Wayne Muller

Revised approach to assessing traffic speed deflectometer data and field validation of deflection bowl predictions

This study presents a revised approach to conceptualising and analysing data from the traffic speed deflectometer (TSD) which enables full deflection bowl predictions. The approach was successfully applied to TSD surface velocity measurements collected at seven test sites as part of recent Australian trials. More than 1500 deflection bowls produced from the TSD data were validated against approximately 600 40- and 50-kN falling weight deflectometer (FWD) deflection bowl profiles. Overall, the results showed a clear correlation between the shape and magnitude of deflection bowls predicted by both methods. Estimates of maximum deflection (d 0) and structural curvature index (SCI300) from both methods were also compared, showing a strong correlation. The results suggest that the TSD device has significant potential to be used to collect measurements of pavement deflection bowls at highway speeds which are comparable with FWD deflection bowl measurements.

Quantitative moisture measurement of road pavements using 3D noise-modulated GPR

Within Queensland, Australia around 90% of state controlled roads are constructed from unbound granular materials with thin bituminous surfacings. These pavements are significantly influenced by moisture. However currently there is no rapid, quantitative method of assessing in-place pavement moisture suitable for use at large scales. This makes it difficult for road engineers to diagnose and treat moisture ingress mechanisms in failing pavements and to properly assess and protect flood weakened roads from heavy vehicle damage. This research focuses on quantitative moisture measurement of unbound granular road pavements using rapid 3D multi-offset ground penetrating radar (GPR) techniques. The work will use an update of an existing 3D noise-modulated GPR (NM-GPR) system to collect multiple wide angle reflection and refraction (WARR) profiles across the road lane, while moving along the road at highway speeds. The intention is to use geophysical methods on this data to determine pavement layer permittivity values and from this estimate in-place pavement moisture. While the NM-GPR update is being finalised, preliminary research has commenced. Synthetic data have been produced to model the expected multi-offset data from the new system. This data have been used to test methods of identifying and tracking subsurface layers and the application of geophysical methods to determine pavement layer permittivity values. Preliminary laboratory investigations of moist pavement materials have also commenced using a vector network analyser (VNA) and a phase-shift measurement technique. This has been done in order to develop the necessary moisture-permittivity relations to calibrate the moisture predictions from the GPR permittivity measurements. This paper describes the current state of research, preliminary simulations and laboratory testing undertaken toward the goal of developing a robust, high-speed method of quantifying in-place road pavement moisture.

A comparison of phase-shift and one-port coaxial cell permittivity measurements for GPR applications

The coarse and loose nature of unbound granular road materials presents a number of challenges for conventional permittivity characterisation approaches. An alternative that appears better suited to these materials involves measuring the phase-shift at discrete frequencies through a sample of known thickness. To validate this approach against more established methods, a comparison is required on materials that can be easily measured using either method. To this end phase-shift measurements were undertaken on a range of solid dielectric slabs including various types of stone, plastic and an artificial material. Permittivity predictions from this method were then compared to results from a one-port coaxial cell. As an additional comparison, and to better understand the results, the phase-shift test setup was also modelled using GPRMax software. To improve the predictions, reverberations within the test apparatus were minimized by isolating the direct wave using time-domain Blackman windowing. However, the narrow window necessary for this particular test setup also degraded the ability to detect frequency-dependent permittivity changes. Overall the phase-shift approach produced real relative permittivity predictions similar to that from the one-port coaxial cell. Despite limitations in the current approach, the results validate the phase-shift approach as a simple and rapid method of characterizing the permittivity of larger dielectric material samples of constant thickness.

Traffic-speed 3-D noise modulated ground penetrating radar (NM-GPR)

A new type of high-performance ground penetrating radar is presented based on a variant of the noise-modulation technique. Benefits of the approach include high operational efficiency; simplified and more robust electronics; stable performance; compatibility with a wide range of antenna types and better compliance with regulatory emission restrictions. A prototype NM-GPR system has been in operation since 2008, tailored to road pavement investigations. That system consists of 24 radar channels in a 3-D configuration with arrays of ground coupled antennas. Each radar channel can operate simultaneously, collecting a 256 point GPR trace every 65mm at a collection speed of 100 kilometers per hour. This equipment has been used extensively within Australia for road investigations. To date more than 100 projects totaling over 5,000 lane-kilometers of road scanning have been completed with the prototype, producing excellent results. The resolution of data produced with the current antennas is similar to commercial 1.5GHz ground coupled impulse system, but with a typical penetration depth of around one metre for most Australian road pavements. This paper describes this new approach to GPR hardware and its use to date for investigating Australian road pavements.

A comparison of modified free-space (MFS), GPR, and TDR techniques for permittivity characterisation of unbound granular pavement materials

This paper reports on a laboratory experiment comparing permittivity measurements using a modified free-space approach to results using common-offset ground-penetrating radar and time-domain reflectometry on moist and compacted samples of unbound granular road pavement materials. In the first part of the experiment, unbound granular samples from the same source were prepared to varying moisture contents and a fixed target density. Separate samples were prepared for modified free-space and time-domain reflectometry testing, all of which were also measured using ground-penetrating radar. In the second part of the experiment, samples were mixed to a consistent gravimetric moisture content and varying densities before undertaking the modified free-space, time-domain reflectometry, and ground-penetrating radar measurements. Reasonably good agreement was found between modified free-space and ground-penetrating radar measurements, which also compared well with literature relations for crushed rock pavement materials. The time-domain reflectometry results were relatively consistent with those literature relations, although they appeared to deviate from the ground-penetrating radar and trend of modified free-space results for lower density and drier samples.

Optimising a modified free-space permittivity characterisation method for civil engineering applications

Measuring the electrical permittivity of civil engineering materials is important for a range of ground penetrating radar (GPR) and pavement moisture measurement applications. Compacted unbound granular (UBG) pavement materials present a number of preparation and measurement challenges using conventional characterisation techniques. As an alternative to these methods, a modified free-space (MFS) characterisation approach has previously been investigated. This paper describes recent work to optimise and validate the MFS technique. The research included finite difference time domain (FDTD) modelling to better understand the nature of wave propagation within material samples and the test apparatus. This research led to improvements in the test approach and optimisation of sample sizes. The influence of antenna spacing and sample thickness on the permittivity results was investigated by a series of experiments separating antennas and measuring samples of nylon and water. Permittivity measurements of samples of nylon and water approximately 100 mm and 170 mm thick were also compared, showing consistent results. These measurements also agreed well with surface probe measurements of the nylon sample and literature values for water. The results indicate permittivity estimates of acceptable accuracy can be obtained using the proposed approach, apparatus and sample sizes.

Permittivity Characterization of Unbound Granular Pavement Materials with a Modified Free-Space Approach

Moisture has a considerable influence on the structural performance of unbound granular (UBG) pavements. Multioffset ground-penetrating radar has the potential to quantify moisture within these materials by measuring the permittivity of pavement layers. To enable moisture estimates from these field measurements, relationships between permittivity and volumetric moisture are required. This paper describes the use of a modified free-space (MFS) laboratory approach to measure the permittivity of compacted UBG material samples and to develop these relationships. Material samples were compacted within form ply boxes; various density and moisture conditions were targeted. A vector network analyzer was used to measure the phase shift between a fixed pair of ground-coupled dipole antennas as a result of sample insertion and an initial ply sheet reference measurement. Frequency-dependent permittivity values for the samples were then determined over the range of 1.0 to 2.0 GHz on the basis of these measurements. Mean values over this range were then related to the volumetric moisture content of the samples. An indicative moisture–permittivity relationship was proposed on the basis of MFS measurements of samples from several quarries. Overall, the permittivity results from the MFS approach showed reasonably good agreement for samples at higher moisture contents compared with relationships in the literature that are based on time domain reflectometry. However, for drier samples, the MFS permittivity values were higher than the literature predictions. Possible reasons for these differences are discussed, and an overview of the advantages and limitations of using the technique for characterizing UBG materials is given.