Speed and temperature superposition on traffic speed deflectometer measurements

The traffic speed deflectometer (TSD) is capable of conducting deflection surveys at traffic speeds and is an efficient tool for collecting network-level pavement structural data. Due to the viscoelastic nature of asphalt concrete (AC), the raw TSD deflections need to be corrected to the reference temperature and speed for baseline comparisons with data from different tests. Although the topic of temperature correction for deflections has been extensively discussed, the correction of traffic speed on pavement deflection has rarely been investigated. According to the principle of time–temperature superposition, the time (speed) effect on pavement responses can be evaluated by the concept of “pseudo temperature”. Combined with the test temperature, the time and temperature dependence of AC materials can be integrated into a single factor, referred to as the effective temperature. By correcting raw TSD deflections from the effective temperature to the reference temperature, speed correction and temperature correction can be achieved simultaneously. It also provides some insights into eliminating the need for temperature correction by artificially presetting the TSD speed so that the TSD deflection collected at the preset speed and test temperature is equal to the TSD deflection at the reference speed and reference temperature. Results from the present analysis suggest that TSD deflections are more sensitive to test temperatures than to test speeds. The equivalence between speed and temperature depends on the activation energy of the AC material. For normal TSD operating speeds, a 10 mph increase in test speed is approximately equivalent to a 1 °C drop in temperature. In general, a practical preset speed range of 20–60 mph can accommodate temperature variations of approximately 4 °C.