Evaluation of the Moisture Susceptibility of WMA Technologies (2014)

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51 This chapter presents overall findings from the three exper- iments described in the previous chapter, a summary of WMA performance compared to HMA, guidelines for evaluating WMA for moisture susceptibility during mix design and QA, and suggested research based on the results of this project. Findings WMA Laboratory Conditioning The following are findings from the results of the WMA laboratory-conditioning experiment that included evaluation of almost 250 LMLC specimens, onsite and offsite PMLC spec- imens, and PMFC cores from the Iowa, Texas, and Montana field projects: • MR results showed that the stiffness of LMLC specimens increased with higher conditioning temperatures and lon- ger conditioning time and that WMA was more sensitive to conditioning temperature than conditioning time. Among the five selected conditioning protocols for LMLC speci- mens, 2 hours at Tc was more representative in terms of the stiffness of HMA and WMA pavements in their early life. Considering the difficulty in accurately defining Tc in the field and the common range of Tc for HMA and WMA, 2 hours at 275°F (135°C) and 240°F (116°C)—instead of 2 hours at Tc—are proposed as the standard laboratory- conditioning protocol for HMA and WMA LMLC speci- mens, respectively. • MR results for PMLC specimens subjected to different conditioning protocols versus PMFC cores at construction showed that onsite PMLC specimens were more represen- tative in terms of stiffness of HMA and WMA pavements in their early life. In contrast, the conditioning protocols used on the offsite PMLC specimens yielded specimens with statistically higher stiffness as compared to the PMFC cores at construction, showing that reheating loose mix had a significant effect on the stiffness of offsite PMLC specimens. Even in the case of HMA and WMA with only reheating to Tc, the stiffness was higher than the stiffness of PMFC cores at construction. Considering the difficulty in accu- rately defining Tc in the field and the common range of Tc for HMA and WMA, Tc in the proposed conditioning protocols for preparing PMLC specimens is standardized at 275°F (135°C) and 240°F (116°C), respectively. • Offsite PMLC specimens of WMA prepared with foaming processes required a different conditioning protocol as com- pared to WMA with additives because the foaming effect during production was assumed lost after mixing and cool- ing of the loose mix. Therefore, the conditioning protocols proposed for preparing PMLC specimens onsite are as fol- lows: (1) 1 hour at 275°F (135°C) for HMA, and (2) 1 hour at 240°F (116°C) for WMA. When compacting PMLC specimens on site is not viable, the proposed condition- ing protocol for offsite PMLC specimens is to (1) reheat to 275°F (135°C) for HMA and WMA with foaming process, and (2) reheat to 240°F (116°C) for WMA with additives. WMA Moisture Susceptibility The following are findings from the results of the WMA moisture-susceptibility experiment that included evaluation of more than 850 LMLC specimens, onsite and offsite PMLC specimens, and PMFC cores from the Iowa, Texas, Montana, and New Mexico field projects: • The selected laboratory-conditioning protocol simulates the early life of the pavement and produces laboratory- compacted mixtures with performance in terms of mois- ture susceptibility equivalent to that of PMFC cores at construction, after a winter, or both, as indicated by the selected laboratory tests. • Based on laboratory moisture-susceptibility tests, WMA can be more moisture susceptible in early life (prior to C H A P T E R 4 Findings, Discussion and Guidelines, and Suggested Research

52 • Equivalent dry MR stiffnesses between LMLC specimens with up to 2 weeks LTOA at 140°F (60°C) and PMFC cores at construction were shown for most Iowa and Texas mix- tures, indicating the similar initial stiffness in the labora- tory as compared to the initial field conditions. In addition, the laboratory LTOA protocols at 140°F (60°C) for 4 to 16 weeks were representative of the field aging experienced by PMFC cores after the first summer. • As with dry MR stiffness, results from other standard labora- tory tests (dry and wet IDT strengths, wet MR stiffnesses, and HWTT parameters) indicated that PMFC cores acquired after the first summer in service had statistically better per- formance than those acquired at construction. However, the difference between PMFC cores acquired after the first win- ter in service versus the ones acquired at construction was not significant. The laboratory LTOA protocols used in this study also had a significant effect on performance, improv- ing the MR stiffness, IDT strength, and moisture susceptibil- ity of the mixtures. In addition, the comparison of mixture performance measured in the laboratory between PMFC cores after several months in service and LMLC specimens with LTOA indicated that laboratory aging of 16 weeks at 140°F (60°C) as well as 5 days at 185°F (85°C) were rep- resentative of the early-life field aging that PMFC cores experienced after construction. • Based on the dry and wet IDT strengths, dry and wet MR stiffnesses, and HWTT test results, HMA had higher stiff- ness and strength and better moisture resistance than its WMA counterparts did at the initial field and laboratory stages. This was indicated by the comparisons of results from these laboratory tests for PMFC cores at construction and LMLC specimens without LTOA. However, the differ- ence between HMA and WMA was reduced as PMFC cores and LMLC specimens experienced field aging and labora- tory LTOA, respectively. For most of the cases, after aging, better or equivalent mixture performance in laboratory tests was achieved by WMA. Thus, WMA pavements are more likely to be susceptible to moisture-related distresses during their early life as compared to HMA pavements. Therefore, measures such as adding anti-stripping agents or ensur- ing summer aging prior to wet and cold winter conditions should be considered to prevent moisture-related pavement distresses from occurring. Performance Summary The trends discussed in the previous chapter focus on a com- parison of WMA and HMA in terms of the selected laboratory performance parameters, the effects of adding anti-stripping agents, and the differences in results for different specimen types and after different conditioning and LTOA protocols. In this section, a discussion of the overall performance of the summer aging) as compared to HMA, but equivalent per- formance is shown after a summer of aging. • WMA may be moisture susceptible in early life (prior to summer aging), and the use of anti-stripping agents may reduce this susceptibility. WMA technologies exhibiting the greatest moisture susceptibility in laboratory tests will show the greatest benefit with the use of anti-stripping agents. Compatibility of the anti-stripping agent with the WMA technology and component materials should be considered. • Onsite and offsite PMLC specimens differ in terms of laboratory-measured moisture susceptibility, with the arti- ficial aging due to reheating producing offsite PMLC speci- mens that exhibit improved resistance to moisture damage. • Agreement between laboratory and field performance based on pavement condition was shown for the Montana and New Mexico field projects with good field performance for all mixtures, and for the Iowa field project with poor field performance for the two WMAs with RAP. Agreement was mixed across specimen types and across the three stan- dard laboratory tests for the Texas field project. WMA Performance Evolution The following are findings from the results of the WMA performance-evolution experiment that included evaluation of more than 500 LMLC specimens, onsite PMLC specimens, and PMFC cores from the Iowa, Texas, and New Mexico field projects: • HMA and WMA PMFC cores experienced significant increase in dry MR stiffness with field aging. The increase in stiffness during the summer months was more signifi- cant than during the winter, probably because of aging dur- ing the high in-service temperatures. For both the Iowa and Texas field projects, HMA had a higher initial stiffness than its WMA counterparts at construction, but the WMA experienced an increase in stiffness at a higher rate with field aging than the HMA mixtures for these field projects. Con- sequently, equivalent dry MR stiffnesses between WMA and HMA were achieved by Iowa PMFC cores after summer at 12 months in service and by Texas PMFC cores after sum- mer at 8 months in service. • The comparison in dry MR stiffness between HMA LMLC specimens with different LTOA times at the same LTOA temperature illustrated the effect of laboratory aging on mixture stiffness. The laboratory aging protocol of 2 weeks at 140°F (60°C) was able to represent the time period where the stiffness of WMA was equivalent to the initial stiffness of HMA without LTOA (for the Iowa pavement) or where the dry stiffness of WMA and HMA converged (for the Texas pavement).

53 HMA and WMA mixtures from the different field projects in the context of these laboratory results, limited field per- formance data, traffic, climate, and materials is provided. The laboratory results were compared to common thresholds, including minimum 80% for TSR and MR-ratio, minimum SIP of 10,000 based on the Iowa specification, and minimum wet IDT strengths of 65 psi and 80 psi for mixtures with unmodi- fied (Iowa) and modified (Montana, New Mexico, and Texas) binders based on averages from the Nevada, Tennessee, and Texas specifications. In general, as detailed in Appendix D, the four field proj- ects are performing well to date (through March 2013) after 18 months (Iowa), 17 months (Montana), 14 months (Texas), and 5 months (New Mexico). The Montana field project is not exhibiting distress related to moisture susceptibility to date, despite construction in October 2011 without experiencing a summer of aging prior to winter conditions, heavy traffic on an interstate highway (Figure 2-4), and an extreme climate for moisture susceptibility (cold and multi-F/T) (Figure 2-1, Fig- ure 2-2, and Figure 2-3). This field project did not use RAP but did include an anti-stripping agent (lime) and a relatively ele- vated high-temperature performance grade binder (PG 70-28) (Table 2-1). In addition, this field project was treated with a seal coat friction course in July 2012. The field performance for the HMA and three WMAs from this field project was in agree- ment with all of the results from the three standard laboratory tests that indicated adequate resistance to moisture suscepti- bility when compared to common thresholds, with the only exceptions being wet IDT strengths and TSR values for onsite PMLC specimens. The relatively recently constructed New Mexico field proj- ect is also not exhibiting distress related to moisture suscep- tibility to date, despite construction in October 2012 without experiencing a summer of aging prior to winter conditions, heavy traffic on an interstate highway (Figure 2-4), and a cli- mate that mirrors aspects of different extreme climates for moisture susceptibility (dry, cold during the winter, and rela- tively hot during the summer, as shown in Figure 2-1, Fig- ure 2-2, and Figure 2-3). This field project used RAP with a relatively low high-temperature performance grade binder (PG 64-28) and included an anti-stripping agent (Versabind) (see Table 2-1). Similar to the Montana field project, agree- ment was shown between the field performance and the labo- ratory performance for all three standard laboratory tests for the HMA and two WMAs from this field project. All of these tests indicated adequate resistance to moisture susceptibility when compared to common thresholds, with the only excep- tions being wet IDT strengths and TSR values for all LMLC specimens and MR-ratios for all LMLC specimens and WMA foaming for onsite PMLC specimens. The Texas field project is also generally performing well to date and not exhibiting distress related to moisture sus- ceptibility, despite winter construction in January 2012 in an extreme climate for moisture susceptibility (hot and wet) (Figure 2-1, Figure 2-2, and Figure 2-3) with heavy truck traffic on a farm-to-market (FM) road (Figure 2-4). This field project did not use RAP or an anti-stripping agent but did use a relatively elevated high-temperature performance grade binder (PG 70-22) with a relatively lower-quality aggregate (SAC-B), as categorized by the Texas Department of Transportation’s (TxDOT’s) Surface Aggregate Classifica- tion System (TxDOT, 2012). For this field project, agreement between the field and laboratory performance as predicted based on common thresholds was not as complete across the different specimen types or across the three standard labo- ratory tests as it was for the Montana and New Mexico field projects. For example, the HWTT results indicated that the two WMAs may have been moisture susceptible in early life, as indicated by onsite PMLC specimens, LMLC specimens, and PMFC cores at construction but that their resistance improved after a summer of aging. Wet IDT strength and TSR values and wet MR stiffness and MR-ratios also indi- cated that the WMA foaming and WMA Evotherm DAT™ may have been moisture susceptible in their early life based on results for LMLC and some PMLC specimens and PMFC cores at construction. The Iowa field project also did not include an anti-stripping agent (Table 2-1) and is now exhibiting some distress related to moisture susceptibility for some of the mixtures. The Iowa field project is exhibiting some raveling in both WMAs (WMA Sasobit® with RAP and WMA Evotherm® 3G with RAP) that was likely exacerbated by paver segregation at the crown and subsequent snow plow damage (Appendix D). The Iowa HMA with RAP is not exhibiting raveling to date. In addi- tion to possible construction issues, the Iowa field project was constructed in September 2011 without experiencing a sum- mer of aging prior to winter conditions in an extreme climate for moisture susceptibility (wet and F/T) (see Figure 2-1, Fig- ure 2-2, and Figure 2-3) and sustains moderate traffic on a U.S. highway (Figure 2-4). This field project used RAP with a relatively low high-temperature performance grade binder (PG 58-28). Although part of the aggregate fraction typically requires the use of an anti-stripping agent, the project did not use it (Table 2-1) because adequate TSR results were obtained during mix design. After construction, Iowa DOT QA results did indicate the need for an anti-stripping agent based on TSR results, and these results agree with those obtained herein that an anti-stripping agent would likely have been beneficial and may have been able to offset the moisture susceptibility of this project in its early life. As for the Montana and New Mexico field projects with good performance for all mixtures, the WMAs with RAP in the Iowa field project indicated inadequate resistance to moisture sus- ceptibility for all tests when compared to common thresholds

54 Discussion and Guidelines Figure 4-1 shows proposed guidelines for evaluating WMA for moisture susceptibility during laboratory mix design based on an analysis of the results of this project. If appropriate labo- ratory equipment is not available to fabricate LMLC specimens with the WMA technology, testing may be conducted on PMLC specimens fabricated on site or off site with minimal reheating from plant trial batch materials as shown in Figure 4-2 and Fig- ure 4-3, respectively. Figure 4-2 and Figure 4-3 can also be used as guidelines for QA of WMA with respect to moisture suscep- tibility. All of these proposed guidelines are incorporated in the revised draft AASHTO R 35 appendix presented in Appendix F. Figures 4-1 through 4-3 were produced as a set of guidelines, and state DOTs can modify them to suit their needs based on their experience. After mixing WMA LMLC specimens according to the AASHTO R 35 appendix, loose mix is subject to STOA for 2 hours at 240°F (116°C) prior to compaction. Next, a test to evaluate moisture susceptibility is selected based on available equipment, costs, and prior experience from the following three choices: wet and dry IDT strengths at 77°F (25°C) and TSR by AASHTO T 283, wet and dry MR stiffnesses at 77°F (25°C) and MR-ratio after moisture conditioning by AASHTO T 283, or HWTT SIP and stripping slope per AASHTO T 324 at 122°F (50°C). Two criteria for each test for these STOA specimens are shown in Figure 4-1. These criteria were developed by sepa- rating the results from the relatively good field and laboratory performance of the Texas WMAs and the relatively poor field and laboratory performance of the Iowa WMAs, as shown in Table 4-1. Mixtures from the Iowa field projects contained a relatively low high-temperature performance grade binder (PG 58-34) and RAP, and those from the Texas field proj- ect contained a relatively elevated high-temperature perfor- mance grade binder (PG 70-22) without RAP. In Table 4-1, red shading indicates that the STOA mixture did not meet the criteria and would likely be moisture susceptible in early life, and green shading indicates that the STOA mixture met the criteria and would likely not be moisture susceptible. These thresholds were verified through examination of the WMAs from the Montana and New Mexico field projects, as shown in Table 4-2. For the Montana field project where LMLC specimens were not available, onsite PMLC specimens were used as proposed in Figure 4-1. Again, both types of mix- tures (with and without RAP) with different high-temperature performance grade binders were used. Mixtures from the New Mexico field project contained a relatively low high-temperature performance grade binder (PG 64-28) and RAP, and those from the Montana field project contained a relatively elevated high-temperature performance grade binder (PG 70-28) with- out RAP. As shown in Table 4-2, this verification predicted adequate performance in terms of moisture susceptibility for based on results from LMLC specimens and PMFC cores in the early life at construction and after winter at 6 months in service. The agreement was not as clear for the HMA with RAP, because all three standard laboratory tests indicated marginal or inadequate performance for some specimen types in contrast to the relatively good field performance. In addition, at least one of the parameters for each of the three standard laboratory tests was able to discriminate this difference in field performance between WMA and HMA by finding that the two WMAs with RAP exhibited reduced performance as compared to HMA with RAP based on PMLC, LMLC, or both types of specimens and PMFC cores in early life at construction or after winter with 6 months in service. For wet IDT strength and TSR values, improved performance of the two WMAs with RAP for PMFC cores after a summer of aging at 12 months in service was exhibited such that equivalent performance as compared to HMA with RAP was attained. WMA Evotherm with RAP also showed this improved performance in terms of the HWTT stripping slope for PMFC cores after a summer of aging at 12 months in service. Based on the overall laboratory results for specimens that were STOA to represent early life and those that were LTOA to represent the effects of a summer aging period, all of the WMA mixtures from all of the four field projects exhibited either adequate moisture susceptibility initially or after a sum- mer of aging as compared to HMA. Unfortunately, all four field projects were constructed in fall or winter and did not experience a summer of aging prior to winter conditions, and thus the overall hypothesis that WMA will exhibit adequate moisture susceptibility after a summer of aging was not fully tested. In addition, based on the results of the survey, most field sections in the United States are not exhibiting signs of moisture susceptibility. Based on the data from the few field projects included in NCHRP Project 9-49, construction with WMA technologies that will not sustain a summer of aging prior to multiple freeze/ thaw cycles or wet and cold days in the first winter may involve some risk of moisture susceptibility, but the addition of anti- stripping agents may mitigate this risk. The use of either a rela- tively elevated high-temperature performance grade binder or a relatively low high-temperature performance grade binder with RAP appears to provide adequate performance in terms of moisture susceptibility with or without an anti-stripping agent. Compatibility of the anti-stripping agent with the WMA technology and the component binder and aggregate materials is crucial, and the laboratory results from this proj- ect indicate that the use of a liquid anti-stripping agent in concert with Evotherm may be unnecessary or even coun- terproductive. The guidelines provided in the next section address this issue of changing performance during the early life through a two-step WMA laboratory evaluation process for moisture susceptibility.

55 grade or inclusion of RAP); or any combination of these modifications is then proposed prior to a second evaluation of the modified WMA with these same criteria. As shown in Table 4-1, even with the addition of two different anti- stripping agents, most of the Iowa STOA mixtures were likely to be moisture susceptible in early life, while the addition of hydrated lime to the WMA foaming mixture from the Texas field project that failed the criteria without anti-stripping agents exhibited adequate performance in terms of both wet IDT strength and TSR and wet MR stiffness and MR-ratio. most of the standard laboratory tests for the STOA WMAs from both of the Montana and New Mexico field projects that agrees with field performance to date. If the WMA passes both criteria for the selected test, the mixture is expected to have adequate performance in terms of moisture susceptibility. If the WMA does not pass one or both criteria for the selected test, early-life moisture suscepti- bility is probable. Mixture modifications in terms of (1) add- ing, modifying the dosage of, or changing anti-stripping agents; (2) changing other mixture components (e.g., binder Note a: select a single test method and use it throughout the mix design verification 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 a a a a a a Figure 4-1. Proposed WMA moisture susceptibility evaluation for mix design with LMLC specimens. Figure 4-2. Proposed WMA moisture susceptibility evaluation for mix design or QA with onsite PMLC specimens. PMLC, Loose Mix Stabilize to Compaction Temperature Note a: select a single test method and use it throughout the mix design verification 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 Moisture Susceptibility at Early Life Add/modify anti-stripping agent and/or other mixture components a a a

56 Figure 4-3. Proposed WMA moisture susceptibility evaluation for mix design or QA with offsite PMLC specimens. PMLC, Loose Mix Reheat to Compaction Temperature Note a: select a single test method and use it throughout the mix design verification 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 100 psi & 70% 300 ksi & 70% 6,000 cycles & 2.0 m/cycle OK OK No No No OK Moisture Susceptibility at Early Life Add/modify anti-stripping agent and/or other mixture components a a a WMA Sasobit® with RAP from the Iowa field project also improved in terms of wet MR stiffness and MR-ratio with the addition of hydrated lime. As shown in Table 4-2, two of the three STOA WMAs from the Montana field project (WMA Evotherm® 3G and WMA Sasobit®) did not meet the criteria for wet IDT strength and TSR, and unfortunately, data were not available to assess the effect of anti-stripping agents. If the original WMA or modified WMA still does not pass one or both criteria for the selected test, early-life moisture susceptibility is probable. To evaluate if the WMA will over- come this vulnerable period, a second evaluation is proposed after LTOA of LMLC compacted specimens for 5 days at 85°C per AASHTO R 30. After long-term aging, the same selected laboratory test is proposed but with changed criteria that reflect the stiffening effects of oxidative aging, as shown in Figure 4-1. For this second evaluation of aged specimens, only wet properties are specified for IDT strength and MR stiffness, but two criteria remain for the HWTT. If the WMA passes all criteria for the same selected test, moisture susceptibility in early life is probable and a summer of aging is needed prior to the occurrence of multiple freeze- thaw cycles or wet and cold days. If the WMA does not pass one or both criteria for the selected test, the mixture is consid- ered moisture susceptible. As shown in Table 4-2, two of the three STOA WMAs from the Montana field project (WMA Evotherm® 3G and WMA Sasobit®) did not meet the criteria for wet IDT strength and TSR, and unfortunately data were not available to assess the effect of LTOA in the laboratory or field aging with PMFC cores after a summer to assess if any possible moisture susceptibility was confined to early life. As shown in Table 4-1, one of the two STOA WMAs from the Texas field project (WMA foaming) marginally failed the criteria for MR-ratio, but after LTOA in the laboratory, this mixture passed the criteria for aged specimens. As shown in Table 4-1, one of the two STOA WMAs from the New Mexico field project (WMA Evotherm® 3G with RAP) also marginally failed the criteria for MR-ratio, but after LTOA in the labora- tory, this mixture also passed the criteria for aged specimens. Finally, as shown in Figure 4-3, if the alternative offsite PMLC specimens are used to evaluate WMA moisture sus- ceptibility, the thresholds are increased for wet IDT strength, wet MR stiffness, SIP, and stripping slope. Suggested Research This section presents suggestions for future research based on results of this project, additional analyses conducted dur- ing the project (see Appendix G), and other ideas for improv- ing moisture susceptibility evaluation of WMA. Based on the WMA laboratory-conditioning experiment, suggestions for future research are as follows: • In this study, standard laboratory-conditioning protocols to prepare LMLC specimens and PMLC specimens for perfor- mance tests were proposed based on MR results. Additional mixture properties need to be considered for validation. These properties may include performance-related prop- erties that indicate moisture susceptibility or resistance to rutting or cracking. • The effect of the total AV in the asphalt mixture specimen on mixture stiffness was verified in this study using LMLC specimens of a single WMA technology prepared with one specific conditioning protocol. Future research into the comprehensive effects of AV on the stiffness of asphalt mixtures prepared with various WMA technologies is nec- essary, with a particular emphasis on exploring the differ- ence in AV between PMFC cores and LMLC specimens and PMLC specimens. • Several WMA additives are available to reduce the pro- duction temperature of asphalt mixtures. In this study,

Aging Protocol (+anti- stripping) Conditioning/Testing Protocol Test Parameter Iowa Texas Evotherm Sasobit Evotherm Foaming LMLC STOA 2 h @ 240°F (116°C) Moisture Conditioning AASHTO T 283 Wet IDT (psi) 50 47 88 77 TSR (%) 84 77 79 66 Conclusion MS @ Early Life MS @ Early Life OK MS @ Early Life Wet MR (ksi) 133 164 281 239 MR-ratio (%) 72 77 80 62 Conclusion MS @ Early Life MS @ Early Life OK MS @ Early Life HWTT AASHTO T 324 SIP (cycles) 1,677 2,176 6,256 4,111 Stripping Slope ( m/cycle) 10.0 6.6 1.7 2.9 Conclusion MS @ Early Life MS @ Early Life OK OK LMLC STOA 2 h @ 240°F (116°C) (+ Hydrated Lime) Moisture Conditioning AASHTO T 283 Wet IDT (psi) 48 55 – 81 TSR (%) 81 92 – 77 Conclusion MS @ Early Life MS @ Early Life – OK Wet MR (ksi) 138 215 – 301 MR-ratio (%) 66 79 – 83 Conclusion MS @ Early Life OK – OK LMLC STOA 2 h @ 240°F (116°C) Moisture Conditioning AASHTO T 283 Wet IDT (psi) 41 51 – 87 TSR (%) 72 91 – 84 Conclusion MS @ Early Life MS @ Early Life – OK (+ LAS) Wet MR (ksi) 93 171 – 238 MR-ratio (%) 53 96 – 69 Conclusion MS @ Early Life MS @ Early Life – MS @ Early Life LMLC STOA 2 h @ 240°F + LTOA 5 days @ 185°F (85°C) Moisture Conditioning AASHTO T 283 Wet IDT (psi) N/A N/A – – Conclusion – – – – Wet MR (ksi) N/A N/A – 475 Conclusion – – – OK Key Does Not Meet Criteria Meets Criteria Note: MS: moisture susceptible. Table 4-1. Threshold Development for WMA Moisture Susceptibility Evaluation Based on the Iowa and Texas Field Projects.

58 result in both mixture types having adequate performance at the onset of freeze-thaw cycles and wet and cold days in the winter. The next step is to characterize any early-life weakness as compared to HMA and tie thresholds for laboratory test parameters to field performance. To date, three of the four field test sections report good performance of WMA in the field over time with respect to moisture susceptibility. LTOA methods need to be evaluated to find better ways to simulate field conditions in the laboratory for WMA with time. • The moisture-conditioning protocol is critical for charac- terizing moisture susceptibility, and the most commonly used protocol in AASHTO T 283 is severe due to vacuum saturation. Besides, achieving the target degree of saturation commonly used WMA additives were used and evaluated. Future research may include other WMA technologies and verify the general applicability of the standard condition- ing protocols proposed in this study. Based on the WMA moisture-susceptibility experiment, several issues regarding WMA remain unclear, and future research is suggested in the following areas: • Moisture affects mixtures over time, but, if with a summer of aging, WMA that is initially more moisture susceptible as compared to HMA can improve resistance to moisture damage, construction that allows for a summer of aging may Table 4-2. Threshold verification for WMA moisture susceptibility evaluation based on the Montana and New Mexico field projects. Aging Protocol Conditioning/Testing Protocol Test Parameter Montana* New Mexico Evotherm Sasobit Foaming Evotherm Foaming LMLC STOA 2 h @ 240°F (116°C) Moisture Conditioning AASHTO T 283 Wet IDT (psi) 76 74 77 81 72 TSR (%) 59 57 72 73 70 Conclusion MS @ Early Life MS @ Early Life OK OK OK Wet MR (ksi) 261 321 234 296 320 MR-ratio (%) 83 86 80 69 76 Conclusion OK OK OK MS @ Early Life OK HWTT AASHTO T 324 SIP (cycles) >20,000 >20,000 >20,000 >20,000 >20,000 Stripping Slope ( m/cycle) 0 0 0 0 0 Conclusion OK OK OK OK OK LMLC STOA 2 h @ 240°F (116°C)+ LTOA 5 days @ 185°F (85°C) Moisture Conditioning AASHTO T 283 Wet IDT (psi) N/A N/A – – – Conclusion – – – – – Wet MR (ksi) – – – 585 653 Conclusion – – – OK OK Key Does Not Meet Criteria Meets Criteria Note: MS: moisture susceptible. * For Montana, no LMLC specimens were available, and thus values of onsite PMLC specimens are used in lieu of LMLC STOA 2 h @ 240°F.

59 duce a stripping number (SN) and crack speed index that also incorporates IDT strength analysis and AV. Remaining life (LCR) in terms of number of cycles prior to stripping failure can also be determined from this repeated load test. • Adhesive bond energy between the binder and aggregate for both dry and wet conditions can be indirectly calculated from a short monotonic load test (IDT strength), a nonde- structive load test (MR), and AV, or directly calculated from cataloged surface energy values of the component materials. • Both of these alternative analyses produce indices that directly incorporate AV that greatly affects mixture charac- terization in any of the standard laboratory tests evaluated in this project. • A different shorter repeated load test (repeated direct ten- sion [RDT] test) in a nondestructive mode followed by a destructive mode allows for the incorporation of healing and determination of an endurance limit. This test and its associ- ated analysis method can also provide input for determining adhesive bond energies in both dry and wet conditions. • Extensive data generated in this project could be reanalyzed according to these proposed alternative analyses to set thresh- olds that separate the mixtures from the Iowa field projects with relatively poor performance and those from the Texas field project with relatively good performance as was done in Table 4-1 for the standard laboratory tests. Validation with the mixtures from both the Montana and New Mexico field projects could also be conducted as was done in Table 4-2 for the standard laboratory tests. Before being considered for adoption, the proposed revi- sions to the appendix to AASHTO R 35 (suggested on the basis of a limited number of field projects) should be used on a trial basis. This will provide additional data to further refine the moisture-susceptibility criteria and the laboratory-con- ditioning and -aging protocols that capture the time when WMA may be most susceptible to this type of distress. Data from additional field projects will provide increased confi- dence in the guidelines provided, along with possible revi- sions to the framework proposed in this report. In addition, further information will be gathered toward resolving any differences between generally adequate field performance and laboratory assessment that indicates potential for mois- ture susceptibility for some mixtures. Continued field per- formance monitoring of the field projects used in NCHRP Project 9-49 is also suggested in order to further improve the guidelines produced. specified in AASHTO T 283 is sometimes challenging, with some mixtures requiring only a few seconds under vacuum and others several minutes. Further research is proposed to assess the differences in saturation that result from differ- ent processes such as high relative humidity, water immer- sion, and use of the Moisture-Induced Stress Tester (MIST) equipment. The use of a relatively small container and steam could create a more realistic and faster moisture-condition- ing method where there would not be concern about pore liquid water pressure in wet specimens during testing. • Another limitation of AASHTO T 283 is that the moisture- conditioned specimens are tested in a wet condition, and concerns remain that the water still present inside the speci- mens is a source of error, especially for a repeated load test- ing such as MR. Wet specimens may need to be dried before testing so that water does not affect the behavior of the spec- imens during testing. Based on the WMA performance-evolution experiment, suggestions for future research are as follows: • For the Texas field project, compacted LMLC specimens of HMA after LTOA of 5 days at 185°F (85°C) exhibited signifi- cantly higher stiffnesses and improved moisture susceptibil- ity in MR and HWTT tests as compared to corresponding PMFC cores after summer at 8 months in service. Based on this observation, future research on LTOA protocols with shorter periods (less than 5 days) is proposed in order to produce LMLC specimens with properties more representa- tive of those for PMFC cores after summer aging in the field. • In this project, STOA of loose mix plus LTOA of compacted LMLC specimens was used to represent field aging of PMFC cores after summer aging. Future research on simulating field aging via only STOA of loose mix at higher temperatures is suggested to reduce the time required to evaluate mixtures. In addition to the suggestions for future research based on the results of the three experiments included in this project, Appendix G discusses other advanced testing and analyses conducted during the project, with promising results shown for the mixtures from the Texas field project that generated the following ideas for improving moisture-susceptibility evaluation of WMA: • An alternative analysis of the results from a repeated load test in the presence of water (HWTT) can be used to pro-