Asphalt concrete

Asphalt concrete is a material often called asphalt, blacktop, or pavement in North America, and tarmac or bitmac in the United Kingdom and Ireland. It is used to cover roads, parking lots, airports, and the core of embankment dams. People have been using asphalt mixtures for building roads since the nineteenth century.

Asphalt concrete is made from mineral aggregate bound together with bitumen, a substance also known as asphalt, pitch, or tar. It is laid down in layers and then compacted to make a strong, smooth surface. In engineering and construction, terms like "asphalt concrete" or "bituminous mixture" are used to describe this composite material made from mineral aggregate and a binder.

History

Natural asphalt has been used since ancient times in places like Mesopotamia, Phoenicia, Egypt, Babylon, Greece, Carthage, and Rome. People used it to make things waterproof, like baths and tunnels, and to build strong roads.

Over time, many places started using asphalt for roads. In 1824, France began using natural asphalt to cover stones for roads. In 1829, a special mix of asphalt was used for a pathway in Lyon, France, and this idea spread to other cities. By the mid-1800s, experiments with asphalt continued in places like Nottingham and Huntingdon. Later, new methods were invented to mix asphalt with other materials to make even better road surfaces.

Mixture formulations

Asphalt concrete can be made in different ways by mixing asphalt and aggregate (small pieces of rock). One common way is called hot-mix asphalt concrete. This is made by heating the asphalt and drying the aggregate before mixing them together. It must be laid while still hot and is often used on busy roads, racetracks, and airfields.

Another way is warm-mix asphalt concrete, which uses special additives to let the mixture be made at lower temperatures. This saves energy and helps the environment. Cold-mix asphalt concrete uses a special liquid to make the asphalt easier to work with, and it is often used for small repairs or less busy roads. There are also other special types of asphalt mixtures made for specific uses, like letting water drain through the pavement.

Roadway performance characteristics

Different kinds of asphalt concrete work differently on roads. They can affect how long the road lasts, how tires wear out, how well cars can stop, and how much noise the road makes. When picking the right kind of asphalt, it’s important to think about how many cars will drive there and what the road needs to do.

Asphalt can shape into a curved surface that helps water run off to the sides. While concrete can also be shaped this way, asphalt is often chosen because it’s cheaper. Asphalt roads are also quieter than roads made from Portland cement concrete or chip seal. As cars go faster, they make more noise because of how their tires interact with the road. Thinking about noise when designing highways became more common in the early 1970s.

How well asphalt roads hold up depends on many things, like the materials used, how the road is used, and the weather. Over time, asphalt roads change, so how they work in the long run is different from how they work at first. The LTPP is a research program by the FHWA that studies how roads behave over many years.

Degradation and restoration

Asphalt roads can get damaged in many ways, such as having cracks, potholes, or uneven parts. Cold weather can make the road break apart, even in just one winter. To fix these problems, workers can fill in the cracks, but the best way to keep roads lasting longer is to build them well and keep water away.

There are three main reasons roads get worse over time: how they are built, the weather, and how much traffic they have. If a road is not built correctly, it can last much longer. Water under the road can also cause problems, especially when it freezes and thaws. Heavy traffic can hurt roads a lot, especially big trucks. Keeping roads clean and well-maintained helps them stay in good shape for many years.

Recycling

Asphalt concrete can be recycled and reused in new roads and other projects. The most common recycled material is reclaimed asphalt pavement (RAP), which is used more than any other recycled material in the United States. Many roofing shingles also contain asphalt, and these can be reused too. Studies show that recycled materials can replace some or even all of the new materials needed for asphalt mixes, though this is usually limited by rules and performance needs. In 2019, new asphalt in the United States contained about 21% RAP on average.

Recycled asphalt can be processed at plants or reused directly on the road. At plants, recycled materials are crushed, dried, and mixed into new asphalt. There are different ways to recycle asphalt directly on roads, such as breaking up the old surface and mixing it again.

Over time, the material that holds asphalt together hardens, which can affect how well recycled asphalt works. But by carefully mixing recycled and new materials, these issues can be managed. Some additives, called rejuvenators, can help restore the properties of older asphalt, allowing mixes with a lot of recycled content to perform well.

Besides RAP, other recycled materials like crumb rubber from tires, steel slag, and even waste plastics can be used in asphalt. Research continues to find new ways to use waste materials in asphalt mixes.