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3 Recent Relevant Literature This section provides a review of recent literature on field performance of WMA pavements, laboratory moisture suscep- tibility tests, and moisture conditioning procedures. The infor- mation gathered was considered when designing the laboratory experiment for evaluating additional moisture conditioning protocols and specimen-drying methods (Chapter 5). A study by Azari and Mohseni (2013) evaluated a new prac- tice for determining resistance of asphalt mixtures to moisture damage. A number of shortcomings for the AASHTO T 283 test were identified including long testing time, high sample- to-sample variability, inappropriate moisture conditioning components (i.e., vacuum saturation and freeze-thaw cycle), and non-uniform moisture conditioning due to specimen shape and size. To overcome these shortcomings, an experi- ment was performed to investigate sample shape and size for improved moisture accessibility, evaluate conditioning meth- ods to improve effectiveness and reduce conditioning time, and explore the use of a different mechanical test to remove sample-to-sample variability. The incremental repeated load permanent deformation (iRLPD) test was proposed to evaluate moisture susceptibility of asphalt mixtures by comparing the permanent deformation before and after moisture condition- ing. The iRLPD test is damaged-based, and thus, the level and duration of the load applied during the test is selected to avoid failure. This allows running the test on the same sample before and after moisture conditioning, significantly reduc- ing sample-to-sample variability. The Minimum Strain Rate (MSR), which is the parameter obtained from the iRLPD test, shows high sensitivity to moisture induced damage. Laboratory test results indicated that smaller specimens (i.e., 100-mm IDT and 150-mm semi-circular bend [SCB]) were preferable over the 150-mm IDT specimens for the follow- ing reasons: a better distinction between moisture-susceptible and moisture-resistant mixtures, a lower load level required for achieving damage, and a greater number of specimens obtained from a single gyratory compacted specimen. In addition, the following two proposed moisture conditioning protocols showed effectiveness in causing moisture damage: (1) 30-minute vacuum suction at 15 mmHg followed by a 300-cycle incre- ment of repeated load and (2) 3,500-cycle Moisture Induced Stress Tester (MIST) at 104°F (40°C) and 40 psi. Finally, com- plete drying of the moisture conditioned specimens before the mechanical test was recommended to reflect the true weak- ening of mixtures due to moisture conditioning, as water present in the pores of the mixture with incomplete drying artificially increased the specimen resistance to applied load. Schram and Williams (2012) also indicated that agencies specifying the IDT strength test per AASHTO T 283 for field acceptance had logistical and practical challenges, including unavailability of a compression machine, tedious condition- ing processing, and poor correlations with field pavement performance. Therefore, an alternative moisture susceptibil- ity test was needed that had good repeatability and allowed for prompt reporting of results. In their study, laboratory tests including dynamic modulus (E*), flow number (FN), IDT strength, HWTT, and MIST were performed on PMLC specimens collected from 13 WMA pavements. The moisture susceptibility parameters obtained from the laboratory tests for all mixtures were ranked and compared against the rank- ing based on the field pavement performance. The difference in ranks for laboratory test parameters and field pavement performance was used to quantify the effectiveness of each moisture susceptibility parameter. Test results indicated that the percent swell after MIST and the submerged FN were the most effective moisture susceptibility parameters, fol- lowed by HWTT parameters including ratio of stripping slope over creep slope, stripping inflection point, stripping slope, and creep slope, and TSR after MIST. Considering the turn- around time and simplicity, the MIST and HWTT tests were recommended for further evaluation as alternatives to the IDT strength test per AASHTO T 283. Another study by Bennert (2010) evaluated the moisture damage potential of WMA mixtures. Higher potential was identified in WMA as compared to HMA due to its method of C H A P T E R 2
4production including inadequate drying at reduced production temperature and introduction of water in the foaming process. For the first WMA project implemented by New Jersey DOT, a TSR of 88% was obtained for Sasobit-modified WMA mix design specimens, while a significantly lower value of 56% was shown by the field cores obtained after construction. The dif- ference in TSR values between laboratory specimens and field cores was attributed to differences in the moisture content of the aggregates. To address this issue, a modified laboratory mix- ing procedure was proposed to better simulate plant produc- tion that involved utilizing predetermined moisture content and drying aggregates with a propane ârosebudâ torch. Addi- tionally, various moisture conditioning and testing methods were investigated in the study for moisture damage evaluation. Laboratory test results indicated that the 4,000-cycle MIST at 104°F (40°C) and 40 psi produced equivalent moisture damage to the modified Lottman protocol per AASHTO T 283. A more recent study by El-Hakim and Tighe (2014) evalu- ated the impact of freeze-thaw cycles on mechanical proper- ties of asphalt mixtures. Four different asphalt mixtures were obtained from Highway 401 in southwestern Ontario, Canada, in the first year of service. E* testing was performed on those mixtures and corresponding E* master curves were con- structed. Afterwards, the specimens were stored at the Center for Pavement and Transportation Technology test track and subjected to one complete winter of freeze-thaw cycles prior to retesting. Test results showed a significant reduction in E* values of all mixtures after one winter exposure in the Canadian cli- mate, especially for E* values at 14°F (-10°C) and 39°F (4°C). Based on the results obtained, the authors recommended that the effect of freeze-thaw cycles on mixture property deteriora- tion should be considered for developing perpetual pavements with adequate performance. Kentucky Transportation Center surveyed 12 southeastern states regarding the use of WMA technologies and performance evaluation of WMA as compared to HMA in that region of the United States. According to the survey results (Graves, 2014), WMA technology had been used in all of the southeastern states and certain changes in standard specifications and special provi- sions were made to permit the use of WMA technologies. Labo- ratory experience with IDT strength test per AASHTO T 283 and HWTT per AASHTO T 324 indicated that WMA mixtures, in most cases, exhibited slightly higher moisture susceptibility than HMA mixtures; however, no moisture-related pavement distress had been observed to date on any of the WMA pave- ments placed in those states. NCHRP Project 9-47A âProperties and Performance of WMA Technologiesâ evaluated the field performance of WMA technologies (West et al., 2014). Field cores after construction and plant loose mix were sampled from 6 existing and 8 new pavements. Each of the pavements included a HMA control section and at least one WMA section. For moisture suscepti- bility evaluation, the IDT strength test per AASHTO T 283 was performed on field cores and PMLC specimens. For most cases, the IDT strengths of WMA and HMA field cores obtained after construction were not significantly different, and remained statistically equivalent through the first 2 years of service. How- ever, a different trend was observed for IDT strength results on PMLC specimens, where the IDT strengths were statistically lower for WMA as compared to HMA for more than half of the comparisons. For the TSR results, 27 out of 33 mixtures passed the standard minimum criteria of 80%, and only two mixtures would have failed a TSR limit of 75%. Since all pavements have performed well with no evidence of moisture damage observed to date, West et al., recommended that the TSR specification be reduced from 80% to 75%.
