Since 1978, Caltrans has used recycled tire rubber in 17 asphalt overlays; a technical review found less cracking, distress and maintenance than conventional mixtures, including thicker ones

Every time you invest in a new set of tires, you need to dispose of your old ones. When you factor in every driver across America, the amount of waste created is staggering. Around 273 million tires are disposed of every year in the US. However, according to the U.S. Tire Manufacturers Association’s 2025 End-of-Life Tire Market Report, around 79% are put into secondary markets such as fuel, mulch, and construction materials. One of the oldest, least exciting ways these old tires are used is in the formation of asphalt.

The 48-year-old pilot nobody talks about

In 1978, California’s Department of Transportation began using crumbed tires. This involved crushing old tires, mixing them into the asphalt binder, and using the new mixture to make their roads. Over the following years, Caltrans went on to create 17 ‘wet process’ overlays. Overall, the overlays performed better than traditional dense asphalt mixtures, exhibiting less cracking, less distress, and requiring less maintenance overall. A technical review from the Recycled Materials Resource Center, which is a federally affiliated research center based at the University of Wisconsin-Madison, goes on to describe how the overlays are typically better than traditional asphalt mixtures of higher thicknesses. This is an incredible achievement considering that part of the material is coming from waste going into landfill.

Since 2007, due to California law, Caltrans is required to use rubberised asphalt in an increasing percentage of their projects. Since 2013, they have been required to use at least 35% of their overall asphalt tonnage. They have always managed to meet or exceed this. The reason is that there are fewer tires in circulation. Every tonne of rubberised pavement used means fewer tires end up in storage or landfill: the program’s core environmental payoff. Plus (in theory), taxpayers are getting a surface that requires less re-surfacing. However, many other states, including Arizona, Texas and Florida, have tried this to varying levels of success.

The not-so-shiny side

It is not always a great improvement over traditional asphalt. The results have varied greatly between states. Some overlay projects between layers in states like Kansas and Minnesota have not justified the higher costs. There is also another method of producing rubberised asphalt called the ‘dry process’. This involves mixing the crumbed rubber into the aggregate. This has shown to be less reliable, with issues of raveling.

The researchers in California have also been carrying out many tests to improve the formula. A 2024 peer-reviewed study titled “Evaluating the effect of reclaimed asphalt pavement on rubberized hot mix asphalt in pilot projects,” conducted by the Pavement Research Centre at UC Davis and published in the journal Construction and Building Materials, looked at 5 pilot projects created by Caltrans in 2022 and 2023. The results showed that adding recycled reclaimed asphalt pavement had little to no improvement in cracking performance.

The bigger picture

This is not a dramatic solution for climate change. However, it is an incredibly useful example of how circularity can be achieved on a large scale. The rubber from your tires that you replaced on your sedan could well be used to help smooth over the potholes you hit on your commute. As many sustainability campaigns require you to do more, this is a rare case where it is done for you. Next time you see a maintenance team resurfacing a section of motorway, remember they may well be using a recycled surface.