Every year 1–2 million tons of old asphalt paving is recycled in Sweden in conjunction with maintenance or the building of new roads. Asphalt laid before the 1970s may contain a binder, road tar, originating from coal tar. Road tar contained, amongst other things, Polycyclic Aromatic Hydrocarbons, PAH, and other poisonous or carcinogenic compounds, which means that it is no longer used for asphalt paving. As long as the asphalt containing tar remains in the road, the risk of contamination is low. This has been shown by tests taken from fixed matter and groundwater next to, and underneath, roads with asphalt containing tar. Nor does there appear to be any increased risk to the work-environment for workers handling the sphalt. Asphalt containing tar becomes dangerous to the surrounding environment when it is crushed or heated-up, something which usually occurs in the process of recycling. It is therefore necessary to carry out comprehensive evaluations of the methods used, something that VTI has been working on within several projects.
There is a number of methods for recycling
asphalt containing tar. In brief, the material can be crushed into an unbound road base in roads where one would otherwise have used crushed rock. Over time, and as a result of increased traffic, the road base consisting of crushed asphalt will be
bound together into a cohesive, asphalt
resembling layer with positive properties. This type of recycling should, however, only be used for recycled tar-mass with a low PAH-content and in roads with compact
paving.
If the tar-mass contains a high level of PAH it should be mixed with a bitumen based binder, which is the type of binder used in asphalt paving today. Bitumen is a highly
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viscous (at room temperature) binder which is extracted from petroleum. It has the capacity to encapsulate the tar, which considerably reduces the amount of carcinogenic PAH. The recycled asphalt can then be used as a bound road base below the paving in new roads.
When the tar-mass is to be mixed with the binder the substances are not heated, instead cold or warm processes are used. If the material is not heated above approximately 110°C the emissions of airborne PAH are considered to be low. The smell is however more palpable during the warm process compared to the cold one. During the so called warm recycling the material is heated to between 60–80°C and soft bitumen is added. Warm masses can be laid later on in the season than the cold and is easier to pack. During cold recycling the tar-mass is not heated at all, and instead of soft bitumen, bitumen emulsion or foamed bitumen is added.
If the tar-masses can not be recycled the remaining alternatives are for it to remain where it is, to be deposited or incinerated. Various investigations have shown that the recycling has minor environmental consequences and has substantial advantages in terms of cost, energy, preventing resource depletion, and air emissions. There are currently national guidelines concerning how to handle and recycle asphalt containing tar. At VTI, researcher Torbjörn Jacobson has been active in this field in close cooperation with the Swedish Geotechnical Institute, SGI.
Sandra Johansson, VTI, Sweden
Contact
Andreas Waldemarson |
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Asphalt samples in the road laboratory at VTI. |
