The common term “tar” used when referring to roads misidentifies the material used in modern construction. The dark, sticky substance binding most roads today is not true tar but a petroleum product called bitumen, often referred to as asphalt cement in North America. This distinction is important because the source, composition, and performance of these materials differ significantly. Bitumen provides the flexibility and durability required for contemporary roadways, replacing true tar which was phased out due to inferior performance and environmental concerns.
The Historical Binder True Tar
True tar is a complex black material produced by the destructive distillation of organic substances, primarily coal or wood, by heating them in the absence of air. The most common historical form was coal tar, a byproduct from producing coal gas and coke. It was widely available and cheap during the 19th and early 20th centuries, making it an attractive binder for early pavement designs.
The term “tarmac” originates from “tar macadam,” a road surface that mixed macadam (crushed stone layers) with coal tar to create a more stable surface. While effective at holding aggregates, coal tar had inherent weaknesses that limited its long-term viability. It softened excessively in hot weather and became brittle, cracking easily, in cold temperatures. Petroleum byproducts eventually replaced it with a more stable and cost-effective alternative.
The Modern Road Binder Bitumen
The binder that holds modern roads together is bitumen, a highly viscous, black, sticky material derived from crude oil refining. Bitumen is a residual byproduct left over after lighter products like gasoline, kerosene, and diesel have been distilled. This process transforms it into the specialized binder used for paving and roofing applications.
Chemically, bitumen is a complex mixture of hydrocarbons, mainly consisting of asphaltenes and maltenes. Asphaltenes are large molecules that provide stiffness and cohesive strength, while maltenes are smaller molecules that provide flexibility and workability. This chemical structure results in bitumen’s viscoelastic properties, meaning it behaves like a viscous fluid at high temperatures and a more elastic solid at lower temperatures. This characteristic is necessary for a durable road surface. In the United States, this petroleum-based binder is called asphalt cement or asphalt binder.
The Final Product Asphalt Concrete
The material used to pave roads is not pure bitumen but a composite known as asphalt concrete, often colloquially referred to as “asphalt.” This final product is a precise mixture of two main components: aggregate and the bitumen binder. The aggregate provides the structural strength of the road, while the bitumen acts as the adhesive to hold the structure together and provide waterproofing.
In a typical asphalt concrete mix, the aggregate (crushed stone, gravel, and sand) accounts for 90% to 95% of the total weight. The bitumen binder makes up the remaining 5% to 10% by mass. These materials are heated and mixed at high temperatures (usually between 300°F and 350°F) to create hot mix asphalt. The mixture is then transported and compacted onto the road base. The bitumen coats the aggregate particles, sealing the mixture and preventing water penetration, which ensures the road’s longevity.
Environmental and Health Impacts
The primary health concern distinguishing true coal tar from modern bitumen is the concentration of Polycyclic Aromatic Hydrocarbons (PAHs). Coal tar is a known hazardous substance with high levels of carcinogenic PAHs, posing a significant risk when disturbed or when it leaches into the environment. Its use as a road binder largely ceased in the late 20th century due to these toxicity concerns.
Modern petroleum-based bitumen contains significantly lower levels of PAHs, making it a safer material for widespread use in infrastructure. Asphalt concrete is highly sustainable environmentally, as it is one of the most recycled materials globally, with high percentages of old pavement incorporated into new road mixes. However, the dark surface of asphalt contributes to the urban heat island effect, and stormwater runoff from roads can carry pollutants, which are ongoing challenges for pavement engineers.