Long-Term Field Performance of Warm Mix Asphalt Technologies (2017)

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3 Background Increased awareness of environmental sustainability has encouraged the use of low energy and low emission technolo- gies, such as WMA, in the pavement industry. Compared with traditional HMA, WMA technologies can reduce the mixing and compaction temperatures by approximately 30°C to 50°C (55°F to 85°F), resulting in lower energy consumption and reduced emissions of greenhouse or other gases harmful to the environment (D’Angelo et al. 2008). The use of WMA technologies can offer many other benefits, such as creating a better work environment; helping to achieve appropriate mixture densities, especially for cold season paving and long hauling distances; and improving the overall sustainability of the transportation industry. Numerous WMA technologies are available in the mar- ket. Generally, they can be categorized into three groups: (1) organic additives (e.g., Sasobit®); (2) chemical additives (e.g., Evotherm® and Rediset WMX®); and (3) water-based foaming products [e.g., Astec Double Barrel Green® (DBG) and AquaBlack®] and water-containing foaming products (e.g., Advera® and Aspha-min® zeolite). Many researchers and state departments of transportation (DOTs) have conducted studies of WMA pavements with respect to both laboratory and short-term field performance (Aschenbrener et al. 2011; Cooper 2009; Corrigan et al. 2010; Diefenderfer and Hearon 2008, 2010; Epps et al. 2016; Estakhri et al. 2010; Hurley and Prowell 2005, 2006a, 2006b; Jones et al. 2008; Kvasnak et al. 2010; West et al. 2014; and Bower et al. 2016). National studies of WMA have focused on the mixture design (Bonaquist 2011), short-term engineering properties and performance as well as environmental emissions (West et al. 2014), moisture susceptibility (Martin et al. 2014), charac- teristics of foamed WMA technologies (Newcomb et al. 2015), and WMA best practices (Prowell et al. 2012). The decision to adopt a particular technology, such as WMA, in pavement engineering should be based ultimately on long-term field performance, which can only be observed after many years of service. In addition, with the increased usage of WMA across the nation, ascertaining significant determinants of field performance and identifying the best practices for the use of WMA are of paramount importance for designing a successful WMA pavement. Objectives and Scope Objectives The following are the overall objectives of this project: • Evaluate the long-term field performance of WMA by comparing distresses (transverse cracking, wheel-path lon- gitudinal cracking, and rutting) between HMA and WMA pavements. • Determine significant determinants for each type of dis- tress by comparing the HMA and WMA field performances of various mixtures and asphalt engineering properties. • Propose best practices for the use of WMA technologies. Scope This study incorporates 23 in-service pavements [i.e., 22 field pavements and 1 heavy vehicle simulator (HVS) project] that were constructed before 2011 and 5 newer pavements that were constructed in 2011 and 2012 across the United States. The pavement age of in-service projects ranged from 4 to 10 years by the time of the second-round distress survey in 2014–2015. Each project contains at least one WMA technology type and a corresponding HMA con- trol pavement, thereby creating one or more HMA-WMA pairs for comparative purposes. A control pavement is defined as the HMA pavement section that has the same (or similar) material, climate, traffic, and pavement conditions as the corresponding WMA pavement section(s). C H A P T E R 1 Introduction

4This study was conducted in three phases. The work in Phase I identified a list of candidate WMA projects and devel- oped an experimental plan for field performance measure- ments and laboratory characterization. The work in Phase II executed the laboratory and field testing program, analyzed the laboratory and field test results, and determined the sig- nificant determinants of the field performance of WMA. The work in Phase III developed conclusions and proposed best practices for the use of WMA. The overall work in this study focused on monitoring the field performance and ascertain- ing the performance differences between the WMA pavements and their control HMA pavements, as well as identifying criti- cal engineering and material properties that can be used to characterize WMA pavements. Summary of Literature Review and Survey Results Previous Studies of Field Performance for WMA Many researchers have studied the performance of WMA. For example, Bonaquist (2011) proposed a mixture design procedure for WMA and concluded that similar volumet- ric properties could be achieved for WMA and HMA using appropriate design procedures for WMA. Earlier research into volumetric properties in the field had shown that WMA sec- tions have density values that are equal to those of control sections (Hurley and Prowell 2005, Chowdhury and Button 2008). More recent WMA research has focused mainly on short-term or early age performance (Kvasnak and West 2009, Kvasnak et al. 2010, Medeiros et al. 2012, Guo et al. 2014, West et al. 2014, Rushing 2013) and WMA pavements at accelerated pavement testing facilities (Jones et al. 2008, Mejías-Santiago et al. 2014). However, understanding the ways that WMA pavements perform over the long term and in the field com- pared with HMA pavements is critical in order to identify best practices for the use of WMA. The following sections summarize relevant previous studies of WMA in terms of transverse cracking, wheel-path longitudinal cracking, and rutting and moisture susceptibil- ity that are pertinent to the understanding of the long-term field performance of WMA observed in this study. Transverse Cracking Transverse cracking in HMA overlays can be low- temperature cracking, reflective cracking, or a combination of both. Lytton et al. (2010) proposed mechanistic models to predict reflective cracking, but the material properties of the HMA overlay in the prediction models were estimated indi- rectly rather than being measured directly. Transverse cracking has been observed in the field for both HMA and WMA pave- ments (West et al. 2014); however, performance tests that are able to characterize transverse cracking in the field have not been identified. So far, researchers have used many methods to evaluate the low-temperature cracking resistance of HMA mixtures; these methods include the semi-circular bending test (Li et al. 2007), disc-shaped compact tension test (Amirkhanian et al. 2011), indirect tensile (IDT) test (Bower et al. 2016), and acoustic emission test (Hill et al. 2012), which also could be used to evaluate WMA mixtures’ low temperature properties. Estakhri et al. (2010) used an asphalt overlay tester to evaluate the reflective cracking resistance of HMA and WMA mixtures based on laboratory-compacted specimens. Whether these tests can be used to evaluate WMA and HMA pavement per- formance in the long-term for transverse cracking requires validation in the field. Research into the effects of transverse cracking of WMA and HMA pavements has focused on short-term or early age performance (Medeiros et al. 2012, Guo et al. 2014, West et al. 2014, Rushing 2013), and most conclusions regarding per- formance comparisons between WMA and HMA are based on laboratory-fabricated specimens (Medeiros et al. 2012, Guo et al. 2014) or field cores extracted from the pavement less than 3 years after construction (West et al. 2014, Rushing 2013). West et al. (2014) found that 8 of 14 monitored projects exhibited minor amounts of transverse cracking after 2 years of service, and the WMA and HMA pavements showed simi- lar amounts of transverse cracking. However, the literature on systematic results of how WMA and HMA pavements perform in terms of transverse cracking for a longer service period is limited. Wheel-Path Longitudinal Cracking As a result of multiple rehabilitation efforts (e.g., overlays) over the past few decades in the United States, asphalt pave- ments have often become thick and act like deep-strength pavements. As a result, wheel-path longitudinal cracking is often observed as a primary distress. The occurrence of wheel- path longitudinal cracking is associated with fatigue and may take several years to appear. Wheel-path longitudinal cracking is often coupled with effects related to the pavement material (Malan et al. 1988, Wambura et al. 1999), climate (Rolt 2001), and traffic loading (Myers et al. 2001). Jones et al. (2008) used a HVS facility to investigate the fatigue of WMA pavements. They observed no fatigue cracking because of the very strong pavement structure of the test track. A field study in Florida found no practical difference between HMA and WMA sections based on the results of pavement condition surveys conducted 3 years after construction (Sholar et al. 2009). Due to the fact that little longitudinal cracking was observed in several field projects that were monitored for 1 or

5 2 years, West et al. (2014) used Mechanistic-Empirical Pave- ment Design Guide (MEPDG) models to quantify long-term performance of top-down longitudinal cracking for HMA and WMA pavement sections. They found that, numerically, the HMA sections had slightly more predicted cracking than the WMA sections, but the sections did not differ statistically. Most studies have focused on the fatigue cracking of WMA based on laboratory tests and early field performance. For example, Goh and You (2011) used four-point beam fatigue tests to evaluate the fatigue life of foamed WMA mixes and found that WMA had a longer fatigue life than HMA based on laboratory-controlled specimens. Haggag et al. (2011) applied the simplified viscoelastic continuum damage (S-VECD) model to WMA mixtures and found no significant difference in fatigue resistance between the HMA and WMA mixtures. Roque et al. (2010) proposed a VECD model and fracture mechanics model to predict top-down cracking initiation and propagation in HMA overlays, respectively. Pellinen et al. (2004) evaluated top-down longitudinal cracking in field projects and examined the material properties (e.g., modu- lus, strength), but their study was based on only three projects and did not identify any significant determinants for top-down cracking. Overall, the material properties obtained from experiments that are conducted to characterize top-down cracking still require extensive validation by field performance, especially long-term field performance, before the widespread imple- mentation of materials and adoption of significant determi- nants to guide design and practice. Rutting and Moisture Susceptibility The use of WMA technologies, especially foaming technolo- gies, has raised some concerns about the potential for pre mature failure of pavements in the long term. The reduced aging that is due to WMA’s low production temperatures and possibly to incomplete drying of the mix might lead to rutting and mois- ture damage in WMA pavements. Research into the rutting and moisture susceptibility of WMA has focused mainly on laboratory-fabricated specimens (Zhao et al. 2012, Malladi et al. 2015, Guo et al. 2014, Mogawer et al. 2009, Ulloa et al. 2013). The test methods commonly used to evaluate rutting and moisture susceptibility are the flow number test (AASHTO TP 79), the modified Lottman test (AASHTO T 283), the Asphalt Pavement Analyzer (APA) test (AASHTO T 340), and the Hamburg wheel tracking (HWT) test (AASHTO T 324). However, the various test methods may yield results that are inconsistent with one another. Ultimately, WMA’s rutting and moisture damage resistance should be judged based on field performance. The test method that can provide results that are the most consistent with the field performance should be used for future performance evaluations. Diefenderfer and Hearon (2008) evaluated the long-term rutting effects of WMA using the MEPDG prediction model and found that WMA’s performance did not differ signifi- cantly from that of conventional HMA. Goh and You (2008) performed a field study to evaluate the rutting of a WMA mixture with Sasobit additive and found that the WMA and HMA control mixture showed similar rutting results. Sargand et al. (2012) investigated field projects in Ohio and observed no measurable rutting for either the HMA or WMA pavement sections after 46 months of service. Similar findings also were reported by Estakhri et al. (2010) and Button et al. (2007). West et al. (2014) investigated six WMA in-service pave- ments and eight new pavements. They found that moisture damage was not evident for any of the field projects and that the rutting of the HMA and WMA pavements was the same. Martin et al. (2014) assessed the moisture susceptibility of WMA using different types of specimens, including laboratory- mixed laboratory-compacted (LMLC) specimens, plant-mixed laboratory-compacted (PMLC) specimens, and plant-mixed field-compacted (PMFC) cores, as well as a field distress sur- vey. They found no moisture damage in the field. The WMA specimens were more susceptible to moisture damage than the HMA specimens in the early life of the pavement, but they became increasingly comparable with the HMA control speci- mens after aging, based on the field aging and laboratory aging of the PMFC cores and LMLC specimens, respectively. Martin et al. (2014) also recommended the use of an anti-stripping agent to address concerns about WMA’s moisture susceptibility in the early life of the pavement. Most of these studies of WMA pavements were based on relatively short-term field performance and small sample sizes. The evaluation results for WMA pavement performance could have been confounded by other factors, such as variations in pavement structure, subgrade conditions, pre-overlay con- ditions of the existing pavement, and so forth. Therefore, a thorough understanding of WMA technologies calls for com- prehensive studies of the long-term performance of WMA pavements based on a relatively large sample population of field experiments, so that a statistically sound comparison of HMA and WMA field performance can be conducted and conclusive findings of the significant determinants of specific performance characteristics (transverse cracking, wheel-path longitudinal cracking, rutting, and moisture damage) can be proposed. Survey Results The research team surveyed the state DOTs for information about WMA projects with HMA control sections. Responses were received from 18 DOTs. The requested information included location, traffic, pavement structure, availability of materials, production temperatures for HMA and WMA,

6and occurrence of distresses. In the survey, the research team also requested basic selection criteria, such as permission for coring by the agencies, presence of a HMA control section, and age of pavements. Old projects were preferred because they had an increased probability of developing distresses during the study, which could represent long-term field per- formance. Another selection criterion was that a minimum difference of 40°C in the production temperatures between HMA and WMA was preferred. Figure 1.1 presents the distribution of WMA technologies for those WMA pavements that have HMA control sections, based on the survey results. The use of foaming technologies is shown to be more popular than the other two technology categories. Figure 1.2 shows that, in terms of individual tech- nologies, Evotherm is the most widely used technology, fol- lowed by Sasobit, Astec DBG, and Advera. Figure 1.3 shows that, based on pavement structure, several WMA projects are overlays of existing Portland cement concrete (PCC) pave- ments. This information served as the basis for selecting the WMA projects used in this study. Organization of the Research Report This report consists of six chapters and seven appendices. Chapter 1 presents the research background, objectives, and scope of work; a summary of the literature review and agency survey results; and the organization of this research report. Chapter 2 briefly describes the research approach that was applied during the execution of this project. Chapter 3 pre- sents the findings for the transverse cracking of the HMA and WMA pavements in the field, including field performance com- parisons, significant material properties determination, and predictive model development. Chapter 4 presents the find- ings for the wheel-path longitudinal cracking of the HMA and WMA pavements in the field. Chapter 5 presents the findings for the rutting and moisture susceptibility of the HMA and WMA pavements. Chapter 6 concludes the research report and summarizes the research findings. Appendix A provides a summary of mix design information for the selected projects. Appendix B details the field perfor- mance of the WMA and HMA pavements in terms of trans- verse cracking, wheel-path longitudinal cracking, and rutting. Appendix C summarizes the significant material properties for the WMA and HMA mixtures. Appendix D provides pro- posed test methods in the AASHTO standard format for the fracture testing of mixtures and binders. Appendix E pre- sents the Pavement ME Design analysis results for all the proj- ects. Appendix F summarizes the evolution of the material properties (mixtures and extracted binders) and field per- formance for the four new projects (MT I-15, TN SR 125, IA US 34, and LA US 61) by comparing the first-round and second-round test results. Appendix G describes the predic- tion model develop ment procedure based on the partial least squares method. Figure 1.1. Distribution of WMA technologies based on WMA categories. Foaming, 53.2% Chemical Additive, 29.9% Organic Additive, 16.9% Figure 1.2. Distribution of WMA technologies based on individual technologies. Evotherm, 23.4% Sasobit, 16.9% DBG, 15.6% Advera, 10.4% Gencor, 6.5% Water Injection, 3.9% Maxam Aquablack, 3.9% Aspha-min, 3.9% Other, 3.9% LEA, 2.6% Rediset TM WMX, 2.6% Meeker, 1.3% Terex, 1.3% Revix, 1.3% Thiopave, 1.3% Cecabase RT, 1.3% Figure 1.3. Distribution of pavements based on pavement structure. WMA+HMA>6", 38.5% WMA+HMA≤6", 30.8% WMA+Concrete, 30.8%