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LACROIX DEFLECTOGRAPH

How It Works

The Lacroix deflectograph measures the vertical deformation, known as deflection, of a surface

under the action of a heavy weight moving at constant speed.

Deflection is measured between each pair of wheels of the rear axle of the vehicle using boom

probes fitted with rotating sensors and articulated on a reference beam. This reference boom,

allowed to move freely with regard to the carrier vehicle, rests on the road surface by means of

three points located outside the load's zone of influence. As the vehicle moves forward,deflection is recorded until the vehicle's wheel pairs draw level with the sensors. The reference

beam is then controlled by the vehicle and moved forward into its initial position on the road

surface with regard to the carrier vehicle, ready for a new measuring cycle, without the vehiclestopping or slowing.

Correlation with the static deflection measurements (by means of sensors embedded in the roadsurface) is of very good quality, even for very slight road surface deformation.

Description

The device comprises the following elements:

a two-axle truck with a wheelbase approximately 5 m; the single rear axle with wheel pairs must

be able to carry a load of 13 t.

a reference boom fitted with two sensor arms.

an electronically controlled traction and guide mechanism protects the ends of the boom from

damage by crushing.

a control console for the electronic measuring and recording equipment.

cast iron ballast weights are fixed to the rear of the truck.

CHARACTERISTICS

Measurement speed: 3 km/h +/- 0,5 km/h Distance between measurement increments: 3 to 6 m depending on the vehicle used and the

testing speed (81 data capture points per measurement increment).

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APPLICATIONS

The Lacroix Deflectograph is mainly used for:

Monitoring road network as well as its changing condition under traffic;

detecting defective areas requiring to be reinforced;

Monitoring performance and effectiveness of surface reinforcement;

Winter road network monitoring (closing or opening roads in the presence of heavy thawing).

Using the LACROIX DEFLECTOGRAPH device, there are two methods for calculating the

bending radius:

using an inclinometer fitted on a beam; however this measurement is relatively

uncommon nowadays because of the lack of equipment.

by recording road surface deformations.

Bending radius values as calculated nowadays are more difficult to interpret, and only have

limited usefulness, and in certain cases (mixed structures, bitumen treated sub grades).

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Depending on the type of road surface, the usefulness of bending radius data may vary:

On conventional flexible road surfaces, it varies inversely with the deflection, and isfairly well correlated with it; thus it provides little new information concerning the road

surface;

On semirigid road surfaces, calculation of bending radius is sensitive to thicknessvariations in the layers, and to temperature variations in the bitumen surface layers. It is

also affected by any tension cracks in materials that have been treated with hydraulic

binders. As a result it displays high rates of non-significant values, in addition toconsiderable value dispersion.

This makes it difficult to use, even though it may serve in certain cases.

In the case of mixed structures, or those with fairly homogenous thick bitumen layers, it

is possible to detect adhesion failure or detachment, or damage in intermediate bitumenlayers, using bending radius values.

Nevertheless it is advisable to confirm the real state of the layers by means of one or two core

samples.