Asphalt

From Pavement Interactive

Asphalt is one of the two principal constituents of HMA. Asphalt functions as a waterproof, thermoplastic, viscoelastic adhesive. In other words, it acts as the glue that holds the road together (Anderson, Youtcheff and Zupanick, 2000). But just what is asphalt and how is it characterized? Like many engineering substances, a vernacular definition of "asphalt" is rather imprecise. For engineering purposes, the definition needs to be more unequivocal. ASTM D 8 provides the following definitions:

asphalt

A dark brown to black cementitious material in which the predominating constituents are bitumens, which occur in nature or are obtained in petroleum processing.

asphalt cement

A fluxed or unfluxed asphalt specially prepared as to quality and consistency for direct use in the manufacture of bituminous pavements, and having a penetration at 25° C (77° F) of between 5 and 300, under a load of 100 grams applied for 5 seconds.

bitumen

A class of black or dark-colored (solid, semi-solid or viscous) cementitious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, of which asphalts, tars, pitches, and asphaltenes are typical.

flux

A bituminous material, generally liquid, used for softening other bituminous materials.

This section uses the generic term, "asphalt binder", to represent the principal binding agent in HMA. "Asphalt binder" includes asphalt cement as well as any material added to modify the original asphalt cement properties. The term "asphalt cement" is used to represent unmodified asphalt cement only.

Contents 1 Background 2 Petroleum Refining 2.1 Process 3 Chemical Properties 3.1 Basic Composition 3.2 Behavior 4 Physical Properties 5 Grading Systems 6 Asphalt Binder Modifiers 7 Other Forms of Asphalt

Background

The first recorded use of asphalt by humans was by the Sumerians around 3,000 B.C. Statues from that time period used asphalt as a binding substance for inlaying various shells, precious stones or pearls. Other common ancient asphalt uses were preservation (for mummies), waterproofing (pitch on ship hulls), and cementing (used to join together bricks in Babylonia). Around 1500 A.D., the Incas of Peru were using a composition similar to modern bituminous macadam to pave parts of their highway system. In more modern times, asphalt paving use first began with foot paths in the 1830s and then progressed to actual asphalt roadways in the 1850s. The first asphalt roadways in the U.S. appeared in the early 1870s (Abraham, 1929).

In the U.S., Trinidad (near the coast of Venezuela) was the earliest source of asphalt binder. Trinidad supplied about 90 percent of all asphalt (worldwide) from 1875 to 1900 (Baker, 1903). The asphalt was produced from a "lake" (Figure 1) with a surface area of 465,000 m² (46.5 hectares or 115 acres) and a depth of about 24 meters (75 feet). In 1900, Tillson estimated that this "lake" contained about 8,000,000 tonnes of "asphalt" (compare this against 1990 consumption in Europe and the U.S. of approximately 40,000,000 tonnes (tons)). This asphalt, once free of water, was too "hard" to use in paving (Krchma and Gagle, 1974). In fact, Trinidad Lake asphalt, when loaded into a ship’s holds for transport, would fuse to the point that removal required chopping.

Typically, producers added flux, created from petroleum distillation, to Trinidad Lake asphalt to soften it for use in early pavements. It appears that the earliest use of asphalt binder in the U.S. was about 1874 for a project built in Washington, D.C. This binder was a combination of Trinidad Lake asphalt and a flux distilled from crude oil. Without question, these early asphalt binders were quite variable, making pavement mix and structural design somewhat challenging. By the 1880s, asphalt binders were regularly produced from crude oil in California and by 1902 in Texas as well. In 1907, crude oil-based asphalt production surpassed "natural" asphalt production (Krchma and Gagle, 1974). Today, asphalt binder for HMA pavements is produced almost entirely from petroleum refining.

Petroleum Refining

In the simplest terms, asphalt binder is simply the residue left over from petroleum refining. Thus, asphalt binders are produced mainly by petroleum refiners and, to a lesser extent, by formulators who purchase blending stock from refiners. The composition of base crude oil from which asphalt is refined can vary widely and thus the asphalt yield from different crude oil sources can also vary widely.

The American Petroleum Institute (API) classifies crude oils by their API gravity. API gravity is an arbitrary expression of a material’s density at 15.5° C (60° F) and is obtained in the following equation:

API gravity can be used as a rough estimate of asphalt yield with lower API gravity crude oils producing more asphalt (Table 1). Figure 2 shows the composition of three very different crude oils and their associated API gravities.

Table 1. API Gravities of Some Typical Substances

Substance	Typical API Gravity
Water	10
Asphalts	5 – 10
Gasoline	55
Low API gravity crude oil	< 25 (yields high percentages of asphalt)
High API gravity crude oil	> 25 (yields low percentages of asphalt)

Process

Crude oil is heated in a large furnace to about 340° C (650° F) and partially vaporized. It is then fed into a distillation tower where the lighter components vaporize and are drawn off for further processing. The residue from this process (the asphalt) is usually fed into a vacuum distillation unit where heavier gas oils are drawn off. Asphalt cement grade is controlled by the amount of heavy gas oil remaining. Other techniques can then extract additional oils from the asphalt. Depending upon the exact process and the crude oil source, different asphalt cements of different properties can be produced. Additional desirable properties can be obtained by blending crude oils before distillation or asphalt cements after distillation.

Asphalt binder specifications used to be relatively lenient, and gave refiners a high level of production flexibility. Therefore, refiners tended to view asphalt as a simple, convenient way to use the residual material from the refinery operation. Partially as a result of Superpave specifications, asphalt binder specifications are now more stringent and asphalt refiners increasingly perceive asphalt as a value-added product. Superpave specifications have also caused many refiners to reevaluate their commitment to asphalt production; some have made a strategic decision to de-emphasize or cease asphalt production, though others have renewed their efforts to produce high-quality binders (Anderson, Youtcheff and Zupanick, 2000).

Chemical Properties

Asphalt binders can be characterized by their chemical composition although they rarely are for HMA pavements. However, it is an asphalt binder’s chemical properties that determine its physical properties. Therefore, a basic understanding of asphalt chemistry can help one understand how and why asphalt behaves the way it does. This subsection briefly describes the basic chemical composition of asphalts and why they behave as they do.

Basic Composition

Asphalt chemistry can be described on the molecular level as well as on the intermolecular (microstructure) level. On the molecular level, asphalt is a mixture of complex organic molecules that range in molecular weight from several hundred to several thousand. Although these molecules exhibit certain behavioral characteristics, the behavior of asphalt is generally ruled by behavioral characteristics at the intermolecular level – the asphalt’s microstructure (Robertson et al., 1991).

The asphalt chemical microstructure model described here is based on SHRP findings on the microstructure of asphalt using nuclear magnetic resonance (NMR) and chromatography techniques. The SHRP findings describe asphalt microstructure as a dispersed polar fluid (DPF). The DPF model explains asphalt microstructure as a continuous three-dimensional association of polar molecules (generally referred to as "asphaltenes") dispersed in a fluid of non-polar or relatively low-polarity molecules (generally referred to as "maltenes") (Little et al., 1994). All these molecules are capable of forming dipolar intermolecular bonds of varying strength. Since these intermolecular bonds are weaker than the bonds that hold the basic organic hydrocarbon constituents of asphalt together, they will break first and control the behavioral characteristics of asphalt. Therefore, asphalt’s physical characteristics are a direct result of the forming, breaking and reforming of these intermolecular bonds or other properties associated with molecular superstructures (Little et al., 1994).

The result of the above chemistry is a material that behaves (1) elastically through the effects of the polar molecule networks, and (2) viscously because the various parts of the polar molecule network can move relative to one another due to their dispersion in the fluid non-polar molecules.

Behavior

Robertson et al. (1991) describe asphalt behavior in terms of its failure mechanisms. They describe each particular failure mechanism as a function of an asphalt’s basic molecular or intermolecular chemistry. This section is a summary of Robertson et al. (1991).

Aging

Some aging is reversible, some is not. Irreversible aging is generally associated with oxidation at the molecular level. This oxidation increases an asphalt’s viscosity with age up until a point when the asphalt is able to quench (or halt) oxidation through immobilization of the most chemically reactive elements. Reversible aging is generally associated with the effects of molecular organization. Over time, the molecules within asphalt will slowly reorient themselves into a better packed, more bound system. This results in a stiffer, more rigid material. This thixotropic aging can be reversed by heating and agitation.

Rutting and permanent deformation

If the molecular network is relatively simple and not interconnected, asphalt will tend to deform inelastically under load (e.g., not all the deformation is recoverable). Additionally, asphalts with higher percentages of non-polar dispersing molecules are better able to flow and plastically deform because the various polar molecule network pieces can more easily move relative to one another due to the greater percentage of fluid non-polar molecules.

Fatigue cracking

If the molecular network becomes too organized and rigid, asphalt will fracture rather than deform elastically under stress. Therefore, asphalts with higher percentages of polar, network-forming molecules may be more susceptible to fatigue cracking.

Thermal cracking

At lower temperatures even the normally fluid non-polar molecules begin to organize into a structured form. Combined with the already-structured polar molecules, this makes asphalt more rigid and likely to fracture rather than deform elastically under stress.

Stripping

Asphalt adheres to aggregate because the polar molecules within the asphalt are attracted to the polar molecules on the aggregate surface. Certain polar attractions are known to be disrupted by water (itself a polar molecule). Additionally, the polar molecules within asphalt will vary in their ability to adhere to any one particular type of aggregate.

Moisture damage

Since it is a polar molecule, water is readily accepted by the polar asphalt molecules. Water can cause stripping and/or can decrease asphalt viscosity. It typically acts like a solvent in asphalt and results in reduced strength and increased rutting. When taken to the extreme, this same property can be used to produce asphalt emulsions. Interestingly, from a chemical point-of-view water should have a greater effect on older asphalt. Oxidation causes aged (or older) asphalts to contain more polar molecules. The more polar molecules an asphalt contains, the more readily it will accept water. However, the oxidation aging effects probably counteract any moisture-related aging effects.

In summary, asphalt is a complex chemical substance. Although basic chemical composition is important, it is an asphalt’s chemical microstructure that is most influential in its physical behavior. Although most basic asphalt binder failure mechanisms can be described chemically, currently there is not enough asphalt chemical knowledge to adequately predict performance. Therefore, physical properties and tests are used.

Physical Properties

Asphalt binders are most commonly characterized by their physical properties. An asphalt binder’s physical properties directly describe how it will perform as a constituent in HMA pavement. The challenge in physical property characterization is to develop physical tests that can satisfactorily characterize key asphalt binder parameters and how these parameters change throughout the life of an HMA pavement.

The earliest physical tests were empirically derived tests. Some of these tests (such as the penetration test) have been used for the better part of the 20^th century with good results. Later tests (such as the viscosity tests) were first attempts at using fundamental engineering parameters to describe asphalt binder physical properties. Ties between tested parameters and field performance were still quite tenuous. Superpave binder tests, developed in the 1980s and 1990s, were developed with the goal of measuring specific asphalt binder physical properties that are directly related to field performance by engineering principles. These tests are generally a bit more complex but seem to accomplish a more thorough characterization of the tested asphalt binder.

This more common U.S. asphalt binder physical properties are:

• Durability
• Rheology
• Asphalt Safety
• Purity

Grading Systems

Rather than refer to an extensive list of its physical properties, asphalt binders are typically categorized by one or more shorthand grading systems. The following grading systems range from simple to complex and represent an evolution in the ability to characterize asphalt binder:

• Penetration Grading
• Viscosity Grading
• Superpave Performance Grading

Asphalt Binder Modifiers

Often, it is necessary to add something to asphalt cement in order to achieve desired properties. Items added to asphalt cement are broadly called "modifiers" and there are literally thousands of them. A general listing of modifiers is located at:

• Asphalt Modifiers

Other Forms of Asphalt

Although asphalt cement is probably the most well known type of asphalt, other forms of asphalt that are used prominently in the paving industry are:

• Emulsified Asphalt
• Cutback Asphalt
• Foamed Asphalt

Note: These types of asphalt are not used in HMA pavements but are used extensively in pavement repairs, supporting layer or subgrade stabilization, bituminous surface treatments (BSTs), slurry seals, tack coats, fog seals, hot in-place recycling (HIPR), cold in-place recycling (CIR) and full depth recycling (FDR).