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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.
backcalculation, nondestructive testing, near field impulse, far field impulse, analysis methods, closed-form methods, graphical methods, least-squares search, global minimum, local minimum, microcomputer backcalculation, random error, systematic error, non-linearity, transfer function, complex moduli, creep compliance, permanent deformation, fracture properties, systems identification methods
Professor, Texas Transportation Institute, Texas A&M University, College Station, TX