tures at 135Â°C for efficient laboratory long-term aging (e.g., Braham et al. 2009, Dukatz 2015, Blankenship 2005). However, the literature indicates that the oxidation reaction mechanism can change when the temperature exceeds 100Â°C, thus suggesting that accelerated aging at 135Â°C may lead to a fundamentally different aged asphalt binder than asphalt aged in the field (at a lower temperature). Therefore, the performance implications of aging laboratory loose mixtures at 135Â°C were evaluated by comparing the dynamic modulus values and the cyclic fatigue performance of mixtures subjected to long-term aging at 95Â°C and 135Â°C to yield the same rheology. In this study, 95Â°C was selected instead of 100Â°C in order to avoid the aging temperature reaching close to 100Â°C due to possible temperature fluctuations in the oven. Although the rheology of the mixtures aged at 135Â°C and 95Â°C matched, their chemistry dif- fered, and thus, the experiments allowed for the assessment of the significance of the chemical differences that are caused by aging at 135Â°C. The results then were used to inform the selection of the laboratory aging temperature. Determination of Project-Specific Aging Durations After selecting the most promising aging method, the aging procedure was applied to some selected component materials for a prolonged duration. Samples were removed periodically and subjected to extraction and recovery, after which the binder AIPs were measured and used to derive the oxidation kinetics. In addition, binders were extracted and recovered from vary- ing depths of a selected group of field cores obtained from in-service pavements. The AIPs of the field-aged binders were measured and compared to the laboratory-aged oxidation rates to determine the laboratory aging duration that is required to match the target field AIPs for spe- cific projects. Climate-Based Determination of Predefined Aging Durations The project-specific aging durations were used to calibrate a kinetics-derived climatic aging index (CAI) that can be used to determine the required laboratory aging duration to match the field aging at any location and depth of interest using EICM hourly pavement temperature data. The CAI, which can be used to relate the pavement temperature history to the required laboratory aging duration, was derived using a simplification of a rigorous oxidation kinetics model that retains the exponential relationship between the oxidation rate and temperature. A diffusion correction factor is included within the CAI to allow the required laboratory aging duration to match different pavement depths of interest. The CAI analysis was used to gener- ate maps of the United States that allow the visual determination of the required laboratory aging durations that match 4, 8, and 16 years of field aging at depths of 6 mm and 20 mm from the surface. Development of Pavement Aging Model Kinetics Modeling of Field Aging Using Mix-Specific Kinetics Parameters An existing kinetics model was applied successfully to predict loose mix aging rates at dif- ferent temperatures using rheology-based AIPs. The kinetics model was then validated using
16 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction a separate set of mixtures. The validated kinetics model provides a basis for the future devel- opment of a methodology that integrates the effects of long-term aging on performance in Pavement ME Design and other mechanistic design and analysis methods. Field aging levels were measured at different depths from field cores obtained at different service lives. Pavement temperatures obtained from the EICM as a function of depth were used along with the devel- oped kinetics model to predict field aging levels as a function of pavement depth. The predicted aging levels were compared against measured field aging in order to calibrate the predictive model. The calibrated kinetics model can be coupled with a diffusion model to enhance the prediction capabilities of that model to account for the effects of mixture morpho logical proper- ties on field aging. Determination of Mix-Specific Kinetics Parameter from Universal Simple Aging Test (USAT) Binder Aging Aging loose mixture in the oven allows the physicochemical effects of filler on asphalt binder oxidation rates to be captured. Thus, this aging method is assumed to provide a good representa- tion of field aging. However, loose mix aging requires the cumbersome extraction and recovery of the binder from the aged mixture prior to the AIP evaluation. Development of a model that can predict the effect of filler on asphalt binder oxidation would negate the need to perform loose mixture aging in order to relate laboratory aging to field aging levels. In order to evalu- ate the idea of obtaining the binder aging rate from USAT (Farrar et al. 2015) and relating it to the loose mix aging rate at a given temperature, binder and loose mix samples were short- term aged at 135Â°C for 4 hours followed by long-term aging in the same oven at 95Â°C. Then, both aged binders from the USAT as well as extracted and recovered binders from loose mix aging were tested using a dynamic shear rheometer (DSR) and FTIR spectroscopy. The aging rates obtained from this binder aging and loose mix aging were compared in order to find a relationship between the obtained aging rates. Test Materials and Field Projects The experimental plan involves two groups of materials. The Group A materials are a set of laboratory-prepared materials for which relatively large quantities were available. The main purpose of investigating these materials was to evaluate different long-term aging scenarios. Also, the Group A materials allowed for a systematic sensitivity study to investigate the effects of changes in the AIPs on the performance of asphalt concrete. The Group B materials are the original component materials (i.e., binder and aggregate) and field cores extracted from in-service pavements. The Group B materials were used to develop the interim long-term aging procedure. Group A Materials Table 3 presents a summary of the Group A materials. Several binders are included to cover a wide range of aging characteristics. The limestone aggregate and PG 58-28 and PG 76-16 binders used in the Asphalt Research Consortium (ARC) study were used in the sensitivity Material Source MaterialAggregate Asphalt Binder ARC Limestone PG 58-28, PG 76-16 North Carolina Granite PG 64-22 SHRP Granite PG 58-28 (AAD-1), PG 58-10 (AAG-1) Table 3. Summary of Group A materials.
Research Approach 17 study. The PG 64-22 binder and granite aggregate were acquired from North Carolina and used in the sensitivity study and for the evaluation of candidate aging methods. The SHRP binders AAD-1 and AAG-1 were used to study the effects of aging temperature due to their known dif- ferences in chemistry. Group B Materials/Projects Table 4 summarizes the Group B materials that are composed of the original binders and aggregate and field cores from the selected pavement sections. These sections cover a wide range of pavement design, climatic conditions, ages, binder and aggregate characteristics, air void contents, asphalt contents, and gradations. The Group B materials were used to develop the long-term aging procedure and for validation and calibration. Figure 3 summarizes the geographic coverage of the selected materials/projects. The figure indicates that a broad range of geographic locations across the United States and Canada were used to develop the long-term aging procedure. Sample Preparation Methods Asphalt Mastic Preparation The composition of the mastics was determined based on the asphalt mixture design for each source. Also, the specific gravity of each aggregate source in the gradation was measured. Trials were conducted to determine the optimum binder content for the mastics. Asphalt binders aged in the PAV for different durations were mixed with the prepared gradations and mixed to form a uniform mix. Samples were taken from the mastic to test in the DSR. Site ID Location Binder/Modification Date Built Date Core Extracted FHWA ALF Virginia Control, slices, and aged binders styrene butadiene styrene (SBS)-LG 2001 2013 Manitoba Manitoba, Canada Control, Foam WMA, Evotherm WMA 2010 2014 National Center of Asphalt Technology (NCAT) Alabama Control, Foamed WMA 2009 2013 LTPPa New Mexico, Grant County, I-10 Frontage Rd., MP 51 Asphalt Cement AC-20 1996 2006, 2014 South Dakota, Campbell County, 101st St., MP 400.1 Asphalt Cement 120-150 pen 1993 2006, 2014 Texas, Brazos County, Old Cameron Ranch Rd., MP 404.2 Asphalt Cement AC-20 1996 2007, 2014 Wisconsin, Marathon County, Apple Ln. unknown 1997 2005, 2014 WesTrack Fine Sections (1, 2, 3, 4, 14, 17, 18) Dayton, Nevada PG 64-22 1995 1995, 1997, 1999, 2014 WesTrack Coarse Sections (36, 39) Dayton, Nevada PG 64-22 1997 1999, 2014 aLTPP stands for long-term pavement performance. Table 4. Selected sections for development of long-term aging protocol and field calibration under Group B materials.
18 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction FAM Preparation The compositional design was developed for FAM based on the mixture design for each source. The maximum specific gravity, Gmm, was calculated at the aging level of 0 year. The aged binder for the FAM samples was pre-aged in the PAV and then blended with aggregate in a conventional bucket mixer. The blended FAM mix was poured into a compaction mold and kept inside the oven until it reached the compaction temperature. After reaching the correct temperature, FAM plugs were compacted in the SuperpaveÂ® gyratory compactor and then left at room temperature to cool. The Ã20-mm Ã 50-mm test specimens were extracted by coring the Superpave gyratory plugs. The target FAM air void contents correspond to mixture air void contents of 4.5% and 8.5% were 6.5% and 11%, respectively. Figure 4(a) and 4(b) show a FAM plug with a cored sample and the test set-up, respectively. Figure 3. Locations of materials/projects included in the study (MB = Manitoba). (a) (b) Figure 4. (a) FAM plug with cored and cut sample and (b) instrumented FAM sample.
Research Approach 19 Asphalt Mixture Aging All of the asphalt mixtures aged in the laboratory were prepared using component materials that were used in constructing the pavements from which field cores were obtained. All of the component materials were prepared based on the original mix design. All HMA mixtures were subjected to short-term aging at 135Â°C for 4 hours in accordance with AASHTO R 30 prior to long-term aging. The WMA mixtures were subjected to short-term aging at 117Â°C for 2 hours prior to long-term aging. Aging of Compacted Specimens For all the compacted specimen aging trials, the short-term aged mixtures were compacted in a Superpave gyratory compacter to fabricate Ã150-mm Ã 178-mm specimens. Subsequently, large specimens for aging were prepared through coring to obtain Ã100-mm Ã 178-mm speci- mens. To fabricate small specimens, Ã38-mm cylindrical specimens were horizontally cored from initial gyratory specimens (Ã150-mm Ã 178-mm). To fabricate the specimens for the WMA aging trials, the specimens were cored to obtain Ã100-mm Ã 178-mm specimens. Then, the inner 150 mm of the Ã100-mm specimens were sliced to obtain 25-mm thick disks. For the oven aging of the large compacted specimens, the ends of the cores were not sawn before aging because of the high probability of an aging gradient that could affect the per- formance test results. Also, wire mesh supports were utilized to minimize distortion under self-weight. A single aging temperature of 85Â°C, as specified by AASHTO R 30, was used for the compacted specimen aging trials. For the small specimens, the ends were sawn to obtain Ã38-mm Ã 100-mm specimens. Figure 5 shows large and small specimens aging in the oven. For the WMA compacted specimen aging trials, thin disk specimens were placed on end in the oven, as shown in Figure 6, to maximize oxygen exposure. Pressure aging trials using the compacted specimens were conducted in a standard asphalt binder PAV. The pressure level of 300 kPa was selected for the PAV aging trials because SHRP researchers (Bell et al. 1994a) had found that higher pressure levels damage specimens upon pressure release. To reduce the stress on the large specimens under self-weight in the PAV, a wire mesh hammock-like support was developed to allow the specimenâs weight to be distributed over a larger area on its sides than if it were positioned vertically, as shown in Figure 7. Pressure Aging of Loose Mix The oven aging of the loose mix was accomplished by separating the mix into several pans such that each pan had a relatively thin layer of loose mix that was approximately equal to the Figure 5. Long-term aging of large and small specimens in the oven.
20 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction nominal maximum aggregate size (NMAS) of the aged mix, as shown in Figure 8. The loose mix was agitated several times during oven aging, and the pans in the oven were rotated sys- tematically to minimize any effects of an oven temperature gradient and/or draft on the degree of aging. After long-term aging, the materials were taken out of the oven and mixed together in order to obtain a uniform mixture, and then the mixture was left to cool to room temperature. The loose mixture was then reheated to the compaction temperature for 75 minutes. Speci- mens were compacted following aging for performance testing. Because the compactability of the aged loose mix was a potential concern, the effect of increasing the compaction tempera- ture on the compactability of the aged loose mix was investigated. The pressure aging trials for the loose mix utilized the standard binder PAV pressure of 2.1 MPa, because pressure was not anticipated to induce any integrity concerns regarding the loose mixture samples, as compaction follows the aging process. Figure 9 shows the specimen Figure 6. WMA compacted specimens aging in an oven. Figure 7. Simple set-up for holding large specimens during aging in the PAV.
Research Approach 21 set-up. The loose mix was dispersed in thin layers, consistent with the process for oven aging. The size of the binder PAV prohibited aging a large quantity of mix efficiently, and thus, any gain in the oxidation rate had to be balanced with the amount of material that could be aged at one time or, conversely, the associated costs of developing a mixture-specific pressure aging device. Due to the capacity constraints of the PAV, the long-term aging trials of the loose mix in the PAV were limited to simply assessing how much pressure would expedite the oxida- tion of the loose mix. Insufficient quantities of material were aged to produce compacted specimens. Asphalt Binder Aging RTFO Aging RTFO aging was conducted using selected original asphalt binder samples according to AASHTO T 240 to simulate short-term aging. Figure 8. Long-term aging of loose mix in thin layers for long-term aging in oven. Figure 9. Aging rack developed for long-term aging of loose mix in PAV.
22 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction PAV Asphalt binder residue obtained from the RTFO aging was subjected to PAV aging based on AASHTO R 28 at 100Â°C for 20 hours. USAT The USAT, developed by Farrar et al. (2015) at the Western Research Institute (WRI), was applied in this study to derive the oxidation kinetics of asphalt binders as part of the aging model development. In the USAT, the binder is placed in grooved plates to achieve a film thickness of 300 micrometers. The USAT plates were placed in an oven at 135Â°C for 4 hours to simulate the short-term aging of loose mixtures. After this binder short-term aging process, the USAT plates were placed in an oven at 95Â°C to simulate long-term aging. Field Core Preparation Full depth cores were acquired from in-service pavements and from the FHWA Materials Reference Library. The acquired field cores were wrapped with plastic wrap and placed in a temperature-controlled room to minimize further aging during storage. Following storage, the upper 2 inches of each field core was sliced into 0.5-inch thick sections. The remainder of the field core was sliced into 1-inch thick slices. Special attention was given to avoid the tack coat and prime coat layers when slicing the field cores. Figure 10 provides a depiction of the field core slicing pattern used. Micro-Extraction and Recovery The micro-extraction and recovery of the asphalt binder from the asphalt mixtures was per- formed following the procedure proposed by Farrar et al. (2015) at the WRI. This procedure uses a solvent mixture of toluene and ethanol (85:15) for extraction and recovery. The mixture sample size is limited to 200 g to produce approximately 10 g of asphalt binder per extraction, which is adequate for both FTIR spectrometry testing and DSR testing. In order to prevent further aging of the binder samples during the extraction and recovery procedure, a distillation flask was subjected to vacuum pressure of 80.0 Â± 0.7 KPa (600 Â± 5 mm Hg) under nitrogen gas. FTIR spectrometry testing was conducted following extraction and recovery to ensure that no detectable solvent was present. Figure 10. Depiction of field core slices used to determine oxidation gradient with depth.
Research Approach 23 Test Methods Asphalt Binders FTIR Test Procedure ATR FTIR spectroscopy collects absorbance data within a wide spectral range (400 cm-1 to 4000 cm-1). The ATR spectra were collected using 64 scans at a resolution of 4 cm-1 using a minimum of two replicates. For each binder, all replicates under different conditions were normalized to the same absorbance value at wave number 1375 cm-1. This wave number was selected for normalization because absorbance at this wave number is not affected by the level of oxidation. Changes in the C + S peaks were tracked at wave numbers 1702 cm-1 and 1032 cm-1, respectively. Additionally, C + S areas were measured as the areas under the FTIR absorbance curves at wave number ranges 1650â1820 cm-1 and 1000â1050 cm-1, respectively. The trap- ezoidal rule was used to numerically determine the area under the band between the specified ranges of wave number. DSR Test Procedure Frequency sweep testing was conducted at frequencies ranging from 0.1 Hz to 30 Hz and multiple temperatures (5Â°C, 20Â°C, 35Â°C, 50Â°C, and/or 64Â°C) using asphalt binders in the DSR with 8-mm parallel plate geometry. A strain amplitude of 1% was applied at all frequencies and temperatures of testing. The rheological properties analyzed included the dynamic shear modulus (G*) at 64Â°C and 10 rad/s, the crossover modulus (Gc*) [Farrar et al. 2013, defined as the G* value that corresponds to the reduced frequency where the storage modulus (Gâ²) and loss modulus (Gâ³) master curves cross (i.e., where the phase angle equals 45Â°)], the zero shear viscosity (ZSV), defined as the viscosity when the shear rate approaches zero (Binard et al. 2004, Brio et al. 2009), and the Glover-Rowe (G-R) parameter (Rowe et al. 2014), which has been proposed as an indicator of ductility and is equal to G*cos2d/sind evaluated at 15Â°C and 0.005 rad/s. Asphalt Mixtures Dynamic Modulus Test Procedure Frequency sweep tests were conducted at multiple temperatures in accordance with AASHTO PP 342 to build dynamic modulus master curves. The initial test temperatures used to build these master curves were -10Â°C, 5Â°C, 20Â°C, 40Â°C, and 54Â°C. However, the dynamic modulus test results from the first set of specimens revealed insufficient overlap between the dynamic modulus values at the different test temperatures for highly aged materials. This lack of overlap in the dynamic modulus values precluded an accurate application of timeâtemperature super- position in order to construct the master curves. Therefore, to enable the successful construction of dynamic modulus master curves, most of the long-term aged mixture dynamic modulus tests were conducted at -10Â°C, 5Â°C, 15Â°C, 27Â°C, 40Â°C, and 54Â°C. At least two test replicates were conducted for each mixture and condition evaluated. Asphalt Mixture Performance Tester (AMPT) Cyclic Fatigue Test Procedure Cyclic fatigue testing was conducted in accordance with AASHTO TP 107. The test tem- perature was determined using either the binder PG or designated regional PG estimated from the LTPPBind program as per AASHTO TP 107. The testing frequency was 10 Hz. Three tests were conducted for each mixture at three different actuator displacement amplitudes (low, medium, and high).