Validation of Guidelines for Evaluating the Moisture Susceptibility of WMA Technologies (2016)

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17 Conclusions and Recommendations Economic, environmental, and engineering benefits moti- vate the reduction of production and placement temperatures for asphalt mixtures. The latest technology that has been rap- idly adopted for this purpose is WMA. WMA technologies offer many benefits such as improved workability and com- pactability, reduced aging, and better resistance to cracking and raveling. However, barriers to the widespread implementation of WMA include the potentially increased moisture suscep- tibility and reduced rutting resistance due to the incomplete drying of the aggregates, reduced binder absorption by the aggregates at lower production temperatures, or the incorpo- ration of additional moisture in the foaming process. NCHRP Project 9-49B “Performance of WMA Technologies: Stage I— Moisture Susceptibility Validation” focused on validating the thresholds in the flow chart for identifying and minimizing moisture susceptibility of WMA, which were initially proposed in NCHRP Project 9-49 “Performance of WMA Technologies: Stage I–Moisture Susceptibility” as a set of guidelines for mix design and QA of WMA mixtures. A web-survey of state DOTs and contractors was per- formed to identify WMA mixtures with available field per- formance plus mix design and/or QA data including wet IDT strength and TSR by AASHTO T 283, wet resilient modulus (MR) and MR-ratio by modified ASTM D 7369, and HWTT per AASHTO T 324. Additionally, reports from related NCHRP Projects 9-47A “Properties and Performance of WMA Technologies” and 9-49A “Performance of WMA Technologies: Stage II–Long-Term Field Performance” were reviewed to identify additional WMA mixtures for which this same type of information was available. In total, 64 WMA mixtures from 44 field projects with moisture susceptibility data available from mix design, construction, production, and field/forensic evaluation as well as technical reports and papers were identified. The results were compiled and used to validate the proposed moisture susceptibility thresholds in the flow chart, which were initially developed using a limited number of mixtures. Table 7 presents the correlations between WMA moisture susceptibility parameters and their corresponding field pave- ment performance. As illustrated, acceptable correlations of 89%, 83%, and 87% were achieved for HWTT stripping inflection point (SIP), and TSR and wet IDT strength for on- site and off-site PMLC specimens, respectively. Therefore, the proposed thresholds in the flow chart could be considered for implementation by state DOTs and contractors to identify and minimize moisture susceptibility in WMA mixtures. A laboratory experiment was performed to assess addi- tional moisture conditioning protocols as alternatives to the modified Lottman protocol per AASHTO T 283 included in the flow chart, and to explore various specimen-drying methods and their effects on moisture susceptibility param- eters. The selected mixture corresponded to a field project on State Route 196 in Wyoming. Three moisture condition- ing protocols were evaluated besides the modified Lottman protocol consisting of vacuum saturation (70 to 80% degree of saturation) plus one freeze-thaw cycle (16 hours at 0°F [–18°C] and 24 hours at 140°F [60°C]) per AASHTO T 283; these included 1,000-cycle and 2,000-cycle MIST protocols at 140°F (60°C) and 40 psi and three-day HWB at 140°F (60°C). A set of laboratory fabricated specimens were subjected to various moisture conditioning protocols prior to being tested for MR stiffness, IDT strength, and APA RRP values. In addi- tion, four different specimen-drying methods were evaluated in the MR and IDT strength tests after the modified Lottman protocol, including SSD per AASHTO T 166, 48-hour air dry at 77°F (25°C), CoreDry per AASHTO PP 75, and 24-hour oven dry at 104°F (40°C). Test results obtained in the experi- ment were used to determine if an equivalent level of mois- ture damage to the modified Lottman protocol was achieved by the MIST or HWB protocols, and to evaluate the effects of various specimen-drying methods on moisture susceptibility parameters. The laboratory test results obtained for various moisture conditioning protocols demonstrated a significant reduction C H A P T E R 6

18 in mixture properties for all moisture conditioned specimens as compared to the dry control specimens. According to the MR stiffness and IDT strength results, the 2,000-cycle MIST protocol produced the most severe moisture damage, while no significant difference was observed for other protocols. How- ever, the APA RRP results showed a distinct trend; the modi- fied Lottman protocol produced the most severe moisture damage, followed by 2,000-cycle and 1,000-cycle MIST pro- tocols, and then the three-day HWB protocol. Based on these results and the moisture susceptibility parameters included in the flow chart, the 1,000-cycle MIST protocol at 140°F (60°C) and 40 psi and three-day HWB protocol at 140°F (60°C) were recommended for use in the moisture susceptibility guidelines as alternatives to the modified Lottman protocol per AASHTO T 283 prior to MR and IDT strength tests. The laboratory test results obtained for various specimen- drying methods after the modified Lottman moisture condi- tioning indicated that SSD and CoreDry specimens had higher MR stiffness and IDT strength values than air dry and oven dry specimens, although the difference was insignificant. The testing of SSD specimens in the MR and IDT strength tests was prob- lematic as the water occupying the permeable pores of the speci- mens could artificially increase mixture load-carrying capacity due to pore pressure and incompressibility of water. Therefore, the other three specimen-drying methods of CoreDry, 48-hour air dry at 77°F (25°C), and 24-hour oven dry at 140°F (60°C) were recommended to dry the moisture conditioned specimens prior to MR and IDT strength measurements, with the CoreDry method being preferred due to its shorter time requirement. Figure 19 presents the revised flow chart for evaluating moisture susceptibility of WMA during mix design or QA based on the results obtained in this project. If appropriate laboratory equipment is not available to fabricate LMLC specimens with the WMA technology, testing may be con- ducted on PMLC specimens fabricated on-site or off-site with minimal reheating from plant trial batch materials. After mixing WMA LMLC specimens according to AASHTO R 35, loose mix is subject to short-term oven aging (STOA) for 2 hours at 240°F (116°C) prior to compaction. Next, a per- formance test to evaluate moisture susceptibility is selected based on available equipment, costs, and prior experience from the following three options: wet and dry IDT strengths at 77°F (25°C) and TSR per AASHTO T 283, wet and dry MR stiffness at 77°F (25°C) per modified ASTM D7369, or HWTT SIP and stripping slope per AASHTO T 324 at 122°F (50°C). For the IDT strength and MR tests, three moisture condition- ing protocols including the modified Lottman per AASHTO T 283, 1,000-cycle MIST at 140°F (60°C) and 40 psi, or 3-day HWB at 140°F (60°C) are available. Depending on the avail- able equipment, the moisture conditioned specimens should be dried using one of the following methods prior to wet MR and IDT strength measurements: CoreDry, 48-hour air dry at 77°F (25°C), or 24-hour oven dry at 104°F (40°C). Two criteria for each performance test for these STOA specimens are shown in Figure 19. These criteria were initially proposed in NCHRP Project 9-49 by discriminating between the results of WMA mixtures with good versus poor field and laboratory performance, and then verified by 64 additional WMA mixtures evaluated in this project. If the WMA passes both criteria for the selected test, the mixture is expected to have adequate performance in terms of moisture susceptibil- ity. Otherwise, early life moisture susceptibility is probable. Mixture modifications in terms of (1) adding, modifying the dosage of, or changing anti-stripping agents; (2) changing other mixture components (i.e., binder grade or inclusion of recycled materials); or (3) any combination of these modifi- cations is recommended prior to a second evaluation of the modified WMA with the same criteria. If the modified WMA still fails at least one criterion for the selected test, another evaluation is proposed for LMLC speci- mens after both STOA and long-term oven aging (LTOA) of 5 days at 185°F (85°C) per AASHTO R 30 to evaluate if the WMA performance improves with aging. After long-term aging, the same selected laboratory test is used but with revised criteria that reflect the stiffening effects due to oxidative aging, as shown in Figure 19. If the LTOA WMA passes all criteria for the same Moisture Susceptibility Parameters Minimum Thresholds Number of WMA Pavements for Validation Performance Correlation HWTT SIP 3,500 (LMLC & on-site PMLC) 36 89% 6,000 (off-site PMLC) TSR & Wet IDT Strength 70% & 65 psi (LMLC & on-site PMLC) 30 83% 70% & 100 psi (off-site PMLC) 23 87% Table 7. Summary of guideline threshold validation.

Note a: if WMA LMLC is not available, use trial batch prior to production for verification: on-site PMLC or off-site PMLC with minimal reheating Note b: select a single moisture conditioning protocol and use it throughout the mix design verification Note c: select a single test method and use it throughout the mix design verification Note d: If trial batch off-site PMLC specimens are used, employ the following thresholds (TSR and MR-ratio remain unchanged): Wet IDT ≥ 100 psi, Wet MR ≥ 300 ksi, SIP ≥ 6,000 cycles, stripping slope ≤ 2.0 µm/cycle HWTT per AASHTO T 324 IDT Strength per AASHTO T 283 MR per modified ASTM D7369 ≥ 65 psi & ≥ 70% ≥ 200 ksi & ≥ 70% OK OK No No No OK LMLC Loose Mix STOA 2 h @ 240°F (116°C) ≥ 115 psi ≥ 450 ksi ≥ 12,000 cycles & ≤ 1.4 µm/cycle Moisture Susceptible OK with Summer Aging OK with Summer Aging OK with Summer Aging Moisture Susceptible Moisture Susceptible Moisture Susceptibility at Early Life Add/modify anti-stripping agent and/or other mixture components Re-evaluate mixture with LTOA ≥ 3,500 cycles & ≤ 5.3 µm/cycle Moisture Conditioning Protocols Laboratory Tests Moisture Susceptibility Parameters Thresholds (STOA Specimens) Wet IDT Strength & TSR Wet MR & MR-ratio SIP & Stripping Slope AASHTO T283 1,000-cycle MIST at 60°C and 40 psi 3-day HWB at 60°C LMLC Loose Mix STOA 2 h @ 240°F (116°C) Wet IDT Strength Wet MR SIP & Stripping Slope HWTT per AASHTO T 324 IDT Strength per AASHTO T 283 MR per modified ASTM D7369 AASHTO T283 1,000-cycle MIST at 60°C and 40 psi 3-day HWB at 60°C Compacted Specimen LTOA per AASHTO R 30 Moisture Conditioning Protocols Laboratory Tests Moisture Susceptibility Parameters Thresholds (LTOA Specimens) a b b b c c c d d d b b b c c c Figure 19. Revised WMA moisture susceptibility evaluation for mix design or quality assurance.

20 selected test, moisture susceptibility in early life is probable and scheduling construction is recommended so that a summer of aging occurs prior to multiple freeze-thaw cycles or wet and cold days. Otherwise, the mixture is considered moisture sus- ceptible. Finally, if the alternative off-site PMLC specimens are used to evaluate WMA moisture susceptibility, the thresholds are increased for wet IDT strength, wet MR stiffness, SIP, and stripping slope as noted at the bottom of Figure 19. Figure 19 was produced as a set of guidelines for identify- ing and minimizing moisture susceptibility of WMA. Before being considered for implementation, the guidelines should be used on a trial basis. This will provide additional data to further refine the proposed moisture susceptibility thresholds and laboratory aging and moisture conditioning protocols to particular needs and conditions of individual agencies. Data from additional WMA mixtures will provide increased con- fidence in the guidelines. In addition, continuous monitor- ing of the WMA mixtures used for proposing and validating moisture susceptibility thresholds in NCHRP Projects 9-49 and 9-49B is suggested.