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43 SASW can identify variations in the top 7 in. of the pave- ment, provided the analysis uses a reasonable value for the stiffness of the pavement. Like IE, the SASW measurement has limited ability to provide the degree of severity and cannot measure pavement condition below the top of the discontinuity. GPR, IE, and SASW can be valuable project-level tools used independently or in series. Table 4.1 provides a summary of the features of the NDT technologies that were further devel- oped as a part of this study. As the NDT industry continues to improve the hardware and software, these NDT tools will become more effective tools for pavement evaluation. For example, data analysis software development is needed to make NDT a network-level tool for detecting delamination in HMA pavements. Recommendations On the basis of results of this study, the following recommen- dations can be made. These recommendations are based solely on the NDT technologies available but should not preclude other innovative approaches as they develop. 1. GPR demonstrated full lane-width testing and IE/SASW improved its lane width capability from a width of 2 ft to a width of 6 ft. NDT manufacturers should continue to develop their equipment to provide testing hardware that can measure full-lane width. The hardware development must include the ability to narrow the width of the equip- ment during transport from site to site for the safety of both equipment and other vehicles. 2. GPR and SASW technologies require an extensive amount of manual data analysis. NDT manufacturers should continue to improve the data analysis software with the goal of providing real-time results that would be valuable for project- and network-level pavement Conclusions NDT hardware to measure changes in pavement response is available. GPR is the only NDT technology that is currently capable of testing full lane width and at moderate testing speed. Advances in mechanical wave NDT equipment significantly reduce the testing time but are limited to testing at speeds of less than 5 mph. Software to analyze the data is available and requires a highly trained technician. The level of automated data analy- sis must be improved to reduce the manual time required to obtain results. None of the NDT technologies can conclusively distinguish between types of pavement discontinuities. The measurement identifies a discontinuity, or change, in the pavement condi- tion, but cannot determine from the measurement why the change occurred. GPR, IE, and SASW each use a unique signal measurement that is influenced by the pavement condition. The technician will need to understand how the pavement condition influences the signal. Coring will still be required to confirm the nature of the discontinuity. None of the NDT technologies is capable of identifying partial or no bond due to inadequate tack coat during construction. GPR can identify variations in the pavement, isolate the depth of a discontinuity in the pavement, and provide a rela- tive degree of severity. Severe conditions, such as stripping, can be observed with conventional analysis software. Detect- ing debonding between asphalt layers is only possible when there is moisture trapped in the debonded area between the layers using current analysis methodology. Software to quantify the extent of discontinuities over a large survey area is needed. IE can identify variations in the pavement below a 4-in. depth, but confident analysis requires the HMA to be cool and stiff. The measurement has limited ability to provide the degree of severity and cannot measure pavement condition below the top of the discontinuity. Conclusions and Recommendations C h a p t e R 4
44 assessment. Software to provide real-time IE results is available. 3. If highway agencies expressed an interest in apply- ing NDT for pavement evaluation, the NDT industry would see market potential and continue to develop its equipment. 4. GPR equipment should have an array of antennae and fre- quency sweep pattern ranging up to 3 GHz. Use GPR with- out a lane closure to locate discontinuities in a pavement. Operate the GPR equipment full-lane width at moderate travel speed to perform a preliminary assessment of HMA pavement condition. The results of the GPR analysis will assist engineers in identifying pavement sections that require a more intensive evaluation. 5. IE/SASW equipment can be built as multiple unit pairs in a towing package. Use IE and SASW within a lane closure to locate the depth of pavement discontinuities. This equipment costs less than a lane-width GPR antenna full package does, but the data collected are also less compre- hensive. IE and SASW can be used to supplement GPR analysis. 6. Highway agencies might consider research funding to support development of software for project-level and network-level analysis. 7. Many highway agencies operate FWD equipment. Research funding should be considered to explore further the use of FWD time history data as an NDT tool for identifying pavement discontinuities. Table 4.1. Summary of Improved NDT Technologies Technology GPR SASW IE Measurement width Full-lane width Half-lane width Half-lane width Measurement speed Up to 40 mph Less than 5 mph Less than 5 mph Effective measurement depth Lower than top 2 in. 0 to 7 in. 4 to 12 in. (for cold-stiff HMA) Analysis software Manual = complicated; some automated features Manual = complicated Automated = real time
