In the Netherlands, when recycling asphalt mixtures, one should check first of all whether there is tar present in the asphalt mixture. Mixtures that contain tar cannot be used for recycling and have to be burned completely. Ultraviolet (UV) blue light is often used to detect the tar contamination in asphalt mixtures. The so called Pak-Marker was used for this purpose (see ). It is especially developed to detect Polycyclic Aromatic Hydrocarbons in asphalt products and is used in the following way.
Figure 4‑1 Pak-Marker for detecting tar-containing
After shaking the spray can, a layer of white spray is sprayed on the asphalt mixtures. Then it is left to dry before further testing. If there is tar in the mixture there will be a discoloring of the white substance. This discolor will be light brown or yellow. If there is no clear visible discoloring, the UV lamp can be used. Under UV light the spray will light up and get a yellow/blue color. Yellow color indicates there is tar in the mixture while blue color shows there is no tar. Figure 4‑2 shows the color difference under UV light between a mixture contaminated with tar and a mixture without tar. Without tar, there are no visible images except gray aggregates and binder, while the sprayed white material has a blue color. When the sample is tar-contaminated, many yellow areas can be observed.
Figure 4‑2 Difference between tar-containing mixture and mixture without tar under UV light
Referred from My publication.