Backcalculation of Pavement Layer Properties

Abstract

This was prepared for the symposium as a state-of-the-art paper. The paper summarizes the history and present use of nondestructive testing (NDT) and speculates on future developments and uses of NDT in backcalculating pavement layer properties. The paper concentrates on the backcalculation of the elastic stiffness of pavement layers. Measurement methods include a variety of methods of applying loads to the pavement and a number of sensors of the pavement response. Loading methods include static or slowly moving loads, vibration, near field impulse, and wave propagation which applies to far field measurements. A fundamental distinction is drawn between near field impulse loads which closely simulate traffic loads and far field impulse loads, the results of which must be corrected substantially to provide realistic pavement layer moduli. Analysis methods are surveyed, including some historical methods, microcomputer methods, impulse and response analysis methods, and systems identification methods. The historical methods include two-layer closed form and graphical methods, equivalent layer methods, and a closed-form multilayered method that searches for the best set of moduli to satisfy a leastsquares criterion. Microcomputer methods are described generically, noting all of their common features, including methods of dealing with both random and systematic errors in measuring and computing layer moduli. Random errors can be reduced by repeating the measurements. Systematic errors include the assumptions used in the backcalculation process, including nonlinearity. The results of an exercise conducted to estimate the size of systematic errors is reported. Impulse and response methods analyze the complete signals to determine a “transfer function” of the pavement from which the complex moduli of the layers can be backcalculated. Some speculation is offered on how this information may be used to determine fatigue and permanent deformation properties of pavement layers. A systems identification method of analyzing far field measurements is illustrated. Uses of nondestructive testing in the future are envisioned to include layer thickness, asphalt or water content, density, elastic stiffness, creep compliance, fracture, and permanent deformation properties. A strategic objective of NDT methods is for them to become the primary method of measuring pavement material properties.

Estimation of Remaining Service Life of Flexible Pavements from Surface Deflections

“…The main drawback of the seismic technique is the difference between the test and real pavement loading conditions (Lytton, 1989), although it is a sophisticated method of pavement evaluation. Some of the drawbacks are the time required to run the test and complexities in data analysis; provides horizontal properties of the pavement components instead of vertical properties; ignores the anistropic nature of the materials; and applies to only low-strain levels.…”

A study of ground‐structure interaction in dynamic plate load testing

“…As an illustration of the assumed viscoelastic model, variation of j EÃj with frequency stemming from (42) is plotted in Figure 8. The granular base and subgrade layers in Figure 6 are assumed to have a small amount of hysteretic damping, typical of granular soils (x r =0, x s =1%), with their complex moduli given by (1).…”

“…In recent years, the foregoing problem has become a subject of increasing importance to non-destructive characterization of geotechnical sites and pavements by means of a dynamic plate load test. One of the devices that are commonly used for such purpose is the falling weight deflectometer (FWD) [1] wherein the dynamic load is generated via a falling weight impacting on an annular contact plate which is buffered on its bottom for enhanced sitting conditions [2,3].…”

Fast error estimates for indirect measurements: applications to pavement engineering

“…We have 3 parameters z~ that have to be determined from the measurements: El, Eu, Es. Corresponding algorithms F/ and F are given in [9,16] (here, F is also a two-stage algorithm: given za and tt, we first compute tensile and compressive strains ~t and ¢v, and then use these strains to predict y).…”