Relationship between Fatigue Condition of Asphalt Pavements and Deflection Lag from Traffic Speed Deflectometer

The phase angle is a good indicator of the current fatigue condition of asphalt concrete (AC) layers, but estimating phase angles from drilled core samples is destructive, expensive, and unsuitable for large-scale applications. Similar to the phase angle, a lag between load and response has recently been observed in the deflection basin of the nondestructive traffic speed deflectometer (TSD), i.e., the lag between the loading point and the maximum deflection point, and the deflection lag may be closely related to the phase angle. This study investigated the potential of TSD deflection lag as a nondestructive indicator of pavement fatigue conditions. The relationship between AC phase angle and TSD deflection lag was investigated using the 3D-Move program, and the effects of pavement structure, TSD speed, and test temperature on the deflection lag were also investigated. Field TSD data collected in Tennessee were used to verify the relationship between deflection lag and fatigue cracking. The results show that the lag distance increases uniformly with increasing fatigue levels (phase angles) until fatigue failure occurs. However, the lag distance is also closely related to the asphalt thickness and subgrade modulus; therefore, only the lag distance of pavements with the same structures can be compared to identify the fatigue sections. Overall, the lag distance can be used as an implicit indicator of the fatigue condition of the pavement to predict the initiation and growth of fatigue cracks. Fatigue cracking is expected to occur where the lag distance is relatively large.