Utilization of the Light Weight Deflectometer for Flexible Pavement Construction and Performance

Abstract

At the National Center for Technology (NCAT) Pavement Test Track in Auburn, Alabama, accelerated pavement testing occurs in three year cycles on 46 unique 200 foot pavement sections that are subjected to 10 million equivalent single axle loads (ESALs). Typically, at this facility, the focus is testing on the asphalt layer itself, however it was noticed in previous research cycles that premature failure of the sections may have been caused by the unbound foundational layers (i.e., subgrade, aggregate base) of the pavement structures. The current standard practice for quality control (QC) of these foundational layers during construction is monitoring in situ density and moisture content with a Nuclear Density Gauge (NDG). While the moisture and density values may meet the specified criteria, they are not parameters that directly affect pavement design. Another device, the Light Weight Deflectometer (LWD), measures the deflection and modulus of the material and more fundamentally characterizes the engineering properties of the in situ materials and has potential to improve construction QC by establishing and testing for target modulus values of the unbound materials. To develop an understanding of how an LWD could help improve construction controls, a study relating to these unbound layers was created by introducing the use of a commercially-available LWD at the Test Track in the 2024 research cycle. The LWD is a portable device similar to a small scale Falling Weight Deflectometer (FWD). The desire to use the LWD for testing on unbound foundational layers has been on the rise due to the relationship modulus values have related to structural capacity during pavement construction as opposed to density and moisture values from typical QC devices such as the NDG. Testing was performed with the LWD alongside the NDG during the 2024 Test Track reconstruction on six fully rebuilt sections at twelve locations throughout each section. Four out of those sections were used for an in depth study to relate NDG values to LWD results and create standard target values for the material tested through different equations relating to field testing. The main finding from this research was by using the optimum moisture content related to laboratory Proctor testing and LWD data collected in reconstruction, a minimum modulus value and maximum deflection value were set for the base material used at the Test Track. For the Test Track Base materials a QC limit of maximum deflection of 0.020 inches (0.504 mm) and a minimum modulus value of 6,908 psi (47.64 MPa) was determined. This also established a method that could be used for other unbound materials based on the optimum moisture content. Furthermore, since construction is typically done every three years at the Test Track, there was interest in using the LWD in other ways apart from construction monitoring. Additional research was performed on three existing asphalt sections that were built in the 2021 test cycle. These three sections were part of an ongoing additive group experiment section and were experiencing varying levels of cracking distress (high, medium, and none). This allowed for a relationship between cracking distress level and average modulus values of the asphalt layer produced by the LWD to be established. Areas with heavy distress levels had an average modulus of 18,319 psi, areas with medium distress levels had an average of 27,270 psi, and areas with light to none had an average of 35,942 psi. These values were found from an average of all three test dates with no temperature corrections. These average values can help to understand the quality of distress a section is experiencing. Also, this additional LWD testing was performed alongside the FWD to create comparisons between the two devices. It was determined that the FWD produced significantly higher deflection and modulus values than the LWD, however the trends of the two results were similar despite the magnitude difference. Two additional LWDs were also tested alongside the main LWD used at the Test Track to understand differences between different LWD devices. Overall, from the performed research, a primary result was determining preliminary modulus and deflection values for QC for the Test Track Base material. A testing method was also determined for thin asphalt pavement sections to determine the structural integrity of a pavement section based on modulus values produced by the LWD. The overall purpose of this thesis was to determine how the LWD could be used in two different, practical and beneficial ways at the NCAT Test Track for QC and other evaluations.