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Pavements...In this section, geophysical or nondestructive test (NDT) methods used for condition evaluation of new and existing pavements are discussed.
Pavement condition monitoring is an essential component of pavement management, and geophysics has proven useful for objective, network-level pavement condition evaluation as well as early stage deterioration detection, whether the deterioration manifests itself through material degradation or defect generation. Geophysics is also beginning to play a more critical role in quality assurance verification of new construction and in project-level condition evaluations of existing pavements.
Geophysics can yield significant useful information for a pavement condition assessment, particularly if the proper method(s) are selected and implemented at the appropriate intervals. The more successful and/or promising methods are discussed here in order of their current overall value toward solving a variety of pavement-related problems.
A geophysical survey method that provides useful, objective information about the relative condition of the various areas on a pavement structure is generally most valuable when it is used prior to making other important decisions. These decisions include deciding whether other NDT, destructive sampling, and laboratory testing and analyses are needed to provide a better evaluation, or choosing between complementary evaluation techniques. If approached in this manner, geophysical techniques can and will supplement any traditional pavement structure evaluation. These techniques add little to the overall price of diagnosing the structure, and the information provided will help minimize the overall inspection, maintenance, or replacement costs associated with the structure during its service life. Geophysics also helps ensure that the appropriate maintenance and preventive maintenance measures are applied at the right time.
Methods used in pavement condition assessment should be accurate, rapid, and nondestructive whenever possible, whether QA verification testing or existing pavement evaluations are the desired goal.
The current practice of pavement condition inspection by chain dragging meets only the last two criteria, and it is not any more efficient than some other, more objective, nondestructive geophysical evaluation techniques. The accuracy of chain dragging is significantly compromised by the fact that it can only be used to identify delaminations at stages where the deterioration has already progressed to such an extent that major rehabilitation or repair measures are needed. Similarly, the pachometer barely meets the last two criteria when it is used as a primary instrument for verifying that QA specifications are being achieved, because it cannot typically achieve requirements that mandate measurement accuracy within 0.25 to 0.64 cm. In addition, this method does not provide repeatable results to the accuracy desired.
Identifying the appropriate NDT method, or methods, prior to some verification ground-truth is always the best approach toward effective use of these geophysical methods. When used as a primary investigative method, either as a baseline condition assessment tool early on in the life of a structure or as a deterioration-mapping instrument, geophysics often proves beneficial if used as a “reconnaissance tool” in the early stages of an evaluation. The results of the geophysical survey can be used to help (a) select complementary inspection methods—destructive or nondestructive—that would best round out the analysis, (b) reduce the quantity of destructive testing or NDT investigations that are more time-consuming and require lane closures—even if they are essential to the overall evaluation, and (c) limit these other tests to much smaller areas. Information obtained by these other techniques will generally complement, validate, and/or “calibrate” the initial geophysical survey method.