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SUMMARY RELATIONSHIP BETWEEN CHEMICAL MAKEUP OF BINDERS AND ENGINEERING PERFORMANCE A number of factors and variables influence the chemical composition and, ultimately, the engineering properties of a binder used in pavement applications. It is important that the users most closely working with binders have a strong working knowledge of the relation- ship between binder chemistry and engineering properties. In addition, the relationship among standard process control actions and final chemical composition needs to be under- stood so that correlation to physical performance can be made. This synthesis can provide the chemical composition knowledge to help departments of transportation (DOTs) and practicing engineers make more informed choices with regard to selection of binders and post-production additives and modifiers and avoid potentially expensive mistakes. Also, researchers might develop more informed methods to design binder modifiers and additives that will result in low-cost and high-performance binders for specific applications. The information in this synthesis was gathered by a thorough review of the available U.S. and international literature. A comprehensive literature search was performed using the TRID database, SciFinder, Bing, and Google. In addition, a survey of U.S. state DOTs and Canadian ministries of transportation was conducted to determine the current status of asphalt chemical analysis. The U.S. state and District of Columbia response rate to the survey was 88.2% (45 of 51). Only eight of the 14 Canadian provinces and territories responded, but their comments were enlightening. This synthesis includes an updated literature review on binder chemistry and the rela- tionship between binder chemistry (including modifiers and additives) and engineering properties. Analytical methods that can be used to qualitatively identify the chemical makeup of asphalt binders based on modern analytical instrumentation are comprehen- sively reviewed. Composition information is useful for understanding asphaltâwhat 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 fail- ures, and provide information needed for corrective measures. This synthesis reviews our current understanding of asphalt composition. Only a limited number of crude oils yield quality asphalt. The price of liquid asphalt has increased dramatically within the past decade mainly as a result of the advent of cok- ing technologies, which enabled refineries to decrease their production of asphalt (residua from crude oil refining processes) by converting the residua into synthetic fuel. This devel- opment has led to a shortage of asphalt binders and an increase in its price independent of the crude oil price. Asphalt usage, as reported by the Asphalt Institute, is summarized
2 according Petroleum Administration for Defense Districts. Trends in net production of asphalt and road oil by refineries and blenders as reported by the U.S. Energy Information Administration show a decline in production over the past 5 years based on 2010 production figures. Demand for asphalt in paving applications is forecast to advance 3.1% annually to 19.6 million tons based on improving economic conditions and a pressing need to repair and expand the nationâs infrastructure. Rising use of recycled asphalt pavement and increas- ing interest in rehabilitating and repairing older or worn surfaces will serve as a check on asphalt demand advances. Asphalt (or asphalt cement) is the carefully refined product derived from selected crude oils. Outside the United States, the product is often called bitumen. Asphalt is no longer just the residua from crude oil refining. It is now known as an asphalt binder and is part of an engineered system. All molecules in asphalt are hydrocarbons with small amounts of sulfur, nitrogen, and oxygen and traces of metals such as vanadium and nickel. The hydro- carbons may consist of polyaromatic structures containing different numbers of fused rings, saturated polycyclic structures also with different numbers of rings, and combinations of these. All these core structures contain saturated hydrocarbon side chains of different chain lengths and different substitution patterns. Many of these side chains are lost during the refining process, so the properties of the asphalt produced today are different from those of the asphalt that was simply the residue of a crude oil distillation. One school of thought is that asphalt could be considered a colloidal or micellar system. The hydrocarbon insoluble components, asphaltenes and resins, are dispersed in a hydro- carbon blend. The overall behavior of asphalt cement is controlled by the compatibility and the relationships of the different components in this macroscopically homogeneous mixture rather than by the quantitative amount of any single component. Historically, the study of asphalt chemical composition has been facilitated by the separation of asphalt into compo- nent fractions based on the polarity of the molecular components present or their adsorp- tion characteristics, or both. The component fractions, sometimes called generic fractions, although useful in classifying and characterizing asphalts and in providing simplified mix- tures for further study, are still complex mixtures, and their composition is a function of asphalt source. The component fractions are, however, sufficiently unique to identify their particular contribution to the complex flow properties of asphalt. A proper balance of com- ponent types is necessary for a durable asphalt. The method for fractionating asphalts into the generic fractionsâsaturates, aromatics, resins, and asphaltenesâhas evolved recently, and the new developments are discussed in this synthesis. Asphaltenes are the insoluble fraction precipitated from a toluene solution of asphalt binder by a nonpolar solvent such as pentane or heptane. The precipitating solvent affects the quantity of asphaltenes precipitated. The nature of the asphaltene molecules in the precipi- tate depends on the precipitating solvent; greater amounts of a precipitant are produced using pentane. In general, the asphaltenes are defined as the mixture of materials precipitated by heptane. The asphaltene molecules are very complex and exhibit a very high tendency to associate into molecular clusters. The amount and characteristics of asphaltenes vary from one asphalt source to another. They play a significant role in the rheology of asphalt binders. The current status of our understanding of asphaltene structure is discussed in chapter two. Recently, Mullins (2011) published a comprehensive review on asphaltenes. Virtually all asphaltene chemical properties, except elemental composition, had been the subject of debate; their molecular weight had been estimated at values spanning six orders of mag- nitude. Molecular weight determination has been, and still is, a challenging problem in asphaltene chemistry. The source of complications is attributable to three basic properties of asphaltene, namely, compositional variance, size polydispersity, and, most important, the high propensity of the covalent asphaltene molecules to form molecular aggregates.
3 In contrast, a second school of thought is that different molecular species in the asphalt binder exist in a colloidal dispersion of asphaltene micelles in the maltenes. Lesueur has published a detailed review of the current asphalt structure concepts that defines links between chemistry, structure, and mechanical properties in a framework of an updated col- loidal picture of asphalt. The resinsâthat is, the polar components of the maltenesâwere thought to stabilize the asphaltene micelles. Extensive analytical procedures are support- ing this model. Traditional analytical techniques like ultraviolet spectroscopy, Fourier transform infra- red spectroscopy (FTIR), nuclear magnetic resonance (NMR), or mass spectroscopy have provided substantial information about the average chemical composition of asphalt. Most asphalts give more or less identical spectra, and there are no known general correlations between physical properties and any particular functional group as identified by the tech- niques listed. The only technique that is frequently used is FTIR, which permits identifica- tion of carbonyls and sulfoxides formed as a result of aging. Modern analytical techniques allow more complete definition of the typical asphalt molecules. These techniques include thermogrammetric analysis (TGA), differential thermal analysis, gel permeation chro- matography (GPC), scanning electron microscopy, and atomic force microscopy (AFM). These modern techniques provide insight into the molecular interactions that govern the physical properties of the asphalt matrix. Changes in the physical properties produced by specific additives can be ascertained to estimate their effectiveness. These techniques are described in chapter three. Renewed interest in asphalt morphology has been promoted by thermal analysis research. TGA and differential scanning calorimetry are used to characterize petroleum bitumens and their chromatographic fractions, including the glass transition temperature and the percentage of crystalline fractions. The influence of the different constituents on the thermal stability of bitumen can be studied by TGA. TGA measurements provided a simple means to determine the thermal stability of bitumen and the presence of volatile species in binders. The advent of modulated differential scanning calorimetry provides new insight on asphalt microstructure. The development of bitumen microstructure and the calculations of the entropy and enthalpy of transitions suggest that bitumen is a structured amorphous phase containing small crystalline phase. 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 the binder, an understanding of its origin and history can be obtained. The FTIR method is an efficient technique for identifying polymer additives in a binder. Determination of polymer content is essential for quality control and quality assurance during the processing and application of polymer-modified asphalts. Differences in the molecular structures of asphalt components have prompted efforts to separate these components using size exclusion chromatography, more commonly known as gel permeation chromatography. GPC is a method of separating molecules based on their size and shape in solution. GPCâs ability 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 has made GPC a useful alternate technique for frac- tionating complicated mixtures such as crude oil residua, asphalts, and asphaltenes and a vital contributor to understanding asphalt mixes. The differences in molecular weights and molecular weight distributions identified by the GPC data are used to predict aging, viscosity aging index, viscosity number, and penetration. Because the polymer and asphalt components of polymer-modified asphalt cements can be separated completely, GPC is an excellent tool for measuring polymer content in modified binders.
4 Proton nuclear magnetic resonance (1H NMR) spectroscopy has emerged as a very pow- erful and versatile tool for bitumen characterization. Using 1H and 13C NMR can yield infor- mation on average structural parameters of asphalt and asphaltenes, such as percentages of aromatic carbons, aliphatic carbons, bridged carbons, methyl carbons, ring carbons, naph- thenic carbons, paraffinic chain lengths, and other parameters. The specific environment of the different types of hydrogens and carbons can be defined. Thus, NMR spectroscopy is a powerful tool for predicting the structure of complex organic molecules. When information from NMR and GPC is combined, possible structures for asphalt and mechanisms of aging can be suggested. Recently, researchers combined atomic resolution imaging using AFM with molecular orbital imaging using scanning tunneling microscopy to show the actual atomic arrange- ments in an asphaltene molecule. Identifying molecular structures provides a foundation to understand all aspects of petroleum science, from colloidal structure and interfacial interac- tions to petroleum thermodynamics, thus enabling a first-principles approach to optimiz- ing resource utilization. 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 an asphalt binder is immense. Certain asphalt chemical parameters have a consistent and measurable effect on the asphalt microstructure that can be observed with AFM. As new efficiency allows refiners 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 and styrene-butadiene rubber, antistripping agents, softening agents, and polyphosphoric acid). The properties and contri- butions of each of these nonbituminous additives are reviewed in chapter four. Polymers are used to enable a wider spread of high and low performance grading (PG) values for binder blends. The benefit of the polymer modifiers will depend on the concentra- tion, morphology, molecular weight, chemical composition, and molecular structure of the material. The crude source, refining process, and grade of the neat asphalt binder are equally important. The residual reactivity of the polymer is useful for improving compatibility of the additive with the binder. Bio-based alternatives, which are being developed across the industry in various coun- tries, could be a solution to reduce the asphalt industryâs dependence on petroleum resources. In addition to efforts to create alternative binders, bio-based additives and extenders are being developed. Recent developments in this field are described in chapter four. Low molecular weight modifiers are specifically used to improve the asphalt bindersâ ability to perform in a given environment. These materials are often proprietary, but they can be classified by their intended application; that is, antioxidants, hydrocarbon supple- ments, antistripping agents, and stiffening agents. The hot-mix recycling operation for bitu- minous mixes commonly uses a modifier to restore the aged asphalt cement to a condition that resembles virgin asphalt cement. The contribution of each of these modifiers to asphalt binder chemistry is discussed in chapter four. The advantages and disadvantages of their employment are elaborated. The survey results in chapter six cover numerous aspects of asphalt chemistry. State DOTs do not monitor the sources of their asphalt binders. Only 13.3% of the respondents could identify their 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 37.8% of the respondents. Binders appeared to be stiffer and exhibit poor low-temperature properties, and some pavements aged prematurely. Although binders
5 met specifications, experienced engineers could detect quarterly changes as crude feeds changed. Changes associated with seasonal or market fluctuations were reported by 35.5% of the respondents. The dominant analytical procedure used for binder testing is rheological; that is, the PG protocol (100% of respondents). FTIR is employed by 24.4% of the DOTs, but this appears to be primarily used for research. The third most common procedure was X-ray fluores- cence. A number of modifiers/additives are currently employed, including polyphosphoric acid (42.2% of respondents), antistripping agents (26.7%), and softening agents (22.2%). Comments on specific DOT procedures can be found in Appendix A. More than 95% of the 40 respondents replying to the questionnaire do not measure the compatibility of modifiers in the final binder. It is hoped that this review will bring about a better understanding of the chemical compositional factors that control the properties of asphalt and will assist in providing direction to both research and application, and lead to improved asphalt products with bet- ter performance and durability.