Investigation into a long-term interlayer bonding of asphalt pavements

In recent years adhesion testing of pavement layers has gained more and more importance throughout Europe. In a lot of projects the bonding properties are determined after construction and many countries have developed standard requirements which the obtained bonding values have to fulfil. The paper focuses on the question of long-term behaviour of bonding properties and presents the results of an extensive Swiss study. The study compares the bonding properties determined with the Layer Parallel Direct Shear Test (LPDS) according to Leutner of 14 high volume roads from the years 1993-97 to the values for the same road sections determined 9-13 years later. In addition, a second study conducted on a circular track investigated the differences between the bonding properties in the wheel path and outside the wheel path. It could be shown that in most cases the bond improves due to compaction and settlement caused by the traffic. Problems arise, resulting in a reduction of shear forces, when the pavement shows distress phenomena. The difference between the bonding properties in the wheel path and outside it could be demonstrated in the circular track study, whereas the differences on the road depend on many factors and often seem to be eliminated over the years.