Evaluation of the Moisture Susceptibility of WMA Technologies (2014)

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4History and Definition HMA is a well-established paving material with proven performance used on 94 percent of the more than 2.5 mil- lion mi (4.0 million km) of paved roads in the United States (FHWA 2008, NAPA 2010). HMA is produced by mixing asphalt binder and aggregate at an elevated temperature in either batch or drum mix plants and then placed by compact- ing the mixture at temperatures ranging from 275°F (135°C) to 325°F (163°C) (Kuennen 2004, Newcomb 2005a). These high production and placement temperatures are neces- sary to ensure complete drying of the aggregate, coating and bonding of the binder with the aggregate, and workability for adequate handling and compaction. All of these processes contribute substantially to good pavement performance in terms of durability and resistance to permanent deformation and cracking. Recent advances in asphalt technology (includ- ing polymer-modified binders and stiff HMA mixtures with angular aggregate that improve resistance to permanent deformation—such as stone matrix asphalt [SMA]) and an emphasis on compaction for QA and subsequent good per- formance have resulted in further increases in HMA mixing and compaction temperatures up to a limit of 350°F (177°C) where polymer breakdown in the binder can occur. These high temperatures are linked to increased emissions and fumes from HMA plants (Stroup-Gardiner et al. 2005). In addition, the HMA production process consumes consider- able energy in drying the aggregate and heating all materials prior to mixing and compacting. Economic, environmental, and engineering benefits moti- vate the reduction of production and placement temperatures for asphalt concrete paving materials. Past efforts to reduce placement and production temperatures date back to the late 1950s and include binder foaming processes (using either steam or water), asphalt emulsification, and incomplete aggregate drying (Kristjansdottir 2006, Zettler 2006). The latest technology adopted to reduce placement and pro- duction temperatures of asphalt concrete paving materials is WMA. This technology was first introduced in Europe in the mid-1990s in order to reduce gas emissions. The technology was transferred to the United States in the early 2000s, largely through the efforts of the NAPA. WMA is defined as an asphalt concrete paving material produced and placed at temperatures approximately 50°F (28°C) cooler than those used for HMA. Several technologies satisfy this definition through different mechanisms and provide economic, environmental, and engi- neering benefits in terms of reduced viscosity of the binder, mixture, or both to allow for complete coating of the aggregate by the binder, sufficient adhesion between the aggregate and binder, and mixture compactability at lower temperatures. Widespread use of this technology and realization of its ben- efits requires production of WMA with similar performance and durability as HMA at substantially reduced production and placement temperatures (Button et al. 2007, Jones 2004, Prowell et al. 2011). Benefits and Issues WMA offers the following benefits (Button et al. 2007, Jones 2004, Koenders et al. 2002, McKenzie 2006, National Center for Asphalt Technology [NCAT] 2005, Newcomb 2005a, Newcomb 2005b): • Short-Term Benefits: – Decreased energy consumption of 30 to 40 percent (Jenkins et al. 2002, Kuennen 2004). – Reduced emissions and odors at the plant (30 percent reduction in CO2) (Kuennen 2004). – Reduced fumes and improved working conditions at the plant and construction site (fumes below detection lim- its and significant dust reduction) (Newcomb 2005a). – Decreased plant wear and costs. C H A P T E R 1 Background

5 Project Objectives and Scope NCHRP has committed funding for the following six research projects to address the major remaining issues asso- ciated with WMA: • Mix design (NCHRP Project 9-43—Mix Design Practices for WMA). • Overall mixture performance, engineering properties, and emissions (NCHRP Project 9-47A—Properties and Perfor- mance of WMA Technologies). • Moisture susceptibility (NCHRP Project 9-49—Performance of WMA Technologies: Stage I—Moisture Susceptibility). • Overall long-term field performance (NCHRP Project 9-49A—Performance of WMA Technologies: Stage II— Long-Term Field Performance). • Laboratory specimen preparation for mix design and perfor- mance testing (NCHRP Project 9-52—Short-Term Labora- tory Conditioning of Asphalt Mixtures). • Foaming properties of binders and laboratory specimen preparation (NCHRP Project 9-53—Properties of Foamed Asphalt for Warm Mix Asphalt Applications). NCHRP Project 9-43 is now complete, and the final NCHRP Reports 691 (Bonaquist 2011a) and 714 (Advanced Asphalt Technologies, LLC 2012) document the results that include laboratory specimen fabrication procedures specific to each WMA technology type and volumetric mix design proce- dures with selection of optimum binder content at 4 percent air voids (AV). The mixture design is also evaluated based on workability, compactability with new initial numbers of gyrations, aggregate coating, moisture susceptibility in terms of tensile strength ratio (TSR) as defined in AASHTO M 323 with AASHTO T 283 testing, and stability in terms of flow number as defined in AASHTO TP 79. The interim report for NCHRP Project 9-47A is available, and the extensive field experiment to establish relationships between engineering properties of WMA binders and mixtures and corresponding field performance is ongoing. NCHRP Project 9-49A began in spring 2011, and NCHRP Projects 9-52 and 9-53 began in summer 2012. In addition to these WMA projects, NCHRP is also sponsoring NCHRP Project 9-48—Field Versus Labora- tory Volumetrics and Mechanical Properties, where different specimen types are being evaluated, including laboratory- mixed laboratory-compacted (LMLC) specimens, PMLC specimens, and PMFC cores. The interim report for NCHRP Project 9-48 is also available (Mohammad and Elseifi 2010). Information and data between this NCHRP Project 9-49 and these related NCHRP projects were shared through NCHRP quarterly reports and other communication in terms of (1) mix design and specimen fabrication protocols, (2) relationships – Extended haul distances, a longer pavement construc- tion season, and a longer construction day than if pro- duced at typical HMA temperatures (Kristjansdottir 2006, NCAT 2005). – Reduced construction time for pavements with mul- tiple lifts (Kuennen 2004). – Improved workability and compactability. – Reduced initial costs (in some cases). • Long-Term Benefits: – Reduced aging and subsequent susceptibility to crack- ing and raveling. – Decreased lifecycle costs. Although WMA technology is successfully used in other countries, where the environmental benefits and high energy costs motivate implementation, many questions remain as it is adopted in the United States, where, in addition to the reduced emissions and lower energy demand benefits, reduced plant wear and associated costs, extended haul distances, and a longer pavement construction season and construction day provide additional driving forces (Barthel et al. 2004, Cervarich 2003, Kuennen 2004, McKenzie 2006). Some technologies result in an increase in initial costs ($3 to $4 per ton premium). However, these costs have decreased (to $0 to $3 per ton premium) as demand has increased and additional equipment required for some WMA technologies has become readily available. Other barriers to implementation include the following specific per- formance and mix design issues (Kuennen 2004, NCAT 2005, Newcomb 2005a, Rand 2008): • Short-Term Issues: – Conditioning/curing in the laboratory and field prior to compacting specimens. – Compaction in the laboratory (including mixing and compaction temperatures) and field. – Coating of aggregates with binder (some WMA tech- nologies). – Mix design (including selection of binder grade and opti- mum binder content with or without additives). – Possible increased susceptibility to permanent deforma- tion due to reduced aging. • Long-Term Issue: – Possible increased moisture susceptibility due to incom- plete drying of aggregate and differences in aggregate absorption of binder. In summary, although there has been a surge in WMA research and implementation in the United States, the effect of WMA technologies on mixture performance is still being evaluated.

6between laboratory tests and field performance, and (3) evalua- tion of the differences in volumetric and mechanical properties measured on different specimen types. Further coordination was facilitated by an invited national workshop in May 2011 and the subsequent NCHRP Research Results Digest 370: Guide- lines for Project Selection and Materials Sampling, Conditioning, and Testing in WMA Research Studies (Harrigan 2012b). NCHRP Project 9-49 focused on the moisture susceptibil- ity of WMA. LMLC specimens, PMLC specimens, and PMFC cores were evaluated to develop guidelines for identifying and limiting moisture susceptibility in WMA pavements. To meet these objectives, this project designed and completed WMA laboratory conditioning, WMA moisture susceptibility, and WMA performance evolution experiments, as described in Chapter 2, which resulted in a series of technical reports that documented the following: • Identification and preliminary assessment of current WMA pavements with evidence of moisture susceptibility (as available) and a work plan for further investigation of these pavements (available as an interim report). • Evaluation of conditioning protocols for WMA prior to mois- ture susceptibility testing to propose protocols for WMA and HMA (available as an interim report with results available in Appendix A). • Evaluation of standard test methods to predict moisture susceptibility and ability of materials and methods to mini- mize this distress (available as an interim report with results available in Appendix B). • Comparison of WMA moisture susceptibility for LMLC specimens, PMLC specimens, and PMFC cores (avail- able as part of an interim report with results available in Appendix B). • Evaluation of WMA pavements to identify possible reasons and evolution of performance with time (available as an interim report with results available in Appendix C). These technical reports are documented in this final proj- ect report and its appendices, along with the following: • Proposed guidelines for identifying and minimizing mois- ture susceptibility in WMA. • A work plan for a future research project to search further for an effective laboratory test method and performance-related criteria for precluding moisture susceptibility in WMA. • Proposed revisions to the appendix of AASHTO R 35. Relevant Literature and Survey Results Despite the attractive economic, environmental, and safety advantages of WMA, several changes in the production pro- cess as compared to HMA have raised concerns regarding the long-term performance of WMA pavements. Bonaquist (2011b), who evaluated mix design practices for WMA through laboratory and field study, indicated that the effect of WMA processes on moisture susceptibility is mixture and process spe- cific. He pointed out that different WMA processes have dif- ferent effects on moisture susceptibility and that most of them provide the mixture with less resistance to moisture damage, although some processes, such as low emission/energy asphalt (LEA), may be beneficial in terms of moisture susceptibility. Thus, moisture susceptibility of WMA mixtures should be evaluated comprehensively. To meet the objectives of this project, the WMA labora- tory conditioning, WMA moisture susceptibility, and WMA performance evolution experiments, described in Chapter 2, were designed and completed. Literature relevant to these three experiments is summarized in this section, including a discus- sion of factors that could increase the moisture susceptibility of WMA, with additional discussion in Appendices A, B, and C. A summary of the national survey conducted at the beginning of this project is also provided. WMA Laboratory Conditioning To simulate the binder absorption and aging that occurs during construction, the standard practice for laboratory mix design of asphalt concrete paving materials is to conduct short- term oven aging (STOA) or condition the loose mix prior to compaction for a specified time at a specific temperature. For HMA, the proposed procedure when preparing samples for performance testing is 4 h at 275°F (135°C); for mix design, when aggregate absorption is less than 4 percent, the condition- ing time can be reduced to 2 h (AASHTO R 30). In the past few years, several studies were conducted to evaluate the effect of different conditioning protocols on WMA mixture properties. These studies are summarized in Table 1-1. In general, most studies performed to understand the effect of conditioning prior to compaction on the performance of WMA have concluded that an increase in laboratory condi- tioning temperature, time, or both may reduce the difference in performance between WMA and HMA. However, no standard conditioning protocol for WMA has been established to date. WMA Moisture Susceptibility Several factors are related to the lower production tempera- ture of the WMA and the use of certain foaming and additive technologies that could increase the moisture susceptibility of WMA. These factors include • Introduction of additional moisture with the WMA tech- nologies that introduce water to produce a foamed binder. • Use of wet or damp aggregates in the production process.

7 Authors Year Conditioning Protocols Laboratory Tests Conclusions Al-Qadi et al. 2010 Reheat to Tc for offsite PMLC E*, Flow Number HWTT IDT Creep and Strength Semi-Circular Bending - Increased stiffness, strength, and rutting resistance with loose mix reheating - Reheating is sensitive to temperature - Effect of reheating: HM A > WMAs Bonaquist 2011a 2 h @ Tc (W) Volumetrics IDT Strength - Equivalent Gmm and dry IDT strength of WMA LMLC with 2 h @ Tc and PMFC cores Clements 2011 0.5, 2, 4, and 8 h @ 240°F (W) and @ 275°F (H) Flow Number Disc-Shaped Compact Tension - Mixture properties: WMA = HMA for each conditioning time Clements et al. 2012 0.5, 2, 4, and 8 h @ 240°F (W) and @ 275°F (H) E* Flow Number HWTT Disc-Shaped Compact Tension - Lower stiffness and resistance to rutting of WMA vs. HMA - Better fracture performance of WMA vs. HMA at 28°F test temperature - Increased stiffness and rutting resistance of WMA and HMA with increased conditioning Estakhri et al. 2010 2 h @ 220°F (W) 2 h @ 250°F (H) 2 h @ 275°F (H & W) 4 h @ 275°F (H & W) HWTT - Increased performance with higher temperature and longer time - Equivalent performance of different WMAs conditioned at 220°F - 4 h @ 275°F is proposed for WMA Estakhri 2012 2 h @ 275°F (W) 2 h @ Tc-HMA (H & W) HWTT Overlay - Increased resistance to rutting for WMA with higher temperature and longer time - Overlay results are sensitive to curing time and temperature - Significant decreased cracking resistance with curing time increased from 2 to 4 h Jones et al. 2011 No conditioning (H & W) (H & W) 4 h @ Tc HWTT Full-Scale Accelerated - Equivalent resistance to rutting of WMA and HMA after 4 h @ Tc - Resistance to rutting, without conditioning: Load Test WMA < HMA Note: W: WMA; H: HMA; HWTT: Hamburg Wheel-Tracking Test; IDT: Indirect Tensile. Table 1-1. Previous research on WMA laboratory conditioning.

8moisture susceptibility as compared to HMA, with mixed conclusions with regard to rutting of WMA as compared to HMA. WMA Performance Evolution Results from the laboratory conditioning experiment indi- cated that the initial stiffness of WMA is less than the stiffness of conventional HMA but that this gap may be reduced over time in the field. In the past few years, several studies were • Reduced binder absorption by the aggregates at lower pro- duction temperatures. • Reduced binder-aggregate coating and bond strength in the presence of certain WMA additives. Only the first factor has not been addressed extensively in previous research. Table 1-2 summarizes selected research studies on the remaining factors. From the performance evaluation of various WMA technologies, the conclusion of several laboratory studies is that WMA has increased Authors Year WMA Technologies Topic Laboratory Tests Conclusions Bennert et al. 2011 Evotherm 3G Sasobit Rediset Aggregate Moisture Content Overlay Tester Fatigue resistance decreased when moisture content increased and decreased as production temperature increased 10 F (5.6 C). Gong et al. 2012 Sasobit Resilient Modulus (MR), Creep Compliance, IDT Strength, Calculated Energy Ratio Moisture susceptibility is aggravated for mixtures that contained incompletely dried aggregates. Hurley and Prowell 2006 Aspha-min Sasobit Evotherm IDT Strength The use of moist aggregates decreased the IDT strength in all cases versus the HMA control. Xiao et al. 2009 Aspha-min Sasobit IDT and TSR Different WMA technologies do not alter IDT strength values significantly. TSR decreased with increase in aggregates moisture content. Austerman et al. 2009 Advera Sasobit Moisture Susceptibility and Rutting Potential HWTT WMAs are more moisture susceptible than HMAs. Advera is more susceptible than Sasobit. Hurley and Prowell 2006 Aspha-min Sasobit Aspha-min: less rutting resistant than HMA; lime improves rutting resistance. Sasobit: anti-stripping agent improves rutting resistance; improved rutting resistance with limestone but not with granite aggregate. Mogawer et al. 2012 Sonne Warmix Use of RAP or polymer- modified binder may improve moisture susceptibility and rutting. Table 1-2. Previous research on WMA moisture susceptibility.

9 (continued on next page) Authors Year WMA Technologies Topic Laboratory Tests Conclusions Hearon and Diefenderfer 2008 Sasobit Moisture Susceptibility IDT Strength and TSR Improved TSR after long-term aging of the mixtures. TSR improved with higher mixing temp. TSR > 80% in all cases where anti-stripping additives were used. Hurley and Prowell 2006 Aspha-min Sasobit All WMA TSR below 0.8 threshold (no anti-stripping agent). Improved IDT and TSR at higher short-term aging temperature. Aspha-min: lime improves TSR. Sasobit: anti-stripping agent improves TSR. Prowell et al. 2007 Aspha-min Evotherm Sasobit Aspha-min: shows TSR below 0.8. Sasobit and Evotherm: results depend on aggregate type. Sasobit: increased TSR with limestone. Evotherm: increased TSR with granite. Alavi et al. 2012 Synthetic Zeolite Surfactants Viscosity Reducers Bond Strength Bitumen Bond Strength (BBS) and Dynamic Modulus Ratio BBS, production at reduced temperatures has the potential to increase moisture susceptibility. Optimize WMA additive/aggregate type combinations for better results in term of moisture resistance, proposed BBS ratio 0.70. Estakhri et al. 2010 Evotherm Surface Free Energy (SFE) and Decreased binder-aggregate bonding with inclusion of WMA Table 1-2. (Continued).

10 participate in the project. Assistance in identifying candidate pavements was also sought from WMA industry groups, including contractors, equipment manufacturers, and addi- tive suppliers. The list of agency representatives and con- tact information was compiled with input from the NCHRP panel, NCAT, the internal and external advisory groups, the FHWA WMA Technical Working Group (TWG), and the RAP Expert Task Group. This section summarizes the informa- tion gathered as a result of the web-based survey and phone interviews. The detailed survey, interview questionnaires, and responses are documented and available as an interim report. State DOT Web-Based Survey To identify WMA pavements with evidence of distress, a brief web-based survey was conducted among the state DOTs. The following information was requested: • Current use of WMA. • Quantity of WMA placed. • WMA use requirements. • Types of WMA technologies. • Use of anti-strip additives. • Moisture susceptibility tests in WMA design practice. • WMA pavements failure or distress. conducted to quantify the evolution of WMA performance- related properties with time in an effort to understand the difference between HMA and WMA and, more importantly, when (or if) the properties of the two types of mixtures con- verge. This is particularly significant when evaluating mois- ture susceptibility, which can occur early in the life of the pavement or after several years in service, depending on envi- ronmental and loading conditions. These studies are summa- rized in Table 1-3. In general, most of these studies performed to understand the effect of long-term oven aging (LTOA) on the performance of asphalt mixtures have concluded that LTOA can significantly increase mixture stiffness. In addition, reasonable correlations between laboratory LTOA and field aging have been proposed based on laboratory test results. Summary of National Survey and Interviews A web-based survey of state DOTs was conducted at the beginning of the project to (1) document the performance of existing WMA pavements with an emphasis on moisture sus- ceptibility and (2) identify candidate pavements for inclusion in the work plan. Follow-up phone interviews were conducted with state DOTs that indicated availability of information regarding the performance of previously placed WMA pave- ments, upcoming construction projects, and willingness to Authors Year WMA Technologies Topic Laboratory Tests Conclusions Sasobit Rediset Work of Adhesion additives. In presence of water, negative work of adhesion meaning de-bonding between materials is likely to occur. Nazzal and Qtaish 2013 Advera Evotherm M1 Sasobit Foaming Adhesive and Cohesive Bond from Atomic Force Microscopy (AFM) For unconditioned samples, all WMAs increase in adhesive bond as compared to HMA. After AASHTO T 283, Evotherm performs better than other WMA and equivalent to HMA. Wasiuddin et al. 2008 Aspha-min Sasobit SFE Aspha-min shows no significant effect on SFE and no improvement in wettability. It shows increased adhesion for PG 70-28 but no effect for PG 64- 22. Sasobit shows increased wettability, decrease in dry cohesive strength and binder- aggregate adhesive bond. It reduced total SFE of the binder. Table 1-2. (Continued).

11 Authors Year Aging Stages Laboratory Tests Conclusions Bell et al. 1994 LTOAs (4 days at 212°F and 8 days at 185°F) MR - LTOA on stiffness: 8 days at 85°C = 4 days at 100°C - Equivalent aging: lab 8 days at 85°C; lab 4 days at 100°C; 9-year field aging Brown and Scholz 2000 LTOA (120 h at 185°F) IDT Modulus - Increased mixture stiffness with LTOA Bueche and Dumont 2011 Long-Term Aging (0, 1, 2, 4, and 12 weeks at room temp) HWTT IDT Strength - No effect on mixture resistance to moisture susceptibility Diefenderfer and Hearon 2008 LTOAs (4 and 8 days at 185°F) IDT Strength - Improved TSR of mixtures produced at 110°C and 130°C - Insignificant effect on TSR of mixtures produced at 150°C - Improved moisture resistance of WMA with LTOAs Estakhri et al. 2009 Field Aging HWTT Dry IDT Strength Strength - Initial stiffness: WMA < HMA - Increased stiffness with field aging - HWTT results for 1-year PMFC cores: WMA = HMA - IDT strengths for WMA: 1-month PMFC core > offsite PMLC - IDT strengths for HMA: 1-year PMFC core = 1-month PMFC core = offsite PMLC Estakhri 2012 Field Aging HWTT Overlay Dry IDT - Comparable performance of WMA and HMA - HWTT, Overlay, IDT Strength results: WMA 1-year PMFC core > PMFC at construction - No effect on WMA cracking resistance after 1 year in service Mogawer et al. 2010 LTOA (16 h at 140°F) HWTT - Improved performance in HWTT with LTOA - Increased stiffness with LTOA Xiao et al. 2011 LTOA (5 days at 185°F) Dry/Wet IDT Strength - Insignificant effect on dry IDT strength - Increased wet IDT strength with LTOA - Improved moisture susceptibility of WMA with LTOA Table 1-3. Previous research on WMA performance evolution.

12 former Aspha-min® product), Astec DBG®, Aquablack™, EvothermDAT™, Sasobit®, and Terex®. About 48 percent of the respondents required the use of anti-stripping agents in WMA due to the use of moisture-susceptible aggregates, results of moisture-susceptibility tests, or both. Concerning moisture-susceptibility testing, 76 percent of the responding state DOTs indicated that their agency specifi- cations included related criteria as part of the HMA or WMA design procedure. The TSR of AASHTO T 283 is the moisture- susceptibility test preferred by 68 percent of the state DOTs. The next preferred test is the HWTT (AASHTO T 324), with 19 percent of the responses. Others tests, such as the Asphalt Pavement Analyzer (APA) (AASHTO TP 63) and the Immersion- Compression Test (AASHTO T 165), accounted for only 10 per- cent of the responses. Finally, all of the state DOTs indicated that their WMA pavements had not experienced failure or distress from moisture damage. State DOT Follow-Up Phone Interviews Based on the knowledge acquired through the web-based survey and input from the internal and external advisory groups, 15 state DOTs were identified as candidates for fol- low-up phone interviews. These states were selected because of their prior experience with WMA technologies via trial or routine projects, existence of WMA pavements planned dur- • Availability of technical data. • Upcoming WMA pavements. • Availability to further participate in NCHRP 9-49 research activities. The web-based survey was launched in November 2010 with an invitation e-mail containing a brief description of the objec- tives of the project and the purpose of the survey. The invitation was sent to DOT representatives from all 50 states in addition to the District of Columbia and Puerto Rico. Thirty-five agen- cies responded to the survey (i.e., a 67 percent response rate). In general, more than half of the responding state DOTs (i.e., 54 percent) indicated current use of WMA in trial proj- ects, approximately 40 percent routinely used WMA, and only 6 percent had no experience with WMA. Figure 1-1 shows the distribution of WMA use in the United States based on the responses. In addition, 44 percent of the respondents indi- cated past or planned use of WMA in 2-5 projects, 21 percent in between 5-10 projects, and 23 percent in more than 10 proj- ects (i.e., routine use). Also, most of the responding state DOTs (i.e., 73 percent) allow the use of WMA as an option; of these, 6 percent require it, 6 percent allow it as a separate bid item, and 12 percent do not allow its use. With regard to specific WMA technologies, the survey results showed that the preferred types, which accounted for 70 per- cent of the responses, included Advera® WMA (including Figure 1-1. WMA use in the United States.

13 mix design stage was based primarily on TSR using AASHTO T 283 or the HWTT. Only two agencies indicated using the Immersion-Compression Test (AASHTO T 165). Two agen- cies did not perform any moisture-susceptibility tests, while one agency required the use of AASHTO T 283 results for mix design approval. Agencies used between three and six Super- pave gyratory specimens to determine TSR, with specimens varying from 4 to 6 inches (100 to 150 mm) in diameter and 4 inches (100 mm) in height. The results of the moisture susceptibility tests varied from agency to agency. For most of the agencies, the WMA TSR test results were lower than for the control HMA. In addition, for one agency, the TSR values of PMFC WMA specimens were less than 80 percent, while for PMLC WMA (i.e., after reheating), the TSR was greater than 80 percent. Another state DOT also indicated observing differences between the WMA TSR results of PMFC versus PMLC specimens. Other agencies indicated that the TSR results of LMLC WMA specimens were above 80 percent for all WMA technologies. One agency used WMA versus HMA (instead of unconditioned versus condi- tioned) to compute the TSR and required a value greater than 85 percent. Construction. The reported WMA production tem- peratures varied from 230 to 270°F (110 to 132°C) depend- ing on the technology being used. The maximum reported production temperature was 280°F. The respondents indi- cated that the mixing process for WMA was no different than that for HMA. With regard to compaction temperatures, the state DOTs indicated that the usual range was around 230°F (110°C) with special instances being as low as 190°F (88°C) or as high as 275°F (135°C). Besides the temperature, the only other reported difference in compaction was the roller pat- tern, with the roller positioned closer to the paver for WMA due to the reduced temperature of the mixture. The type and weight of the rollers were the same for both HMA and WMA. The QA measures required on the WMA pavements were the same as the ones prescribed for HMA construction: volumet- rics, aggregate gradation, binder content, etc. Performance. All state DOTs indicated that, to date, no distress related to moisture damage had been observed or reported in the WMA pavements. However, these pavements are relatively new, and yearly condition monitoring is planned to track performance. One agency reported thermal cracking appearing in the WMA pavements during the first winter sea- son after construction. Another two agencies reported prob- lems with compaction. In one case, it was sheen effects and high densities, and in the other, it was poor compaction and difficult handwork after long haul distances. Two other agen- cies reported observing cracking and other minimal distresses occurring on all pavements, including the HMA control. ing the 2011 construction season, and willingness to partici- pate in the NCHRP 9-49 research efforts. The state DOTs were asked to identify past pavements as part of their responses to the follow-up interview questions. The questions addressed pavement location, structure, traffic level, environmental conditions, type of materials and WMA technologies used, laboratory tests performed, construction procedures, QA measures, pavement performance, planned maintenance and rehabilitation, and WMA quantity and cost. Ten state DOTs were available to complete the follow- up phone interview. Some of these DOTs proposed contact- ing researchers in charge of studying various performance aspects of the WMA pavements in their respective states. Six researchers were interviewed to complement the answers of the state DOTs. The responses of state DOTs and research- ers are summarized next, and the summary is organized by questionnaire topics. Materials and WMA Technology. The technologies most commonly used in the selected WMA pavements, which were built between 2006 and 2010, were Evotherm™, free water foaming systems, and Sasobit®. The most common aggregate type used in these WMA pavements was limestone with minor use of other materials (e.g., gravel, quartzite, dolomite, and basalt). The quartzite and specific sources of limestone in some states were classified as moisture suscep- tible. The predominant mixture type used was a 12.5 mm Superpave dense-graded mixture. The types of binders used were all performance graded, including PG 58-28, PG 64-34, PG 64-28, PG 64-22, PG 70-22, PG 76-22, and PG 76-16. The use of anti-stripping additives was mandatory for three agencies, three agencies did not require it, and the others pre- scribed use only when employing aggregates prone to stripping or mixtures prone to moisture damage based on moisture- susceptibility test results. With regard to material availability, all the state DOTs indicated that virgin materials (i.e., binder, aggregates, and additives) from past WMA pavements were not available; a few state DOTs had plant loose mix or cores. Mixture Design and Location. All selected pavements were built during dry and mild to hot weather, except for one done after a heavy rain. Regarding WMA mix design practices, only one state DOT used separate HMA and WMA mix design specifications; the rest of the agencies stated that they followed the Superpave volumetric criteria used for HMA when design- ing WMA. Thus, the WMA design was done following HMA practices with the only difference being that the mixture was produced at reduced temperatures based on additive producers or equipment manufacturers’ recommendations. Some agencies did not consider any critical distresses as part of the WMA design, and others used the same criteria applied to HMA. Moisture-susceptibility testing during the

14 especially when including crumb-rubber modified asphalt. The responses also indicated that the purpose of using WMA in 85 to 90 percent of the pavements built with WMA was to achieve temperature reduction. In the other instances, the purpose stated was to extend haul distances/times, obtain bet- ter density, achieve cold in-place recycling, control emissions, provide a cleaner and safer construction environment, achieve cost/fuel savings, or accelerate construction placement. Two contractors indicated that their technology was used in the laboratory as part of the WMA mix design. As far as changes in plant operations are concerned, the pri- mary modification (besides the temperature reduction) was introducing hardware and controls to introduce the additives. With respect to field operations, the contractors indicated the differences were the lower compaction temperature and the location/timing of the compaction rollers, which were placed closer to the paver because of the reduced temperature of the mixture and thus have limited time to achieve the required density and finish the surface. One contractor answered that when using free water foaming technologies, adjustments to the rolling pattern and timing of the rollers had to be made because placement of the WMA under shaded areas caused the mixture to become tender due to the lower temperatures. The typical compaction temperatures the respondents had used varied with WMA technology. The reported tempera- tures were as low as 205°F (96°C) to as high as 275°F (135°C). In terms of QC measures, all contractors followed regular HMA practice (i.e., volumetrics). One contractor allowed the compacted loose mix to cool down before compaction in the laboratory to replicate agency practices. Another contractor included moisture-susceptibility tests as part of its quality assessment and obtained lower TSR for WMA (i.e., 39 percent) versus HMA (i.e., about 90 percent). After reheating the mixture in the laboratory, the TSR of the WMA increased to around 50 percent but was still below the desired threshold of 80 percent. The respondents reported no significant difference in the layer thicknesses prescribed for WMA versus conventional HMA. Regarding cost, the contractors noted that the major cost difference of producing WMA versus HMA was the initial capital investment on equipment, additives, or both. However, they also indicated that as the use of WMA becomes more prevalent, the capital and production costs will probably be offset by the energy savings obtained by producing at reduced temperatures. With respect to performance, the contractors indicated that, to date, they had not observed any distresses on any of the pavements, even ones built 3 years ago. The contractor that obtained the low TSR values in the laboratory also observed that this particular WMA pavement had not shown signs of stripping (moisture damage) in the field. Finally, upcoming projects were investigated with the respective state DOTs. All agencies were expecting the same or better service life out of the WMA pavements compared with the HMA pavements. In addition, the maintenance and rehabilitation options being considered for the WMA pavement sections were the same ones being used for HMA pavements. Other. The cost of WMA was handled in different ways by the various agencies. For some, because the WMA pavements were trial or demonstration projects, the cost was subsidized by the additive supplier, equipment manufacturer, or contrac- tor. For others, the cost was very similar to typical HMA prices. One agency required the contractor to reduce the price per ton of the WMA based on value engineering, under the principle that energy savings generated by producing WMA should be shared with the agency. General. The state DOTs and researchers were asked to give additional information, ideas, or comments useful to the study of moisture susceptibility of WMA. Input to this final question touched on the following topics of interest: • Measure the change in WMA performance with time and versus HMA. • Evaluate the sensitivity of current tests to quantify mois- ture susceptibility of WMA. • Validate/calibrate current tests to accurately quantify mois- ture susceptibility of WMA and reflect field performance. • Clarify negative aspects associated with the production of foamed asphalt. • Develop a process to identify well-performing WMA addi- tives and methods in the future. • Establish a unified laboratory mix design process using WMA additives and foaming. • Study the effects of wet/moist aggregates on WMA. Contractors’ Phone Interviews With the input of the internal and external advisory groups and the outcome of the state DOTs’ web-based survey and follow-up interviews, a list of contractors was consolidated to collect information via phone interviews regarding candidate WMA pavements as well as current WMA practice. Interviews for contractors included questions about construction prac- tices using specific WMA technologies such as mix design, changes in plant and field operations, QC measures, place- ment temperatures, compaction, mat thickness, and costs. They were also asked to share information about upcoming construction of WMA pavements. Six contractors were interviewed. These contractors used technologies that included an array of foaming and additive types. The mix type most commonly used in all instances was dense-graded with minor use of open or gap-graded mixtures,

15 technologies ranged between 260 and 285°F (127 and 141°C) with higher temperatures sometimes required when crumb- rubber asphalt was incorporated in the mixture. When pro- ducing WMA using the foaming systems at temperatures below 32°F (0°C), some precautions needed to be taken to prevent the water supply from freezing. In addition, burner adjustments were also necessary when decreasing the tem- perature and thus increasing the production. Regarding the cost difference of producing WMA using the equipment/additive versus HMA, the price was compa- rable, especially for the foaming systems. Some cost savings resulted from using less energy when mixing at reduced tem- peratures, requiring less compaction effort to achieve den- sity, and being able to open the pavement to traffic sooner. Concerning the quantity produced and application time of WMA versus HMA, all respondents indicated that there was no difference. No specific list of upcoming pavements was available from the interviewees. However, based on their insight as to which states were likely to use their equipment/additive, additional inquiries were sent to the respective state DOT representatives. The equipment manufacturers and additive supplier pointed out these topics of interest: • Improve current laboratory tests to accurately quantify moisture susceptibility of WMA. • Develop guidelines to limit maximum moisture content of aggregates used in production. • Dispel negative opinions associated with the production of foamed asphalt. Some topics of interest that the contractors pointed out at the end of the interview included • Quantify the differences in material properties between WMA versus HMA. • Improve moisture-susceptibility laboratory tests to corre- late with field observations. • Validate moisture-susceptibility laboratory test criteria for WMA. • Measure moisture content of WMA in the field and com- pare with HMA. WMA Equipment Manufacturers and WMA Additive Suppliers Phone Interviews Interviews for equipment manufacturers and additive suppliers included questions aimed at identifying primary customers of the equipment/additive, pavements where the equipment/additive was used, technical information on the WMA technology process (e.g., temperature, cost, application time, and quantity produced), and upcoming WMA pave- ment construction. Two WMA equipment manufacturers of free water foaming systems and one WMA additive supplier were interviewed. With respect to primary customers, the equipment manu- facturers indicated that contractors were their main clients, while the additive supplier’s customers consisted primar- ily of state DOTs. Although the foaming and additive pro- cesses have a different approach in lowering the viscosity of the binder, the production temperatures of WMA for both