The purpose of this study was to identify the currently available nondestructive evaluation technology that holds the greatest potential to detect moisture in flexible pavements and then apply the technology in multiple locations throughout Virginia. Ground-penetrating radar (GPR) was chosen for use in a field investigation because of its ability to measure large areal extents and reports of successful implementation by other researchers. This technology was used to determine the moisture content of the subgrade beneath five flexible pavement sections in Virginia. The GPR survey was conducted at normal driving speeds, and data were collected at a sampling rate of 1 scan per foot. For each site, three scans were collected in the travel lane (in the right wheel path, the center of the lane, and in the left wheel path). Existing passing lanes were also scanned. Initial data processing subdivided each pavement section into a three-layer system composed of the hot-mix asphalt layers, the aggregate base layers, and the subgrade. The processing also included calculating the dielectric constant of each layer. These raw data were used to conduct further analyses considering data from only the subgrade. The data were normalized to highlight those areas with the highest dielectric constants since it is known that moisture will have the greatest influence on the dielectric properties of the material. This study showed that GPR can identify areas of varying dielectric constant attributed to variations in the moisture content of the subgrade of various pavement sections. The use of the GPR offered a safe and rapid means for nondestructively surveying large areas of pavement as the survey was conducted at normal driving speeds. In addition, the use of a statistically based data normalization procedure allowed GPR to be used to assess qualitatively the moisture condition of the subgrade of flexible pavements. Two advantages of GPR testing are that it can be used to provide a continuous reading of subgrade moisture conditions (rather than a point location) and can be performed at highway speeds with no traffic control. A typical network-level study involving one subgrade bore per mile would cost $1,200 per location. The cost for the current study averaged $0.04 per data point. Assuming one data point every foot and three scans per lane, the resultant cost was approximately $680 per mile. VDOT maintains approximately 225 lane-miles of flexible pavement with subsurface drainage layers and does not routinely inspect the condition of the outlet pipes. In approximately 1 workday, a two-person crew could use a push camera to inspect approximately 5 lane-miles worth of outlet pipes for subsurface drainage layers or the GPR system could scan approximately 330 lane-miles (assuming operation at 55 mph for 6 hours). Thus, the entire system of flexible pavement with subsurface drainage could be inspected in approximately 2% of the time.

As with the previous two symposia, the 32 papers from the June/July, 1999, Seattle symposium present advances in the nondestructive testing of pavements using conventional falling weight deflectometer techniques and other promising techniques such as ground penetrating radar, rolling weight deflecto

A review of literature concerned with the nondestructive testing of flexible pavements, and two types of testing methods are presented in this report. The first method deals with the measurement of pavement response through the deflection of the pavement surface. Effect of the loading characteristics, pavement structure, and environment on measured deflections are discussed. Experience has shown that deflections of certain magnitudes are associated with failure. Bending strain has also been associated with failure through field measurements of radius of curvature of the deflection basin and through laboratory fatigue studies. The second method involves the application of a vibratory load to the pavement surface and determination of pavement response through deflection of the surface under a low-frequency vibration or by measurement of wave propagation at higher frequencies. The deflection of the pavement under the vibratory loading is used to determine dynamic stiffness, or type of spring constant, for the pavement structure. The wave velocity is used to arrive at a shear modulus and compression modulus of each pavement layer and the subgrade as well as thickness of each layer. Theoretical calculations involved and typical results are presented. Conclusions are made from findings in the literature, and recommendations are given for developing a method of nondestructive evaluation of flexible pavements. (Author).

The proceedings of June 1993 international symposium held in Atlanta, Georgia, called specifically to develop and standardized evaluation procedures for non-destructive methods of testing pavements. The 29 papers discuss analytical models and techniques, measurement and calculation techniques in the field and laboratory, problems and errors associated with backcalculation methods and design parameters, and testing for other pavement uses. Also includes a history of the quest for a standard and the status of that effort. Reproduced from typescripts. Annotation copyright by Book News, Inc., Portland, OR