Long-Term Pavement Performance Program Determination of In-Place Elastic Layer Modulus: Backcalculation Methodology and Procedures

Chapter 1. Introduction

Background

Deflection data have been measured at periodic intervals with a falling weight deflectometer (FWD) on all rigid, flexible, and composite pavement test sections included in the Long-Term Pavement Performance (LTPP) program. These data or deflection basins have been measured to determine the load-response properties of the pavement structure and subgrade. Currently, there are 16,364 FWD testing days and more than 2,400 test sections in the LTPP database.

Deflections are measured approximately every 2 years for sections included in Specific Pavement Studies (SPS) experiments and every 5 years for sections included in General Pavement Studies (GPS) experiments. There are 64 test sections included in the LTPP Seasonal Monitoring Program (SMP), and deflection data were measured every month for 1 to 2 years for these sites. The SMP sites were used to determine the change in structural properties throughout the year and how changes in moisture and temperature affect the in-place
structural response properties.

A common use of deflection data is to backcalculate in-place layered elastic modulus values. Layered elastic modulus values and how the values change over time are used as inputs for estimating remaining life and deciding on an appropriate rehabilitation and design strategy. In addition, many agencies have used the LTPP deflection data for use in calibrating mechanistic-empirical distress transfer functions, including those in the Mechanistic-Empirical Pavement Design Guide (MEPDG).⁽¹⁾

The Federal Highway Administration (FHWA) sponsored earlier studies to backcalculate elastic layer modulus values from deflection basins and included these computed values in the LTPP database. (See references 3, 4, 11, and 12.) Some of the methods used in the earlier studies, however, have been advanced within the past decade. In addition, the amount of deflection data in the LTPP database has increased substantially, especially for the SPS sites. As such, FHWA sponsored a follow-up project to revisit the methods used in the first round of backcalculation and to calculate the elastic layered modulus values for the deflection data that did not exist during the first round of backcalculation.¹

Project Objective

The objective of this study was to select one or more methods to determine the in-place elastic layered modulus from deflection basin measurements for the LTPP test sections and execute those methods for all flexible, rigid, and composite pavement sections included in the LTPP program. These backcalculated elastic layer modulus values, simulated pavement structures for each LTPP test section, and related parameters were integrated into the computed parameter tables (CPTs) of the LTPP database. The project was divided into the following two phases:

• Phase I: Phase I included selecting methods and demonstrating those methods and associated pre- and post-processing tools for estimating in-place elastic layered modulus values.² As part of phase I, the recommended procedures to be used in the next phase were demonstrated through a series of case studies.³
  
  
• Phase II: Phase II included executing the methods selected from phase I and backcalculating the elastic layered modulus from deflection basin data stored in the LTPP data for the simulated flexible, rigid, composite, and rehabilitated pavement structures. This phase also included uploading the calculated values and associated parameters into a set of CPTs in the LTPP database.

Scope of Report and Organization

This report is divided into seven chapters, including this introductory chapter, as follows:

• Chapter 2 describes the process used to select candidate backcalculation methodologies and programs. It summarizes the work completed under phase I of this project.
• Chapter 3 summarizes the case studies that were used to select the backcalculation methods and develop the pre- and post-processing utility tools as well as the
  automated procedure.
• Chapter 4 provides an overview of the backcalculation process. This includes the rules of simulation, deflection data evaluation to identify problem basins, and acceptance criteria used to judge and interpret the results of the backcalculation process.
• Chapter 5 summarizes the results from the backcalculation process and the data included in the computed parameter database. It also identifies the evaluation factors used to judge the results of test sections that had moderate to high errors and were excluded from the statistical summaries in the CPTs.
• Chapter 6 includes examples to demonstrate application of the backcalculated elastic layered modulus results.
• Chapter 7 summaries the findings and recommendations from this project.

This report also contains three appendices that provide detailed information on the computed parameter database, the tools and macros written to facilitate the entry of required data, and the backcalculation process so that similar results can be obtained by others not directly involved in this project.

¹ In this report, “backcalculation packages” refers to both forward calculation and backcalculation software programs for estimating the in-place layered elastic modulus values from deflection basin data.

² The project team reviewed multiple methods to determine the in-place elastic layered modulus values, and the results were reported to FHWA in an unpublished technical memorandum, the contents of which are summarized in this report.

³ The project team reported the phase I results to FHWA in an unpublished interim report. The contents of the interim report are summarized in this report.