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

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7 CHAPTER ONE INTRODUCTION This chapter introduces background information and highlights the objectives, organization, and key definitions used in the report. This synthesis includes an updated literature review on binder chemistry and the relationship between binder chem- istry (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 comprehensively reviewed. Compositional information is useful in helping to understand asphalt—what makes it behave as it does and what makes one asphalt behave differently from another. The efficiency of the refiners and producers to extract crude oil for reservoirs increases the source of crudes, but the number of crudes that yield quality asphalt residua is limited. As refiners extract more gasoline and other petroleum products from crude oil, the need for additives to upgrade straight-run asphalts increases. The properties and contributions of nonbituminous additives are reviewed. BACKGROUND Asphalt pavements were first constructed in the 1870s using asphalt mined from a lake of native asphalt in Trinidad. The asphalt was diluted with petroleum residuum to make an asphalt cement of the desired consistency (Welborn 1984). As early as 1892, scientists realized that the durability of asphalt pavements depended on an asphalt cement capable of adhering to the aggregate while remaining elastic. In 1903, an ASTM committee on Road and Paving Materials Procedures was formed to develop test methods and specifications for highway materials. Test methods, which included volatilization, penetration, and bitumen quality, were adopted by ASTM in 1911. As asphalt characterization techniques continued to evolve, three national specifications—federal, AASHTO, and ASTM—for asphalt cements to be used in hot-mix asphalt (HMA) were published. With minor exceptions, the requirements for physical and chemical properties were essentially the same for the three national specifications. The specification tests included penetration, flash point, ductility, and loss on heating (Welborn 1984). The specifications proved to be inadequate for predicting the field hardening of the HMAs. Laboratory accelerated tests were developed to assess age and oxidative hardening. Relating these tests to field performance became possible when Abson developed a procedure for recovering asphalt liquid from asphalt-aggregate mixture in 1933. This procedure has evolved to the standard used today (AASHTO 2011). Various thin-film oven (TFO) and rolling thin-film oven (RTFO) tests were developed to assess the extent of hardening, since hardening was judged to be the one property most closely related to asphalt performance (AASHTO 2013). These tests predict the properties of asphalt at the time of construction, but they do not provide sufficient information on the change in properties of the asphalt during service in a pavement. SUPERIOR PERFORMING ASPHALT PAVEMENTS Strategic Highway Research Program (SHRP) research had a significant impact on characterization and specification of asphalt binders through development of more fundamental rheology-based test methods. Advances in instrumentation and personal computers permitted routine application of rheological methods in conducting and determining specification compli- ance. In 1987, the SHRP began developing a new system for specifying asphalt materials; that is, Superpave, which stands for Superior Performing Asphalt Pavements. The goal was to develop an improved system for specifying component materials, asphalt mixture design and analysis, and pavement performance prediction (SHRP 1993a, b). As a result of SHRP research, the 1990s saw the introduction of a new binder purchase specification now known as the Superpave binder specification. The Superpave binder specification is based on rheological properties of the asphalt binder measured over a wide range of tempera- tures and aging conditions. Measuring binders’ rheological properties over a wide range of temperatures, loading conditions, and aging conditions allows performance relationships to be established between the test results and the pavement. The details of this asphalt binder testing are described in an AASHTO Specification (AASHTO 2016).

8 The new system for specifying binders is performance-based. It specifies binders on the basis of climate and attendant pavement temperatures in which the binder is designed to serve. Physical criteria remain the same, but the temperature at which the binder must attain the properties changes. Performance graded (PG) binders are designated by high and low tem- perature limits. For example, in the designation PG 64-22, the first number, 64, is the high temperature grade, which desig- nates the highest temperature in centigrade at which the binder exhibits adequate physical properties. This would correspond to the average high temperature for the climate where the binder is expected to serve. The second number (-22) is the low temperature grade; that is, the binder will meet specified physical properties in pavements down to at least −22oC. A key feature of binder evaluation in the Superpave system is that physical properties are measured on binders. After test- ing the unaged binder, the physical properties are also measured on binders exposed to rolling thin-film oven aging to simulate oxidative hardening that occurs during hot mixing and installation. A pressure aging vessel (PAV) is used to expose the binder to severe aging expected after it has served many years in the field. Various pieces of equipment are used to measure stress strain relationships in the binder at the specified test temperatures. The equipment includes the dynamic shear rheometer (DSR) and bending beam rheometer (BBR). These devices and others that may be used to characterize a binder, along with the purpose of each test, are summarized in Table 1. The procedures focus on the physical properties of asphalt binders and their respective asphalt concretes (HMAs). TABLE 1 SUPERPAVE ASPHALT BINDER TESTING EQUIPMENT AND PURPOSE Device Purpose Performance Parameter Rolling Thin-Film Oven Simulate binder aging during hot-mix asphalt (HMA) production and construction Resistance to aging and adequate stiffness during construction Pressure Aging Vessel Simulate binder aging during HMA service life Resistance to aging (durability) in the field Rotational Viscometer Measure binder properties at high and intermediate construction temperatures Facility in handling and pumping Dynamic Shear Viscometer Measure binder properties at high and intermediate service temperatures Resistance to permanent deformation (rutting) and fatigue cracking Bending Beam Rheometer Measure binder properties at low service temperatures Resistance to thermal cracking Direct Tension Tester Measure binder properties at low service temperatures Resistance to thermal cracking Understanding the chemistry of asphalt binders is complicated because asphalt composition is complex. Asphalt is the residue from fractional distillation of crude oil, so its composition varies with the crude oil source (Corbett 1984). The distil- lation process may be followed by further separation of the vacuum residue by solvent deasphalting in order to extract high boiling fractions for lube oils or as a feedstock for catalytic cracking. The insoluble fraction (precipitated asphalt) may be used as a blending component for asphalt cements. Residuum oil supercritical extraction uses low molecular weight hydrocarbon solvents (propane or pentane) for production of asphaltenes, resins, and oils by extraction of the nonvolatile constituents of petroleum crudes. Solvent selection depends on the nature of the feedstock and product usage (Gearhart and Garwin 1976). If the viscosity of the vacuum residuum must be increased or the temperature sensitivity improved, a limited air-blowing process may be used. The process involves continuous pumping of an asphalt resid (flux) through an oxidation tower while air is passed through the flux stream (Corbett 1984). CHEMICAL ANALYSIS OF ASPHALT Efforts to use chemical analysis to predict the performance of asphalt as a construction material have been thwarted by the complexity of the material. The atomic composition of asphalt is predominately carbon (83%–87%) and hydrogen (10%–11%) (Peterson 1984). Asphalt is a mixture of a wide variety of chemical compounds that include aliphatic hydrocarbons and aromatic ring (polypericyclic) systems. The hydrocarbon component consists of straight and branched chains referred to as aliphatic or paraffinic molecules, simple or complex hydrogen saturated rings called naphthenic molecules, and simple or complex unsaturated ring structures classified as aromatic molecules. The aromatic molecules could include benzene or naphthalene, but more commonly consist of more complex multiring structures (polycyclic molecules). The smallest size of the hydrocarbons is defined by the processing conditions of the crude oils (cut point in the vacuum distillation tower) and the largest size is defined by the crude oil source. The hydrocarbons are considered to be nonpolar components.

9 Polycyclic molecules may contain one or more heteroatoms; that is, sulfur (1%–5%), nitrogen (0.3%–1.1%), and oxygen (0.2%–0.8%). The heteroatoms are attached to the asphalt molecules in various forms, but each of the functionalized mol- ecules is considered a polar component of an asphalt cement. These polar components vary depending on the source of the asphalt. The nonpolar components of the asphalt cements act as solvents or dispersing agents of polar components and control the compatibility of the asphalt cement. Depending on the crude source, trace metals such as vanadium (4–1,400 ppm), nickel (0.41–110 ppm), and iron (6–250 ppm) occur in porphyrins present in the polar components. The molecular interactions among these polar molecules strongly influence the physical properties and the performance of the asphalt. SYNTHESIS APPROACH The information in this synthesis was gathered through 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. The TRID database and SciFinder complement each other in that TRID covers engineering subjects and SciFinder provides access to the chemical literature. Journals not covered by either of these search engines can be accessed directly using Bing or Google. In addition, a survey of U.S. departments of transportation (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.