Shear fatigue behaviour of tack coats in pavements

For many years, pavement engineers use mechanistic design methods that are based on algorithms to compute stresses, strains and displacements in a flexible pavement structure, or in a multi-layered structure in general. Most of these methods assume, to ease the modelling of the structure and the computation process, that the layers are fully bonded or completely unbounded to each other. Moreover, determining the effectiveness of the bonding between two layers is difficult since there is no standard test procedure to assess it. Hence, proper modelling of the interface bonding condition will represent an important finding in understanding the real behaviour of road structures, and will lead to reduce maintenance and rehabilitation costs. Asphalt emulsion is commonly used, as tack coat, to ensure the bond between two pavement layers. This study deals with an experimental characterization of interfaces shear fatigue behaviour through laboratory tests. A testing device for determining the shear fatigue behaviour is described in this paper. Shear fatigue tests, at a temperature of 5 degrees C and a frequency of 1 Hz, were performed on asphalt concrete mix layers interface with tack coat. Samples were subjected to a cyclic symmetrical alternate shearing load at the interfaces, aiming interfaces failures within the range of 10(4) to 10(5) cycles of loading. Within this range of number of cycles, the fatigue results indicate a linear evolution of the initial shear stress half-amplitude with the decimal logarithm of the number of cycles to failure. These results are used to derive a linear fatigue law, which will serve to model the real behaviour of the interface with tack coat studied in this research.