Determining pavement structural number with traffic speed deflectometer measurements

Traffic Speed Deflectometer (TSD) overcomes the slow operating speed of the conventional Falling Weight Deflectometer (FWD), but it also introduces new difficulties in estimating the effective structural number (SN) of existing pavements. Due to the different loading mechanisms of TSD and FWD, the FWD-based SN calculation procedure proposed in the AASHTO Pavement Design Guide is incompatible with TSD measurements. To match the existing FWD-based SN records, the authors proposed a new approach to determine the SN with TSD measurements by modifying the AASHTO method. Simulation results show that the difference in AASHTO SN between TSD and FWD essentially came from the difference in the equivalent loading frequency. The asphalt concrete (AC) layer exhibited lower stiffness under TSD than under FWD. The TSD-based AASHTO SN was generally smaller than the FWD-based AASHTO SN (error of 0.74), and the deviation increased with increasing AC thickness. Therefore, an enhanced AASHTO method was proposed for calibrating TSD-based SN by the AC thickness, which made great progress in estimating SN from TSD deflections with an error of 0.17. The proposed method was verified by filed TSD and FWD measurements and was recommended for pavements with AC thickness greater than 4 in.