Asphalt is a complicated mixture
By Dr. Selim Erhan, TLT Editor | TLT From the Editor October 2025
Road engineers specify the amount of gravel, type of gravel, the amount of asphalt and the adhesion chemistries based on the road’s load and weather requirements.
We as lubrication-related professionals usually concentrate on our issues, which is plenty. But our health and lives are intertwined with all the other areas of application, so it is important that we look into some of these other areas to protect ourselves. One such area is asphalt! There are internal lubricants in it, so it is somewhat involving lubrication, too. It is an important area because we all drive on it, we pay for it and we breathe it in. I will come to this a bit later.
Crude petroleum has several fractions that are held together with natural surfactants. There are hydrocarbons making up the fluid section. There are also benzene rings and many other types of molecules. I want to mention the benzene rings from a structural point of view. If we look at the road-making process from a 30,000 feet level, we see that the mixture that is laid on the roads is composed of aggregate, which is gravel that is coated with asphalt. The asphalt holds the gravel together giving the road the strength to carry the weight it needs to carry. The amount of gravel, type of gravel, the amount of asphalt and the adhesion chemistries are all well known. Specifications are set by road engineers. These specifications are designed after many years of studies to optimize the mixture factoring in the load that road is going to carry, the weather conditions it will live through and the length of time it is designed to last.
Roads are designed to last for about 10 years. They are also very expensive. A mile of road costs about 10 million dollars. These specifications were prepared in the 1950s but revised as new materials become available. I drove in Finland where the whole country is under snow and ice during their very, very long winters. I did not see a single crack or pothole! I drove in Dubai, the other opposite where the heat is intense during their very, very long summers, and again did not see ruts and waves on their asphalt. These countries are not rich enough to lay their roads every year. Or they prefer to spend their tax dollars on other useful areas. It is sad to hear the asphalt joke in the U.S., which asks, how do police catch drunk drivers? Because drunk drivers go straight and do not weave to avoid potholes! I guess the U.S. is able to pave roads every year. We evidently also have the extra time to sit in road construction traffic for hours. When one time I was ranting and raving in a taxi, the driver, who at one time had worked in road construction, said, “If we made roads like they did back in the 1950s, we would go bankrupt!”
Coming back to chemistry, how the asphalt will coat the aggregates is very important because during use, if the stone surface is exposed to water, it will adhere to the stone and when it freezes and expands cracks, it will take in more water to make matters worse. The stone surface is inorganic and polar, but asphalt is organic and mostly nonpolar. So wetting agents are used to adhere the organic surfaces onto inorganic surfaces. The final mixture is then tested with very expensive and sophisticated rheometers and load instruments to see if it passes very low to very high temperature tests and load tests.
Once the asphalt is in place, with time, the liquid hydrocarbon fraction slowly starts seeping into the layers below or evaporates. The millions of benzene rings that had been emulsified then start aggregating. A six-membered ring has a chair-like shape but benzene rings have three conjugated double bonds which flatten the ring to allow free movement to the electron clouds above and below the ring. These flat benzene rings easily stack together to form brittle layers that we call asphaltene. The asphalt starts losing its flexibility and cracks easily under load.
Roads are expensive so it is beneficial to at least recycle used asphalt. The problem is that it is very difficult to separate the now aggregated asphaltene layers. Nonpolar solvents cannot penetrate and separate the benzene rings so they can be re-emulsified into their original consistency. The solvents are too nonpolar and the electron density in the asphaltene aggregated makes them too polar. The industry has developed rejuvenating agents that can penetrate and separate the benzene rings and when they are used, the percentage of the recycled asphalt pavement (RAP) goes from 10% to 50%. Quite a bit of savings.
The structure and lifetime of asphalt is important from another point of view. The tire must somewhat adhere to the road to get traction. As the tire rolls very small pieces of rubber tear off the tire, also tearing a piece of asphalt together with it, and now the particle with mixed amounts of asphalt and rubber becomes airborne. It stays in the air and travels a significant distance so there is plenty of chance for people to breathe it in. The rubber industry is now spending a significant amount of time increasing the tear resistance of tires to reduce wear and lower the wear particles that become airborne. Hopefully the asphalt industry will also look into what can be done to improve the flexibility of asphalt and reduce tearing. If they do not, then someone must ask for it.
Dr. Selim Erhan is president of Erbur Solutions in Trout Valley, Ill. You can reach him at selim.erhan@outlook.com.