Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging (2021)

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B-1   A P P E N D I X B Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary

B-2 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder 1. SCOPE 1.1. This standard practice covers 0.8 mm thin film conditioning of asphalt binders to simulate short-term (from production to placement) and long-term (in service) aging asphalt binders. 1.2. The aging of asphalt binders is affected by plant operations, construction operations, climate, and several mixture associated variables. This practice is intended to provide an evaluation of the extent that different asphalt binders age during construction and the service life of the pavement. 1.3. This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concerns associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO STANDARDS • M 231, Weighing Devices Used in Testing of Materials • M 320, Performance-Graded Asphalt Binder • M 332, Performance-Graded Asphalt Binder Using Multiple Stress Creep Recovery (MSCR) Test • T 240, Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test) • R 28, Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV) 3. TERMINOLOGY 3.1. Definitions 3.1.1. asphalt binder – an asphalt-based cement that is produced from petroleum residue either with or without the addition of organic modifiers.

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-3   3.1.2. aging- the change in the properties of asphalt binder due to the combined effects of time, traffic, and the environment. 3.1.3. conditioning- the process of simulating aging in the laboratory. 4. SUMMARY OF PRACTICE 4.1. Nominal 0.8 mm thin films are formed in 10 pans by weighing 12.5 g of asphalt binder into 140 mm diameter flat bottom stainless steel pans, then placing the pans in a 163°C oven to allow the binder to flow and cover the flat portion of the pan. 4.2. The 10 pans of binder are first short-term conditioned at 163°C for 90 minutes to simulate the aging that occurs during construction. The 10 pans are removed from the 163°C oven. One pan is set aside for rheological testing of the short- term conditioned residue. 4.3. The remaining nine pans are transferred to a pressurized aging vessel system and long-term conditioned for 20 hours under 2.1 MPa air pressure at specified temperatures based on the environment where the binder will be used to simulate aging that occurs during the service life of the pavement. 4.4. The PAV-conditioned residue is then vacuum degassed before rheological property testing of the long-term conditioned. 5. SIGNIFICANCE AND USE 5.1. Residue of asphalt binders conditioned using this practice are used to determine specification properties for AASHTO M 320 and AASHTO M 332. 5.2. The short-term conditioning step produces residue with rheological properties similar to residue from AASHTO T 240. 5.3. The long-term conditioning step produces residue with rheological properties similar to binder recovered from the top 1 inch of the pavement after 10 years in service. 6. APPRATUS 6.1. Balance- A balance conforming to the requirements of M 231, Class G 2. 6.2. Stainless Steel Pans- Ten flat bottom stainless steel pans 9.5 mm deep with an inside diameter of 140 mm and having a thickness of 1.0 mm. Note X1 – Flat pans are needed to form and maintain a 0.8 mm thin film. Pans made from 1.0 mm stainless steel will not warp at the temperatures used in this

B-4 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging practice. The pans may warp if subjected to higher temperatures for cleaning. The pans should be cleaned using mineral spirits, mineral oil, citrus-based solvents, toluene, or similar solvent followed by acetone to remove the solvent residue from the pans. 6.3. Pressure Aging Vessel System (PAV)- A PAV system meeting the requirements of sections 6.1 of AASHTO R 28, except the pan holder, when installed in the PAV, must hold the pans level to within 0.025 degrees. The PAV system shall have a method for rapidly leveling the sample holder while it is installed in the PAV and verifying levelness using a precision machinist level. 6.4. Pressure/Vacuum Oven- A combination low pressure and vacuum oven to: (1) short-term condition binders and (2) vacuum degas long-term conditioned residue. The pressure/vacuum oven shall meet the following requirements. 6.4.1. Pressure/vacuum vessel- The pressure/vacuum vessel shall be designed for internal gauge pressure ranging from −100 to +100 kPa and temperature up to 180°C. The pressure/vacuum vessel and shall have: (1) a vacuum port, (2) a pressure port, (3) a vent port with flow valve, and (4) a pressure relief valve. The pressure relief valve shall not permit pressure inside the pressure/vacuum vessel to exceed 100 kPa. The pressure/vacuum vessel shall be sized to accept the loaded 10-pan holder used in the PAV system. The pan holder when installed in the pressure/vacuum vessel must hold the pans level to within 0.025 degrees. The pressure/vacuum vessel shall have a method for rapidly leveling the sample holder while it is installed in the pressure/vacuum vessel and verifying levelness using a precision machinist level. The pressure/vacuum vessel shall have a sight port and lighting for observing the film in the top pan of the pan holder. 6.4.2. Temperature control system- The pressure/vacuum oven shall have temperature control between 135°C and 180°C. Temperature shall be controlled by a temperature sensor accurate to ±0.5°C mounted inside the pressure/vacuum vessel. The temperature control system shall maintain the temperature at any point inside the pressure/vacuum vessel within ±1.0°C of the set point temperature. The temperature control system shall be capable of bringing the temperature in the pressure/vacuum vessel back to the set point temperature ±1.0°C within 10 minutes after loading the pan holder and pans loaded with binder into the pressure/vacuum vessel. The temperature control system shall be capable of recording the temperature inside the pressure/vacuum oven at intervals of 1 minute or less. 6.4.3. Vacuum System- A vacuum system consisting of: (1) a vacuum pump, (2) a vacuum gauge, and (3) piping, valves, and fittings. The vacuum system shall be designed to handle air at temperatures up to 180°C. The vacuum system shall be capable of reducing the pressure in the pressure/vacuum vessel to

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-5   15 kPa absolute or less and maintaining the pressure at 15 kPa absolute ±2.5 kPa for at least 30 minutes. The vacuum system shall have a vacuum gauge or electronic pressure sensor with ±0.5 kPa accuracy and readable to 2 kPa or less. 6.4.4. Pressure Control System- A pressure regulator and absolute pressure measuring device capable of controlling the pressure in the pressure/vacuum vessel at 110 ± 1.0 kPa absolute. 6.5. Precision Machinist Level- A precision machinist level for verifying the levelness of the pans in the pressurized aging vessel and the pressure/vacuum oven. The level shall have sensitivity of 0.00042 mm/mm (0.024 degrees) or better per division. Note X2 – Starrett No. 98 Precision Machinist Level 6 inches in length has been used for this purpose. 7. CALIBRATION AND STANDARDIZATION 7.1. PAV Temperature Sensor- Per Section 9 of AASHTO R 28, standardize the PAV temperature sensor to 0.1°C at least every 6 months using a calibrated thermometer. 7.2. PAV Pressure Gauge- Per Section 9 of AASHTO R 28, standardize the PAV pressure gauge to 1 percent at least every 6 months. 7.3. Pressure/Vacuum Oven Temperature Sensor- Standardize the vacuum oven temperature sensor to 0.5°C at least every 6 months using a calibrated thermometer. 7.4. Pressure/ Vacuum Oven Vacuum Gauge- Standardize the pressure/vacuum oven vacuum gauge or electronic absolute pressure sensor to ±0.5 kPa using a calibrated pressure sensor or a manometer to 4 mm Hg at least every 6 months. 7.5. Pressure/Vacuum Oven Absolute Pressure Device- Standardize the pressure/vacuum oven pressure measuring device to ±0.5 kPa using a calibrated pressure sensor. 7.6. Precision Machinists Level- Verify the level at least weekly, any time the level is mishandled, or when changes must be made after previous leveling of the equipment using the following procedure. 7.6.1. Select a stable, flat bearing surface. The bearing surface does not have to be perfectly level but must be level enough for the bubble to be within the measuring range of the level.

B-6 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging Note X3 – The penetrometer stand is a good surface for this procedure. 7.6.2. Clean the bearing surface and the underside of the level to remove any asphalt binder, dust, or other material. 7.6.3. Place the level onto the bearing surface, mark the perimeter of the level base, and note the position of the bubble after it has settled. 7.6.4. Turn the level by 180 degrees and place the level back onto the surface at the same location as 7.6.3. Note again the position of the bubble after it has settled. 7.6.5. The bubble should be in the same position in 7.6.3 and 7.6.4. If not, adjust the adjustment nut on the level and repeat the verification until the bubble is in the same position when the level is turned 180 degrees. 8. PREPARATION OF OVENS 8.1. Before each run, verify the levelness of the pans in the pan holder when installed in the pressure/vacuum oven using the precision machinist level. Check levelness along perpendicular axes. Adjust as necessary to achieve a level plane within 0.025 degrees. 8.2. Before each run, verify the levelness of the pans in the pan holder when installed in the PAV using the precision machinist level. Check levelness along perpendicular axes. Adjust as necessary to achieve a level plane within 0.025 degrees. 8.3. Preheat the pressure/vacuum oven with the pan holder without the pans for at least 2 hours to 163 °C ±1.0°C. 8.4. Select the long-term conditioning temperature from Table 1 and preheat the PAV without the pan holder and pans to the selected long-term conditioning temperature. Note X4 – Preheating the PAV to 10 °C to 15°C above the conditioning temperature can be used to reduce the drop in PAV temperature during the loading process and minimize the time required to stability the system after loading to attain the required temperature. 9. PROCEDURE 9.1. Short-Term Conditioning 9.1.1. The asphalt binder sample, as received, shall be free of water. Heat the sample in its container with a loosely fitted cover in an oven not to exceed 163°C for the

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-7   minimum time necessary to ensure that the sample is completely fluid. Manually stir the sample to avoid incorporating air bubbles. 9.1.2. Designate one pan for short-term only conditioning. Record the mass of the pan designated for short-term only conditioning to the nearest 0.001 g. Table 1. Long-Term Conditioning Temperatures Average 98% Reliability High and Low Pavement Temperature Long-Term Conditioning Temperature ⁰C PG Grade Based on Environment −6 85 PG 40-52; PG 46-52; PG 40-46 −3 0 90 PG 52-52; PG 46-46; PG 40-40; PG 46-40; PG 52-46; PG 40-343 6 95 PG 58-46; PG 52-40; PG 46-34; PG 40-28; PG 58-40; PG 52-34; PG 46-28; PG 40-229 12 100 PG 64-40; PG 58-34; PG 52-28; PG 46-22; PG 40-16; PG 64- 34; PG 58-28; PG 52-22; PG 46-16; PG 40-10; PG 64-28; PG 58-22; PG 52-16; PG 46-10 15 18 21 105 PG 70-28; PG 64-22; PG 58-16; PG 52-10; PG 70-22; PG 64- 16; PG 58-1024 27 110 PG 70-16; PG 64-10; PG 70-10 30 33 115 PG 76-10 9.1.3. Load 10 pans with asphalt binder. Pour 12.5 ±0.1 g of the asphalt binder onto the center of each of the 10 pans. 9.1.4. Allow the pans to cool for at least 60 minutes, but no more than 180 minutes before loading them into the pressure/vacuum oven. 9.1.5. After cooling, record the mass of the loaded pan that was designated for short- term only conditioning to the nearest 0.001 g. 9.1.6. Open the pressure/vacuum oven, remove the pan holder, place a loaded pan into each slot of the pan holder, load the pan holder into the pressure/vacuum oven, and close the oven. 9.1.7. Apply 110.0 ±1.0 kPa of air pressure to the pressure/vacuum oven and start timing the short-term conditioning.

B-8 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging 9.1.8. Maintain the pans in the oven under pressure at 163 ±1.0°C for 90 minutes ±1 minute. The temperature of the pressure/vacuum oven shall reach 163 °C ±1.0°C within the first 10 minutes; otherwise, discontinue the conditioning and discard the material. 9.1.9. At the end of the 90 minute conditioning period, reduce the internal pressure of the pressure/vacuum oven to atmospheric pressure using the flow valve on the vent port. 9.1.10. Open the pressure/vacuum oven, remove the pan holder, remove the pan designated for short-term conditioning only, and place it on the cooling rack. 9.1.11. Leave the nine remaining pans in the pan holder and proceed to long-term conditioning. 9.1.12. Allow the short-term only pan to cool for at least 60 minutes, but no more than 180 minutes. Record the mass of the pan that was designated for short-term only conditioning to the nearest 0.001 g. 9.1.13. Heat the pan designated for short-term only conditioning in an oven set at 163°C for a minimum time until sufficiently fluid to pour. Pour and scrape the residue into a container for subsequent testing. 9.1.14. From the residue generated, prepare test specimens directly from the residue in the container or set the container aside for future testing. 9.2. Long-Term Conditioning 9.2.1. Open the PAV. Place the holder loaded with nine pans into the PAV and close the PAV. 9.2.2. Connect the temperature transducer and air pressure supply to the PAV. 9.2.3. Wait until the temperature inside the PAV is within 20°C of the conditioning temperature; apply an air pressure of 2.1 ±0.1 MPa and then start timing the test. If the temperature inside the vessel has not reached the desired temperature for applying pressure within 2 hours of loading the pan holder and pans, discontinue the procedure and discard the samples. Note X5 – Pressures in excess of 2.1 MPa do not substantially increase the rate of aging. Therefore, higher pressures are not warranted. Note X6 – Once pressurized, the temperature inside the pressure vessel will equilibrate rapidly. The time under pressure, not to include any preheating time at ambient pressure, is the conditioning time. Relatively little aging occurs at ambient pressure during the time that the vessel is being reheated to the test

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-9   temperature given that asphalt binder residue under test has been exposed to 163°C during short-term conditioning. 9.2.4. Maintain the temperature and air pressure inside the pressure vessel for 20 hours ±10 minutes. 9.2.5. Preheat the pressure/vacuum oven to 170 °C ±5°C. The pressure/vacuum oven should be preheated to allow the pan holder with PAV-conditioned residue to be transferred directly to the pressure/vacuum oven after PAV conditioning. 9.2.6. At the end of the 20-hour test period, slowly begin reducing the internal pressure of the PAV using the air pressure bleed valve. Adjust the bleed valve to an opening that requires 9 minutes ±1 minute to equalize the internal and external pressures on the PAV, thus avoiding excessive bubbling and foaming of the asphalt binder. During this process, it may be necessary to adjust the setting of the needle valve as the pressure drops in order to maintain an approximate linear rate of pressure decrease. Do not include the pressure release and equalization time as part of the 20-hour conditioning period. 9.2.7. If the temperature indicated by the temperature-recording device falls above or below the target conditioning temperature ±0.5°C for more than 60 minutes during the 20-hour aging period, declare the test invalid and discard the material. 9.2.8. Open the PAV, remove the pan holder with the PAV-conditioned residue from the PAV, and immediately transfer it to the pressure/vacuum oven. 9.2.9. Close the pressure/vacuum oven and maintain the temperature in the vacuum oven at 170°C ± 5°C for 15 minutes ±1 minute without vacuum applied. 9.2.10. After 15 minutes ±1 minute of equilibration, reduce the pressure in the vacuum vessel to 15 ±2.5 kPa absolute and maintain this pressure for 30 minutes ±1 minute. After 30 minutes ±1 minute, vent the vacuum oven to atmospheric pressure. Note X7 – If the pressure gauge or sensor in the vacuum oven reads absolute pressure, use 15 ±2.5 kPa. If the pressure gauge or sensor reads gauge pressure, use Table 1 in AASHTO R 28 to determine the gauge reading for the elevation of the laboratory. 9.2.11. Open the vacuum oven; remove the pan holder and pans with degassed PAV residue, and transfer the pans to an oven set at 170°C. Transfer the PAV residue by scraping into containers for testing. Combine degassed PAV residue to a single container followed by gentle stirring to blend the residue.

B-10 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging 9.2.12. Prepare test specimens directly from the residue in the container; subdivide the residues into smaller containers for future testing or set the container aside for future testing. 10. MASS CHANGE CALCULATION 10.1. Calculate the mass change to three decimal places using the following equation. The mass change may be negative (mass loss) or positive (mass gain): mass change, % = (B − A) (A − C) × 100% where: A = mass of binder plus pan before conditioning, g B = mass of binder plus pan after short-term conditioning, g C = mass of binder, g 11. REPORT 11.1. Report the following Information: 11.1.1. Sample identification; 11.1.2. Maximum and minimum temperature during short-term conditioning, nearest 1.0°C; 11.1.3. Maximum and minimum absolute pressure during short-term conditioning, nearest 1.0 kPa; 11.1.4. Mass change of short-term conditioned residue to three decimal places; 11.1.5. Long-term conditioning temperature, nearest 0.5°C; 11.1.6. Maximum and minimum long-term conditioning temperature, nearest 0.1°C; 11.1.7. Maximum and minimum long-term conditioning pressure, nearest 0.1 MPa; 11.1.8. Total long-term conditioning time, hours, and minutes; and 11.1.9. Any deviations from the specified procedure. 12. PRECISION AND BIAS 12.1. Precision- The research required to develop precision estimates for tests performed on short- or long-term conditioned residue from this practice has not been conducted.

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-11   12.2. Bias- The research required to establish the bias for tests performed on short- or long-term conditioned residue from this practice has not been conducted. 13. Key Words 13.1. Accelerated aging, short-term conditioning, long-term conditioning, mass change, pressurized aging vessel

B-12 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging COMMENTARY FOR DRAFT PROVISIONAL STANDARD PRACTICE FOR 0.8 MM STATIC FILM SHORT- AND LONG-TERM CONDITIONING OF ASPHALT BINDER INTRODUCTION This proposed standard practice describes short- and long-term conditioning of asphalt binder using 0.8 mm thin films. This commentary was prepared to summarize the research from National Cooperative Highway Research Program (NCHRP) Project 09-61, “Short- and Long- Term Binder Aging Methods to Accurately Reflect Aging in Asphalt Mixtures” that supports this proposed practice. It is intended for those who will be responsible for adoption and future revision of the proposed standard practice. The sections in this commentary follow those in the proposed practice. When appropriate, references are cited. If additional development work is needed for implementation, it is also described. 1. SCOPE Research conducted in NCHRP Project 09-61 concluded that static 0.8 mm thin film conditioning of asphalt binder could be used to simulate both short-term (from production to placement) and long-term (in service) aging of asphalt binders. This conclusion offers the potential to simplify laboratory conditioning of asphalt binders for performance grading. This practice was developed to combine short-term and long-term conditioning into a single procedure. 2. REFERENCED DOCUMENTS Documents referenced in the proposed standard practice. 3. TERMINOLOGY Definitions for various terms used in the proposed standard practice. 4. SUMMARY OF PRACTICE In NCHRP Project 09-61, static thin film conditioning of 12.5 g of asphalt binder in a 140 mm diameter pressurized aging vessel (PAV) pan was evaluated as an alternative to AASHTO T 240. This mass of binder in a PAV pan yields a nominal 0.8 mm thick film. The aging index defined as G*/sinδ after short-term conditioning divided by the unconditioned G*/sinδ was compared to that for binder recovered from mixture that was short-term conditioned in accordance with the recommendations from NCHRP Project 09-52, 2 hours at 135ºC. A paired difference statistical analysis was conducted using eight binders. The results are summarized in Table B-1 for AASHTO T 240 and the static 0.8 mm thin film. The conclusion for the paired difference statistical analysis was there was not a significant difference for either of the short- term binder conditioning procedures compared to short-term oven conditioning. The negative average difference for the static 0.8 mm film conditioning indicates a conditioning time longer

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-13   than 85 minutes is probably needed. For the draft provisional standard, the short-term conditioning time was increased to 90 minutes. Table B-1. Summary of Paired Difference Analysis for Simulated HMA Aging. Binder/Statistic Test Temp ºC Aging Index Difference T 240 Static 0.8 mm Neat PG 52-34 52 0.02 0.11 Neat PG 64-22 64 −0.56 −0.54 Terpolymer PG 64-34 64 0.50 0.40 SBS PG 76-22 76 0.21 −0.19 PG 64-34 64 −0.18 −0.41 PG 76-28 76 0.42 0.05 64-22 Latex 76 −0.58 −0.68 PG 88-22 88 −0.20 −0.03 Average Difference −0.04 −0.16 Standard Deviation of Differences 0.408 0.363 t-statistic −0.311 −1.255 Critical t-statistic, 0.05 Confidence Level 1.895 1.895 Conclusion No significant difference No significant difference A major part of NCHRP Project 09-61 was directed at long-term conditioning. Cracking data from the LTPP Specific Pavement Study 8 (SPS-8), Study of Environmental Effects in the Absence of Heavy Loads, were analyzed to determine the pavement age to target with long-term conditioning. The cracking data are shown in Figure B-1. From this figure, a target age of 10 years was selected. A PAV operating parameters experiment was conducted in NCHRP Project 09-61 using four pavements from the Asphalt Research Consortium (ARC) field sections in Arizona and four pavements from the ARC field sections in Minnesota to determine the operating parameters needed to reproduce the properties of recovered binders from these sections after approximately 10 years in service. The temperature, binder mass, and conditioning time were varied. The conclusion drawn from the PAV operating parameters experiment was it is possible to approximate near-surface field aging by varying the operating parameters of the PAV. For PAV conditioning at 100ºC, 20 hours of conditioning using a mass of 12.5 g is approximately equivalent to 40 hours of conditioning using a mass of 50.0 g. Both are approximately equivalent to 10 years of field aging at a depth of 0.75 in for the pavements at the Arizona and Minnesota sites. A PAV calibration experiment was then conducted to determine appropriate temperatures for 12.5 g, 20-hr, 2.1 MPa PAV conditioning for various climates. The basic approach consisted of comparing rheological and chemical properties of laboratory-conditioned binders to those for binder recovered from cores from in-service pavements. Table B-2 lists the pavements that were

B-14 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging used in the long-term calibration experiment. These pavements were in 23 states and two provinces and cover a wide range of climates. The mean annual air temperatures during the period from construction to coring ranged from approximately 1.7°C to 20.0°C. Figure B-2 shows the geographical distribution of the pavements that were used. The selected pavements include new, full-depth construction and overlays of existing asphalt concrete and Portland cement concrete pavements. The thickness of the asphalt concrete placed during the LTPP construction ranged from 3.7 in to 10.5 in. Recovered binder properties were measured for three 0.5 in thick slices: (1) 0 to 0.5 in, (2) 0.5 to 1.0 in, and (3) 1.5 to 2.0 in from cores stored in the LTPP Materials Reference Library (MRL). Volumetric properties of the cores were also measured. To determine appropriate PAV conditioning temperatures for each pavement, the original binder samples were first RTFOT conditioned. The RTFOT-conditioned residue was then further conditioned in the PAV using 12.5 g in standard PAV pans, 20 hours conditioning time, and 2.1 MPa pressure. Three temperatures were used: 85°C, 100°C, and 115°C. Rheological and chemical properties of the original binder, RTFOT residue, and the residue from the three PAV conditioning temperatures were measured. Dynamic shear rheometer (DSR) frequency sweep testing was conducted to determine Christensen-Anderson master curve parameters and Fourier-transform infrared (FTIR) spectroscopy. FTIR testing was conducted to determine carbonyl and sulfoxide absorbance. The master curve, carbonyl absorbance, and sulfoxide absorbance data were analyzed to determine equivalent PAV conditioning temperatures for each slice for each pavement. Statistical analysis of the equivalent PAV 0 5 10 15 20 25 30 35 40 45 0 2 4 6 8 10 12 14 16 18 20 Le ng th o f T ra ns ve rs e Cr ac ks , m Pavement Age, yrs AR CA MS NM NC MO NY OH WI Figure B-1. Average Transverse Cracking in LTPP SPS-8 Sections With Sections on Swelling Soil and Sections That Received Maintenance Treat- ment Removed.

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-15   Table B-2. Summary of LTPP Sections to be Used in the Long-Term Calibration Experiment. No. LTPP ID State Age, yrs Region MMAT, °C Binder Type 1 010102 AL 12.4 wet-no freeze 17.9 AC-20 2 050804 AR 7.3 wet-no freeze 17.8 Unknown 3 060603 CA 13.5 wet-no freeze 9.1 AR-4000 4 170603 IL 15.9 wet-freeze 11.1 Unknown 5 180603 IN 12.9 wet-freeze 10.0 Unknown 6 190108 IA 14.5 wet-freeze 11.2 Unknown 7 230504 ME 9.1 wet-freeze 6.1 Unknown 8 240507 MD 13.2 wet-no freeze 11.4 AC-20 9 270504 MN 14.7 wet-freeze 4.2 Pen 85/100 10 280805 MS 11.9 wet-no freeze 17.0 AC-30 11 290507 MO 7.6 wet-freeze 14.8 Unknown 12 300806 MT 9.3 dry-freeze 1.7 Pen 85/100 13 340507 NJ 13.6 wet-freeze 11.9 AC-20 14 350802 NM 9.4 dry-no freeze 17.2 AC-10 15 360802 NY 13.2 wet-freeze 8.6 Unknown 16 370802 NC 8.7 wet-no freeze 16.8 Unknown 17 400603 OK 14.1 wet-no freeze 15.4 AC-20 18 420608 PA 12.7 wet-freeze 8.5 AC-20 19 460804 SD 14.2 dry-freeze 7.7 Pen 120/150 20 480802 TX 11.0 wet-no freeze 20.0 AC-20 21 550806 WI 7.7 wet-freeze 5.8 Unknown 22 810504 AB 15.7 dry-freeze 3.2 Pen 200/300 23 830504 MB 14.7 wet-freeze 2.9 Pen 150/200 24 06A806 CA 7.8 dry-no freeze 18.1 AR 4000 25 48A504 TX 15.9 wet-no freeze 18.9 AC-10 with latex temperatures was performed to develop a model of the equivalent PAV conditioning temperature as a function of pavement age, climate, volumetric properties, depth, and binder properties. This model was then used to recommend PAV conditioning temperatures for AASHTO M 320 and AASHTO M 332 grading. The recommended 12.5 g, 20-hr, 2.1 MPa conditioning temperatures are summarized in Table B-3.

B-16 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging Figure B-2. Geographical Distribution of LTPP Sections to Be Used in the Long-Term Calibration Experiment. Table B-3. Recommended 12.5 g, 20-hr PAV Conditioning Temperatures for Performance Grading. Average 98% Reliability High and Low Pavement Temperature Calculated PAV Temperature °C Recommended Temperature °C % of LTPPBind 3.1 Stations PG Grade Based on Environment −61 84.4 85 1 PG 40-52; PG 46-52; PG 40-46 −31 86.6 01 88.9 90 4 PG 52-52; PG 46-46; PG 40-40; PG 46-40; PG 52-46; PG 40-343 91.1 6 93.4 95 20 PG 58-46; PG 52-40; PG 46-34; PG 40-28; PG 58-40; PG 52-34; PG 46-28; PG 40-229 95.7 12 97.9 100 41 PG 64-40; PG 58-34; PG 52-28; PG 46-22; PG 40-16; PG 64-34; PG 58-28: PG 52-22; PG 46-16; PG 40-10; PG 64-28; PG 58-22; PG 52-16; PG 46-10 15 100.2 18 102.5 21 104.8 105 20 PG 70-28; PG 64-22; PG 58-16; PG 52-10; PG 70-22; PG 64-16; PG 58-1024 107.1 27 109.3 110 13 PG 70-16; PG 64-10; PG 70-10 30 111.6 331 115.0 115 1 PG 76-10 1 Outside range of data used in calibration

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-17   5. SIGNIFICANCE AND USE This practice is intended for conditioning binder for grading in accordance with AASHTO M 320 and AASHTO M 322 after modification of the AASHTO M 320 and AASHTO M 332 low- and intermediate-temperature specification property criteria. As discussed in the commentary for Section 4, the short-term conditioning was selected to provide residue that is approximately the same as short-term conditioning using AASHTO T 240. Therefore, the short-term conditioned criteria in AASHTO M 320 and M 332 do not require adjustment. However, the long-term conditioning provides residue is much more aged than AASHTO R 28. Therefore, adjustments are required to the long-term conditioned criteria for intermediate- and low-temperature grading in AASHTO M 320 and AASHTO M 332. Table B-4 compares intermediate- and low- temperature continuous grade properties for 10 binders tested in NCHRP Project 09-61 using AASHTO R 28 long-term conditioning and 12.5 g, 20-hr, 2.1 MPa long-term conditioning. From this table, it is clear that this practice produces binder that is significantly more aged, and adjustment to performance grading criteria is needed to use this practice for performance grading. Table B-4. Intermediate Temperature Continuous Grade Data for the Sensitivity Experiment Binders. Binder Continuous Intermediate Temperature Grade Continuous Low Temperature Grade AASHTO R 28 This Practice AASHTO R 28 This Practice AAK-1 20.8 28.9 −27.0 −20.4 AAM-1 19.0 20.1 −22.0 −16.5 AZ1-1 22.0 25.8 −20.3 −11.0 AZ1-3 28.1 38.2 −20.2 −7.2 MN1-2 10.7 13.2 −34.6 −33.3 MN1-4 11.2 13.3 −32.3 −30.9 MN1-5 16.0 17.3 −29.3 −29.0 ME3 16.2 19.3 −31.4 −28.1 AC1928 6.4 8.9 −34.7 −23.9 AC1879 21.4 28.2 −25.7 −18.1 6. APPRATUS 6.1 Balance Taken from AASHTO R 28. 6.2 Pans The 1.0 mm thick pans that are specified are thicker than those specified in AASHTO R 28, Standard Practice for Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel, and AASHTO T 179, Standard Method of Test for Effect of Heat and Air on Asphalt Materials (Thin-Film Oven Test). Section 6.2 of AASHTO R 28 specifies the dimensions of the pans to be used for PAV conditioning. This section specifies “… thickness of approximately 0.635 mm

B-18 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging (0.025 in).” This language came from AASHTO T 179. Note 2 in AASHTO T 179 indicates 24 gauge stainless steel (0.6 mm thick) pans are acceptable; and 26 gauge stainless steel (0.5 mm thick) pans are also acceptable but have a greater tendency to warp. Note 2 states “… in no case shall the thickness of the metal be less than 0.381 mm (0.015 in).” Therefore, pans meeting AASHTO T 179 requirements do not necessarily meet AASHTO R 28 requirements. Through initial testing of 0.8 mm films in NCHRP Project 09-61, it was determined that 0.6 mm thick pans did not remain flat enough to consistently form a film coating the entire pan. Thicker 1.0 mm thick pans were purchased from Infratest, USA and used for the 12.5 g PAV conditioning. When leveled to 0.00042 mm/mm (0.024 degrees), using a precision machinist level, 0.8 mm films were successfully formed using these pans. The set of 10 pans was used for approximately 100 cycles to condition 0.8 mm thin films at temperatures ranging from 85ºC to 120ºC without warping. The pans were cleaned with mineral spirits and then rinsed with acetone. 6.3. Pressure Aging Vessel System All PAV conditioning for NCHRP Project 09-61 was done in an ATS Pressure Aging Vessel 3 (PAV3). Through initial testing, it was determined that more precise leveling than normally performed for AASHTO R 28 conditioning was needed to form and maintain the 0.8 mm thin film. For each PAV run, the ATS pan holder was leveled using a certified, 6-inch Starrett No. 98 Precision Machinist Level that has a sensitivity of 0.00042 mm/mm (0.024 degrees). Leveling the ATS PAV3 is somewhat cumbersome. The leveling system for the pan holder consists of set screws in a ring at the bottom of the PAV as shown in Figure B-3. The set screws cannot be accessed with the pan holder in place, and the resistance temperature detector (RTD) projects above the leveling ring making it difficult to verify the levelness of the pans. Additionally, the set screws have coarse threads. The method used in NCHRP Project 09-61 to verify the levelness was to check levelness by placing the machinist level across the holder at the top while the holder was in place. The holder was then removed, and small adjustments were made to the screws. The holder was replaced, and the levelness checked. This process was repeated several times until the holder was level within the sensitivity of the machinist level. It should be noted that ATS’s specifications for the PAV3 shows the slots holding the pans to be parallel to within 0.05 mm. Over the pan diameter of 140 mm, this is 0.00036 mm/mm (0.020 degrees), which is a tighter tolerance than the sensitivity of the level used to check levelness. PAV holders from other suppliers may not have shelves meeting this tolerance. With PAVs available from several suppliers (Prentex, ATS, Infratest, Matest, Controls, and Cox / Cooper), the proposed standard practice for 0.8 mm thin film conditioning specifies the PAV system must provide a method to rapidly level the sample holder while in the PAV to within 0.025 degrees, and verify the levelness using a precision machinist level. For the ATS PAV, this can be accomplished by modifying the leveling ring to move the leveling screws outside of the holder, using fine thread screws, and providing a long handle adjustment wrench. Providing this capability may be more difficult for other PAV manufacturers. Additionally, it may be possible to build a verification device that uses electrolytic tilt sensors or use these sensors to provide automatic leveling. One manufacturer has an electrolytic tilt sensor with ± 3 degree range, and ±0.001 degree accuracy/repeatability (Fredericks Company, 2020). If 0.8 mm thin film conditioning is considered for adoption, PAV manufacturers should be surveyed to determine the cost of modifying existing equipment to meet the levelness requirement in this appendix of 0.025 degrees. Modified equipment from various manufacturers

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-19   should then be evaluated to confirm compliance with the levelness requirement and evaluate the time and effort required to level the pans. Figure B-3. Photograph of ATS PAV3 Leveling Ring. 6.4. Pressure/Vacuum Oven The pressure/vacuum oven specified in this section is needed for short-term conditioning and to degas the residue after conditioning. In NCHRP Project 09-61, several binders with a wide range of consistencies were successfully conditioned as 0.8 mm thin films at 163ºC and 135ºC. Table B-5 presents properties of the binders. The equipment specified in this section was not used. The binders were conditioned in PAV pans in a PAV rack in a forced-draft oven. The PAV rack was painstakingly leveled using a precision machinist level. The trial and error leveling process with shims used in NCHRP Project 09-61 is unacceptable for production work. Leveling Ring Adjustment Screw RTD

B-20 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging Table B-5. Properties of Original Binders. Binder AASHTO T 315 Properties AASHTO T 316 Viscosity, Pa•s Test Temp, ºC G*, kPa Phase Angle, º G*/sinδ, kPa 135ºC 163ºC Neat PG 52-34 52 1.39 85.8 1.39 0.22 0.07 Terpolymer PG 64-34 64 1.04 67.6 1.12 0.73 0.21 Neat PG 64-22 64 1.65 86.9 1.65 0.43 0.13 SBS PG 76-22 76 1.14 75.5 1.18 1.11 0.30 GTR-Modified 82 1.17 79.5 1.19 NT* 0.99 SBS PG 64-34 64 1.99 63.4 2.23 1.21 0.37 SBS PG 76-28 76 1.46 57.9 1.72 2.36 0.54 PG 64-22 w 3% Latex 76 1.09 79.8 1.10 3.06 0.74 SBS PG 88-22 88 1.42 51.5 1.81 7.65 1.64 * GTR-Modified Binder Not Conditioned at 135ºC The differences between the specified pressure/vacuum oven and the approach used in the NCHRP Project 09-61 research are: (1) the ability to form and short-term condition 0.8 mm thin films simultaneously on 10 pans in the holder to be used for PAV conditioning, (2) the ability to apply a low pressure during short-term conditioning to condition binder at the same pressure regardless of the elevation of the laboratory, and (3) the ability to degas the 0.8 mm, long-term conditioned residue by returning the holder loaded with conditioned residue to this oven for degassing under vacuum. Using the same pans and holder for short- and long-term conditioning and degassing simplifies the conditioning process and eliminates binder transfer and loss issues. NCHRP Project 20-07 / Task 400 concluded that there was an elevation effect on the rheological properties of RTFOT-conditioned residue, and the elevation effect was large enough that different conclusions on the acceptability of a binder could be reached depending on the elevation of the laboratory testing the binder (Advanced Asphalt Technologies LLC 2018). The preferred method for eliminating the elevation effect is to condition binders at the same absolute pressure in all laboratories. This requires conditioning under a small positive pressure, which is impossible with AASHTO T 240. However, with static thin film conditioning, it is feasible to condition the binder under a small positive pressure. An oven designed for 1 atm positive pressure is also acceptable as a vacuum oven; therefore, it is feasible to have a combination pressure/vacuum oven that can be used for short-term, static thin film conditioning and vacuum degassing. The temperature range specified is based on the short-term conditioning temperatures of 135ºC for warm mix and 163ºC for hot mix used in NCHRP Project 09-61 and the temperatures specified in AASHTO R 28 for vacuum degassing. A standard conditioning pressure somewhat above the maximum atmospheric pressure in the United States was selected to avoid cycling between pressure and vacuum. Atmospheric pressure varies with elevation and weather. The elevation effect, however, is much larger. Standard atmospheric pressure at sea level is 101.325 kPa (14.70 psi). Based on the relationship for the variation in atmospheric pressure with elevation (U.S. Department of Commerce 1963), the atmospheric pressure at 7,000 ft elevation, the elevation of the highest labs participating in the AASHTO re:source proficiency sample testing, is 78.18 kPa (11.34 psi). The historical range of maximum and minimum atmospheric pressures corrected to sea level recorded in the United States is from 93.40 kPa (13.56 psi) to 106.33 kPa (15.42 psi) (Weather Underground

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-21   n.d.). Therefore, the short-term conditioning pressures should be set to 110.00 kPa (15.95 psi) absolute, which is a small gauge pressure of 8.67 kPa (1.25 psi) at sea level and 16.6 kPa (4.61 psi) at an elevation of 7,000 ft. Based on work completed in NCHRP Project 20-07 / Task 400, a significant difference in AASHTO T 240 residue occurs when the elevation difference between labs exceeds about 1,000 ft which corresponds to a pressure difference of about 3.50 kPa (0.51 psi). Therefore, pressure control should be more accurate than this value. Pressure gauges and closed-loop electronic air pressure regulators are available with an accuracy of 0.25 percent of full scale. A 210 kPa (30 psi) full scale device with an accuracy of 0.25 percent of full scale is accurate to 0.52 kPa (0.08 psi). If this practice is considered for adoption, PAV manufacturers should be asked to develop prototype pressure/vacuum ovens and demonstrate their use. An issue for short-term conditioning under pressure that was not addressed in NCHRP Project 09-61 is how much will the low pressure during short-term conditioning suppress volatile loss? The changes to the binder that occur during AASHTO T 240 conditioning and the construction process include oxidation and volatile loss. The short-term conditioning research in NCHRP Project 09-61 documented lower mass loss for the static 0.8 mm thin film compared to AASHTO T 240. Applying a pressure to the thin film during short-term conditioning may suppress volatile loss. Additional experimental work is needed to determine if volatile loss suppression is significant. 6.5. Precision Machinist Level The accuracy of a digital level or sensitivity of a glass vial level is the important characteristic. Many digital levels have resolution better than the sensitivity specified for the precision machinist level, but their accuracy is much poorer. The precision machinist level specified offers excellent sensitivity at a reasonable price. It can be purchased with certification that it complies with the sensitivity of 0.00042 mm/mm. 7. CALIBRATION AND STANDARDIZATION This section of the proposed practice describes standardization of the temperature sensors, pressure sensors, and vacuum gauge. The format for these is the same as used in AASHTO R 28 and AASHTO T 240. This section also describes a procedure for verifying the precision machinist level. This is a well-known procedure for verifying levels. It does not require a perfectly level surface. It requires a stable flat surface that is level within the range of the level being verified. It involves placing the level on the surface, noting the position of the bubble, rotating the level 180 degrees, and noting the position of the bubble. If the level is true, the bubble should be at the same position when the level is rotated. The precision machinist level has an adjustment screw that can be used to make changes to the level. This verification should be performed any time there is a concern about the level. 8. PREPARATION OF OVENS This section of the proposed practice describes tasks that should be accomplished before starting to condition binder samples. This includes: (1) verifying the levelness of the pan holder in the pressure/vacuum oven, (2) verifying the levelness of the pan holder in the PAV, (3) preheating the pressure/vacuum oven to the short-term conditioning temperature of 163ºC, (4)

B-22 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging selecting the appropriate long-term conditioning temperature from Table 1 of the proposed practice, (5) preheating the PAV to the long-term conditioning temperature. The temperatures for 12.5 g, 20-hr, 2.1 MPa long-term conditioning were a major product of NCHRP Project 09-61. A summary of the research leading to these temperatures was presented earlier in Section 4 of this commentary. 9. PROCEDURE This section of the proposed practice describes the procedure for short-term and long-term conditioning of asphalt binders using static 0.8 mm thin films. The basic steps are: (1) 10 pans are loaded with 12.5 g of binder, (2) one pan is accurately weighed for mass change determination, (3) the 10 pans are then short-term conditioned in the pressure/vacuum oven for 90 minutes at an absolute pressure of 110 kPa and temperature of 163ºC, (4) one pan is removed, cooled for mass change determination then reheated and scraped into a container for short-term conditioned rheological property testing, (5) nine pans are transferred to the PAV and long-term conditioned at the appropriate temperature under 2.1 MPa pressure for 20 hours, (6) the pans are transferred to the pressure/vacuum oven for degassing, (7) the long-term conditioned residue is then scraped into a container for long-term conditioned rheological property testing. Many of the steps in the procedure are extracted from AASHTO T 240 and AASHTO T 28 as discussed below. 9.1 SHORT-TERM CONDITIONG 9.1.1. Taken from AASHTO T 240. 9.1.2. One pan provides sufficient short-term conditioned residue for high-temperature grading. That pan is also used for mass change determination. 9.1.3. Taken from AASHTO R 28 with adjusted mass of 12.5 g. The tolerance of ±0.1 g was determined from a film thickness study completed during NCHRP Project 09-61 that investigated the change in various rheological properties as a function of film thickness. That study included short-term and long-term conditioning of binder films between 0.8 and 3.2 mm for SHRP binders, AAC-1 and AAF-1. Plots of rheological properties as a function of binder film thickness were prepared and the slope at 0.8 mm was used to determine the change in film thickness that would result in rheological property changes equal to the within-laboratory coefficient of variation for various tests. The results are summarized in Table B-6. The allowable film thickness variation was then converted to an allowable mass variation by multiplying the allowable film thickness variation by the pan area and the density of the binder. From Table B-6, short-term conditioned binder is most sensitive to film thickness changes. Since ±0.1 g was determined to be achievable with reasonable effort in testing done during NCHRP Project 09-61, it was included as the tolerance in the proposed practice.

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-23   Table B-6. Summary of Film Thickness Tolerance Calculations for Properties Measured on Short- and Long-Term Conditioned Residue. Property Binder Slope Property at 0.8 mm Film Thickness %/mm Allowable Variation Film Thickness, mm Mass, g G*/sinδ AAC-1 −2.050.0 kPa/mm 3.27 kPa −62.6 0.05 0.50 AAF-1 −3.485 kPa/mm 3.26 kPa −106.8 0.03 0.29 G·sinδ AAC-1 −526.4 kPa/mm 3143 kPa −16.8 0.29 2.83 AAF-1 −1705.1 kPa/mm 7156 kPa −16.4 0.30 2.90 Creep Stiffness AAC-1 −9.84 MPa/mm 91.5 MPa −10.8 0.23 2.25 AAF-1 −24.43 MPa/mm 80.4 MPa −30.4 0.08 0.80 m- value AAC-1 0.010400 /mm 0.362 2.9 0.35 3.37 AAF-1 0.024100 /mm 0.373 6.5 0.15 1.50 9.1.4. Taken from AASHTO T 240. 9.1.5. Mass change measurement similar to AASHTO T 240. 9.1.6. First step in short-term conditioning process. 9.1.7. See Section 6.5 of this commentary for a discussion of the selection of the standard pressure of 110 kPa absolute and the tolerance of ±1.0 kPa. 9.1.8. The temperature and temperature tolerance were taken from AASHTO T 240. The time is based on the 0.8 mm thin film conditioning research in NCHRP Project 09-61, which showed 85 minutes generally produced less aging than oven-conditioned loose mix. See Section 4 of this commentary. The tolerance of ±1.0 minute is based on experience gained in NCHRP Project 09- 61. 9.1.9. Step in short-term conditioning process. 9.1.10. One pan provides sufficient short-term conditioned residue for high pavement temperature rheological property testing. 9.1.11. Nine pans will proceed to long-term conditioning. 9.1.12. Taken from AASHTO T 240. Mass change measurement similar to AASHTO T 240. 9.1.13. Adapted from AASHTO R 28. 9.1.14. Adapted from AASHTO R 28.

B-24 Asphalt Binder Aging Methods to Accurately Reflect Mixture Aging 9.2 LONG-TERM CONDITIONG 9.2.1. First step in long-term conditioning. 9.2.2. Taken from AASHTO R 28. 9.2.3. Taken from AASHTO R 28. Notes X5 and X6 taken from AASHTO R 28. 9.2.4. Taken from AASHTO R 28. 9.2.5. Work in NCHRP Project 09-61 found that vacuum degassing is necessary for 0.8 mm thin film conditioned in the PAV to remove the air from the stiffer film. The vacuum oven should be preheated before PAV conditioning is finished. 9.2.6. Taken from AASHTO R 28. 9.2.7. Taken from AASHTO R 28. 9.2.8. The pressure/vacuum oven specified permits direct transfer of the holder with conditioned PAV residue from the PAV to the vacuum oven for degassing. This should speed the overall conditioning process. Note, the pressure/vacuum oven specified for this process was not available during NCHRP Project 09-61. Additional development work is needed to verify the acceptability of the process described. 9.2.9. Taken from AASHTO R 28. It may be possible to reduce the time or eliminate this heating step because the PAV residue is being transferred directly from the PAV. 9.2.10. Taken from AASHTO R 28. Note X7 refers to Table 1 in AASHTO R 28 for equipment that uses gauge pressure. The first column heading in Table 1 is incorrect. This heading should read “Corrected Gauge Readings (inHg).” 9.2.11. Taken from AASHTO R 28. 9.2.12. Taken from AASHTO R 28. 10. MASS CHANGE CALCULATION This section of the proposed practice presents the equation for calculating the mass change. AASHTO T 240 does not present an equation for mass change. 11. REPORT This section of the proposed practice describes the reporting requirements. Basic report items are similar to AASHTO T 240 and AASHTO R 28. Additionally, maximum and minimum

Proposed AASHTO Practice, 0.8 mm Static Film Short- and Long-Term Conditioning of Asphalt Binder and Commentary B-25   temperatures and pressures during short- and long-term conditioning are reported to aid in troubleshooting. 12. REPORT Additional research is needed to establish precision estimates for tests performed on short- and long-term conditioned residue from this practice. This research should be conducted after the equipment described in this practice is developed and subjected to ruggedness testing. It is probably not possible to establish the bias of the conditioning recommended in this proposed practice. 13. KEYWORDS This section presents appropriate keywords for the proposed practice. REFERENCES Advanced Asphalt Technologies LLC. NCHTRP Project 20-07 / Task 400, Effect of Elevation on Rolling Thin Film Oven Aging of Asphalt Binder, Final Report. 2018. U.S. Department of Commerce, Weather Bureau. Manual of Barometry (WBAN), Vol 1, First Edition, Washington, DC, 1963. Fredericks Company. 0703-0711-99 ±3° TrueTILT™ Single-Axis Electrolytic Tilt Sensor, https://www.frederickscompany.com/products/0703-0711-99/#1476383581128-d8e57e33-4ae7, last accessed 8/6/2020. Weather Underground. n.d. “U.S. City Barometric Pressure Records,” available at https://www.wunderground.com/resources/pressure_records.asp. Accessed September 7, 2019.

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America’s Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TDC Transit Development Corporation TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S. DOT United States Department of Transportation

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