Relationship Between Chemical Makeup of Binders and Engineering Performance (2017)

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66 CHAPTER SEVEN CONCLUSIONS An updated literature review on binder chemistry and the relationship between binder chemistry (including modifiers and additives) and engineering properties reveals extensive progress in understanding asphalt materials. Composition informa- tion is useful for understanding asphalt—what makes it behave as it does and what makes one asphalt behave differently from another. With the asphalt sources available, composition information can be used to improve the product through modification with additives, by blending, and so on, or to alter use design procedures to accommodate specific properties. Composition information can be used to match asphalt and aggregate, provide clues as to what modifications are necessary to make an asphalt-aggregate system more serviceable under a given environment, diagnose failures, and provide information needed for corrective measures. The survey reveals that DOTs’ current understanding of asphalt composition is very limited. Asphalt binder supplies are shrinking because only a limited number of crude oils yield quality asphalt. Asphalt is no longer a by-product of crude oil distillation, but rather it is a designed product produced at market demand rates and priced accordingly. Currently, approximately 16.5 million tons are used in road construction each year. The demand for asphalt is increasing at approxi- mately 3.1% per year based on improving economic conditions and a pressing need to repair and expand the nation’s infrastructure. Asphalt is considered a colloidal or micellar system. The hydrocarbon insoluble components, asphaltenes and resins, are dispersed in a hydrocarbon blend. The overall behavior of asphalt cement is controlled by the compatibility and the relation- ships of the different components in this microscopically homogeneous mixture, rather than by the quantitative amount of any single component. A proper balance of component types is necessary for a durable asphalt. Methods for fractionating asphalts into the generic fractions—saturates, aromatics, resins, and asphaltenes (SARA)—have evolved and new developments pro- vide valuable information on changes in asphalt composition according to both source and date of production. APPLICATIONS OF ANALYTICAL INSTRUMENTATION TO BINDER CHARACTERIZATION The advent of modulated differential scanning calorimetry provides insight on asphalt microstructure. The development of bitumen microstructure and calculations of the entropy and enthalpy of transitions confirm that bitumen is a structured amor- phous phase containing a small crystalline phase. Recently, researchers combined atomic resolution imaging using atomic force microscopy (AFM) to show the actual atomic arrangement in an asphaltene molecule. Identifying molecular structures provides a foundation for understanding all aspects of petroleum science, from colloidal structure and interfacial interactions to petroleum thermodynamics, enabling a first-principles approach to optimizing resource utilization. Particularly, the findings contribute to a long-standing debate about asphaltene molecular architecture. The impact of AFM imaging on understanding the microstructure and performance characteristics of asphalt binders is immense. Each SARA chemical fraction influences asphalt phase structuring, most nota- bly the well-known asphalt “bee” structures. Certain asphalt chemical parameters have a consistent and measurable effect on the asphalt microstructure that is observed with AFM. Particular microstructures that emerged through chemical doping were then discovered to have unique chemical polarity, which explicitly impacts the durability and performance of asphalt. Gel permeation chromatography (GPC), a method of separating molecules based on their size and shape in solution, facili- tates the analysis of polymer and asphalt components of polymer-modified asphalt cements. The ability of GPC to separate mixtures by molecular size rather than by some complex property such as solubility or absorptivity is one of the great advan- tages of the technique. This feature makes GPC a useful alternate technique for fractionating complicated mixtures, such as crude oil residua, asphalts, and asphaltenes. Only one DOT appears to be using GPC on a routine basis, but the prevalence of polymer-modified asphalts suggests that this technique could be employed more widely. The presence of recycled asphalt pavement (RAP) and recycled asphalt shingles (RAS) in modified asphalt binders can be analyzed by carefully deconvoluting the asphaltene/polymer region of the GPC chromatograms.

67 Fourier transform infrared spectroscopy (FTIR) is one of the more important methods for fingerprinting asphalt materials and quantifying the distribution of asphalt components. By determining the various chemical functional groups in a binder, an understanding of its origin and history can be obtained. The FTIR method is employed to identify some antistripping agents and to a more limited extent specialized additives in a binder. Amine-based antistripping agents are difficult to observe. Quantitative redetermination of polymer content can be achieved with proper calibration techniques, but this is not a routine procedure. FTIR is able to quickly assess aliphaticity, aromaticity, and extent of oxidative binder aging. Relatively low-cost ($20,000 to $40,000) portable devices have become available for FTIR. These can be employed in the field to test the chemical composition of the delivered materials. These are point-and-shoot applications that could potentially be used by field techni- cians with accuracy similar to that obtained by using traditional stationary laboratory equipment. Proton nuclear magnetic resonance (1H NMR) spectroscopy has emerged as a very powerful and versatile tool for bitumen characterization, but it is limited to the laboratory by the complexity of the instrumentation employed. Using 1H and 13C NMR can yield information on average structural parameters of asphalt and asphaltenes. When information from NMR and GPC is combined, possible structures for asphalt and mechanisms of aging are suggested. As refiners’ efficiency allows them to extract more gasoline and other petroleum products from crude oil and as the source of crudes that yield quality asphalt residua decreases, the need for additives to upgrade straight-run asphalts increases. The most common additives to asphalt binders are polymers [styrene-butadiene-styrene (SBS) and styrene-butadiene rubber (SBR), antistripping agents, softening agents, and polyphosphoric acid (PPA)]. Understanding the properties and contribu- tions each of these nonbituminous additives to asphalt is critical for good binder design. SURVEY FINDINGS The survey results in chapter three cover numerous aspects of asphalt chemistry. State DOTs do not monitor the sources of their asphalt binders. Only 13% of the respondents could identify their crude sources. Similarly, most DOTs are not familiar with the process used to produce their binders. Significant differences in asphalt binder properties over the past 10 years were reported by 42% of the respondents. Binders appear to be stiffer and exhibit poor low-temperature properties, and some pave- ments aged prematurely. Although binders met specifications, experienced engineers could detect quarterly changes as crude feeds change. Changes associated with seasonal or market fluctuations were reported by 38% of the respondents. The dominant analytical procedures used for binder testing are rheological; that is, the performance grading (PG) proto- col (100% of respondents). FTIR is employed by 23.9% of the DOTs, but this appears to be primarily used for research. The third most common procedure was X-ray fluorescence spectroscopy (XRF). A number of modifiers/additives are currently employed, including PPA (42.2% of respondents), antistripping agents (26.7%), and softening agents (22.2%). Documentation of the use of modifiers depends on the binder suppliers; therefore, these data may not be complete. The addition of compaction aids (waxes, wetting agents, and antistripping agents) is paving related and dictated by the job, aggregate, and plant. With the exemption of polymers and PPA, the additives in a binder are usually not specified or quantified, because that is considered the responsibility of the supplier. Some concern over the excessive use of PPA is reflected in the upper-limit specifications imposed by 12 DOTs. Comments on specific DOT procedures can be found in Appendix A. More than 95% of the 40 respon- dents replying to this question do not measure the compatibility of modifiers in the final binder blend. Failure to understand this issue may lead to problems with long-term aging and pavement performance. Prediction of asphalt cement aging is primarily accomplished using combined rolling thin-film oven and pressure again vessel (PAV) procedures as defined in AASHTO M320. These procedures were developed for straight-run asphalts and they may not apply to modified asphalts. The use of additives and chemical and polymer modifiers to enhance binder properties has also greatly increased; in some cases, the oxidation kinetics of such modified binders are significantly different from those of conventional binders. Stiffer binder grades may experience insufficient oxidation in the laboratory aging process. Studies have confirmed that a better understanding of binder aging and oxidation can improve our ability to predict damage in asphalt pavements. The most common suggestions to improve SHRP performance grading procedures focus on improving laboratory aging procedures so that results relatable to field experience can be obtained. A one-point aging test such as PAV does not tell the whole story; multiple PAVs provide some improvement, but an effective routine aging test is still lacking. Measurement of aging rate would allow more flexibility in determining the long-term properties of binders and mixtures. Efforts to understand the impact of recycled materials by extracting binder from mixtures are focused on RAP (65.2%) and RAS (47.8%). A limited number of DOTs (fewer than 10) are using recycling (rejuvenating) agents; the most common

68 materials employed are soft asphalt fluxes and abietic acid esters. The recycling agents appear to be used on an experimental basis for specific projects, and nine DOTs expressly state that they do not use them. A majority of the respondents (69%) did not consider the current PG criteria satisfactory in identifying compositional/ chemical properties of a given binder. PG criteria, which are physical measurements, currently appear to fail to capture some poorly performing binders. To better identify compositional properties of polymer-modified asphalt, Illinois DOT specifies PG+ tests (force ductility, elastic recovery, and separation of polymer). Procedures used to quantify the presence of modifiers/additives in final asphalt are surveyed. Polymer-modified binders are checked using multiple stress creep recovery (MSCR) (30.4%), XRF (15.2%), viscosity (8.7%), and FTIR (6.5%). Soften- ing agents such as refined engine oil bottoms (REOB) are checked using XRF. The blend compatibility with antistripping agents, polymers, and PPA is tested in a limited number of cases using modern variants of the “cigar tube test.” In one case, evidence for incompatibility in blends containing PPA was reported. A specification defining an upper limit on PPA is used by 12 DOTs. SUGGESTED FUTURE RESEARCH Given the dearth of chemical information provided by DOTs and ministries of transportation in this study, a more diligent effort to assess binder chemistry on a routine basis should be considered. More data are required to establish trends and the proper analytical focus to prepare a practical guide for chemical analysis of binders. The relationship between the asphalt chemistry of the various rheological properties and performance indicators of asphalt binders has yet to be defined. The results of this synthesis indicate that the primary need for research is to identify more practical and cost-effective tests that truly reflect field performance of asphalt mixtures. Better communication between asphalt producers and DOTs with regard to asphalt cement composition would allow better understanding of the factors critical for long-term field performance. Practical procedures to quantitatively confirm the presence of modifiers/additives in final asphalt cements are needed. In addition to MSCR, viscosity, and FTIR, polymer-modified binders are readily characterized using GPC. Routine GPC analy- sis of binder blends also provides useful information on asphaltene content and size distribution. Building a library of GPC chromatograms will give DOTs a valuable resource for monitoring changes in the binder during field aging and identifying the contribution of binder composition to field failures. Louisiana and Virginia DOTs are employing GPC to confirm the presence and concentration of polymers, but a quick, easy field identification of modifiers (polymer and rubber) remains to be developed. It is important that protocols for ensuring the presence and effectiveness of antistripping agents be defined. The blend capability of mixes with antistripping agents, polymers, and other additives needs to be evaluated more pre- cisely. Several laboratories are developing alternate techniques for estimating binder performance. Connecticut DOT cur- rently specifies the ductility test (AASHTO T-51), toughness and tenacity test (Colorado CP-L 2210), and elastic recovery test (AASHTO T-301) as indicators for polymer modification on some grades of binder. Ontario Ministry of Transportation (MOT) has already implemented the double-edge notch tension test, MSCR, and ash content test (to limit REOB). It is phasing in the extended bending beam rheometer test and looking at XRF and FTIR tests to quantify REOB. MOT has also developed a modified PAV procedure that is currently being evaluated. Quebec MOT has developed a method to evaluate the adhesion between a binder and an aggregate. As these tests evolve, their utility could be shared with all DOTs and MOTs. Asphalt pavement is America’s most recycled material. Despite the massive use of RAP in hot-mix asphalt production, the chemico-physical phenomena that characterize the blending of these mixtures have not yet been completely explored. The detection and understanding of these mechanisms, as well as the study of the heterogeneity that characterizes high-RAP mix production, are fundamental to improving the approach to recycling, because they represent the source of the mixture’s characteristics and performance. It is important that the variability of binder rheology with different aging levels be studied to evaluate RAP binder heterogeneity and its evolution with time. Protocols and blending charts defining the dosage of virgin bitumen required in asphalt mixtures with or without RAP need further development. Investigating the specific contribu- tions of all the components of the mixtures (virgin aggregates, fillers, recycling agents, and RAP) should be evaluated. The overall goal of the study will be to provide guidance on the design and specification of RAP mixes and to reduce uncertainty surrounding the performance of asphalt mixes designed and manufactured with RAP. Similar guidance is needed for mixes containing RAP/RAS blends and ground tire rubber (GTR).

69 Long-term aging of binder blends to ascertain the effect of modifiers and additives is needed. Continued research on recycling agents and related additives to confirm their long-term efficacy could be pursued. The use of bio-based products introduces new parameters to studies on compatibility and stability of asphalt mixtures. The issue is further complicated by the addition of RAP, RAS, and GTR to mixtures. Although it is desirable for a transportation agency to pursue the recycling of asphalt pavements, the economic assessment to determine the best value among asphalt pavement recycling options remains a challenge. This issue has been long debated at agency and industry levels. The problem is that there is no “one size fits all” answer to this issue. A number of considerations can influence the outcome of an economic analysis of this issue. Several methods are available for recycling existing HMA pavement materials into new pavements. Among these are inclusion as RAP in HMA mix designs, cold in-place recycling, cold plant recycling, and hot in-place recycling. Continued research on life-cycle analysis of asphalt cements is needed. It is important that studies of the morphology of asphalt binders using transmission electron microscopy be continued. The various forms of microstructures in different types of asphaltenes and asphalt binder samples can be related to the impact of additives as well as the progress of field aging. The prevalence, size, and shape of the microstructures in field-aged asphalt binders may significantly affect binder engineering properties in pavement use as well as binder behavior in recycling or rejuvenation. Hence, the microstructures are believed to be the key to understanding asphalt binder aging and aging control/ reversal methods. Conventional rheological and chemical tests provide a global view of asphalt property and composition changes upon aging, but offer few details on the changes at the microscopic level. Using atomic force microscopy (AFM), the micromechani- cal properties of asphalts can be analyzed after different aging conditions. Aging was found to significantly increase the spa- tial variations of the sample properties. It generally increases the ratio between the dissipated energy and total work to deform the sample during the indentation process by AFM probe. It also appears to increase the adhesive and/or cohesive strength of the sample. The asphaltene content and the size of microstructures both appear to affect the micromechanical properties of the binders. It is also possible to use the AFM technique to distinguish the effects of additives on the morphological and micromechanical properties of bitumen films. Despite the progress made on morphological characterization of bitumen using AFM, the fundamental question of whether the microstructures observed on bitumen surfaces represent its bulk structure remains to be addressed. Combining AFM with other chemical analytical tools that can generate high resolution comparable to AFM would provide an avenue to linking bitumen’s chemistry to its microscopic morphology and mechanical properties, and consequently benefit efforts to develop structure-related models for bituminous materials across different-length scales.