Asphalt is not a single chemical but a highly complex, sticky mixture. The composition is primarily derived from the residue left after crude oil refining, known as bitumen or asphalt cement, combined with crushed rock. Understanding the chemicals in asphalt requires looking at the bulk physical components and then diving into the molecular structure of the bituminous binder itself. This intricate blend is designed to provide durability and flexibility to withstand heavy traffic and varying weather conditions.
The Physical Components: Aggregate and Binder
The asphalt mixture, or asphalt concrete, consists of two main physical components: the aggregate and the binder. The aggregate forms the bulk of the material, making up approximately 95% of the total mass. This portion is composed of chemically inert materials such as crushed stone, sand, and gravel, often derived from silica or limestone.
The asphalt binder, also known as bitumen, typically comprises only 5% to 6% of the final product by mass. This binder acts as the glue, coating and holding the aggregate particles together to create a cohesive, load-bearing pavement. Although a small percentage of the total, the binder’s properties determine the asphalt’s overall performance, including its resistance to cracking and rutting.
The Complex Hydrocarbon Classes of Bitumen
The asphalt binder is a colloidal system composed mainly of carbon and hydrogen, with trace amounts of sulfur, nitrogen, and oxygen. Scientists use a method called SARA fractionation to categorize its components based on their polarity and molecular size. The SARA acronym stands for Saturates, Aromatics, Resins, and Asphaltenes, which are the four main chemical classes in the binder.
Saturates are the least polar fraction, consisting of linear, branched, and cyclic saturated hydrocarbons, which contribute to the fluidity and workability of the asphalt. Aromatics are slightly more polar and act primarily as the solvent or continuous phase, helping to dissolve the more complex components. Resins are highly polar compounds that are miscible in the aromatic solvent, playing a crucial role by stabilizing the most complex fraction.
Asphaltenes are the largest and most polar molecules, forming dispersed solid-like particles within the binder that provide the material’s hardness and viscosity. The balance between these four fractions is paramount to the binder’s performance, affecting its flexibility, durability, and resistance to aging. If the ratio is unbalanced, the asphalt can become brittle or too soft.
Volatile and Semi-Volatile Trace Compounds
During the manufacturing and laying of asphalt, trace chemical compounds can be released into the air. These emissions are categorized as Volatile Organic Compounds (VOCs) and Polycyclic Aromatic Hydrocarbons (PAHs). VOCs are compounds that easily turn into gases at typical temperatures and are the source of the distinct odor associated with fresh asphalt.
Examples of VOCs released include toluene, xylene, and ethylbenzene. PAHs are semi-volatile organic compounds, which are larger molecules that consist of multiple fused benzene rings. These compounds are of interest because some types are suspected carcinogens.
The PAHs released from asphalt fumes are typically dominated by smaller molecules with two or three aromatic rings, such as naphthalene and phenanthrene. While these trace compounds are not part of the bulk structural chemistry, their emission factors are closely monitored and generally decrease when lower-temperature mixing methods, like warm mix asphalt, are used.
Intentional Additives and Mix Modifiers
Beyond the natural components of aggregate and bitumen, modern asphalt often incorporates various modifiers to enhance its performance. These substances are added to alter the physical properties of the binder or the overall mix to suit specific environmental or traffic demands. Polymer modifiers are one of the most common additions, with materials like Styrene-Butadiene-Styrene (SBS) block copolymers being used to increase the asphalt’s elasticity and high-temperature stiffness.
Elastomers, such as SBS, help the pavement resist rutting and cracking by making the binder more rubbery and durable over a wider range of temperatures. Other chemical additives include anti-stripping agents, which improve the adhesion between the binder and the aggregate, preventing moisture damage. Mineral fillers, like hydrated lime or cement dust, are also added in small amounts to stiffen the mixture and improve its density.