The black surface you drive on every day is almost certainly not tar at all. The word “tar” stuck around in everyday speech long after the actual substance disappeared from most road construction. True tar is a byproduct of heating coal to produce coke or gas, and it was widely used in road building through much of the twentieth century. What replaced it, and what now covers the vast majority of roads worldwide, is asphalt binder, a thick, dark material refined from crude oil. The two look similar and do similar jobs, but they come from completely different sources, have different chemistries, and carry very different health and environmental profiles.
Why People Still Say “Tar” When They Mean Asphalt
Coal tar was a go-to road material for decades, particularly in Europe and parts of North America. It was cheap, abundant, and a useful way to dispose of an industrial byproduct from gasworks and coke ovens. Over time, though, research revealed that coal tar contains high concentrations of polycyclic aromatic hydrocarbons, or PAHs, many of which are carcinogenic. Coal-tar-based products used as pavement sealants typically contain roughly 20 to 35 percent coal-tar pitch, which itself carries around 200 different PAH compounds.1PubMed Central. Coal-tar-based pavement sealcoat and PAHs: implications for the environment, human health, and stormwater management By the late twentieth century, many countries had moved away from coal tar in road construction, and asphalt containing more than 0.1 percent coal tar is now classified as hazardous waste under European regulations.2ScienceDirect. Tar-containing reclaimed asphalt – Environmental and cost assessments for two treatment scenarios
The language never caught up with the chemistry. In British English, “tarmac” (short for tarmacadam) remains everyday shorthand for any paved surface. In American English, “tar” persists as a casual synonym for asphalt. But modern road surfaces are petroleum asphalt mixed with crushed stone aggregate, not coal tar mixed with anything.
What Petroleum Asphalt Binder Is Actually Made Of
Asphalt binder is the heaviest fraction left over after crude oil has been refined. When a refinery distills petroleum, it pulls off gasoline, diesel, kerosene, and other lighter products at progressively higher temperatures. The thick residue that remains at the bottom of the distillation column is asphalt cement, sometimes called bitumen. It accounts for a small fraction of each barrel of crude but is produced in enormous volumes because the world processes so much oil.
Chemically, this residue is a dense soup of hydrocarbons. Researchers break it into four families of molecules, collectively called the SARA fractions: saturates, aromatics, resins, and asphaltenes.3PubMed Central. Research on the Correlation Between the Chemical Components and the Macroscopic Properties of Asphalt Binder Each fraction plays a different role in how the binder behaves on the road.
- Saturates: Light, waxy molecules that help the binder flow. They have the lowest molecular weight of the four groups and act as a dispersing medium between the heavier components.
- Aromatics: Ring-shaped hydrocarbons that also belong to the lighter fraction. Together with saturates, they keep the binder flexible enough to coat aggregate stones.
- Resins: Heavier, polar molecules that serve as a bridge between the light oils and the heaviest fraction. They help keep asphaltenes suspended rather than clumping together.
- Asphaltenes: The heaviest, most complex molecules in the mix. They give asphalt its stiffness and load-bearing strength. Asphaltenes and resins tend to cluster together because of their high molecular weight and polarity, forming the structural core of the binder’s internal architecture.4ScienceDirect. Impacts of colloidal and interfacial interactions on bonding strength of rubberized RAP asphalt binder
The balance among these four groups determines whether a given batch of asphalt is soft and pliable or hard and brittle. Crude oils from different parts of the world produce binders with different SARA ratios, which is one reason road engineers can’t just use any asphalt interchangeably. A binder rich in asphaltenes will be stiffer; one heavy on aromatics and saturates will be softer and more flexible.
How the Binder Holds Together at a Molecular Level
Asphalt binder behaves as a colloidal system, meaning it has tiny particles (asphaltene clusters) suspended in a continuous oily phase (the maltenes, which are everything that isn’t asphaltene). This colloidal model has been a central framework for understanding bitumen’s microstructure for decades.5PubMed Central. The Structure of Bitumen: Conceptual Models and Experimental Evidences Think of it loosely like a thick salad dressing where solid particles stay suspended rather than settling out. The resins wrap around asphaltene clusters and keep them dispersed, while saturates fill the spaces between. When the system is well balanced, the binder flows when heated, stiffens when cooled, and grips the aggregate stone tightly.
When that balance shifts, problems start. If the lighter fractions evaporate or oxidize away over time, the asphaltene clusters lose their cushion, begin to agglomerate, and the binder gets brittle. That’s essentially what pavement aging is.
How Asphalt Ages and Why Roads Fall Apart
A freshly paved road surface is dark, flexible, and resilient. Over the years, it turns gray and starts cracking. That change is driven by chemistry, not just traffic loads. Ultraviolet light, heat, and oxygen attack the lighter molecules in the binder. UV radiation carries enough energy to break many of the chemical bonds in asphalt, rearranging its composition and accelerating aging.6Case Studies in Construction Materials. Evolution of asphalt performance under the coupled aging of ultraviolet, high temperature and water As oxidation proceeds, sulfur atoms in the binder react with oxygen faster than carbon does, forming sulfoxide groups, while carbon atoms form carbonyl groups. Both reactions make the binder stiffer and more prone to cracking.
Water makes things worse. When moisture infiltrates the thin film of binder coating each aggregate stone, it can wedge itself between the binder and the rock at a molecular level. Research using molecular simulations has shown that water molecules form hydrogen bonds with the aggregate surface, pushing the asphalt aside and reducing adhesion. Aging compounds this problem because oxidized binder becomes more attracted to water (more hydrophilic), so aged pavement is even more vulnerable to moisture damage.7Journal of Cleaner Production. Moisture damage mechanism of asphalt mixtures containing reclaimed asphalt pavement binder: A novel molecular dynamics study The result is what road engineers call “stripping,” where the binder peels away from the stone underneath, leading to potholes and raveling.
Modifiers and Additives That Change the Recipe
Plain refined asphalt binder works fine for many roads, but engineers routinely modify it to handle extreme conditions. The most common modifications involve adding polymers. Styrene-butadiene-styrene (SBS) is the workhorse polymer additive in the industry. It creates a network of rubbery chains within the binder that helps it stretch without cracking at low temperatures and resist flow at high temperatures. Crumb rubber from recycled tires serves a similar purpose, and the two are sometimes combined.8Taylor & Francis Online (Road Materials and Pavement Design). Performance of crumb rubber bitumen and asphalt modified in the wet process alone and in combination with SBS polymer
Beyond polymers, the industry uses a range of other additives. Anti-stripping agents improve the bond between binder and stone so moisture can’t peel them apart.9Innovative Infrastructure Solutions. Evaluating moisture damage resistance in asphalt mixtures using amine-free anti-stripping agent for enhanced durability Warm-mix asphalt technologies, including wax additives, chemical surfactants, and foaming processes, allow the mix to be produced and compacted at lower temperatures, which reduces fuel use, emissions, and fume exposure for workers.10PubMed Central. The Role of Additives in Warm Mix Asphalt Technology: An Insight into Their Mechanisms of Improving an Emerging Technology Some of these additives, like Sasobit (a synthetic wax) or Evotherm (a chemical package), have become standard options that paving contractors can specify depending on the project.
There’s also growing interest in nano-scale additives. Nano-silica, for example, has been shown to improve an asphalt binder’s resistance to rutting at high temperatures, though it can reduce flexibility in the cold.11Journal of Rehabilitation in Civil Engineering. Rehabilitation of Asphalt Binder to Improve Rutting, Fatigue and Thermal Cracking Behavior using Nano-Silica and Synthesized Polyurethane No single additive solves every problem, which is why modified binder formulations are tailored to the climate and traffic loads of a given road.
Coal Tar’s Toxic Legacy
While coal tar largely disappeared from road construction by the 1990s and 2000s, it survives in one stubborn niche: pavement sealcoat. In parts of the central and eastern United States, coal-tar-based sealcoat is still sprayed onto parking lots, driveways, and even playgrounds. These products are the dominant source of PAHs in many urban watersheds, contaminating stormwater runoff, lake sediments, soil, house dust, and air.1PubMed Central. Coal-tar-based pavement sealcoat and PAHs: implications for the environment, human health, and stormwater management
The toxicity profile of coal-tar sealcoat goes beyond the well-known PAHs. When researchers tested coal-tar-based products for PAH derivatives (nitrogen-containing and oxygen-containing versions of PAHs), they found that including these compounds raised the calculated cancer-equivalent concentrations by up to about 39 percent. In mutagenicity testing, coal-tar-based sealcoats caused genetic mutations in bacteria, while asphalt-based sealcoat products did not.12Environmental Science & Technology Letters. Identification and Toxicological Evaluation of Unsubstituted PAHs and Novel PAH Derivatives in Pavement Sealcoat Products Coal-tar-based products also have a far more complex chemical fingerprint than petroleum asphalt. Advanced analysis has identified compounds with 88 unique elemental compositions in coal-tar sealcoat, encompassing a set of 240 individual PAH compounds, making them distinguishable from other PAH sources like diesel fuel or used motor oil.13PubMed. Characterization of Polycyclic Aromatic Compounds in Commercial Pavement Sealcoat Products for Enhanced Source Apportionment
Several U.S. states and municipalities have banned coal-tar sealcoat. The distinction matters for homeowners: if you’re sealing a driveway, asphalt-based sealcoat is not the same thing as coal-tar-based sealcoat, and the health and environmental difference between them is substantial.
Health Risks for Paving Workers
Even with coal tar largely out of the picture, petroleum-based asphalt paving carries occupational health concerns. Heating asphalt to working temperatures (typically above 150°C for conventional hot-mix) releases fumes containing a cocktail of volatile organic compounds. Studies have linked occupational exposure during asphalt paving to reduced lung function and increased lung cancer risk among paving workers.14Annals of Work Exposures and Health. Occupational Exposure during Asphalt Paving—Comparison of Hot and Warm Mix Asphalt in Field Experiments Warm-mix asphalt technologies, which allow paving at temperatures 20 to 40 degrees lower, are one of the most practical tools for reducing fume exposure. The lower production temperature means fewer volatile compounds evaporate into the air at the job site.
What Happens When Old Roads Get Recycled
Asphalt is one of the most recycled materials in the world. When a road is milled up for resurfacing, the resulting material, called reclaimed asphalt pavement (RAP), contains aged binder still coating the aggregate. Mixing RAP into new pavement saves virgin binder and stone, but the old binder is stiff and oxidized. Left as-is, high RAP content makes the new pavement brittle and crack-prone.15PubMed Central. Performance evaluation of rejuvenators in recycled asphalt mixtures based on mechanical and rheological properties
To solve this, engineers add rejuvenators, chemicals that restore the balance of light and heavy fractions in the aged binder. Some rejuvenators are petroleum-based oils, but there’s a strong push toward bio-based alternatives. Epoxidized soybean oil derivatives, for instance, have been shown to chemically break apart clumped asphaltenes in aged binder, enabling pavement with 45 percent recycled content while reducing cost, energy use, and emissions by about 30 percent compared to standard practice.16Fuel. Chemically mediated asphalt rejuvenation via epoxidized vegetable oil derivatives for sustainable pavements Palm oil derivatives are another avenue under investigation, designed to soften aged binder while maintaining compatibility with the existing asphalt chemistry.17Industrial Crops and Products. Palm oil-derived rejuvenator for recycled asphalt binders and mixtures: Molecular simulation and performance evaluation
The appeal of bio-based rejuvenators goes beyond green branding. Conventional petroleum rejuvenators are essentially light oils that dilute the stiff binder, but they don’t reverse the chemical changes caused by aging. Bio-based epoxidized oils, by contrast, appear to interact chemically with the asphaltene clusters, breaking them apart rather than just diluting them. That distinction translates into better cracking resistance over the long term.
Environmental Footprint of Modern Asphalt Roads
Normally constructed asphalt pavements release relatively low levels of contaminants during their working life.18PubMed. Review of the impact of stormwater and leaching from pavements on the environment Studies testing reclaimed asphalt for leaching found that it did not qualify as hazardous waste and did not release PAHs, volatile organic compounds, or most heavy metals above typical groundwater standards. The exception was lead in asphalt from older roadways, which initially leached at levels slightly above the primary drinking water standard but diminished over time.19Environmental Engineering Science. Leaching of pollutants from reclaimed asphalt pavement
The bigger environmental concern today is what comes off the pavement surface during rain. Heavy metals and PAHs can be released from asphalt pavement and carried by stormwater into nearby water bodies. Research exposing zebrafish to asphalt leachate has been used to characterize those ecotoxicological risks.20PubMed. Ecotoxicological risk of asphalt pavements to aquatic animals associated with pollutant leaching Coal-tar sealcoated surfaces, as noted earlier, are a far larger source of PAH contamination than the asphalt itself, so the type of surface treatment applied to a parking lot or road shoulder matters more than the underlying pavement.
Microplastics from Pavement Wear
A relatively new area of concern is microplastic generation from roads. When tires roll across asphalt, both the tire and the pavement surface wear down, shedding tiny particles. Tire wear particles are a major source of microplastics from vehicles, and increased road traffic raises their concentration in road dust. These particles can become airborne or wash into waterways during rain.21PubMed. Quantification of tire wear particles in road dust based on synthetic/natural rubber ratio using pyrolysis-gas chromatography-mass spectrometry across diverse tire types
Field studies have found that asphalt pavement is more susceptible to rutting and releases more microplastics than some alternative surfaces, including a large proportion of tire wear particles in the runoff.22PubMed. Pavement wear generates microplastics in stormwater runoff This has led researchers to argue that microplastic generation should be considered during pavement material selection, a factor that historically played no role in road design decisions. It is still early days for this line of research, but it adds another dimension to the environmental impact of the material beneath your tires.
Bio-Based Binders and the Road Ahead
The search for alternatives to petroleum-based asphalt binder is accelerating. Lignin, a natural polymer found in wood and agricultural waste, can partially replace petroleum binder, though it tends to make the asphalt stiffer and more brittle. Adding bio-oil (a liquid product of heating plant matter) to lignin-modified asphalt has been shown to significantly reduce that stiffness, improving flexibility and fatigue life. At a 10 percent bio-oil addition, the fatigue performance was comparable to unmodified asphalt, and the adhesive bond between binder and aggregate actually improved.23Materials Research Express. The impact of bio-oil on the structure, rheology, and adhesion properties of lignin-modified asphalt
None of these bio-based approaches are ready to fully replace petroleum asphalt at scale. The global road network is staggeringly large, and petroleum refining produces asphalt binder as a natural byproduct whether anyone wants it or not. But as crude oil sourcing becomes more volatile and environmental regulations tighten, the percentage of plant-derived material in road binder will likely grow, especially in applications where recycled content and rejuvenators already make up a significant share of the mix.
Self-Healing Pavement Technology
One of the more futuristic developments in road materials is self-healing asphalt. The concept combines two technologies: tiny capsules of rejuvenator embedded in the pavement, and induction heating using steel fibers mixed into the asphalt. When the pavement develops micro-cracks, applying an electromagnetic field heats the steel fibers, which softens the surrounding binder and allows the cracks to flow shut. Simultaneously, the capsules rupture and release rejuvenator into the crack zone, replenishing the aged binder from within. Laboratory tests have demonstrated that induction heating requires just two minutes of exposure to trigger repair, and the encapsulated rejuvenator restores the binder’s ability to heal itself over time.24Journal of Cleaner Production. A novel self-healing system: Towards a sustainable porous asphalt
Self-healing asphalt has moved from the lab to limited field trials, though widespread adoption is still years away. The extra material cost of steel fibers and microcapsules needs to be offset by longer pavement life and fewer maintenance closures. For porous asphalt pavements, which are especially vulnerable to aging because air circulates through the open voids, the technology is particularly promising. Whether it ever becomes standard practice depends on whether the lifecycle economics work out at full scale, a question that is still being answered.