Numerical Investigation of the Viscoelastic Behavior Effects of the Asphalt Concrete Layer on TSD Deflection Slopes and Back-Calculation Outputs

Abstract A traffic speed deflectometer (TSD) captures pavement surface deflection velocities while traveling at the speed of the traffic. Normalized deflection velocities by travel speeds, known as deflection slopes, can be used for pavement management purposes. Many of the practical approaches currently available for back-calculating pavement layers’ moduli from deflection slopes report either a linear elastic modulus or a constant complex modulus as the output for the asphalt concrete (AC) layer. This study evaluated the effect of the AC layer’s viscoelasticity on deflection slopes over a wide range of temperatures from 4°C to 37°C and travel speeds from 20 to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="80 k m divided by normal h" display="inline" overflow="scroll"> <mml:mn>80</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>km</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">h</mml:mi> </mml:math> using finite-element method (FEM) modeling of a three-layer flexible pavement system. Subsequently, a TSD back-calculation tool was used to back-calculate pavement layers’ moduli from deflection slopes by simplifying the AC layer’s full viscoelastic behavior to either a linear elastic modulus or a constant complex modulus. Both approaches were shown to produce errors of less than 16% across all pavement layers when back-calculation was performed at an AC temperature of 4°C and a travel speed of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="20 k m divided by normal h" display="inline" overflow="scroll"> <mml:mn>20</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>km</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">h</mml:mi> </mml:math> , using a conventional approach for equating the AC design modulus to its viscoelastic master curve based on travel speed. However, it was also found that simplification of AC viscoelasticity to linear elastic behavior or a constant complex modulus could lead to errors as high as 80% and 100%, respectively, in the back-calculation of the AC layer’s modulus at the high temperature of 37°C. These findings highlight the potential to correct deflection slopes to low pavement temperatures and travel speeds, thereby enabling the simplification of the AC layer’s full viscoelastic behavior while still achieving high accuracy in the back-calculation process.