Evaluation of multilayered elastostatic backcalculation using in-situ measurements in geosynthetic-reinforced asphalt pavement

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Weaver, Benjamin Kurt

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Raw output data from geophones embedded in an instrumented pavement section with six different geosynthetic reinforcements used as interlayers in the hot mix asphalt (HMA) were processed to determine deflections occurring at several depths within the pavement under falling weight deflectometer (FWD) testing. This testing took place during loading campaigns that were conducted over the first two years of the rehabilitated pavement design life. The intent of the study was to use these measured deflections to assess the accuracy of elastostatic backcalculation, by comparing predicted deflections occurring within the pavement using backcalculated moduli and layered elastic theory to measured ones under similar loading. Deflection Basin Parameters (DBPs) obtained from consideration of surface deflections only which included the Surface Curvature Index (SCI), Base Damage Index (BDI), outermost deflection (W7), and spreadability were first used to qualitatively compare the change in performance of the seven test sections over the first two years of rehabilitated pavement life. When deflections were load and temperature normalized, the spreadability decreased while the SCI increased in each section over time, which are both indicative of the start of damage accumulation. The overlays appeared to be performing similarly in each section, except for in Section 7 which seemed to be deteriorating at a more rapid rate. Additionally, the baseline spreadability of the Old HMA and overlay in the control section immediately following rehabilitation was highest but compared more closely with the spreadability of several of the reinforced sections two years following rehabilitation, which may provide early indication of a possible effect of geosynthetic reinforcement in slowing the degradation of pavement flexural rigidity. Each of these effects should continue to be investigated over the course of the overlay design life through nondestructive testing and and/or routine PCI surveys. Taking the geophone measured depth deflections into consideration and assessing the agreement between theoretical and in situ depth deflections under FWD loading, it was found that during the first two loading campaigns the overall agreement between the two was acceptable, with 75 percent of all observations having a relative error of less than 9 percent, and with the absolute error occurring within a range of -1.2 to +1.0 mils. This apparent range in absolute error was fairly consistent across all load levels and maximum deflections, which suggests that the error is most likely due to a combination of the signal to noise ratio of the geophones as well as slight imprecision in the location of FWD drops as opposed to being a function of nonlinear behavior of unbound pavement materials. When the effect of anomalous readings during the first loading campaign in Section 3 and second loading campaign in Section 4 were ignored, the presence of a geosynthetic reinforcement in the HMA did not seem to affect the overall accuracy of elastostatic backcalculation, with overall average relative errors per sensor in each section being approximately 4.0 to 6.5 percent. The exception to this was in Section 7, which was also the section that appeared to be deteriorating more quickly than the others and had an average relative error per sensor in excess of 11 percent between the two loading campaigns. However, the average absolute error per sensor increased significantly in all sections over time, with the largest errors observed during the last two loading campaigns included in the analysis. It is unknown if this effect is primarily attributable to degradation of the pavement or to degradation in performance of the instrumentation, since a relatively large number of sensors used in this analysis (19 percent) malfunctioned causing the data to become unusable within the time incorporated in this study. The exact cause of the malfunctioning geophones is still under review but lends to the importance of careful installation techniques and planning for redundancy and possible sensor loss if geophones are to be used to provide in-situ measurements for long-term field monitoring programs. Engineers should be wary of the possibility for non-uniqueness of solution in deflection basin fitting, and if pavement layers need to be combined, they should consist of the materials with as close of a match as possible in engineering properties.


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