Characterisation of interlayer contact behaviour in asphalt pavements via pavement system transfer function analysis

The interlayer contact condition strongly influences pavement performance and maintenance decisions. This study combines Falling Weight Deflectometer (FWD) testing with the Pavement System Transfer Function (PSTF) to assess interlayer contact in semi-rigid asphalt pavements. A finite-element model was established to simulate deflection responses under different structural and interface conditions. Based on linear time-invariant (LTI) system theory, PSTFs were derived from deflection data through time–frequency transformation and used to analyse spectral responses associated with interface deterioration. Results show that changes at the surface–base interface markedly affect PSTF spectra. For a typical structure, the normalised difference (ND) in the real component at 10 Hz reached 54.9% and 70.1% under semi-bonded and debonded conditions, respectively, compared with the bonded condition. The response was most sensitive within 110 cm of the load centre, with sensitivity increasing for lower subgrade modulus and thicker surface layers. By contrast, changes at the base–subgrade interface produced only minor spectral variations, generally below 10%. Based on these spectral features, a rapid identification framework was developed, achieving over 80% accuracy on the theoretical dataset. These findings support PSTF-based non-destructive evaluation of pavement interface conditions.