A core-free calibration method for non-destructive PQI-based asphalt pavement density measurement

The pavement quality indicator (PQI) provides a core-free alternative to conventional core sampling for determining asphalt pavement density. However, its measurements are dependent on testing conditions and require calibration to the standard bulk density determined by the saturated surface-dry method, thus necessitating a dedicated calibration methodology. Conventional calibration methods rely on pavement coring, which compromises the core-free advantage and suffers from poor representativeness due to the limited number of cores. To address this issue, a novel calibration method was proposed in this study to eliminate field coring while explicitly accounting for critical testing conditions. First, the high precision of the PQI instrument was confirmed by repeatability and reproducibility tests. Subsequently, the effects of measurement orientation, testing temperature, and moisture content on PQI measurements were investigated. Results indicated that measurement orientation had a limited effect on accuracy under the testing conditions. In contrast, surface moisture content significantly affected the measurements, and thus the instrument should be placed on a dry pavement surface. Furthermore, testing temperature significantly affected the measurements, with higher temperatures producing higher density values and the effect becoming more pronounced as temperature increased. Consequently, corrections were required when the testing temperature deviated from the reference temperature (25 °C). Based on these findings, a four-step calibration method was developed, consisting of field PQI measurement, temperature correction, inter-method calibration, and minimum difference match to determine the optimal calibrated density. Validation on a test road demonstrated excellent agreement between calibrated PQI measurements and conventional core-derived densities. Overall, this approach provides a core-free and temperature-compensated calibration for the PQI, making it suitable for real-time compaction quality assessment while preserving pavement integrity.