19 The inclusion of recycled asphalt shingles (RAS) in asphalt mixtures has become 20 increasingly common; however, the underlying design principles vary significantly by 21 agency. The primary objectives of this study included: (1) evaluating the ‘binder availability’ 22 concept for RAS mixtures through a carefully designed laboratory experiment; (2) 23 demonstrating a balanced mixture performance testing approach for the design of RAS 24 mixtures; and (3) evaluating the field data of RAS mixtures placed in the Midwest region 25 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 of the U.S. Three asphalt mixture designs with RAS contents of 0.0, 2.5 and 5.0%, which 26 were designed to have nearly identical volumetric characteristics, were investigated. The 27 binder availability was determined to be approximately 100% in the two RAS mixtures 28 considered. In addition, Hamburg wheel tracking and disk-shaped compact tension tests 29 were conducted to evaluate the high- and low-temperature mixture performance. As 30 expected, the addition of RAS significantly improved the rutting resistance. DC(T) test 31 results demonstrated that a soft base binder effectively permitted the design of thermal-32 crack-resistant RAS mixtures. Field investigations indicated that the performance of 33 pavement surfaces containing RAS was similar to that of surfaces containing only 34 reclaimed asphalt pavement or virgin materials. This study also highlights a performance-35 engineered mix design approach, which is currently being adopted by several agencies in 36 the Midwest (e.g. Illinois Tollway, Missouri DOT, etc.) and can provide mix designers a 37 reliable approach for designing innovative asphalt mixtures with higher recycling levels 38 and a modern, heterogeneous composition. Furthermore, the proposed approach may 39 prove to be a simpler, more mixture-centric alternative to the primary method suggested 40 in AASHTO PP78-17, which recommends arbitrary VMA bumping plus binder extraction, 41 recovery, and advanced binder testing. 42 43 Keywords: 44 Recycled Asphalt Shingles; RAS; DC(T); Cracking; Pavement 45 46 *Corresponding author. Tel.: +1 217 974 0659. E-mail address: prath@mail.missouri.edu (P. Rath). Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 1 Introduction 47 The sustainability revolution in the paving industry over the past twenty years has led to a proliferation 48 of approaches toward meeting standards and goals pertaining to asphalt mixture design, performance, 49 and sustainability. Modern asphalt mixtures are considerably more heterogeneous than previously used 50 ones, and they contain multiple recycled materials and other additives such as binder modifiers, 51 rejuvenators, and chemicals to enhance the mixture workability, performance and sustainability. 52 Sustainability in asphalt pavements, in terms of material recycling, focuses on the incorporation of 53 recycled construction materials such as recycled asphalt shingles (RAS), reclaimed asphalt pavement 54 (RAP), and ground tire rubber (GTR) into asphalt paving mixtures. RAS is attractive due to the high 55 content of asphalt cement present within the material (approximately 5 to 6 times that of RAP); however, 56 it must be carefully engineered into the mixture to account for its significant stiffness and high melting 57 point. Approximately 11 million tons of shingle waste is produced in the United States annually (CIWMB, 58 2005; CMRA, 2007). According to Brock (1998), a savings of $1 billion dollars annually could be realized 59 by engineering RAS into asphalt mixtures in a comprehensive manner (Brock, 1998). Owing to the 60 regional use of RAS worldwide (most prevalent in the Midwest and East Coast United States (Buttlar et 61 al., 2019; Rath et al., 2019; Williams et al., 2018; Buttlar and Wang, 2016)), somewhat limited research 62 and field data regarding RAS are available compared to research on RAP. 63 Asphalt shingles are composed of 20–40% asphalt, 40–70% aggregate granules, and 1–25% base 64 materials (Arnold, 2014). Before being manufactured into asphalt shingles, the asphalt binder is oxidized 65 by an air blowing process to increase the viscosity to reduce high temperature deformations. In addition, 66 considering the aging and hardening experienced during service life, tear-off shingle asphalt is 67 considerably stiffer than the asphalt used in the traditional asphalt mix design. For instance, it was found 68 that the Superpave performance grade of shingle asphalt recovered from a particular source of RAS in 69 Illinois was PG 112+2, which represents a stiffness notably higher than that of the PG 64-22 grade 70 commonly used in paving mixtures in Illinois (Lippert and Brownlee, 2012). Aggregate granules in 71 roofing shingles help to shield the asphalt from sun damage, improve the functional characteristics, and 72 facilitate color variation for architectural purposes. Ceramic granular, headlap granules, backsurfacer 73 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 sand and stabilizer are typical granular components in shingles, which possess a similar quality level 74 as fine aggregates in paving materials. Base materials, typically categorized as either organic or 75 fiberglass bases, provide a matrix to support the other component materials. 76 Early RAS studies demonstrated that the addition of RAS did not have an evident effect on the 77 volumetric properties and served to only improve the compactability. Watson et al. (1998) and Mallick 78 et al. (2000) showed that the gradation and volumetric properties of RAS-modified mixtures did not 79 change appreciably in comparison to those of the control mixtures (Watson et al., 1998; Malik et al., 80 2000). Foo et al. (1999) confirmed that voids in the total mixture (VTM), voids in the mineral aggregate 81 (VMA), and voids filled with asphalt (VFA) of RAS mixtures were similar to those of conventional HMA 82 mixtures (Foo et al., 1999). Newcomb et al. (1993) and Sengoz et al. (2005) found that the optimum 83 total virgin asphalt content for RAS mixtures reduced in comparison to control mixtures without RAS, 84 implying that a certain amount of shingle asphalt participates as a binder in the mixture (Newcomb et 85 al. 1993; Sengoz et al., 2005). 86 Binder characterization has been conducted to investigate RAS participation in the asphalt binder and 87 to characterize the effect of RAS on the binder system. Paulsen et al. (1986) evaluated roofing waste 88 from five different states (Paulsen et al., 1986). The binder test results indicated that the inclusion of 89 RAS generally decreased the penetration and increased the viscosity of virgin asphalt, depending on 90 the source. Maupin et al. (2010) found that 25% RAS binder by weight created a 2–3 Superpave PG 91 high temperature ‘grade bump,’ along with an upward bump of one PG low-temperature grade (Maupin, 92 2010). You et al. (2011) investigated the influence of 5% and 10% RAS on PG 52–34 binder (You et al., 93 2011). Bending beam rheometer test results demonstrated that the creep stiffness of the binder 94 increased with the increased usage of RAS. Similar to RAP mixtures, a softer virgin binder is typically 95 used to counterbalance the rheological effects of the stiffer RAS binder. Cooper et al. (2015) evaluated 96 the asphaltene content of the extracted binder from RAS and virgin PG 64–22 binder using gel 97 permeation chromatography (Cooper et al., 2015). As expected, the asphaltene contents of the two 98 studied RAS sources were noted to be considerably higher than that of the virgin binder, and the 99 extremely different molecular weights distribution of the associated asphaltenes from RAS and the virgin 100 binder suggested that binder compatibility should be considered. 101 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 A number of mixture performance studies have been performed to evaluate temperature 102 susceptibility, moisture susceptibility, permanent deformation, low temperature behavior and fatigue 103 cracking resistance in RAS mixes. Testing performed by Newcomb et al. (1993) showed that RAS had 104 no distinct effect on the moisture susceptibility of the asphalt mixtures investigated (Newcomb et al., 105 1993). Wu et al. (2016) conducted a Hamburg wheel tracking test to characterize the rutting resistance 106 of RAS mixtures, and the results showed that the rut depths of RAS-modified asphalt mixtures were 107 lower than those of the control mixture due to the presence of the stiffer RAS binder, which is consistent 108 with the results pertaining to binder research (Wu et al., 2016). Other studies similarly concluded that 109 RAS increases the asphalt mixture rutting resistance (Mallick et al., 2000; Maupin, 2010). 110 While RAS is generally beneficial for high-temperature rut resistance, the higher binder stiffness 111 present in RAS suggests that cracking behavior should be carefully evaluated and addressed in the mix 112 design. The disk-shaped compact tension fracture test, or DC(T) as specified in ASTM D7313, and an 113 acoustic emission technique were utilized by Arnold (2014) to examine the low temperature behavior of 114 RAS mixtures. It was found that the inclusion of RAS decreased the fracture energy and increased the 115 embrittlement temperature of the mixtures evaluated. A softer virgin asphalt binder was required to 116 restore the low temperature properties of the mixture to those of the control mixture (Arnold, 2014). The 117 fatigue cracking resistance of RAS modified HMA was studied by Cascione et al. (2015) and Wu et al. 118 (2016). Cascione et al. (2015) conducted four-point bending beam testing on several asphalt mixtures 119 and found that both RAS and non-RAS mixtures performed well with respect to fatigue performance, 120 and, in fact, the RAS modified asphalt mixtures performed slightly better than the control mix (Cascione 121 et al., 2015). Similarly, Wu et al. (2016) evaluated the fatigue performance of asphalt field cores 122 containing RAS via monotonic fatigue testing and noted that the fatigue resistance of the mixtures was 123 not significantly affected by the addition of RAS (Wu et al., 2016). This finding was consistent with field 124 surveys that showed that the test sections were in excellent condition after three years of service. 125 2 Objectives 126 The primary objectives of this RAS study were to evaluate the availability of asphalt cement from RAS 127 by considering the volumetric properties of RAS mixtures, and to evaluate the effectiveness of RAS 128 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 mixtures using laboratory performance tests. In the initial phase of the study, a total of six asphalt 129 mixtures with RAS contents of 0.0%, 2.5%, and 5.0% under two mixing regimes (standard laboratory 130 mixing, and manual, full binder blending) were designed and evaluated. These mixtures were designed 131 to have identical volumetric parameters so that their pavement performances were comparable. After 132 mixture design phase was completed, the amount of asphalt cement provided by the RAS material was 133 calculated, and the ‘available RAS binder’ percentage was computed. Finally, disk-shaped compact 134 tension (DC(T)) and Hamburg wheel tracking tests were conducted to examine how the designed 135 mixtures performed in relation to performance thresholds, for various combinations of virgin and RAS 136 binder under two mixing regimes. 137 3 RAS Mix Design Approaches and Their Implications 138 Asphalt mix designers have grappled with the question of how to treat the stiffer, higher melting point 139 asphalt materials present in RAP and RAS in the mixture design process. Research studies have 140 demonstrated that neither RAP nor RAS binders fully blend with a virgin binder in production (Buttlar 141 and Dave, 2005; Buttlar, 2005). For RAS mixtures, one of several approaches described below is 142 typically used to ‘discount’ a portion of the RAS binder. This is based on the assumption that RAS binder 143 only partially fulfills the binder’s role to coat the aggregate, to facilitate the consolidation of aggregate 144 during compaction, and to provide mixture durability. These approaches include: (a) the use of a ‘binder 145 availability factor’, or; (b) adjustment of volumetric mix design targets such as raising VMA requirements 146 or lowering the target air void level. Research by Cooper et al. (2014) determined that binder availability 147 of up to 100% from RAS is possible with the introduction of rejuvenators in a polymer modified stone 148 matrix asphalt (SMA) mixture (Cooper et al., 2014). However, research to date has not addressed the 149 asphalt binder availability in a dense-graded asphalt mixture with a grade-bumped neat asphalt binder. 150 AASHTO PP78-14 (2014), developed by a Federal Highway Administration (FHWA) asphalt mixture 151 expert task group (ETG) recommends a binder availability factor of 70% to 85% (AASHTO PP78-14, 152 2014). However, the scientific problem with this approach is that the remaining percentage of RAS 153 asphalt binder is considered as part of the aggregate skeleton, which is physically incorrect and leads 154 to arbitrary aggregate gradation and specific gravity adjustments. As a result, AASHTO PP78-17 (2017) 155 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 was modified to remove the binder availability factor; it now suggests an arbitrary VMA bump of +0.1% 156 for each percent of RAS used, along with advanced low-temperature binder testing on the recovered 157 binder (AASHTO PP78-17, 2017). The advanced low temperature binder testing suite involves the use 158 of the bending beam rheometer to obtain the so-called ‘ ΔT-critical’, or ‘ ΔTc’ parameter (AASHTO PP78- 159 17, 2017). AASHTO PP78-17 includes a provision for the designer to conduct mixture performance 160 testing in lieu of binder extraction, recovery, and advanced low temperature testing. It is possible that 161 mix designers will continue to use a binder availability approach for a number of years before AASHTO 162 PP78-17 is widely adopted. This possibility, along with the impractical and inexact nature of binder 163 extraction, recovery, and advanced low temperature testing in the mix design process, has motivated 164 researchers to evaluate an alternative approach, as presented herein. 165 A central question addressed in this study is: What exactly is ‘binder availability’ from a mix design 166 and performance standpoint? This study seeks to determine the answer through rigorous laboratory mix 167 design and performance testing. The first phase of the study involved the investigation of RAS binder 168 availability for a dense-graded mixture with a neat binder and two commonly used levels of RAS (2.5% 169 and 5% by weight of mixture). These mixtures were designed to have similar VMA, VFA, and percent 170 effective asphalt binder (P be) values. Once the mixture designs were completed, the amount of asphalt 171 cement provided by the RAS material was inferred from the volumetric measurements, and the 172 ‘available RAS binder’ percentage was determined. The second part of the study involved the evaluation 173 of the RAS asphalt mixtures via low- and high-temperature performance tests. DC(T) and Hamburg 174 wheel tracking tests were performed to examine how the designed mixtures perform in relation to 175 specification thresholds, for various combinations of virgin and RAS binder and under two mixing 176 regimes (standard laboratory mixing, and manual, full binder blending). This was accomplished to 177 evaluate a possible mixture performance testing approach for RAS mixtures, as permitted in AASHTO 178 PP78-17, as a rigorous-yet-practical approach to the standard method proposed in the standard. Lastly, 179 field data from RAS mixtures in the Chicago, IL vicinity were used to validate the proposed mixture 180 performance approach for RAS mixture design. Furthermore, the extension of this approach to modern, 181 heterogeneous mixtures with RAP, GTR and other components/additives is discussed. 182 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 4 Testing Methods and Mixture Designs 183 In this study, the development of asphalt mixture designs followed the Superpave and Illinois 184 Department of Transportation (IDOT) standards. The mixing and compaction temperatures satisfied the 185 IDOT standards for laboratory production of PG 58-28 asphalt binder. The bulk and maximum specific 186 gravity measurements for compacted asphalt mixtures followed ASTM D2726 and ASTM D4021, 187 respectively. The aggregate stockpiles were evaluated in terms of the specific gravity, absorption, etc., 188 by considering the IDOT standards. The effective asphalt cement associated with the binder from RAS 189 was determined by maintaining all other mixture variables constant and determining if additional virgin 190 asphalt binder was required to obtain 4% air voids in RAS mixtures as compared to the virgin control 191 asphalt mixtures. 192 4.1 Materials and Experimental Methodology 193 The components of the asphalt mixtures studied included PG 58-28 asphalt cement, RAS sourced in 194 Chicagoland, along with crushed dolomitic limestone coarse aggregate, crushed dolomitic limestone 195 sand and natural sand. The aggregates were sampled from Open Road Paving in Champaign, IL. The 196 RAS mixtures used in the study did not contain any RAP material to allow the evaluation of the 197 availability of the asphalt binder provided by RAS only. The mixture design phase was separated into 198 two parts: 199  First, traditional shingle mixture designs were developed to control volumetric properties such 200 as VMA, VFA, and air voids. The portion of RAS asphalt binder that is active in mixing could be 201 estimated by comparing the RAS mixtures to the virgin asphalt mixture in terms of the total asphalt 202 content of the mixture. 203  Second, initially, the RAS material was subjected to an extraction-recovery process to obtain 204 the RAS binder and RAS pulp. Subsequently, a combination of extracted RAS pulp and virgin asphalt 205 binder was added to the virgin mix instead of directly adding RAS material to the mixture. This 206 substitution and subsequent comparison with the binder content measurements of mixtures following 207 traditional design methods allowed the determination of the amount of available asphalt binder from 208 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 the RAS. 209 A detailed process flow of the abovementioned methodology is shown in Fig. 1. 210 Fig. 1 Process flow of the proposed experimental methodology. 211 4.1.1 Traditional RAS Mixture Designs 212 Mixing and compacting of asphalt mixture specimens was performed at a temperature of 150°C in 213 accordance with IDOT specifications. All aggregate and asphalt cement samples were heated for 214 approximately four hours prior to mixing to ensure temperature consistency. The RAS present in each 215 mixture sample was thoroughly mixed with virgin aggregate prior to placement in the oven to avoid RAS 216 clumping in the mixture. To develop comparable mixtures, it was ensured that the VMA, VFA, and P be 217 in the finalized designs were nearly equivalent. The final mixture design gradations are listed in Table 218 1. 219 220 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 Table 1 Mixture Design Gradations. 221 Sieve Virgin Mix 2.5% RAS Mix 5.0% RAS Mix 25.0 mm 100.0 100.0 100.0 19.0 mm 100.0 100.0 100.0 12.5 mm 100.0 100.0 100.0 9.5 mm 99.2 99.3 99.4 4.75 mm 72.9 74.6 76.3 2.36 mm 45.9 47.9 50.3 1.18 mm 27.7 29.2 31.5 0.60 mm 16.7 17.9 20.1 0.30 mm 9.4 10.9 13.4 0.15 mm 6.2 7.6 10.1 0.075 mm 5.3 6.4 8.5 222 The mixture designs considered in this study satisfied the 3 million equivalent single axle load 223 requirement, owing to the use of 90 design gyrations. The mixtures contained 0.0, 2.5, and 5.0% 224 RAS, which were respectively equivalent to an asphalt binder replacement (ABR) of approximately 225 0, 10, and 20%. The optimum asphalt content of 6.6% was obtained by changing the amount of 226 virgin asphalt content using similar aggregate blends (as presented in Table 1) until the volumetric 227 parameters were obtained. The volumetric properties of the asphalt mixtures are listed in Table 2. 228 These results demonstrate that the total asphalt content of 4.0% air voids remains constant when 229 VMA, VFA, and Pbe are maintained to be nearly constant. Therefore, the asphalt binder provided 230 by the RAS was approximately 100% active in this case. The total activity of the complete virgin 231 plus RAS binder system demonstrates that the material is completely functional as a soft binder in 232 terms of facilitating mixing and compaction. To fully evaluate the binder availability, extracted RAS 233 pulp and 100% virgin asphalt binder were substituted for the RAS product, as discussed previously. 234 It should be noted that the dust-to-asphalt ratios in some mixes slightly exceeded those mentioned in 235 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 the Superpave and IDOT criteria. This aspect was considered acceptable for the purposes of this study, 236 in which maintaining VMA and other mixture volumetrics was assigned priority. The state of Illinois 237 employs a maximum dust-to-total-binder ratio of 1.0 in design and one with a range of 0.6–1.2 during 238 mix production. The slightly high ratio helps reduce VMA such that it remains approximately equal to 239 15.3%. In practice, the control and 5% RAS mixture are likely to be designed in a slightly different 240 manner to satisfy the dust-to-asphalt specification requirement. 241 Table 2 Mixture Volumetrics. 242 Volumetric Property Mixture Virgin 2.5% RAS 2.5% Pulp 5.0% RAS 5.0% Pulp Total Asphalt Content (%) 6.6 6.6 6.6 6.6 6.7 Asphalt Binder Replacement (%) 0.0 10.6 10.6 21.2 21.2 Air Voids (%) 4.0 4.0 4.0 4.0 4.0 VMA (%) 15.2 15.3 15.3 15.2 15.4 VFA (%) 74.0 73.8 73.7 73.7 73.2 Effective Asphalt Content (%) 4.9 4.9 4.9 4.9 4.9 Dust/Total AC 0.8 1.0 1.0 1.3 1.3 Dust/Effective AC 1.1 1.3 1.3 1.7 1.7 243 4.1.2 Pulp Substitution Mixture Designs 244 This portion of the study substituted the RAS material in the asphalt mixture with RAS pulp and PG 58-245 28 virgin asphalt binder to evaluate the RAS binder availability. For e.g. the mix with 2.5% RAS 246 traditionally is modified with 2.5% RAS particles measured by the weight of the mixture; with RAS pulp 247 substitution method, the weight of binder available in 2.5% RAS particles (by weight of the mixture) was 248 computed and that much weight of the virgin PG 58-28 binder was added to the mixture with the 249 corresponding RAS pulp – in other words, this method substitutes the RAS binder with the virgin PG 250 58-28 binder in the mixture. The same proportions of virgin aggregate gradation were used as in the 251 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 traditional RAS mixture designs. This consistency in aggregate proportioning allowed for appropriate 252 volumetric comparisons between all mixtures. Once again, the optimum asphalt content of 6.6% was 253 optimized with the RAS pulp substitution till volumetric and densification parameters were met. The 254 difference between the total asphalt contents for the pulp substitution and traditional RAS mixtures 255 allowed calculation of the binder availability factor. The volumetric properties of the mixtures are shown 256 above in Table 2. 257 As seen in Table 2, even with the pulp substitution method, as asphalt content was optimized to 258 target volumetrics and densification parameters, the total asphalt content remained similar as in 259 the traditional mixing technique, indicating a binder availability factor of approximately 100%. Thus, 260 all RAS binder was active and therefore acted as asphalt binder in the mixtures. As shown in the 261 table, in the RAS pulp mixture total asphalt content was 0.1% higher than the traditional RAS 262 mixture. However, the VMA in the RAS pulp mixture was also slightly higher in this case, which 263 likely led to a slight increase in asphalt content from 6.6 to 6.7%. Had the VMA remained 264 unchanged, the total asphalt content would likely have been 6.6%, which would have yielded 265 identical asphalt contents for the 5.0% RAS and 5.0% RAS pulp mixtures. 266 The 100% RAS availability factor carefully measured in this study does not agree with the 267 suggested binder availability factor range of 70-85% recommended in AASHTO PP78-14. This 268 value is also higher than that presented in the research findings of Cooper et al. (2014). The current 269 study considers a dense graded asphalt mixture with a softer virgin asphalt grade. The results 270 found by Cooper et al. (2014) considered a polymer modified SMA, a different RAS sample, and a 271 different experimental approach, which may have led to the difference in findings (Cooper et al., 272 2014). The results obtained in this study demonstrate that 100% RAS availability is possible and 273 that the elimination of the binder availability factor in AASHTO PP78-17 was appropriate. 274 It must be noted here that there is difference between recycled binder availability and recycled 275 binder blending, and this study is only addressing the binder availability from RAS. Recycled binder 276 blending depends on variables like mixing temperature and contact (mixing) time (Rad, 2013). In 277 this study, the mixtures were carefully designed in a lab environment to achieve optimum blending 278 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 and the results show a 100% availability of the recycled binder. Thus, this study addresses the fact 279 that although incomplete blending of virgin and recycled binder likely occurs in both RAP and RAS 280 mixtures, the recycled binder is ‘available’ from the standpoint of both mixture volumetric design 281 and compaction. After all, a key contribution of the binder from a compaction and mixture volumetric 282 standpoint is its lubricating effect, which is responsible for the concave upward VMA versus asphalt 283 content curve. Although aggregate mass remains constant, additional binder (up to a point) leads 284 to additional densification of the aggregate structure via lubrication. Thus, even if incompletely 285 blended, the recycled and virgin binder appear to work together to create similar compaction and 286 volumetric characteristics as compared to virgin mixtures. This begs the question: should all 287 mixtures with soft, recycled materials, i.e., RAP binder, RAS binder, GTR, etc., be evaluated such 288 that even if a portion of the recycled material exists in an imperfectly blended state, it should 289 nevertheless be treated as a component of the binder system from a volumetric standpoint? 290 Moreover, can a uniform design procedure be developed for mixes with recycled materials? 291 Ultimately, as the industry moves towards adopting Balanced Mix Design methods, performance 292 testing is needed to determine if a mixture, when designed with standard volumetric design 293 procedures, can withstand mechanical and environmental loads (West et al., 2018). The next 294 phase of the study investigated a mixture-performance-based design alternative for RAS mixtures, 295 which was recently introduced in AASHTO PP78-17 as an alternative binder extraction, recovery, 296 and advanced binder testing to obtain ΔTc. 297 4.2 Performance Testing Methods 298 To characterize the rutting and cracking resistance of asphalt mixtures, Hamburg Wheel Tracking and 299 DC(T) fracture testing were performed. The Hamburg Wheel Tracking test was used to evaluate the 300 permanent deformation characteristics of the asphalt mixtures investigated. All gyratory specimens, 130 301 mm in height, were cut in half, and sawn along one edge to produce a flat face to produce a geometry 302 suitable for the Hamburg test (using the cylindrical geometry option). All Hamburg tests were conducted 303 until either 20,000 passes were reached or 20.0 mm of rut depth was achieved (AASHTO T324, 2017). 304 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 Wagoner et al. (2005) developed a DC(T) geometry for asphalt mixtures to create a mode-I fracture 305 (Wagoner et al., 2005). This configuration contains sufficient fractured surface area to produce good 306 repeatability and can be easily fabricated from both laboratory-compacted specimens and field cores. 307 In addition, a low-temperature cracking pooled fund study, along with research by Dave et al. (2008), 308 demonstrated that the DC(T) fracture energy parameter correlates well with the thermal cracking 309 resistance of asphalt concrete mixtures (Marasteanu et al., 2007; Marasteanu et al., 2012, Dave et al., 310 2008). For both tests, specimens were compacted to approximately 7.0% air voids to comply with 311 AASHTO T-324 standards, and four replicates per mixture were tested. 312 The Hamburg test, specified in AASHTO T-324, is conducted in a water immersed state at 50°C 313 to induce both permanent deformation and moisture damage, as shown in the Fig. 2(a). A steel 314 wheel applies a load of approximately 71kg to each specimen and external linear variable 315 differential transducers (LVDTs) measure the rut depths at regular intervals during each pass of 316 the wheel. PG 58-28 mixtures are considered satisfactory in terms of permanent deformation 317 resistance if they can withstand 5,000 wheel passes prior to reaching a 12.5mm rut depth in order 318 to conform to IDOT standards. The presence of stripping can be validated by visually examining 319 the tested material. Finally, the maximum rut depth is defined as the rut depth present at the end 320 of the test. 321 The DC(T) test evaluates the fracture energy associated with propagating a crack perpendicular 322 to the applied load through the asphalt mixture. Fracture energy can be calculated by measuring 323 the area under the load-crack mouth opening displacement (CMOD) gauge curve, shown in Fig. 324 2(b), and normalizing it by the fractured surface area. Researchers in this study tested all 325 specimens at -12°C which corresponded to the ASTM recommendation for asphalt mixtures with 326 a -22°C low temperature PG grade. Furthermore, all tests were run at the standard CMOD opening 327 rate of 1.0 mm/min (ASTM D7313, 2013). 328 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 329 Fig. 2 Performance tests: (a) Hamburg wheel tracking device; (b) Typical DC(T) load-CMOD plot. 330 5 PERFORMANCE TESTING RESULTS 331 The mixtures in this portion of the study included PG 64-22 and PG 58-28 virgin control mixtures, 2.5% 332 and 5.0% traditional RAS mixtures, and 2.5% and 5.0% RAS pulp mixtures. Unlike the mixture design 333 portion of the study, the 2.5% and 5.0% mixed binder mixes (RAS pulp mixtures) contained RAS pulp 334 plus virgin binder and recovered RAS asphalt binder with 10.6% and 21.2% ABR, respectively. This 335 was done to examine the effects of complete RAS binder blending in the asphalt mixtures and compare 336 it to the traditional RAS mixtures. The two control mixtures used in this study allowed examination of 337 the effects of binder grade bumping. 338 5.1 Hamburg Wheel Tracking Test Results 339 As shown in Table 3, the virgin asphalt mixtures did not meet the Hamburg testing requirement 340 developed by IDOT. On the other hand, the RAS mixtures both exceeded the minimum requirement of 341 5,000 wheel passes prior to reaching 12.5 mm (1/2”) rut depth. The improvement in Hamburg wheel 342 tracking performance from 2.5% RAS to 5.0% RAS showed that an increase in ABR (from RAS) from 343 10.6% to 21.2% significantly improved rutting resistance. 344 The 2.5% and 5.0% RAS mixed binder Hamburg results demonstrate the increased rutting 345 resistance created by artificial, complete mixing of virgin and extracted RAS binder. Research by 346 Mogawer et al. (2013) found that the complex modulus of traditional RAP-RAS mixtures yielded a 347 lower modulus as compared to a fully mixed RAP-RAS binder mixture (Mogawer et al., 2013). 348 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 Thus, if it happens that unblended RAS binder continues to mix with virgin binder during service 349 life, this phenomenon would have a positive effect on rutting resistance. 350 Table 3 Hamburg Wheel Tracking Test Results. 351 Mixture No. of Passes to Failure (12.5mm) Required Passes Pass/Fail Virgin PG 58-28 3030 5000 Fail Virgin PG 64-22 5860 7500 Fail 2.5% RAS PG 58-28 5110 5000 Pass 5.0% RAS PG 58-28 14,430 5000 Pass 2.5% RAS Mixed Binder 7370 5000 Pass 5.0% RAS Mixed Binder 15,850 5000 Pass 5.2 DC(T) Fracture Test Results 352 The DC(T) results are shown in Fig. 3. The PG 58-28 virgin asphalt binder was used in this study to 353 evaluate the counterbalancing effect of using a softer base binder with RAS. The use of PG 58-28 and 354 either level of RAS (2.5% or 5.0%) led to passing results in the DC(T) test. As shown, all three PG 58-355 28 mixtures met the medium traffic threshold developed during the FHWA Pooled Fund Low 356 Temperature Cracking of 460 J/m2 (Marasteanu et al., 2012). Furthermore, the RAS mixtures 357 outperformed the PG 64-22 virgin asphalt mixture in terms of fracture energy. It was interesting to note 358 no major difference between the fracture energies of 2.5% and 5.0% RAS-modified mixtures. Perhaps, 359 testing at a lower temperature would bring out differences in those mixtures. 360 Additional DC(T) tests (shown with red bars) were completed on mixtures containing RAS pulp 361 and mixed virgin plus extracted RAS binders. In both of these RAS mixtures produced with 362 idealized, complete binder blending (for the sake of research) led to decreases in fracture energy, 363 as expected. However, the resulting fracture energies measured still met the low traffic level DC(T) 364 requirement of 400 J/m2. Furthermore, the mixed binder mixtures exhibited higher DC(T) fracture 365 energy as compared to the reference virgin PG 64-22 mixture. Thus, even if the asphalt binders 366 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 achieve complete blending with time in the field, grade bumping in this case allowed the mixtures 367 to meet the bracketed performance requirements for the Hamburg and DC(T). 368 369 Fig. 3 DC(T) test results at -12°C. 370 This combination of the Hamburg and DC(T) test represents a possible mixture performance testing 371 approach for RAS mixtures, as permitted in AASHTO PP78-17. This would provide a rigorous-yet-372 practical, mixture-centric alternative to the primary method suggested in AASHTO PP78-17 (arbitrary 373 VMA bump plus extraction, recovery and advanced binder testing/analysis to obtain ΔTc). The use of a 374 mandatory VMA bump specified in AASHTO PP78-17 should be revisited and, if deemed appropriate, 375 dropped. Rather, the mix designer should be given the flexibility to lower the void target, increase VMA, 376 etc., for RAP or RAS mixes, as needed to pass mixture performance test result criteria. 377 The extension of this approach to modern, heterogeneous mixtures with RAP, GTR, and other 378 components/additives is suggested by the results obtained in this study as well as others studies (Buttlar 379 et al., 2019; Rath et al., 2019; Buttlar and Wang, 2016). Currently, RAP and RAS mixtures are designed 380 using different principles by most agencies, for instance, using a binder availability approach or VMA 381 bumping plus advanced binder testing for RAS, but not RAP mixes. The approach explained herein 382 provides a simple, unified design system involving classic mixture volumetric calculations with recycled 383 materials, along with high and low temperature performance tests. The designer may wish to target a 384 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 slightly lower design air void level for high ABR mixes, perhaps 3.0 or 3.5%, applying this principle for 385 mixtures containing both RAP and RAS. 386 387 6 Field Observations 388 In 2014, a 2.5” lift of N30 surface course was paved as an overlay on an existing pavement at a 389 pavement recycling facility in Pulaski, Illinois, near Chicago. The mixture featured nearly two-thirds 390 (66.2%) ABR, which achieved by using 45.3% RAP and 7.5% RAS in the mixture. A very soft base 391 binder (PG 46-34) was used to offset the stiffness imparted by RAP and RAS, and the mixture was 392 designed at 3% air voids to enhance durability. Bracketed performance testing was added to the mix 393 design stage, which involved the aforementioned Hamburg and DC(T) tests. After three years in service, 394 three specimens were fabricated from field cores and tested at -12oC using the DC(T) fracture test to 395 evaluate residual thermal cracking resistance in this highly recycled mixture. The DC(T) reported an 396 average fracture energy of 564 J/m2, with a coefficient of variation of 6.0%, which easily met the 460 397 J/m2 threshold for medium-traffic roads (Marasteanu et al., 2012). A visual examination of the 250-meter 398 test section (Fig. 4) revealed only one transverse crack and two localized areas of load-induced 399 cracking, both likely caused by reflective cracking mechanisms. The section receives heavy truck 400 loading, as fully loaded trucks enter and leave the asphalt and concrete recycling center at regular 401 intervals, carrying 20 or more tons of payload. 402 A study conducted by Buttlar (2014) investigated 12 mixtures placed in locations with low, medium, 403 and high traffic levels and varying age (between 1 and 7 years of service) around Chicago and northern 404 Illinois (Buttlar, 2014). Of the 12 mixtures, six mixtures contained both RAP and RAS, while four 405 contained only RAP recycling content and two used only virgin materials. Similar trends in DC(T) 406 fracture energy values, as measured from field cores taken in 2014 were found, and similar field cracking 407 performance of the various mixture investigated was observed in 2014 field performance evaluations. 408 This demonstrates that careful design of mixtures with RAS can lead to good performing, highly 409 sustainable pavement surfaces, even in demanding climates such as the Midwest United States. The 410 long-term performance of these and more newly constructed sections will continue to be monitored. 411 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 The performance-engineering mix design approach presented herein is being formalized and 412 specified in various agencies in the Chicago area and Midwest, and appears to provide mix designers 413 and agencies a reliable approach for designing innovative asphalt mixtures with higher recycling levels, 414 and moreover, modern, mixtures with heterogeneous binder and mixture compositions. 415 416 Fig. 4 Surface condition of Pulaski test site in July 2017, after 3 years of service. 417 7 Summary and Conclusions 418 This study evaluated mixture designs and performance results for virgin, 2.5% and 5.0% RAS modified 419 asphalt mixtures, which allowed evaluation of the binder availability concept. The test results also 420 demonstrated a robust and attractive mix performance testing approach that can be accomplished by 421 following an allowed provision in the recently approved AASHTO PP78-17 document. A careful 422 volumetric mix design process was used, involving binder extraction and recovery of RAS binder and 423 RAS pulp, along with careful blending and volumetric analysis of virgin and recycled mixtures. 424 Performance testing in the Hamburg wheel tracking and DC(T) device was performed, and field 425 performance data was presented. Several conclusions can be drawn from the findings reported herein: 426 1. The asphalt binder available from RAS was found to be approximately 100%. This result was 427 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 arrived at by creating mixture designs with equal VMA, VFA, and P be and virtually identical total 428 asphalt contents. This supports the decision to drop the binder availability concept in the new 429 AASHTO PP78-17 document. 430 2. The inclusion of RAS led to improvements in Hamburg wheel tracking performance. In 431 particular, relatively weak virgin aggregates such as those used in this study benefitted from the 432 inclusion of RAS to meet the wheel tracking requirements. 433 3. RAS mixtures met and exceeded the 460 J/m2 requirement from DC(T) testing for low and 434 medium traffic roads, and exceeded the fracture energy of the PG64-22 control mixture, with the use 435 of a PG 58-28 virgin binder (softer base binder). The RAS mixtures in the current study were the only 436 mixtures which passed the bracketed performance thresholds set for Hamburg and DC(T) tests. 437 4. This study also serves to demonstrate a balanced mix design approach that is being 438 formalized and now specified in various agencies in the Chicago area and Midwest. This appears to 439 provide mix designers and agencies a reliable approach for designing innovative asphalt mixtures with 440 higher recycling levels, and moreover, those with modern, heterogeneous binder and mixture 441 compositions. This also serves as a simpler, and more mixture-centric alternative to the primary 442 method suggested in AASHTO PP78-17, which entails arbitrary VMA bumping plus binder extraction, 443 recovery, and advanced binder testing to obtain ΔTc. 444 In light of these findings, it is recommended that the next revision of AASHTO PP78 be revised to 445 give priority to the mixture performance-based design approach over the arbitrary VMA bumping and 446 advanced binder testing approach, and not vice-versa. The use of a mandatory VMA bump should also 447 be dropped; rather, giving the mix designer flexibility to lower the void target, increase VMA, etc., for 448 RAS mixes, as guided by mixture performance tests. In addition, it is recommended that designers of 449 modern, heterogeneous asphalt mixes containing RAP, RAS, GTR, and other additives begin to employ 450 modern mixture performance tests as a supplement to volumetric mix design, and that, uniform 451 procedures be developed. Currently, RAP and RAS mixtures are designed using different principles by 452 most agencies, for instance, using a binder availability factor approach or VMA bumping plus advanced 453 binder testing for RAS, but not RAP mixes. The approach explained herein provides a simple, unified 454 design system involving classic mixture volumetric calculations in the design of mixes with recycled 455 Preprint submitted to Journal of Traffic and Transportation Engineering (English Edition) Accepted on 28th September 2019 materials, along with high and low temperature performance tests. The designer may wish to target a 456 slightly lower design air void level for high ABR mixes, perhaps 3.0 or 3.5%, applying this principle in 457 an identical fashion for mixtures containing RAP and/or RAS. 458 Conflict of interest 459 The authors do not have any conflict of interest with other entities or researchers. 460 461 Acknowledgments 462 The authors would like to acknowledge Dr. Brian Hill for his assistance in mixture testing while studying 463 at the University of Illinois. We would also like to thank our colleagues at Southwind RAS, for providing 464 all the RAS materials, and colleagues at S.T.A.T.E. Testing, LLC, for their assistance in wheel track 465 testing and interpretation of mix design results. The findings and opinions contained herein are those of 466 the authors, and not necessarily those of the sponsors. 467