National Cooperative Highway Research Program
NCHRP Project 20-5

Synthesis of Highway Practice 38-15
Falling Weight Deflectometer Usage

FINAL SCOPE

Falling Weight Deflectometers (FWDs) have been in use since the 1970s.  These devices are used to measure pavement deflections in response to a stationary dynamic load, similar to a passing wheel load.  The data obtained are used to evaluate the structural capacity of pavements for research, design, rehabilitation, and pavement management purposes.  Currently, approximately 240 FWDs produced by four different manufacturers are in use in the U.S. The number of FWDs in use and the importance of their role in pavement engineering practice are expected to rise as agencies move toward mechanistically-based pavement design.   The interpretation of FWD data is a key method for estimating the in situ moduli of pavement layer materials.

Calibration protocols suitable for all FWDs currently sold in the U.S. (other than lightweight FWDs) were developed as part of the Long-Term Pavement Performance research and adopted by AASHTO.  FWD calibration centers were established to provide service across the continental U.S.  Calibration center records suggest that many of the FWDs currently being used are not calibrated on a regular basis.  Absent calibration, agencies have no way to be sure that the substantial investment they make in data collection and interpretation is yielding meaningful results.  Similarly, the knowledge and information exchange that takes place at annual meetings of the FWD User Group suggests that many aspects of FWD use and data application are inconsistent among owners and operators. 

The purposes for collecting FWD data have a major influence on the highway agency practices.  This synthesis of highway agency practices for FWDs use will provide information needed to support guidelines for advancing the state-of-the practice.  Information needed to quantify and document the various practices should include, but not be limited to:

·         FWD Equipment

o        Ownership (e.g., agency Vs. consultant)

o        Type, quantity, age

o        Configuration (e.g., truck or trailer mounted, number of sensors and spacing, nominal load levels used)

o        Maintenance practices (e.g., in-house Vs. manufacturer)

·         Equipment Calibration

o        Agency requirements (e.g., frequency, distance traveled, costs)

o        Calibration verification

o        Calibration center support and use

·         Data Collection, Management and Storage

o        File formats (e.g., PDDX, *.fwd)

o        Collection guidelines (e.g., preparation, testing protocols)

o        Safety (e.g., traffic control)

·         Data Analysis (e.g., research, design, pavement management)

o        Methods (e.g., back-calculation)

o        Purpose (e.g., MEPDG, forensic investigations)

o        File formats (e.g., PDDX, *.fwd)

·         Personnel Training

o        Agency and/or consultant FWD operator qualifications

o        Data Analysis personnel qualifications

o        Calibration center operator certifications

o        Participation in FWD User Group meetings

A review of the relevant literature in the field will be combined with surveys of state DOTS and other appropriate entities.  Survey results and selected case studies will be developed to profile innovative and successful practices, as well as lessons learned. Gaps in information and knowledge will be identified along with research needs.

Consultant

Dr. Sirous Alavi