USAF Wave Propagation NDT Method

This paper reviews U.S. Air Force efforts to find a nondestructive testing (NDT) method for evaluating airfield pavement load-carrying capabilities. The paper concentrates its discussion on the wave propagation nondestructive testing method for evaluation of airfield pavements. The method was used by the Air Force from 1983 to 1985. The information given for the wave propagation method includes the theoretical background, test equipment and procedures, methods of test data analysis, and pavement analysis procedures. Also presented are: comparisons of results between the wave propagation NDT and destructive evaluation methods; reasons for the Air Force moving away from the wave propagation method; ongoing research; and possible future use of the wave propagation method.

The wave propagation method used by the Air Force was composed of: a hardware for field testing, data reduction, and analysis; and software for evaluation of allowable gross loads (AGL) through a computer analysis. The test equipment was composed of an impact loader and a data acquisition system, both housed in a self-propelled van.

The impact loader could be operated both by program control and manually. The pavement response to the impact loading was monitored with accelerometers which were mounted on the pavement surface at various distances from the loading plate. A computer was used to perform fast Fourier transform (FFT) of the test data. From the results of the FFT analysis, graphs relating phase angle with frequency, and wave velocity with wave length (dispersion curves), were obtained. From these graphs, appropriate wave velocity values were selected for determination of the elastic modulus of each pavement constituent layer. These elastic modulus values were then used to analyze the critical pavement response to a desired aircraft loading by means of a computer program named AIRPAVE. Based on these critical response data, together with the evaluation criteria, the number of aircraft operations that could be supported by the pavement were determined.

Using the wave propagation NDT method, a number of airfield pavements were evaluated between 1983 and 1985. Results of the NDT evaluations were compared and correlated with the results of destructive testing methods. A good correlation, with a small percent of difference in AGLs between the two methods, was shown. However, because of a number of advantages associated with the falling weight deflectometer (FWD) over the wave propagation method, the Air Force has adopted the FWD as its primary evaluation method.