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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-16-009 Date: March 2017 |
Publication Number: FHWA-HRT-16-009 Date: March 2017 |
The need to accurately characterize the structural condition of existing pavements has increased with the recent development and release of the MEPDG. An integral part of this process is the accurate characterization of material properties of each layer in the pavement structure, which can be determined either through laboratory testing procedures or through the testing of in situ pavement structures using various techniques, such as the FWD. In the past few decades, FWD testing has become a routine pavement evaluation method, and deflection data collected by the FWD can be quickly and easily used to characterize the properties of the paving layers, which are required inputs into the MEPDG for new flexible pavement design, new rigid pavement design, and rehabilitation design.
This document is part of a three-volume report investigating the use of the FWD as part of mechanistic-empirical pavement design and rehabilitation procedures. In this volume, general pavement deflection-testing procedures and commonly used deflection analysis approaches and backcalculation programs are reviewed. Specific procedures for interpreting and analyzing deflection data for flexible, rigid, and composite pavement structures are described, along with specific modeling issues unique to each pavement structure. The relevance of the different procedures and approaches to the current MEPDG are explored, giving rise to the examination of the use of FWD testing results in six case studies. These six case studies used pavement sections from the LTPP database containing sufficient design, construction, and testing data results (laboratory testing and FWD testing) as a means of assessing the way that FWD deflection data are used in the rehabilitation portion of the MEPDG. Specifically, deflection data and backcalculation results were used to characterize the existing HMA, PCC, stabilized and unstabilized bases, and aggregate and subgrade properties in the MEPDG design program. Laboratory testing results were compared with FWD testing results, and the final designs were found to be relatively insensitive to the differences in characterization of existing layer inputs; that is, new material properties tended to control design results. Some of the significant findings and recommendations from the specific case studies are summarized in the following sections.
For this case study, the MEPDG results indicated that surface-down cracking was critical in the rehabilitation design of an HMA overlay over existing HMA pavements. Nevertheless, a 76-mm (3-inch) HMA overlay was satisfactory for nearly all of the input combinations. Within the ranges identified, the selection of inputs was more critical as one approached the lower values for any layer.
Based on the design runs conducted for this case study, it is recommended that the correction factors developed by Von Quintus and Killingsworth for backcalculated properties of laboratory values should be applied to the backcalculation results until additional guidance becomes available.(87,116)
When the existing HMA modulus is based on FWD testing, 30 Hz should be used for the testing frequency input in the design program. This is consistent with the defined equivalent frequency.
For HMA overlays on rubblized concrete pavements, the critical performance measure in the MEPDG software was surface rutting. The required HMA overlay thickness to achieve a 90-percent reliability level was 178 mm (7 inches) for all analyses. However, if surface rutting were addressed through maintenance at some intermediate year (less than 20 years), a thinner HMA overlay would be appropriate.
The surface rutting predictive model was mainly sensitive to HMA overlay thickness and HMA and rubblized layer moduli. Therefore, care should be taken in selecting the modulus for the rubblized layer.
In addition to the recommendations made for case study 1, it may be useful to combine the rubblized layer with the existing unbound base layer (for existing rubblized layers) when using some backcalculation programs.
The studied section was in poor condition, with a large number of transverse cracks. Three kinds of overlays (HMA overlay, unbonded JPCP overlay, and bonded JPCP overlay) were designed for the rehabilitation. The MEPDG produced the thinnest design for the bonded PCC overlay, while it produced an unreasonably thick HMA overlay, even when modifications were made to the HMA mix design properties.
The manually entered k-value was used for unbonded JPCP and bonded JPCP overlay designs but did not appear to be used for the HMA overlay design. No appreciable difference in the design thickness was found among the three design alternatives by varying layer input values from laboratory or backcalculated results, indicating the reliability of using the backcalculated dynamic (or static) elastic modulus for the PCC layer and the dynamic k-value for the supporting layers in the MEPDG design. Furthermore, it was also found that the backcalculated k-value representing the composite stiffness of all layers beneath the slab could be directly entered into the MEPDG without significantly influencing the design thickness for the pavement structure analyzed.
Similar rehabilitation designs (for HMA overlays, unbonded overlays, and bonded overlays) were obtained when either the laboratory or backcalculated modulus values were used. The use of modified values to account for the stabilized base produced a thicker unbonded PCC overlay.
For the HMA overlay design, it seemed impossible to draw a definite conclusion about the constituents contributing to the k-value being reported in the design output (and assumed to be used in the design calculations). The entered dynamic k-value did not appear to be used in the determination of k-value. The difference in the calculated k-values for low and high base stiffnesses was so slight that it appeared that the base layer stiffness was not considered in the calculated k-value.
With the unbonded JPCP overlay design, a noticeable difference was found in the calculated k-value between a low and high existing PCC modulus, which might indicate that the stiffness of the existing PCC was involved in the calculation of the k-value.
The modulus of the base appeared to be considered in the calculation of the k-value for bonded PCC overlay designs, which agreed with the assumptions listed in the MEPDG. It was also apparent that the calculated k-values matched the entered dynamic k-values.
The selection of layer property inputs from backcalculation values had minimal influence on the overall design results. It appeared that the input dynamic k-value was not considered in the design of an HMA overlay of an existing composite pavement. However, the MEPDG documentation indicates the HMA overlay performance is determined using the flexible pavement design, so this was consistent in that the design was controlled by the HMA overlay properties. Although the rigid performance was stated to have been analyzed using that modeling method, the output k-value did not appear to be based on the entered dynamic k-value. Therefore, the entered moduli should be based on the determined values from backcalculation.
It was noted that the resultant design was relatively insensitive to the trial PCC moduli. Therefore, the use of the established dynamic backcalculation adjustment factor (0.8 for the PCC modulus) could continue until new factors are developed or an agency develops more specific values. The MEPDG program appears to use static PCC elastic modulus values as entered inputs but the output files suggest it then reverts back to a dynamic value.
The established modular ratios (reported by Khazanovich, Tayabji, and Darter) to determine layer moduli from the backcalculated composite pavement modulus should continue to be used unless specific testing data are available to determine project specific ratios.(27)
Including an aggregate base layer and determining a corresponding dynamic subgrade k‑value did not appear to have a significant effect on the design results.
More details on the conduct of the case studies are found in chapter 7 of this volume and in volume II.
Based on the analyses that were conducted for the case study investigations, guidelines were developed for the conduct of FWD testing and the interpretation of the results. Specific guidance is provided on establishing FWD testing plans, performing backcalculation of deflection data (including useful tips on dealing with both routine and atypical situations), and using deflection data in the MEPDG. These guidelines are found in volume III of this report.
In addition, findings from the literature review and work on the case studies identified the need for continued improvements and developments in the analysis and interpretation of pavement deflection data. As described in chapter 8, these improvements lie in a number of specific areas, including the more direct consideration of climatic effects and slab size effects, the movement toward dynamic analyses (as warranted), the development of reliable corrections for dynamic loading conditions, and the development of improved models for both forward analysis and backcalculation for composite pavement structures.
