Tarmac is short for “tarmacadam,” and in its original form it was exactly what the name suggests: coal tar poured over layers of crushed stone. The recipe has changed substantially since the early 1900s, though. What most people call tarmac today is actually asphalt concrete, a blend of petroleum-derived bitumen, mineral aggregates, and fine fillers mixed and laid hot. The shift from coal tar to bitumen, and the growing list of additives and recycled ingredients now finding their way into the mix, make the modern version a far more engineered product than the accidental invention it started as.
Tarmac, Asphalt, and Bitumen — Sorting Out the Names
The word “tarmac” started as a brand name. Edgar Purnell Hooley patented the material in 1902 after noticing that a barrel of tar spilled on a macadam road created a smooth, dust-free surface. His product gave the English language a word that stuck, even as the material itself evolved beyond recognition.
Today, “tarmac” in everyday speech usually means the dark surface on roads, airport runways, and driveways. But in the paving industry, the proper term is almost always “asphalt” or “asphalt concrete.” True tarmac made with coal tar is rarely laid anymore in most countries, replaced by formulations that use bitumen — a thick, sticky residue left over from crude oil refining. In everyday British English, people tend to say “tarmac”; in American English, “asphalt” or “blacktop” is more common. They are generally referring to the same material.
Bitumen is the binder, the glue that holds everything together. Asphalt concrete is the finished product: binder plus stone plus fillers. When you stand on a road, the black surface underfoot is asphalt concrete, and the dark glue holding the stones in place is bitumen.
The Binder — Bitumen from Crude Oil
The binder is what makes the mixture cohesive rather than just a pile of gravel. In modern roads, that binder is almost always bitumen, produced during crude oil refining. When refineries distill crude oil at high temperatures to extract gasoline, diesel, and kerosene, the heaviest fraction left behind is what becomes bitumen. During this process, the chemical character of the original crude changes: aromatic compounds become more concentrated, and lighter oil fractions are stripped away.1Fuel. Investigation on the chemical composition evolution from crude oil to asphalt
Bitumen typically makes up about 4–8% of the total asphalt mixture by weight, but its influence on performance is enormous. It needs to be flexible enough not to crack in cold weather, stiff enough not to flow in summer heat, and sticky enough to grip the stones permanently. Refineries and asphalt producers adjust the grade of bitumen — its hardness and viscosity — to suit the local climate and expected traffic loads. A road in northern Scandinavia needs a softer grade than a highway in the Arabian Peninsula.
Mineral Aggregates — The Structural Backbone
Crushed stone and sand make up roughly 90–95% of an asphalt mixture by weight. These aggregates provide the structural skeleton that carries the load of every car and truck passing over it. The type of stone matters more than you might expect. Research has shown that both the strength and the shape of aggregates significantly affect how loads transfer through the pavement. Harder, more angular stones like granite and diabase interlock better than softer, rounder particles, forming a sturdier internal structure.2ScienceDirect. Study on the influence of aggregate strength and shape on the performance of asphalt mixture
Aggregate sizes are carefully graded — a blend of large stones, smaller chips, and fine sand — so the pieces fit together tightly with minimal gaps. On top of this, a small proportion of very fine powder known as mineral filler fills the remaining voids and stiffens the binder. One of the most common fillers is hydrated lime, which does more than just fill space: it improves the bond between the bitumen and the stone, helps the pavement resist moisture damage, slows down the aging of the binder, and acts as an anti-stripping agent.3Construction and Building Materials. Mechanical and durability performance of asphalt mixtures with partial replacement of hydrated lime by calcium carbonate Other mineral fillers include limestone dust, Portland cement, and fly ash, chosen based on local availability and the properties the engineer needs.
Why Coal Tar Disappeared from Most Roads
The original “tar” in tarmac was coal tar, a byproduct of cooking coal at high temperatures to produce coke for steelmaking. Coal tar is a complex mix of organic compounds. Testing has measured its melting point at around 80°C, with a specific gravity of about 1.18 g/cm³.4Journal of Physics: Conference Series. Coal tar, material used in soil improvement for use in road engineering It does an adequate job of binding stone, but the trouble lies in what else it contains.
Coal-tar pitch is a known human carcinogen packed with roughly 200 polycyclic aromatic hydrocarbon (PAH) compounds. In the central and eastern United States, coal-tar-based sealcoat products — applied to parking lots, driveways, and even playgrounds — are typically 20–35% coal-tar pitch. Research has found PAHs from these products contaminating stormwater runoff, lake sediment, soil, house dust, and air.5PubMed Central. Coal-tar-based pavement sealcoat and PAHs: implications for the environment, human health, and stormwater management
The toxicity extends to aquatic life. Studies have shown that coal-tar sealcoat runoff is acutely lethal to some fish species, and even at lower concentrations it causes a range of cardiovascular abnormalities in developing fish.6PubMed. Severe Coal Tar Sealcoat Runoff Toxicity to Fish Is Prevented by Bioretention Filtration These health and environmental concerns are the main reason road construction shifted to petroleum-based bitumen decades ago. Coal tar still turns up in some specialty applications and parking-lot sealcoats in parts of North America, but many states and municipalities have banned coal-tar sealants entirely.
Modern Additives That Go Into the Mix
Bitumen and stone are the foundation, but engineers often add other ingredients to fine-tune how the pavement performs.
Polymers are among the most common modifiers. Styrene-butadiene-styrene (SBS) rubber is widely blended into bitumen to make it more elastic and resistant to cracking. Polymer-modified bitumen costs more per ton but lasts longer on heavily trafficked highways and in climates that swing between extremes of heat and cold.
Warm mix asphalt technologies represent a more recent shift. Conventional hot mix asphalt is produced and laid at temperatures around 150–180°C. Warm mix additives — waxes, chemical surfactants, and foaming agents — allow the mixture to be workable at temperatures 20–40°C lower than that.7PubMed Central. The Role of Additives in Warm Mix Asphalt Technology: An Insight into Their Mechanisms of Improving an Emerging Technology Lower production temperatures mean less fuel burned at the asphalt plant, fewer fumes for workers, and a smaller carbon footprint per ton. Trade names like Sasobit (a synthetic wax) and Evotherm (a chemical surfactant) are among the most widely used warm mix products.
Anti-stripping agents are another routine addition. Water is one of asphalt’s worst enemies — if it works its way between the bitumen and the stone, the bond fails and the pavement crumbles from within. Liquid anti-stripping agents, often amine-based compounds, help the binder cling to stone surfaces even in persistently wet conditions. Hydrated lime serves the same function when used as a filler, which is one reason it remains so popular.
What Happens When Asphalt Ages
Once laid, asphalt does not stay the same forever. The bitumen binder slowly oxidizes, losing the lighter, more volatile compounds that keep it flexible. Over years of sun, rain, and traffic, the surface becomes stiffer and more brittle. That is why old roads develop networks of interconnected cracks, sometimes called “alligator cracking” because of the pattern. UV radiation speeds up the process, and daily temperature cycling — hot days followed by cold nights — stresses the material repeatedly.
This aging is also why you sometimes see fresh, jet-black patches next to faded gray pavement. The gray surface has lost much of its original binder chemistry. Road agencies apply surface treatments like chip seals or thin overlays to extend the life of aging pavement before a full repaving becomes necessary. Understanding the oxidation process has become increasingly important as engineers try to build roads that last longer with less maintenance.
Recycled Asphalt Pavement
One of the biggest trends in road construction is reusing old asphalt. When a road is milled up for repaving, the resulting material — called reclaimed asphalt pavement, or RAP — still contains usable bitumen and perfectly good stone. The challenge is that the old binder has stiffened with age, so it cannot simply be reheated and relaid as-is.
This is where rejuvenators come in. These are chemical additives designed to restore the flexibility of aged bitumen by replenishing the lighter oil fractions it has lost. Researchers are actively evaluating different rejuvenator types to find the best balance of restored flexibility and long-term durability.8PubMed Central. Performance evaluation of rejuvenators in recycled asphalt mixtures based on mechanical and rheological properties In one Norwegian case study, a heavily aged RAP binder was successfully refreshed to match the chemical, physical, and rheological properties of fresh bitumen.9Case Studies in Construction Materials. The classification and reutilisation of recycled asphalt pavement binder: Norwegian case study
Pushing the proportion of RAP in new asphalt mixtures higher is an active area of engineering. Some projects now use enhanced grading methods combined with bio-rejuvenators and reinforcing fibers such as basalt to allow more old material in the blend without sacrificing pavement quality.10Construction and Building Materials. Maximizing the circularity of asphalt pavements by improving the RAP content in recycled asphalt mixtures The economic logic is simple: less virgin bitumen and less newly quarried stone means lower material costs, less mining, and less waste sent to landfills.
Bio-Based and Waste-Plastic Alternatives
Because bitumen is a petroleum product, its cost tracks oil prices and its production contributes to carbon emissions.1Fuel. Investigation on the chemical composition evolution from crude oil to asphalt The road industry has been experimenting with partial replacements from renewable sources, and the results so far are genuinely interesting.11PubMed Central. State-of-the-Art Review on the Behavior of Bio-Asphalt Binders and Mixtures
Lignin, a natural polymer found in wood and other plant material, has shown promise as a partial substitute for bitumen. Adding it at modest dosages — around 3–5% of the binder weight — reduces the binder’s susceptibility to aging, essentially serving as both a bitumen replacement and an antioxidant. The trade-off is that mixing temperatures need to stay below about 180°C to avoid degrading the lignin.12Construction and Building Materials. Comprehensive characterisation of lignin-based bio-bitumen as a sustainable paving material Combining lignin with bio-oils derived from waste biomass is another avenue being explored, with the goal of reducing reliance on petroleum even further.13Construction and Building Materials. Performance and synergistic mechanism of bio-oil/lignin composite modified asphalt based on molecular dynamics
On the waste-plastic front, shredded polyethylene, polypropylene, and other common plastics have been blended into asphalt mixtures in pilot projects around the world. Reviews of the research suggest that incorporating waste plastics can significantly improve how the pavement handles high temperatures, though performance depends heavily on the type of plastic, the dosage, and the blending method.14PubMed Central. Using Waste Plastics as Asphalt Modifier: A Review Recycled rubber from old tires shows acceptable moisture resistance when added to asphalt, but the interaction between plastic or rubber properties and mixture behavior still needs more study before large-scale standardization.15Journal of Traffic and Transportation Engineering (English Edition). Rubber and plastic waste-modified asphalt binders and mixtures: Performance against environmentally induced distresses
Perhaps the most surprising finding involves recycled ethylene-vinyl acetate (EVA), a foam material common in shoe soles and packaging. In lab comparisons, recycled EVA performed similarly to virgin SBS polymer in viscosity and flow behavior, and it actually showed slightly better storage stability, likely because the EVA disperses more evenly through the binder.16Construction and Building Materials. Using waste polymers as a reliable alternative for asphalt binder modification – Performance and morphological assessment Findings like these point toward a future where road surfaces could absorb meaningful streams of post-consumer and industrial waste.
How Asphalt Pavement Absorbs and Releases Heat
If you have ever walked barefoot across a parking lot in July, you already know that dark pavement gets brutally hot. Asphalt concrete’s dark color gives it low albedo — it reflects very little sunlight — and its dense structure lets it store a great deal of heat. On hot summer days, asphalt surface temperatures can climb to around 60°C.17PubMed. The urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete
This property makes asphalt a significant contributor to the urban heat island effect, where cities run several degrees warmer than surrounding countryside. The pavement absorbs solar energy during the day and radiates it back as heat well into the night, keeping nighttime temperatures elevated. Engineers have developed “cool pavement” strategies to address this: reflective coatings or lighter-colored aggregates to bounce more sunlight away, and permeable or porous surfaces that cool themselves through water evaporation. There is a catch, though. Both approaches need to be matched to local conditions. In certain urban geometries, a highly reflective pavement can redirect heat onto nearby buildings or pedestrians, potentially making the heat island worse rather than better.17PubMed. The urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete
Testing the Finished Product
Getting the recipe right is only half the battle. Asphalt mixtures are tested before and during construction to make sure they will perform once traffic starts rolling over them. Engineers measure properties like stiffness, resistance to permanent deformation under heavy loads (rutting), and resistance to cracking at low temperatures.
There has been growing interest in moving beyond simple recipe-based specifications toward performance-related testing, where the mixture is evaluated for how well it actually resists the kinds of damage real roads face. Despite that interest, adoption has been slow. The asphalt industry has frequently pointed to equipment cost, test complexity, and the time and labor required as barriers to routine performance testing during quality control.18Transportation Research Record: Journal of the Transportation Research Board. Indirect Tensile Test (IDT) to Determine Asphalt Mixture Performance Indicators during Quality Control Testing in New Jersey As a result, many agencies still rely primarily on checking that the mixture has the right proportions of binder, stone, and air voids, rather than directly measuring how the finished product will behave under a decade of truck traffic.
A typical flexible road is a layered system. A surface course of asphalt concrete sits on top of a base course, which sits on a sub-base, which rests on compacted soil underneath.19Construction and Building Materials. Investigations on design and durability characteristics of cement treated reclaimed asphalt for base and subbase layers Each layer has to do its job for the whole structure to last, and no amount of perfect surface mix can compensate for a poorly prepared foundation. It is the interaction among ingredients, layers, and construction quality that determines whether a road holds up for five years or twenty-five.