1. EVALUATION OF FWD DATA FOR DETERMINATION OF LAYER MODULI OF PAVEMENTS Dr. Yusuf Mehta, P.E. Rowan University Dr. Reynaldo Roque, P.E. University of Florida Presented at the 2 nd European Pavement User Group June 22 nd, 2002 Cascais, Portugal

2. Acknowledgements Florida Department of Transportation Florida Department of Transportation Federal Highway Administration Federal Highway Administration

3. Presentation Outline Problem Statement Problem Statement Objectives Objectives Scope Scope Background Background  Backcalculation  Falling Weight Deflectometer Testing Results and Analysis Results and Analysis Summary Summary Conclusions Conclusions

4. Problem Statement Backcalculation of layer moduli does not provide a unique solution Backcalculation of layer moduli does not provide a unique solution 95 % of the deflection measured at the surface attributed to subgrade 95 % of the deflection measured at the surface attributed to subgrade Computer programs may match deflection basins but may not “necessarily” give reasonable values Computer programs may match deflection basins but may not “necessarily” give reasonable values  Depends heavily on the seed value

5. Problem Statement Program cannot think for the user! Program cannot think for the user! Factors affecting surface deflection influenced by Factors affecting surface deflection influenced by  damaged layers, thickness variation and temperature A detailed analysis of FWD data necessary to prevent misleading interpretation of data A detailed analysis of FWD data necessary to prevent misleading interpretation of data

6. Objectives To present case histories To present case histories  illustrate problems that can be encountered  interpretation methods that can be used to properly evaluate FWD data Proper interpretation of FWD data Proper interpretation of FWD data  complete evaluation of all available data is necessary for pavement sections that have experienced multiple milling operations and overlays Utility of multiple FWD measurements along the length of the highway Utility of multiple FWD measurements along the length of the highway  identify patterns that are crucial to the proper interpretation of the data

7. Overview of Projects Palm Beach Marion Brevard Duval Hamilton Hamilton MadisonCountyDAB US 301 62387491 AndersonI-7522139971 CouchI-1012134391 WhiteI-7532139961 RangerTurnpike72325941 MacAsphaltI-9552423161 AtlanticI-1042133001ContractorRouteProject ID (UF) Projectno Projects cover a broad range of contractors and materials

8. Backcalculation “Inverse” problem of determining material properties of layers “Inverse” problem of determining material properties of layers No direct closed form solutions available No direct closed form solutions available Heavily depends on “seed” value Heavily depends on “seed” value Combination of moduli value may produce similar deflection basins Combination of moduli value may produce similar deflection basins  In this case elastic layered analysis was used (BISDEF)  The incremental advantage of using other may be compromised by inherent assumptions in the backcalculation process

9. FWD Testing 10.16 cm 30.5 cm 20.3 cm 30.5 cm D0D0 D1D1 D2D2 D3D3 D4D4 Load 40 kN D5D5 D6D6

10. FWD Analysis Various algorithms Various algorithms  Absolute difference, % difference or sum of absolute difference Matching the curvature is better than minimum error method Matching the curvature is better than minimum error method  Representative of stiffness of pavement system

11. FWD Data – Project 1 (I-10WB) AC Modulus: 1100 ksi Tested on Dec 2000LayerDesign Thick., in FC0.75 SP-9.51.57 SP-193.15 ARMI0.39 Binder course 1.57 Limerock9.84 Stabilized base 11.81

12. FWD Data – Project 2 (I-75SB) AC Modulus: 1600 ksi Tested in Jan 2000 Core thickness varied from 6 to 12 inLayerDesign Thick., in FC0.75 SP-12.51.97 SP-192.95 ARMI0.47 Binder course 2.24 Limerock10.51 Stabilized base 11.81

13. FWD Data – Project 5 (I-95NB)LayerDesign Thick., in FC0.75 SP-9.51.57 SP-12.53.15 ARMI0.39 Binder course 1.57 Limerock base 10.51 Stabilized subgrade 11.81 Tested in Nov 1999 Asphalt concrete modulus=1100 ksi Stabilized subgrade= 200 ksi

14. FWD Data – Project 6 (US301SB)LayerDesign Thick., in FC0.79 SP-12.50.97 ARMI0.47 Type I AC 1.10 Binder course 2.05 Limerock10.83 Stabilized base 12.01 The AC modulus correlated well with cracks at the bottom of the cores. Tested in Aug 1999. The modulus of surface course: 350 ksi

15. Findings from FWD data Generally, very stiff pavements with good base and subgrade (some variability noted) Generally, very stiff pavements with good base and subgrade (some variability noted) It appears reasonable to attribute observed rutting to permanent deformation within the surface layer (i.e., good experiment). It appears reasonable to attribute observed rutting to permanent deformation within the surface layer (i.e., good experiment). FWD was useful in identifying the damaged structural layer within the surface layer in project 6 (It may influence future cracking performance). FWD was useful in identifying the damaged structural layer within the surface layer in project 6 (It may influence future cracking performance).

16. Conclusions and Recommendations Although algorithms have improved for interpretation of deflection basin, Although algorithms have improved for interpretation of deflection basin,  FWD interpretations have become increasingly challenging  More and more of the roads experience multiple milling operations, overlays, and complex performance histories  interpretation of FWD data should include complete evaluation of all available data The default values obtained from any program may give a good fit between the measured and computed deflections, but the modulus values obtained from the analysis may be of little value. The default values obtained from any program may give a good fit between the measured and computed deflections, but the modulus values obtained from the analysis may be of little value.

17. Conclusions and Recommendations there is no unique solution for FWD backcalculation analysis, there is no unique solution for FWD backcalculation analysis,  it is very difficult to ascertain a modulus value based on the backcalculation at a single location. analyzing the variation of moduli of all the layers along the length of a section are critical in identifying inconsistencies and assuring reasonableness and accuracy of the interpretation analyzing the variation of moduli of all the layers along the length of a section are critical in identifying inconsistencies and assuring reasonableness and accuracy of the interpretation  understanding the inconsistencies in modulus values along the length of a section is essential in deciding which layer moduli should be fixed or varied along the section

18. Conclusions and Recommendations Independent verification parameters Independent verification parameters  air or pavement temperature during FWD testing  actual asphalt concrete thickness from cores or ground penetrating radar  change(s) in mixture design  variations in water-table, or anomalies from the construction plan (culverts, etc)  the data could also include specific distress like core information and various forms of distress, and rating prior to rehabilitating underlying section

19. Thank you!