Dynatest FWD - 1. Introduction

2017.06.19 FwdMan2.8.17.docx 1-1
1. Introduction
1.1 Intro
A major advantage of analytically based structural design methods over more empirical
methods is that the former may be used with any type of material and structure and under all
climatic conditions, whereas the latter only may be used under those conditions for which the
empirical relationships were developed. Most pavement research effort has therefore
concentrated on developing and perfecting analytically based methods (e.g., the first six Ann
Arbor Conferences).
Analytically based methods can only be used if the moduli of the pavement layers can be
determined. This may be done if the test method allows for:
1. A force amplitude and duration approximating the effect of a heavy moving wheel load in
order to allow for non-linear and visco-elastic stress-strain response, and
2. Very accurate deflection measurements, especially at several large distances (i.e. larger
than the thickness of the pavement) from the centre of the load. This is absolutely essential
in order to get an accurate determination of the (non-linear!) Subgrade modulus, which
generally contributes some 60-80% to the total centre deflection.
The Dynatest 8000 FWD (Falling Weight Deflectometer) Test System, 8000 VMD (Van
Mounted Deflectometer) and 8081 HWD (Heavy Weight Deflectometer) Test System all
satisfy these conditions, whereas most vibrating equipment has a light load capacity, and
Benkelman or other beam type equipment is inaccurate at significant distances from the load
because the deflection gauge must be supported within the deflection basin itself.
The FWD, VMD or HWD may therefore - contrary to most other equipment - be used with
analytically based structural design methods.
1.2 History
Based on early work in France during the sixties, the Technical University of Denmark, the
Danish Road Institute and the Dynatest Group have gradually developed and employed the
Falling Weight Deflectometer (FWD) for use in Non Destructive Testing of highway and
airfield pavements. In 1977, the Dynatest 7800 Registration Equipment was introduced to
obtain digital load and deflection value readouts, although it was still necessary to hand place
the deflection sensors away from the load. Various research projects showed, however, that
simultaneous measurements of at least six deflections (plus the load) were necessary for quick
and precise deflection basin determinations, from which it became possible to evaluate the In
Situ E-moduli or stiffnesses of all structural layers in an existing pavement system through the
use of the Dynatest “ISSEM4” main frame Fortran computer program, and in the recent years
through the ELMOD PC computer program.
In 1979, Dynatest introduced the Model 8001 FWD featuring compact mechanical design
with integrated raise/lower sensor bar and remote control of all operations, including load