Phoenix FWD 1966 531X150

FWD History

The story of Falling Weight Deflectometers - in brief

The present article deals with the story about the PHØNIX FWD, which is today the PRIMAX FWD series now manufactured by Grontmij after the acquisition of Phønix Pavement Consultants, the FWD department of Phønix Contractors A/S. The story starts back in 1964…

Lacrois 20Equipment 20Danish 20Road 20Directorate 201972In the early sixties French engineers were doing research work on bearing capacity measurements and how to perform this kind of measurements. In France the FWD tests were stopped as the engineers preferred the French wandering Benkelman Beam, the Lacroix deflectograph.


In Denmark the possibilities of applying a Falling Weight Deflectometer (FWD) for bearing capacity measurements were examined for the first time in 1964 by two students of the Danish University of Technology. Dr Tech Axel O. Bohn describes this in his article about the first 25 years of the history of FWD “The history of the Falling Weight Deflectometer (FWD)” . You can download his article on this site.

The reason for trying a new method was the realisation that bearing capacity of roads could actually not be measured by means of the static plate-bearing testing method.

The principles of the two measuring methods used parallel in the sixties and the seventies were:

The French method was the “Wandering Benkelman Beam” – the Deflectometer Lacroix. The measuring principle of this method is:

A truck drives along the road and deflection measurements are continuously made at 3 m intervals.

The other method was the falling weight deflectometer, which is also the method applied today:

A variable mass transmits after free fall its impact load to a damped spring system on a circular loading plate placed on the measuring point. The deflection is measured by LVDC sensors (geophones) in the centre of the loading plate and by sensors placed at various distances from the centre geophone on a geophone beam. The system simulates the load from a rolling truck wheel.

A/S PHØNIX takes up the idea

216 Phoenix FWD 1966In 1965 Civil Engineer J. B. Villadsen made his company A/S PHØNIX construct a series of falling weight deflectometers in a more handy and safe version. The first model of the FWD was transported on a truck and was a manual system. The measuring results were written on a wax strip. The spring system was a so-called S-system.

216 Olepoulsen Forsteprototypetrailer 1968In the mid-sixties A/S PHØNIX manufactures the first trailer-mounted FWD. The result was safer measuring equipment. In this way A/S PHØNIX (today Sweco Pavement Consultants) was the first company to commercialise the FWD and in 1968-1969 sixty-five FWDs were produced and sold from the factory of A/S PHØNIX in Vejen, Denmark.

216 FWD CombineddynatestphoenixlenzIn 1979 Dynatest A/S starts producing falling weight deflectometers and interrupts the cooperation with A/S PHØNIX but until then the Danish FWD was a combination of know-how from A/S PHØNIX and Dynatest. However, the start was at DTU in cooperation with A/S PHØNIX in 1965 and up to the time when some of the members of the FWD team establishes Dynatest A/S.

In 1979 Dynatest A/S presented their first FWD and in 1981 A/S PHØNIX presented a FWD with one load cell and 6 geophones as an answer to the Dynatest 8001 FWD from 1979. This equipment was the old ML5000 model.

216 ML5000 PhoenixThe first equipment made by the remaining FWD team with A/S PHØNIX

A/S PHØNIX continued developing and improving its FWD, and in 1984 A/S PHØNIX presented a new FWD under the name of ML10000 with 6 geophones and one load cell and later the ML12000 was introduced. The difference between the ML10000 and ML12000 was that the ML10000 was constructed on a single-axle trailer and the ML12000 was constructed on a bogie-trailer. Both equipments had measuring ranges of up to kN 120.

216 ML10000 IKRAM Malaysia 1986

Later in the eighties the MLY10000 and MLY12000 were introduced - still with 6 geophones.

216 MLY12000 Indien Nov1993       216 MLY10000 Rumania 1993

The difference between the ML-series and the MLY-series was that the ML-series was mechanically operated and the MLY-series was hydraulically operated.

With the MLY-series A/S PHØNIX introduced the use of hydraulic control instead of electric control. Both the ML-series and the MLY-series had electronic recording systems.

The first van-integrated FWDs 
In the early eighties Phønix started to develop the first truck-mounted FWD which was delivered to Norway and later in 1984/85 A/S PHØNIX constructs its first van-integrated FWD. Two van-integrated FWDs were constructed – one built into a T5 VW LT and another into a Toyota Highlux. The purpose of this project was to develop equipment that could be handy and safer to apply on e.g. airfields and in big cities.

Series1 VW LT Van Integrated FWD     216 Vanintegrated Fwdtoyota

The PRI-series 
At the end of the eighties the development work that would later result in the PRI-series was initiated and in 1990 A/S PHØNIX introduces a new generation of the traditional FWDs – the so-called PRI-series which in many ways breaks with earlier FWD versions’ limitations and sets new standards for the capacitities of FWDs. The new generation had from 6 to 9 geophones and was infinitely adjustable from 7-250 kN.

The PRI-series was available in two versions a traditional 7-150 kN FWD under the name of PRI 1506 and PRI 2509 and a Heavy Weight 7-250 kN version under the name of PRI 2509. These FWDs were also constructed both as trailer-versions and van-integrated versions.

Some of the new features were:

  • The number of geophones was increased to 9 geophones
  • The measuring range was increased to 250 kN
  • A time-history facility was introduced as well
  • Safety systems were improved

PRI2100 - for the new millennium
The PRI2100 series is a very flexible version and is constructed as a unit which makes it easy to switch from a trailer-version to a van-integrated and visa versa and from a traditional 7-150 kN to a 7-250 kN. The first versions of this generation were introduced in 1995 by A/S PHØNIX Contractors.

The PRI2100 series offered a lot of new advantages and some of these were:

  • Flexibility – easy to upgrade or downgrade
  • Constructed as a unit – easy to mount on a trailer and to switch to van-integrated and visa versa – and to upgrade from a 7-150 kN to a 7-250 kN
  • Safety – emergency switches, safety transport lock, incorporated alarms displaying on the operator’s screen, the operation is blocked if the operator ignores vital warnings.
216Prilunderskovbroen4094x02       216Pribolivia

Carl Bro A/S acquires PHØNIX pavement consultants
In 1999 Carl Bro A/S acquires PHØNIX Pavement Consultants from A/S Phønix Contractors of Rieber & Sons ASA and PHØNIX Pavement Consultants changes name to Carl Bro Pavement Consultants, but the staff behind and the know-how remained the same.

PRIMA 100 - a portable FWD – Light Weight Deflectometer (LWD)
In 1996/1997 Pavement Consultants, at that time a business unit in A/S PHØNIX Contractors of Rieber & Sons ASA, started the development of the first prototype of PRIMA100 based on the technology applied for the traditional large falling weight deflectometers (FWD) – knowledge that our FWD team has had since the early sixties.

The reasons for starting the development were an increasing amount of inquiries from our customers and our own engineers to have a handy type of equipment for quality control in connection with e.g. testing of unbound structures. Also they needed equipment that could measure in places where the traditional FWDs could not move about.

The goal was to develop equipment that was easy to handle and gave the end-user results on site. Furthermore, the equipment could be a supplement to the

A new generation in 2003

216 Prima100In 2003 Carl Bro develops a new version of the PRIMA100 LWD and today the generation of the PRIMA100 is available with Bluetooth, GPS and a load range of 1-20 kN.




A revolution in 2005 - the first super HWD in the world
In 2005 Carl Bro Pavement Consultants constructs the first Super Heavy Weight Deflectometer in the world – PRI2100-S. This FWD is designed for measurements on pavements subjected to extremely heavy traffic such as Boing 777 or Airbus 380 aircrafts the latter with a width of 80 m and a weight of 560,000 kg. The load range of the Super HWD is 7-300 kN.

Also the Super HWD is available as trailer mounted and van-integrated versions.

216 CB SHWD PRI2100 Van Leaflet        216 CB SHWD PRI2100

The new PRIMAX series with 18 geophones or more
Today the traditional FWDs, HWDs, SHWDs produced and sold by Sweco Danmark A/S, Pavement Consultants are sold under the product name PRIMAX. With this series a totally new data collection software and software for the FWD has been develop. Sweco is the first manufacturer in the world to introduce a van integrated SHWD - Super Heavy Weight Deflectomer - with 18 geophones. A sensation applied on airports around the world.

The PRIMAX series is of course available both as a trailer version and a van-integrated version.

Van        PRIMAX FWD Trailer Video

Quality Assurance of the FWDs from Sweco
- and other isotope methods. Troxler

ISO certification
Sweco is certified under ISO 9001 and all equipment types comply with the EMC regulations (EEC electronic compability regulations - EMC directive). All equipment is CE-marked.

Calibration your assurance of reliable results!
Calibration of equipment is very important to assure the quality of data collected.

Sweco offers a stacking tower for verification of the geophones before a measuring job.

For absolute calibration we have developed portable calibration equipment which allows us to calibrate FWD at our own workshop but also at customers’ sites.

We recommend absolute calibration every 12 months to ensure that the equipment measures correctly.

Traceable to international institute:

  • Physikalisch-Technische Bundesanstalt
  • PTB, Braunscweig

We hope you enjoyed reading about our falling weight deflectometers up through times. We, on our side, enjoyed telling the story about our equipment. We are happy to see that even some of the old ML-units are still in operation through out the world. We take this as a token of that our efforts to develop precise and reliable equipment bear fruit.

So far we have sold more than 300 units around the world.

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