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1 Economic, environmental, and engineering benefits motivate the reduction of produc- tion and placement temperatures for the asphalt concrete paving materials used on most paved roads in the United States. The latest technology that has been rapidly adopted for this purpose is warm mix asphalt (WMA), which is defined as an asphalt concrete paving material produced and placed at temperatures approximately 50°F (28°C) cooler than those used for hot mix asphalt (HMA). WMA was first introduced in Europe in the mid-1990s as a way to reduce greenhouse gas emissions and then transferred to the United States in the early 2000s largely through the effort of the National Asphalt Paving Association (NAPA). WMA technologies offer several benefits, including decreased energy consumption, reduced emissions and fumes at the plant, improved working conditions at the construction site as a result of reduced fumes and emissions, extended haul distances, longer pavement construction season and reduced construction days, improved workability and compactabil- ity, reduced aging, and better resistance to cracking and raveling. However, barriers to the widespread implementation of WMA include (1) the wide variety of WMA technologies and (2) the imprecise correlation between the laboratory and field performance of these technologies. The latter likely results from the lack of standard laboratory conditioning and aging protocols during mix design to better simulate early-life performance, where WMA may be more susceptible to rutting due to reduced aging and moisture susceptibil- ity resulting from incomplete drying of aggregate and differences in aggregate absorption of binder. NCHRP Project 9-49 focused on the moisture susceptibility of WMA. Laboratory-mixed laboratory-compacted (LMLC) specimens, plant-mixed laboratory-compacted (PMLC) specimens, and plant-mixed field-compacted (PMFC) cores were evaluated to develop guidelines for identifying and limiting moisture susceptibility in WMA pavements. To meet these objectives, the research conducted in NCHRP Project 9-49 included the following: ⢠Identification and preliminary assessment of current WMA pavements with evidence of moisture susceptibility and a work plan for further investigation of these pavements. ⢠Evaluation of laboratory conditioning protocols for WMA prior to moisture-susceptibility testing to propose protocols for WMA and HMA. ⢠Evaluation of standard test methods to predict moisture susceptibility and ability of materials and methods to minimize this distress. ⢠Comparison of WMA moisture susceptibility for LMLC specimens, PMLC specimens, and PMFC cores. ⢠Evaluation of WMA pavements to identify possible reasons and evolution of performance with time. S U M M A R Y Evaluation of the Moisture Susceptibility of WMA Technologies
2The results of the experiments on WMA laboratory conditioning, WMA moisture suscep- tibility, and WMA performance evolution were used to produce the primary products from NCHRP Project 9-49. These products include (1) proposed guidelines for identifying and minimizing moisture susceptibility in WMA, (2) proposed revisions to the appendix of the AASHTO R 35 Special Mixture Design Considerations and Methods for WMA, and (3) a work plan for future research to continue the search for an effective laboratory test method and performance-related criteria for precluding moisture susceptibility in WMA. The major conclusions from the research completed in NCHRP Project 9-49 are sum- marized in the figure below, which details the proposed laboratory conditioning and aging protocols and thresholds for three different standard laboratory tests used to assess moisture susceptibility of WMA. These thresholds were developed based on the field and laboratory performance of two of the four field projects used in NCHRP Project 9-49, and then they were verified based on the performance of the other two field projects. This flow chart was produced as a set of guidelines for mix design and quality assurance (QA), and state depart- ments of transportation (DOTs) can modify it to suit their needs based on their experience. The research conducted in NCHRP Project 9-49, based on a limited number of field projects, showed that the use of WMA that will not sustain a summer of aging prior to multiple freeze-thaw cycles or wet and cold days in the first winter should be approached with caution, especially in extreme climates for moisture susceptibility. The addition of anti-stripping agents compatible with the WMA technology and the component binder and aggregate materials will likely mitigate the potential for moisture susceptibility. Based on the field projects evaluated in this project, the use of either a relatively elevated high- temperature performance grade (PG) binder or a relatively low high-temperature PG binder Note a: if WMA LMLC is not available, use trial batch prior to production for verification: onsite PMLC or offsite PMLC with minimal reheating Note b: select a single test method and use it throughout the mix design verification Note c: If trial batch offsite 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 m/cycle LMLC, Loose Mix STOA 2 h @ 240°F (116°C) HWTT per AASHTO T 324 Stripping Inflection Point (SIP) & Stripping Slope Moisture Conditioning per AASHTO T 283 Wet IDT Strength & TSR Indirect Tensile (IDT) Strength per AASHTO T 283 Resilient Modulus (MR) per modified ASTM D7369 Wet MR & MR- ratio 65 psi & 70% 200 ksi & 70% 3,500 cycles & 5.3 m/cycle OK OK No No No OK Compacted Specimen LTOA per AASHTO R 30 LMLC Loose Mix STOA 2 h @ 240°F (116°C) HWTT per AASHTO T 324 Moisture Conditioning per AASHTO T 283 SIP & Stripping Slope IDT Strength per AASHTO T 283 MR per modified ASTM D7369 Wet IDT Strength Wet MR 115 psi 450 ksi 12,000 cycles & 1.4 m/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 c c c a b b b b b b
3 with reclaimed asphalt pavement (RAP) appears to provide adequate performance in terms of moisture susceptibility with or without an anti-stripping agent. WMA was also shown to improve in terms of moisture susceptibility measured in standard laboratory tests after either summer aging in the field or long-term oven aging (LTOA) in the laboratory that simulated this early-life field aging period. Before being considered for adoption, the proposed revisions to the appendix to AASHTO R 35 are based on a limited number of field projects and should be used on a trial basis. This will provide additional data to refine the moisture susceptibility criteria and the laboratory-conditioning and aging protocols that capture the time when WMA is most susceptible to this type of distress. Data from additional field projects will provide increased confidence in the guidelines provided and possible revisions to the framework proposed in this report. In addition, further information will be gathered to resolve dif- ferences between generally adequate field performance and laboratory assessment that indicates potential for moisture susceptibility for some mixtures. Continued field perfor- mance monitoring of the limited number of field projects used in NCHRP Project 9-49 is also suggested so as to further improvement of the guidelines produced.
