In 1979, Minnesota began mixing ground-up scrap tires into two asphalt test roads; years later, FHWA said both experimental sections were still performing well

Each year, millions of tires reach the end of their usable life. Once they can no longer be safely used on vehicles, they become difficult to dispose of properly. But what if one could use some of that rubber in building roads? This is something that was attempted in Minnesota many years ago, when they added crumbled tire rubber into asphalt to see if it would help with winter road performance. It was an interesting way of giving tires a new lease on life under cars’ wheels.

The state's Department of Transportation put to the test two sections of asphalt made using a procedure called PlusRide. Years after the Minnesota experiment began in 1979, the Federal Highway Administration (FHWA) reviewed the PlusRide sections and found that the pavement had performed well structurally, although it did not provide the expected de-icing benefits. However, the pavement did not provide the expected ice-removal advantages that researchers hoped to achieve through the use of recycled rubber. This case is, thus, a good example to learn about road engineering because a recycled product can be used in a pavement but still not make it economically beneficial.

How old tires became part of road pavement

The Minnesota tests used a technique called the dry process, in which ground or granulated tire rubber replaces a small portion of the fine aggregate in an asphalt mix. Aggregate refers to the combination of materials such as stone and sand that provides structure to pavements. In this process, rubber particles are combined with the aggregate before the introduction of asphalt cement, which acts as a binder for the mix. This process is different from the wet process in which crumb rubber is first incorporated into the asphalt binder before mixing it with aggregate. The distinction matters because the two methods produce different mixtures and can lead to different results on the road.

In Minnesota, the PlusRide technology was utilized to create a gap-graded asphalt mixture that had granulated rubber. The idea behind this was to make a road that would shed ice, which was an attractive objective considering that in Minnesota, driving is often dangerous because of the harsh winters.

Why engineers wanted pavement that could resist ice

Winter roads cause numerous challenges for the transportation department. Ice causes loss of traction of tires, which prevents the vehicle from stopping and maneuvering properly. Road crew members can put de-icing agents or use plows on the roads, but this action needs to be taken repeatedly according to changing weather conditions. Asphalt designed to help melt ice or prevent ice formation appears to be another possible solution to this problem. The idea behind this was to create a road surface that could shed ice, an attractive objective considering that driving in Minnesota is often dangerous because of harsh winters.

But changing the composition of asphalt does not guarantee the desired effect. Road performance depends on many factors, including the ratio of components, the placement and compaction of the mixture, weather conditions during construction, and traffic conditions after the road opens. The mixture can perform adequately without having any additional features.

What the later assessment found

The FHWA’s review reported that both Minnesota PlusRide sections had performed well. However, they were found not to have any advantage in de-icing at all, and their performance does not justify the increased price of the road. The experiment showed that the rubberized pavement could function as a road surface, but its ability to provide the intended de-icing advantage was not demonstrated.

The experience of rubber-modified asphalt has also been quite controversial. In the same federal guidelines, there were examples of other states where cracking, surface damage, and performance not better than that of the standard asphalt have been noticed. Various factors, including pavement conditions, have produced varying results, making it difficult to apply the technique successfully from one location to another. The FHWA keeps supporting the usage of crumb rubber in road building in case of proper engineering and economy of the process. It is stressed in the federal guidelines that the material should be evaluated according to its purpose, and not because of recycling the waste product.

The challenge of making recycling worthwhile

Reprocessing old tires as pavement material will provide a second life for the rubber, but there are other things to consider aside from recycling. Transportation agencies have to determine the effectiveness of the road, the cost of construction and maintenance, and whether the material offers any benefits over ordinary asphalt.

For engineers, however, the Minnesota tests point to the need for each of these to be evaluated individually. It could be true that a substance is good enough for building purposes, but lacks the performance benefits that make it more valuable than regular asphalt. Evaluations would allow agencies to distinguish the two situations before making a small test a bigger investment.

The following concern involves finding ways to compare road surfaces throughout their lifetime more effectively. Long-term observation can determine if the initial promise of durability and resilience of the material against cracking and deterioration justifies the expense. This is especially true if the recycled material is used for a specific purpose, such as allowing pavement to withstand ice.