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141 Findings Production and Construction of RAS Mixtures Production and construction information was collected from three WMAâRAS projects built prior to the start of this study and five new WMAâRAS projects constructed and monitored during the course of this study. Findings with regard to production and construction are as follows: â¢ Lower mix production temperatures associated with WMA did not cause any plant issues or construction problems for any of the projects evaluated in this study. There were no problems with burners, baghouses, motor amperages, or mix storage. Excellent mixture coating was achieved with all WMA technologies at the lower mixture production temperatures. â¢ Ignition Oven tests and chemical extractions using TCE were performed to determine the asphalt content of all mixtures obtained during construction. On average, the ignition method indicated a 0.1% higher asphalt con- tent than the solvent extraction method. Differences in asphalt content between the two methods on any project can be influenced by inaccurate correction factors caused by recycled materials and natural variations in virgin aggregates. â¢ In almost all cases, using the same roller patterns resulted in statistically equivalent as-constructed densities for WMA mixes compared to the corresponding HMA, even for projects using much lower temperatures for WMA. Only one of the eight HMA to WMA comparisons had an average as-constructed density of the WMA section statistically higher than its corresponding HMA. This finding is consistent with other research comparing HMA and WMA on projects at numerous sites across the U.S. (West et al. 2014). â¢ For the project that included a comparison of mixtures con- taining PCâRAS and MWâRAS, the mixture with PCâRAS had a statistically higher as-constructed density than the mixture containing MWâRAS. However, this observation was based on limited data from a single project and does not support a broad conclusion with regard to PCâRAS versus MWâRAS mixture densities. Short-Term Field Performance of RAS Mixtures Field performance of all the projects was assessed over the first few years after construction. For the three existing projects, inspections were conducted 24 to 35 months after construction. For the five new projects, inspections were conducted 16 to 29 months after construction. Findings with regard to short-term field performance are as follows: â¢ For all projectsâexisting and newâWMA sections per- formed the same as corresponding HMA sections with regard to rutting. All of the field projects had less than 5 mm of rutting after 2 years of traffic. â¢ None of the test sections had significant amounts of ravel- ing. All of the test sections had similar surface texture depths and texture change after 2 or more years of traffic. â¢ None of the field projects had any evidence of moisture damage. Cores taken from the projects after 1 to 2 years of traffic were inspected for visual evidence of stripping. â¢ Overall, there was a statistical increase in density for the test sections because of traffic. However, the use of WMA did not appear to affect density changes under traffic com- pared to HMA. â¢ Four of the five new projects had minor amounts of low- severity transverse cracking, but many of these cracks were apparent reflection cracks. The three existing proj- ects had different types of cracking after about 3 to 4 years. However, the WMA sections were performing slightly better than the corresponding HMA sections with regard to cracking. C H A P T E R 7
142 Engineering Properties of RAS Mixtures For this study, 15 RAS mixtures (eight WMA and seven HMA mixtures) from the five new projects were tested using several laboratory performance tests to evaluate cracking and rutting susceptibility. In addition, eight WMA to HMA com- parable pairs were statistically analyzed. Findings on engineer- ing properties and laboratory performance are as follows: â¢ Asphalt content of mixtures obtained during construction showed that there were some differences between WMA and HMA mixtures. Although the average difference in asphalt content was only 0.11%, WMA mixtures from the Alabama and Tennessee projects had 0.4% to 0.5% higher asphalt content than their companion HMA mixtures. â¢ Testing of recovered binders from mixes obtained during construction generally showed that the WMA binders had aged slightly less than the corresponding HMA binders. The average difference in the high critical temperatures between HMA and WMA binders recovered from plant-produced mixtures was 1.1Â°C, and the average difference for the low critical temperatures was 0.97Â°C. Such small differences would not be expected to significantly impact pavement performance. â¢ All extracted binders met Level âEâ traffic grade, the high- est traffic grade according to AASHTO M 332. This indi- cates that the mixtures will not be susceptible to rutting under extremely heavy traffic. â¢ Linear Amplitude Sweep Test results on extracted binders from WMA and HMA pairs were not consistent with regard to fatigue resistance. For five of eight WMA to HMA com- parisons, the asphalt binder extracted from HMA mix- tures had higher cycles to failure than binders from WMA mixtures. â¢ Dynamic moduli of WMA mixtures were statistically lower than those of corresponding HMA mixtures in most cases. â¢ For the project that included two types of RAS (MW and PC), no statistical differences in dynamic moduli values were found at Î± = 0.05. â¢ Hamburg Wheel-Tracking Test results showed that WMA mixtures had statistically higher rut depths than HMA mixtures. However, all of the mixtures passed the Texas DOT Hamburg Wheel-Tracking Test criteria. RAS type (MWâRAS versus PCâRAS) did not significantly impact the Hamburg Wheel-Tracking Test results. â¢ Flow Number Test results for plant-produced WMA mix- tures were statistically lower than corresponding HMA mixtures in half of the comparisons. All of the mixtures met the recommended criteria for the expected traffic of the respective projects. â¢ Based on four laboratory mixture cracking tests, most WMA mixtures appear to be more or equally resistant to cracking than their corresponding HMA mixtures. Comparisons of some HMAâWMA pairs may have been influenced by the differences in asphalt content between the mixtures. Bending Beam Fatigue Test results were not statistically different for WMA and HMA from the same project in seven of the eight comparisons. Overlay Test results of WMA mixtures were higher than corresponding HMA mixtures in every case, but only statistically higher in three of the eight comparisons. Overall, energy ratio, flexibility index, and semi-circular bendâJc results were not statistically different for WMA and HMA mixtures, based on paired t-tests. However, six of the eight flexibil- ity index project comparisons were statistically different (higher flexibility index values for WMA mixtures), based on Tukeyâs Test. â¢ The Indirect Tensile Creep Compliance and Strength Test conducted to evaluate thermal cracking potential indicated that WMA mixtures generally had a very small improve- ment in low-temperature cracking compared to their corresponding HMA mixtures. â¢ A hierarchical clustering analysis indicated that the 15 mix- tures could be grouped into three different clusters, based on engineering properties. Cluster 1 included mostly WMA mixtures. Cluster 2 included a combination of WMA and HMA mixtures. Finally, Cluster 3 included HMA mix- tures only. â¢ Analysis of dynamic modulus master curve parameters indicated that the inflection point of the master curve and peak dynamic moduli values and phase angles from black space diagrams generally agree with results obtained from other laboratory cracking tests. â¢ I-FIT was conducted on cores from the existing and new projects at the last inspection. For the existing projects, the pavements were 3 to 4 years old; for the new projects, the pavements were 1 to 2 years old. For both existing projects in Texas, the flexibility index results on the cores were less than 1.0, which indicates that the pavements are highly suscep- tible to cracking. Cores from the existing project in Illinois had flexibility index values of 7.9 and 10.1, which would be marginal to good according to Illinois DOTâs preliminary minimum criteria of 8.0. The Illinois project was the only project in this study to use stone matrix asphalt mixtures. Of the new projects, flexibility index results ranged from 0.11 to 5.92. Like the cores from Texas, all of the Alabama pavements had flexibility index results less than 1.0. Field performance and flexibility index results on field cores indicate that WMA and HMA mixtures are generally not substantially different. Mix Design Verifications of RAS Mixtures Slight differences in the optimum asphalt content were found for all mixtures compared to the contractorâs mix designs. Five of the six mix design verifications had higher optimum asphalt
143 content, ranging from 0.1% to 0.6% more asphalt binder. Some of the differences between contractor and NCAT mix designs were caused by differences in aggregate specific gravities. In most cases, NCAT Gsb values were lower than contractorâs reported values. But all differences were less than 0.030, which is considered reasonable for multilaboratory comparisons. Consistent RAS aggregate Gsb results between different labo- ratories can also be difficult because of the small size of RAS aggregate particles. AASHTO PP 78-17âthe latest revision of the provisional standard for designing mixtures containing RASârequires an increase in the minimum VMA to account for RAS binder that does not activate during the mixing process. The mini- mum VMA requirement is increased by 0.1% for 1% of RAS by weight of total aggregate. Only one of the verified mix designs did not meet the adjusted VMA criteria. However, two of the verified mixtures had VMA results more than 1.5% above the minimum criteria. Mix designs with failing VMA or excessive VMA would have to be altered by adjusting the aggregate blends. None of the binders recovered from the mix design veri- fication samples met the â5.0 DTc requirement for 40-h PAV aged recovered binders in AASHTO PP 78-17. Meeting this criterion would require a reduction in the RAS content, use of a virgin binder with strongly m-controlled low-temperature properties, and/or a well-formulated rejuvenator. Some data in this study showed that PG XX-28 virgin binders with higher DTc values (strongly m-controlled) resulted in better DTc values for the binders extracted from the plant-produced RAS mixtures. However, those results were based on the standard 20-h PAV. The new DTc criteria, based on 40-h PAV, could greatly restrict the use of RAS in asphalt mixtures. Economic Analysis of Asphalt Mixtures Containing RAS The use of RAS is most economically favorable when con- tractors are able to maximize tipping fees and use higher percentages of RAS, when RAP content is not impacted, and when virgin asphalt prices are relatively high. Highway agen- cies are more likely to realize an economic benefit when com- petition among contractors is good, and the performance of mixtures containing RAS is equal to or better than mixtures without RAS. Other Studies Additional analyses of RAS and RAS mixtures conducted in this study are summarized in Appendix B and Appendix C. Appendix B summarizes experiments using differential scanning calorimetry analysis to assess the phase change char- acteristics of the RAS particles compared to a virgin asphalt binder. Phase changes of RAS particles were only evident at temperatures well above the normal mixing temperatures for an asphalt plant. At temperatures up to 150Â°C, partial melt- ing of RAS binder was evident. In addition, scanning electron microscopy (SEM) was used to assess how well RAS was dis- persed in a plant-produced asphalt mixture. In this analysis, inorganic fibers in the RAS were found to provide a unique indicator of RAS dispersion. However, this technique was not practical to quantify dispersion of RAS in an asphalt mixture or to indicate if the RAS binder is activated. Appendix C provides a summary of a parallel research study aimed at evaluating the activation of shingle asphalt binder using three experiments. The first experiment involved laboratory mixing heated aggregate and ambient tempera- ture RAS without any additional asphalt to determine if any RAS binder was transferred to the virgin aggregate. The sec- ond experiment evaluated the compactability of laboratory- prepared mixtures containing 5% RAS at temperatures ranging from 250Â°F to 350Â°F. The third experiment involved laboratory performance tests to evaluate the effect of mix- ing and compaction temperatures, the effect of separate RAS components on mixture properties, and laboratory versus plant mixtures with and without RAS. The results of this study indicate that increasing the mixing temperature to better activate RAS binder results in increased mix stiffness and negatively affects cracking resistance. Increasing the mix- ing time and/or storage time may additionally increase activa- tion of the shingle asphalt. Finally, further aging or increased mixing temperatures of laboratory-produced mixtures con- taining RAS may be needed to match plant-produced proper- ties better. General Conclusion and Proposed Implementation Actions Overall, no detrimental effect of using WMA technologies with mixtures containing RAS was observed. In the labora- tory, mixtures containing RAS and using a WMA technol- ogy had lower stiffness than companion HMA mixtures, and some tests showed an improvement in cracking resistance for RAS mixtures using WMA technologies. Analysis of other cracking test results found that there was no statistical dif- ference between the HMA and WMA mixtures. In the field, the short-term performance of HMA and WMA mixtures containing RAS is practically the same. It is suggested that the field projects be reexamined at 4 to 5 years and the field performance be compared to the laboratory tests and their preliminary or recommended criteria. Based on the literature review and the research study described in Appendix C, there is still no clear method to determine the degree to which RAS binder activates and becomes an integral part of the composite binder in an asphalt mixture. Some evidence indicates that at least par- tial activation of the RAS binder increased by higher mixing
144 temperatures and longer mixture storage time. Other factors likely include the stiffness of the RAS binder, sizes of the RAS particles, and chemical properties of the virgin binder and/or rejuvenator or other additives. Recent modifications to the AASHTO provisional practice for design of asphalt mixtures containing RAS (AASHTO PP 78-17) included several changes, based on the best infor- mation available at the time, that were intended to improve the durability of such mixtures. The two most significant changes in the standard tried to address the quantity and the quality of the composite binder. The quantity of composite binder was addressed by requiring an increase in the VMA proportional to the amount of RAS being used in the mixture. In theory, this should result in a slightly higher virgin binder content for the mixtures, thereby improving their durability. To address the quality of the composite binder, the parameter DTc was introduced with a recommended minimum crite- rion of â5Â°C. DTc is an indicator of how brittle or ductile the binder is using the common Bending Beam Rheometer Test. Some experts question the validity of the DTc parameter and criterion, based on a number of concerns. First, testing and analysis of extracted binders (such as PG grading, black space diagrams, linear amplitude sweep, and DTc) is viewed as a flawed approach since the extraction process causes full blending of virgin and recycled binders, even though they may not actually be fully blended in the mixture. A second concern is that there is very limited field data to support the â5Â°C criteria. A third concern is that the aging of the binder using 40-h PAV or mixture aging of 24 h at 135Â°C is too harsh, especially for mixtures used in lower layers of a pavement structure. In this study, none of the binders recovered from the mix design verification samples met the â5.0 DTc require- ment for 40-h PAV aged recovered binders. The new DTc cri- teria could greatly restrict the use of RAS in asphalt mixtures. Notes in AASHTO PP 78-17 provide three options for agencies to consider with regard to the DTc parameter. First, Note 7 gives specifying agencies the option to adjust the â5Â°C criteria, based on local experience. Second, Note 13 allows mix designers to avoid the DTc testing and analysis when the RAS binder ratio is less than 0.10 (which is typically about 3% RAS by weight of aggregate). This study provides no results to support these exceptions. Further research is needed to vali- date, disprove, or refine the DTc parameter and criterion for use in mix design. Note 9 provides a third option, which is to require mixture performance testing in lieu of the binder testing for DTc. Most asphalt mix design experts agree that the ultimate solution to solving many of the unknown impacts of recycled materials and other additives on asphalt mixtures is to imple- ment the use of mixture performance tests in mix design and quality assurance. Although a few state DOTs have already implemented a balanced mix design approach with some of the performance tests used in this study, most highway agencies are uncertain as to which performance tests and what criteria should be used in their specifications. NCHRP Project 20-07, Task 406, is currently developing a framework for balanced mix design and identifying research to address knowledge gaps. Several research studies are also underway to help deter- mine which performance tests provide a good relationship to field performance and are suitable for implementation. An updated Best Practices for Processing Recycled Asphalt Shingles (RAS) document is provided in Appendix D. This document provides current suggested practices for handling and processing RAS for use in asphalt mixtures. This docu- ment covers topics such as collection of RAS, minimizing deleterious materials, stockpiling of waste shingles prior to processing, grinding, testing, and stockpiling processed shingles.