Evaluating structural characteristics of asphalt pavements by using deflection slopes from traffic speed deflectometer

The Traffic Speed Deflectometer (TSD) overcomes the limitations of the Falling Weight Deflectometer (FWD) in terms of traffic interruption and testing inefficiency, making it an efficient tool for network-level pavement structural evaluation. The current research on TSD is inherited from FWD and mainly focuses on deflection itself. However, the raw measurement of TSD is the deflection slope, i.e., the first-order derivative of deflection. TSD deflection is calculated by curve fitting and integration, and the extensive use of deflection itself may accumulate the error caused by curve fitting. This study explores the potential use of TSD deflection slope in assessing the strength of pavement structures, estimating the location of the inflection point, and calculating the lag distance. The results show that both the TSD deflection and slope are effective in detecting weak asphalt layers and subgrades, but they are not sensitive to damage to the base layer. It is recommended that TSD slopes at 5″ and 60″ (in front of the load center) be used to indicate the strength of the asphalt layer and subgrade, respectively. The location of the inflection point is related to the asphalt thickness and subgrade modulus, and it can be approximated by the location of the maximum slope measurement. In addition, the lag distance can be calculated by determining the maximum deflection point or by determining the zero-slope point. Since the maximum deflection method accumulates errors in curve-fitting and is more sensitive to interpolation methods, the zero-slope method is recommended to calculate the lag distance.