Using Falling Weight Deflectometer Data With Mechanistic-Empirical Design and Analysis, Volume III: Guidelines for Deflection Testing, Analysis, and Interpretation

CHAPTER 1. INTRODUCTION

The need to accurately characterize the structural condition of existing pavements has increased with the ongoing development of mechanistic-empirical thickness design procedures and particularly with the release of the Mechanistic-Empirical Pavement Design Guide (MEPDG) prepared under National Cooperative Highway Research Program (NCHRP) Project 1-37A.^(1,2) In the MEPDG, the performance of the pavement being designed is projected by simulating the expected accumulated damage on a monthly or semimonthly basis over the selected design period. The amount of incremental damage occurring during each computation interval (either monthly or semimonthly) varies as the effects of prevailing environmental conditions, changes in material properties, and effects of traffic loading are directly considered. Ultimately, the incremental damage accumulated during each computation interval is converted into the development of physical pavement distresses and projected roughness levels using calibrated performance models.^(1,2)

An integral part of this process is the accurate characterization of material parameters of each layer in the pavement structure. Deflection data collected by the falling weight deflectometer (FWD) can be used to characterize the parameters of the paving layers through backcalculation, in which the engineering material parameters of the paving layers (elastic modulus, E, or dynamic modulus, E*) and underlying soil (resilient modulus, M_R, or modulus of subgrade reaction, k) are estimated based on the measured surface deflections, the magnitude of the load, and information on the pavement layer thicknesses. In essence, the set of characteristics for the paving layers and subgrade material is determined such that it produces a pavement response that best matches the measured deflections under the known loading conditions.

Taken as a whole, pavement deflection data can be used in a number of ways, including the following:

• Evaluate maximum deflections normalized to a standard load.
• Backcalculate material parameters of pavement layers.
• Evaluate subgrade support conditions.
• Assess potential areas of localized weakness.
• Assess the degree of deterioration in the pavement structure.
• Determine structural enhancement requirements (overlay thickness).
• Determine structural remaining life for projected traffic loadings.
• Evaluate the structural capacity of the pavement structure.
• Develop load limits and/or seasonal load restrictions.
• Determine the presence of built-in upward curling (for rigid pavements).

Over the years, researchers and practitioners have developed numerous approaches to backcalculate pavement layer and subgrade moduli, as well as numerous programs to perform the calculations, in ongoing efforts to better characterize the material properties of the existing pavement structure.

This report provides best practice guidelines for deflection testing of existing pavement structures, as well as recommended backcalculation techniques and data interpretation procedures to analyze those results as part of a mechanistic-empirical pavement evaluation and rehabilitation process. The focus of the report is the use of the FWD because it is the most commonly used deflection testing device.

In addition to this chapter, this guideline report consists of the following:

• Chapter 2: A general overview of deflection testing, including a summary of various factors that affect deflection testing results and general guidelines for conducting a deflection testing program.
  
  
• Chapter 3: General guidelines for backcalculation, including a discussion of the various data input requirements and suggested default values for backcalculation, an overview of backcalculation modeling issues, methods for assessing convergence and verification of backcalculated results, and a presentation of a backcalculation example.
  
  
• Chapter 4: General background information on mechanistic-empirical pavement design, while also providing specific data and testing recommendations for project evaluation, and flexible, rigid, and composite pavement inputs in the new MEPDG.
  
  
• Chapter 5: Overall summary of the report.
  
  
• Glossary: List of terms and their definitions.