Long-Term Pavement Performance Program Determination of In-Place Elastic Layer Modulus: Backcalculation Methodology and Procedures
Chapter 7. Conclusions
The automated backcalculation procedure and tools reported in this report were used to determine the elastic layer properties (or Young’s modulus) from deflection basin measurements for all LTPP test sections. This report summarizes the reasons why EVERCALC© and MODCOMP© were selected for the computations and analyses of the deflection data, provides a summary of the results using the linear elastic modulus for selected test sections, and identifies those factors that can have a significant effect on the results. This chapter includes some of the highlights and findings from this study and recommendations for future activities in support of accomplishing the overall LTPP objectives.
Findings
Backcalculation of elastic properties is not an exact science and requires user interaction in some cases. However, the process was automated through a series of utility functions and tools to reduce the impact of user interaction, bias, and/or inexperience. Results from this automated procedure provide elastic layer load response properties that are consistent with previous experience and laboratory material studies related to the effect of temperature, stress state, and seasonal effects on the material load-response behavior. The following list highlights some of the important findings from this study:
- In 1997, Von Quintus and Killingsworth recommended one software package to be used because of large differences between multiple programs.(46) Similarly, one package was used in the first backcalculation project, MODCOMP4.0©. The present study used three programs: EVERCALC© and MODCOMP© were used to calculate the elastic layer moduli of all pavement sections in the LTPP database, and the best fit method was used for all PCC-surfaced test sections. EVERCALC© and MODCOMP© are available in the public domain, and they provide non-unique solutions. The best fit method is a forward calculation procedure that provides unique solutions.
- EVERCALC© and MODCOMP© resulted in statistically indifferent results for deflection basins classified as typical or in conformance with elastic layer theory as defined by Von Quintus and Simpson and Von Quintus and Killingsworth.(4,46) For type 2 deflection basins, the EVERCALC© and MODCOMP© programs resulted in statistically different elastic moduli for the intermediate layers (the weathered soil layer and thinner aggregate base layers, which accounted for about 25 percent of the deflection basins analyzed). As reported in the first backcalculation study, the pavements exhibiting deflection-softening behavior with type 2 deflection basins were the most difficult to analyze and were generally found to have higher RMSEs.(4) Some of these deflection basin analyses resulted in no reasonable solution, or the solution provided unrealistic layer moduli for the type of material defined in the LTPP database.
- The PCC elastic moduli calculated with EVERCALC© resulted in significantly greater values in comparison to the moduli calculated with the best fit unbonded method. Thus, results from the two methods should not be combined, or a factor needs to be used to adjust the EVERCALC©-generated PCC moduli to the best fit unbonded condition method or laboratory-measured elastic moduli.
- Over 90 percent of the deflection basins analyzed with EVERCALC© and MODCOMP© resulted in layer moduli with an RMSE less than 3 percent and are considered acceptable.
- In the authors’ opinion, there is still no consensus on the best backcalculation package that provides the most reliable and accurate results. However, the case study findings were very similar to the findings documented in the Smith study.(53) Specifically, EVERCALC© consistently resulted in lower error terms and a higher number of successful modulus determinations when considering all deflection basins. When only considering those deflection basins that ran successfully, however, the MODCOMP© program resulted in lower RMSE values.
- Historically, a constant modulus for the PCC rubblized slabs has been used for rehabilitation design projects. The elastic modulus of the rubblized PCC slabs, however, was found to steadily increase over time for some of the LTPP SPS-6 test sections.
- No difference was found in the load-response properties (stiffness) between the RAP and virgin mixtures of the SPS-5 experiment.
- Similar c-factor values were found in this project to the values included in the MEPDG.(1) More importantly, no significant difference or bias was found between the laboratory-derived dynamic modulus and the field-derived values for the HMA layers.
- The use of drop heights 1 to 4 in the LTPP deflection testing program did not result in significantly different elastic moduli for the unbound layers. As such, it was difficult to determine the coefficients of a constitutive equation to estimate the stress sensitivity of an unbound layer because of the variation in moduli over time and along a specific test section. The stress levels resulting from the four drop heights used in the LTPP deflection testing program are too narrow.
- Damage was determined in accordance with the MEPDG procedure for multiple LTPP test sections to demonstrate its use in rehabilitation design.(1) The results were found to be positive and used to support the MEDPG procedure. The damage concept as applied to interpreting the backcalculated moduli can be very useful in explaining low moduli. In addition, the Florida Department of Transportation was one of the first agencies to monitor changes in the deflection basin (similar to this damage concept) over time in planning rehabilitation projects from a pavement management standpoint. From the case study sites and examples of data use, the damage concept can be used to evaluate the condition of the existing pavement and plan future rehabilitation projects.
- The use of four layers generally resulted in lower RMSEs than the use of three layers. In many cases, breaking or separating the subgrade into at least two layers improved the match between the measured and calculated deflection basins.
Recommendations
The results from this study have shown that elastic layer moduli (or load-response properties) can be computed from deflection basins and provide pavement engineers with useful information about the pavement structure and subgrade condition. However, this study only touched on the different ways the backcalculated elastic layer modulus database can be used for improving pavement design and rehabilitation strategies. More detailed analysis can be completed to demonstrate the usefulness of the deflection data and resulting elastic moduli. The following list highlights some specific topics that can be investigated in the future:
- Compare the field-derived elastic moduli of HMA layers to the laboratory-derived dynamic moduli for determining the equivalent load frequency of the FWD for matching the two moduli.
- Compare the PCC laboratory-measured static elastic moduli to the field-derived values to determine the adjustment factor for the EVERCALC©/MODCOMP© backcalculated elastic modulus of PCC layer to be used in the MEPDG.(1)
- Use the LTPP database for preparing a set of guidelines for estimating damage through deflection basin testing and backcalculated elastic moduli between within and outside the wheel paths. Agencies can use this information for planning rehabilitation projects.
- Confirm or revise the default PCC elastic layer moduli of rubblized and crack and seat slabs, as well as define how those values change with time.
- Use the backcalculated results to confirm the default moduli included in the MEPDG or recommend revisions to those default values and determine whether these values depend on season or climate.(1)
