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100 C H A P T E R 5 Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing
Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing 101 Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing AASHTO Designation: TP xxx-xx 1 SCOPE 1.1 This standard practice describes a procedure for the long-term conditioning of uncompacted hot mix asphalt (HMA) for performance testing to simulate the aging that occurs over the service life of a pavement. The procedure for long-term conditioning in performance testing is preceded by a procedure for short-term conditioning in mixture mechanical property testing in AASHTO R 30. 1.2 This standard may involve hazardous material, operations, and equipment. This standard does not purport to address all safety problems associated with its use. It is the responsibility of the user of this procedure 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: PP 3, Preparing Hot Mix Asphalt (HMA) Specimens by Means of the Rolling Wheel Compactor PP 60, Preparation of Cylindrical Performance Test Specimens Using the Superpave Gyratory Compactor (SGC) R 30, Mixture Conditioning of Hot Mix Asphalt (HMA) T 312, Preparing and Determining the Density of Hot Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor T 316, Viscosity Determination of Asphalt Binder using Rotational Viscometer 2.2 Other Document: Draft Final Report for NCHRP Report 9-54, Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction, October 1, 2017.
102 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction 3 ERMINOLOGY 3.1 efinions: 3.1.1 Enhanced Integrated Climac Model (EICM) â A one-dimensional coupled heat and moisture flow model that can be used to determine pavement temperature at any depth of interest. 4 UMMARY OF PracÂce 4.1 mixture of binder and aggregate is condiÂoned in a forced-dra oven at 95°C aÂer prior short-term condiÂoning for mixture mechanical property testing according to R 30. The duraÂon of condiÂoning at 95°C is selected to reflect the Âme, climate, and pavement depth for a given pavement locaÂon in the United States using the climaÂc aging index (CAI). The long- term condiÂoned loose mixture samples are prepared for maximum specific gravity (Gmm) tesÂng and then compacted for subsequent performance tesÂng. 5 IGNIFICANCE AND USE 5.1 The long-term performance of HMA can be predicted more accurately by using condiÂoned test samples rather than uncondiÂoned samples. The long-term mixture condiÂoning in the performance tesÂng procedure is designed to simulate the aging that the mixture will undergo in service. 6 T D S A S APPARATUS 6.1 Oven: A forced-dra oven, thermostatically-controlled with horizontal air flow, and capable of maintaining any desired temperature seÂng from room temperature to 176°C within ± 3°C. 6.2 Thermometers: Thermometers having a range from 50°C to 260°C and readable to 1°C. 6.3 Miscellaneous: A metal pan for heating aggregate, a shallow metal pan for the short-term conditioning of uncompacted HMA, a 1-inch tall metal pan for the long-term conditioning of uncompacted HMA, a metal spatula or spoon, Âmer, and gloves for handling hot equipment.
Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing 103 7 AZARDS 7.1 This standard involves the handling of HMA that can cause severe burns if it contacts skin. Follow safety precauÂons to avoid burns. 8 ONG-TERM MIXTURE CONDITIONING PROCEDURE 8.1 The long-term conditioning for performance tesÂng applies to laboratory- prepared loose mixtures that have been subjected to short-term condiÂoning as part of the mixture mechanical property tesÂng procedure described in R 30. 8.2 H L Determine the required long-term conditioning duraÂon that reflects the desired field aging in terms of age, climate, and depth using EquaÂon 1. 1 exp( / ) / 24 N oven a i i t DA E RT (1) where toven = required oven aging duration at 95°C to reflect field aging (days); CAI = climatic aging index; D = depth correction factor; A = frequency (pre-exponential) factor; Ea = activation energy; R = universal gas constant, or ideal gas constant; Ti = pavement temperature obtained from the EICM at the depth of interest at the hour of interest, i (Kelvin); Table 1 lists the values of D, A, and Ea. Table 1 â Climatic Aging Index Fitting Coefficients Pavement Layer Depth CorrectionFactor (D) Pre-exponential Factor (A) Activation Energy (Ea) Surface Layer (6 mm) 1.0000 1.40962 13.3121 20-mm Depth 0.4565 1.40962 13.3121 Deeper Layers (below 20 mm) 0.2967 1.40962 13.3121 8.2.1 Break down any large chunks of asphalt mixtures in the short-term aged loose mixture sample, taking care to avoid fracturing the aggregate, so that the clusters of the fine aggregate porÂon are not larger than the nominal maximum aggregate size (NMAS). This step needs to be performed shortly aÂer short-term aging to ensure that the asphalt mixture is sufficiently so to be separated into pans for oven aging. If an HMA sample is not sufficiently so to be separated manually,
104 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction then place it in a pan and warm it in an oven unl it can be separated as described. 8.2.2 Apporon the short-term aged loose mixture into several pans such that each pan has a relavely thin layer of loose mix, approximately equal to the mixture NMAS. 8.2.3 Place the pans that contain mixture in a forced-dra oven at 95°C ± 3°C for the duraon determined by Equaon (1). Place the pans on different shelves so that the pans are arranged vercally within the oven as much as possible. Ensure that adjacent pans do not overlap. 8.2.4 Rotate the pans to different shelves at four evenly-spaced me intervals during the long-term condioning period so that each pan has similar exposure to heat and air flow at different locaons within the oven. 8.2.5 AÂer long-term condioning, remove the condioned mixtures from the oven and mix all of the mixtures together in order to obtain a uniform mixture. 8.2.6 Allow the mixture to cool to room temperature. The long-term condioned loose mixture sample is now ready for compacon or subsequent tesng as required. 8.3 reparing Specimens from Loose HMA 8.3.1 Specimens Compacted Using the Superpave Gyratory Compactor 8.3.1.1 ompact the specimens in accordance with PP 60. Cool the test specimens at room temperature for 16 ± 1 hour. Note 4. Extrude the specimen from the compaction mold after cooling for 2 to 3 hours. Note 5. Specimen cooling usually is scheduled as an overnight step. Cooling may be accelerated by placing the specimen in front of a fan. 8.3.2 Specimens Compacted Using the Rolling Wheel Compactor 8.3.2.1 ompact the specimens in accordance with PP 3. 8.3.2.2 ool the test specimens at room temperature for 16 ± 1 hour. 8.3.2.3 emove the slab from the mold, and saw or core the required specimens from the slab. 9 EPORT 9.1 P C C C R R Report the binder grade, binder content (to nearest 0.1 percent), and the aggregate type and gradaon, if applicable.
Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing 105 9.2 eport the following long-term condi oning information for the performance tes ng condi ons, if applicable: 9.2.1 Long-term mixture condi oning temperature in laboratory (nearest 1°C); 9.2.2 Long-term mixture condi oning dura on in laboratory (nearest 5 min); and 9.2.3 Laboratory compac on temperature (nearest 1°C). 10 Keywords 10.1 ondi oning; hot mix asphalt; long-term conditioning. APPENDICES (Nonmandatory Information) X1. R C LABORATORY AGING DURATION MAPS X1.1. The CAI values for various locations in the United States were calculated, as described in Section 9, using hourly pavement temperature history data obtained from the EICM to provide an overview of the proposed laboratory aging durations for various climates, field aging durations, and depths. X1.2. The CAI values and measured durations that are needed to match field aging correlate linearly in a one-to-one relationship. Thus, the CAI values represent the required duration at 95°C that is needed to match the field aging for a given pavement temperature history and depth. X1.3. Laboratory aging durations were calculated for three field ages: 4 years, 8 years, and 16 years. For each field age, the laboratory aging durations were determined at three depths: 6 mm, 20 mm, and 50 mm, and rounded to the nearest day. X1.4. Figure X1.1 shows the CAI-determined loose mixture aging durations at 95°C that are required to match 4, 8, and 16 years of field aging at a depth of 6 mm. Figure X1.1 demonstrates that climate has a significant effect on the required laboratory aging duration that is required to match a given field age.
106 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction (b) (a)
Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing 107 Figure X1.1-Required oven aging duration at 95°C to match level of field aging 6 mm below pavement surface for (a) 4 years of field aging, (b) 8 years of field aging, and (c) 16 years of field aging. X1.5. Figure X1.2 shows the CAI-determined loose mixture aging durations at 95°C that are required to match 4, 8, and 16 years of field aging at a depth of 20 mm. A comparison between the laboratory aging durations presented in Figure X1.1 and Figure X1.2 demonstrates that significantly shorter laboratory aging durations are required to match the field aging at a depth of 20 mm compared to 6 mm, indicating that the temperature gradient and diffusion in pavements significantly affect oxidation levels. X1.6. A depth of 20 mm represents a reasonable depth for the evaluation of surface layer asphalt mixtures because it better reflects bulk behavior within a pavement structure than nearer the surface and avoids the effect of ultraviolet oxidation. (c)
108 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction (b) (a)
Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing 109 Figure X1.2-Required oven aging duration at 95°C to match level of field aging 20 mm below pavement surface for (a) 4 years of field aging, (b) 8 years of field aging, and (c) 16 years of field aging. X1.7. Figure X1.3 shows the CAI-determined loose mixture aging durations at 95°C that are required to match 4, 8, and 16 years of field aging at a depth of 50 mm. The results demonstrate that considerably shorter aging durations are required to simulate aging at a depth of 50 mm compared to depths of 20 mm and 6 mm, thus indicating the presence of a significant oxidation gradient with depth near the surface of the pavements. In Figure X1.3, the required aging duration of zero days in a few cold northern states indicates that long-term aging at 50 mm below the pavement surface in these cold regions are not significant enough to require long- term oven conditioning to mimic field conditions. X1.8. Based on the results presented in the draft NCHRP Project 9-54 report, the researchers concluded that long-term aging does take place below a depth of 50 mm, but does not change appreciably below that depth. Consequently, the evaluation of asphalt mixtures that have been prepared to match field aging at a depth of 50 mm could be useful for evaluating intermediate and base asphalt layers. (c)
110 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction (b) (a)
Proposed Standard Method of Test for Long-Term Conditioning of Hot Mix Asphalt (HMA) for Performance Testing 111 Figure X1.3-Required oven aging duration at 95°C to match level of field aging 50 mm below pavement surface for (a) 4 years of field aging, (b) 8 years of field aging, and (c) 16 years of field aging. (c)
