U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-05-153
Date: December 2006
Long-Term Pavement Performance Program Falling Weight Deflectometer Maintenance Manual
Chapter 1. Introduction
Understanding why some pavements perform better than others is essential to building and maintaining a cost-effective highway system. Since 1987, the Long-Term Pavement Performance (LTPP) program, a comprehensive 20-year study of in-service pavements, has conducted a series of rigorous long-term field experiments monitoring more than 2,400 asphalt and portland cement concrete pavement test sections across the United States, Puerto Rico and Canada.
Established as part of the Strategic Highway Research Program (SHRP) and now managed by the Federal Highway Administration (FHWA), LTPP was designed as a partnership with the States and Canadian Provinces. LTPP’s goal is to help the States and Provinces make decisions based on the performance of existing pavement designs that will lead to better performing and more cost-effective pavements in the future.
One aspect of the LTPP experiments is the long-term and seasonal structural response of pavement. Many pavement engineers rely on falling weight deflectometer (FWD) technology to determine this information. FWDs "thump" the pavement and record information related to its structural integrity.
Falling weight deflectometers are often preferred over destructive methods of testing because they are much faster than destructive tests and do not entail the removal of pavement materials. In addition, the testing apparatus is easily transportable as a trailer, or vehicle-mounted.
The FHWA’s LTPP program operates eight Dynatest® Model 8000 FWDs to collect deflection data on in-service pavement test sections across North America. LTPP has collected pavement deflection data in daily operations for 15 years, and in that time, the FWDs have had very little downtime. Continuous preventive maintenance is necessary to keep the complex hydraulic-electrical-mechanical FWDs operating under demanding conditions to collect high quality data and pass rigorous annual reference calibrations. The owner’s manual from the manufacturer provides guidance on most repairs and troubleshooting; however, eventually FWDs require service beyond routine maintenance—in other words, the time comes for a complete overhaul.
In spring 2003, the LTPP Southern Region support contractor overhauled one of the FWDs operated for the LTPP program. During the overhaul, the contractor documented the process with photographs and captured illustrations of the FWD components and subcomponents. This document provides FWD owners, operators, and technicians information as a supplement to the Dynatest 8000 owner’s manual. Maintenance guidelines are based on continuous operation of FWDs. Less frequent operation may require adjusting maintenance frequency; however, the owner’s manual from the manufacturer should be followed.
Maintenance activity frequencies described as "routine" or "periodical" are roughly equivalent to monthly and semiannually, respectfully. Maintenance frequencies described in distance roughly equate to operating time as 1 hour equals 80.47 kilometers (km) (1 hour equals 50 miles (mi)).
This document is divided into six chapters. Chapter 2 discusses the FWD trailer. Chapter 3 covers the subassembly. Chapter 4 discusses the strike plate, load cell, and the raise/lower bar. Chapter 5 covers the electronics, and chapter 6 is a conclusion. A maintenance log is provided in the appendix at the end of the manual.
Chapter 5 covers the electronics, and chapter 6 is a conclusion. A maintenance log is provided in the appendix at the end of the manual.