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The simple composition of asphalt ensures successful recycling

Published 19.5.2022

Asphalt pavement consists of aggregate and bitumen. The simple composition makes recycling easier, which is very import to the Finnish Transport Infrastructure Agency. We examined the anatomy of asphalt, how to recycle it and potential alternatives to this material.

There are about 50,000 kilometres of paved roads in Finland. The recipe for the pavement material is fairly simple: about 95% of asphalt is aggregate and the remaining 5% is bitumen. The composition is simple, which makes recycling easier.

“Some of the problems with potential bitumen replacements boil down to this. If there are changes to the composition in the future, it must be ensured that the valuable material does not become less recyclable,” says Katri Eskola, Specialist, Road Maintenance at the Finnish Transport Infrastructure Agency, highlighting the issue.

Reuse of the pavement material is essential, especially in Finland where the asphalt on busy roads must be replaced at frequent intervals. The rapid wear is caused by studded tyres, which gradually eat away the road surface. For example, on the busiest motorways, the pavement will only remain in good condition for between four and five years.

“The ability to recycle asphalt is a key issue to us because it combines ecological and economic thinking. We are not only dealing with waste that is our responsibility but also with material for new pavement. Recycling aggregate is as important as recycling bitumen,” Eskola adds.

Two approaches to recycling

There are two ways of recycling asphalt. In the traditional method, which has been used at least since the 1970s, the old asphalt is removed from the road and taken to the asphalt plant. At the plant, the asphalt is crushed to the desired grain size, examined and added to the new asphalt mass, using a recipe prepared in advance. Usually, the old asphalt accounts for between 10% and 30% of the new asphalt but higher amounts can also be used on a case-by-case basis.

“As the priority is shifting towards pavement renewal, recycled asphalt is mainly laid on main roads where more rigid binding agents can be used. On low-traffic roads, pavement materials are reused on site for road structure improvements. In them, using old asphalt for new pavement layers is limited by the need for more fluid binding agents, which cannot be produced with recycled bitumen. There is a surplus of recycled crushed asphalt in areas near large cities because transporting it to other parts of Finland would be uneconomical,” Eskola explains.

The second method is unique to Finland: it is the Remix method, which is mostly used to remove the ruts on main roads caused by studded tyres. In this method, the rutted asphalt is reused on site and the old material accounts for as much as 75% of the new pavement.

“The asphalt produced with the Remix method costs about one third of what we have to pay for completely new asphalt. It has a slightly shorter service life but in relation to its price, it is usually the most cost-effective way to manage rutting resulting from wear,” Eskola adds.

There are differences between pavement materials

Even though the pavement on main roads and low-traffic roads would look the same, they are different in terms of composition. The bitumen used on main roads is more rigid and the pavement material contains more bitumen so that the road surface can withstand heavy traffic. There are several pavement layers, whereas on low-traffic roads, only one or two layers are laid. On these roads, the bitumen must be more flexible so that the pavement material can successfully withstand the movements arising from the base and changes in temperature for longer periods.

The composition of the pavement materials used on the two parallel lanes on a motorway may also be different. The material used on the main lane must be particularly resistant to studded tyres whereas a less wear-resistant pavement material can be laid on the secondary lane.

“The needs in Finland differ from what is required in other countries. This is often overlooked by those suggesting that foreign innovations should be used in our country. Many of the new innovations make the pavement more rigid, which is often a disadvantage on Finnish roads,” Eskola notes.

In fact, designing the composition of the pavement material is a matter of continuously optimising different properties. For example, rigid pavement would tolerate heavier loads but excessive rigidity will make the pavement less resistant to sub-zero temperatures. Resistance to studded tyres could be enhanced by using coarse-grained materials but this would make the pavement noisier. In Finland, the maximum aggregate grain size used in main road pavement materials is usually 16 millimetres, which is a compromise solution.

“Climate change and rising costs have prompted us to pay more attention to the long-term durability of the pavement materials. However, at the same time, it must also be ensured that the other good properties are not lost,” Eskola says, analysing the issue.

Concrete failed to withstand studded tyre wear

In addition to bitumen, other pavement materials have also been tested in both Finland and elsewhere. One of them is concrete, which was still found on some roads in Finland in the early 2000s. However, it failed to withstand studded tyre wear in the manner expected of a material of this price category. As a result, the road sections paved with concrete have now been repaved with asphalt.

A mixture of bitumen and rubber from used car tyres has also been tested. On the test sections, the aim has been to achieve properties similar to those of industrially manufactured rubber bitumen.

“Bitumen is a great material because when heated, it becomes fluid and when cooling, it turns rigid. It has proved difficult to find a pavement material with better technical properties and a better price-to-quality ratio. At some point in the future, bitumen will definitely be replaced with other materials, at least partially, but the new materials must be compatible with the existing ones in terms of recyclability,” Eskola notes.

Mixing crushed bitumen roofing with asphalt has also been discussed recently. So far, it has only been approved for use in lower payment layers on roads and for pedestrian and bicycle paths. It has not yet been permitted for use on the top layer (surface course) on roads.

“The bitumen contained in crushed bitumen roofing differs from the bitumen used on roads, and we have not yet collected much experience on its recycling and extensive reuse in road pavement materials. We are learning more by monitoring and studying test sections. In reuse, we would like to give priority to our own asphalt waste because it is made of high-quality aggregate specifically produced for pavement materials and bitumen purchased for road use. Crushed bitumen roofing might be an option in areas where crushed asphalt is not available provided that we learn how to use it and to manage its impacts,” Eskola adds.