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    Development of a Laser-Based High Speed Deflectograph

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    Today's heavily congested highways provide a difficult setting for bearing capacity measurements using stationary equipment like the FWD resulting in a situation which is dangerous to both drivers and FWD operator. Furthermore, techniques like the FWD method only provide information in discrete points that are often separated several hundred metres apart. To overcome these deficiencies a laser-based high speed deflectograph is being developed which can perform continuous bearing capacity measurements at all driving speeds in the range 20–70 km/h.

    The unique concept of the high speed deflectograph utilizes laser Doppler sensors to provide the deflection speed of the pavement surface. By using this technique the deflection speed is determined in one operation as opposed to the more commonly used distance laser sensor method which determines the deflection as the difference between vertical deformation in two different points. The laser Doppler system has been found capable of determining deflection speeds within the desired range of driving speeds with an accuracy of 0.14 mm/sec, which has been shown to correspond to an accuracy of the deflection of 5 μm.

    While it is important to measure deflection speeds correctly, it is equally important to continuously register and control the position of the laser sensors using secondary measuring systems like servo systems and inertial units.

    The high speed deflectograph consists of a towing truck and a trailer on which the laser sensors are mounted. The trailer is equipped with a system, which allows variations of the applied load.

    In parallel with the development of the physical measuring device a study is carried out concerning the interpretation of results from the high speed deflectograph. Results from work carried out to establish a coupling between deflection speeds and absolute deflections, of which the latter can be compared to FWD results, are presented.


    pavement, bearing capacity, test, nondestructive, apparatus (measuring), deflection, laser, Doppler effect, Denmark

    Author Information:

    Hildebrand, G
    Research Engineer, Road Directorate, Danish Road Institute, Roskilde,

    Rasmussen, S
    Development Engineer, Greenwood EngineeringTechnical University of Delft, BrøndbyGA Delft,

    Andrés, R
    Graduate Exchange Student, Greenwood Engineering, Brøndby,

    Committee/Subcommittee: D04.39

    DOI: 10.1520/STP14784S