In addition to controlling the integrity of piles, the MIMP 16 allows to detect voids by the impedance method. A software specially developed for this method makes it possible to perform a large number of measurements very rapidly.

The principle :

Scanning a paving by the mechanical impedance method aims at detecting the areas of void between the body of the paving and the shape.
Let us briefly remind that it deals with a vibration method. A sound wave is created from a shock of known strength. The velocity of the wave is recorded by the MIMP 16 by means of a geophone positioned upon the element to test.
Studying the signal allows one to know whether the paving rests on its substrate correctly or not. In order to evaluate the vibratory information obtained at best, a post processing is performed.
It includes the analysis :
- of the average mobility (the velocity to force ratio),
- of the low frequency suppleness (slope at the origin of the mobility curve).

In order to limit the number of parameters to process, the mobility x suppleness product is computed. When this value is high, the paving is very likely to include a void.
Plotting a mapping allows to easily point out the areas that need to be repaired by an injection on the spot.
This method can also be used :
- to control the injection of concrete behind tunnel voussoirs,
- to detect void areas behind large pipes (between the pipe and the earth),
- to detect faulty cementing of tiles or pavings on a building façade or on a ground.

Operating the method :

On a floor, a lattice is virtually or physically created, for example by marking the spots of measurement with a chalk.
The operator assigns a number to each spot. The spots will be located 0.5 m apart from each other, for instance.
The operator tells the software the distance between each spot and the number of them in the X and Y directions, before the beginning of the acquisitions.
It then performs an acquisition by setting the geophone onto a spot and hitting with the hammer near it.
The signals are stored by the MIMP 16 automatically ; the operator goes to the next point before he performs another acquisition.
It can thus cover the whole surface of the floor.
The software knows the number of spots in the X and Y directions, it therefore knows the acquisition path of the operator precisely.
The MIMP 16 is so easy to use that it allows a single person to acquire 1500 points in a day.
After the whole area to test has been covered by the operator, a simplified mapping can be displayed on the screen if the MIMP 16.
This mapping is meant to detect the areas whose probability of faulty cementing is high.
After the data have been transferred to a computer, the IMPRO2 software makes it possible to process the data more finely, and to edit a 2 - or 3 - dimensional mapping.

The accessories of the MIMP 16 :

Miscillaneous accessories are available for the void detection method by the MIMP 16 :

• a bar instrumented by a force sensor, which avoids the operator to lean down when giving a hammer stroke,
• a system to set the geophone which also avoids the operator to lean down when setting a geophone,
• a small instrumented hammer to control voids behind thin plates such as façade tiles,
• a horizontal geophone to control voids behind vertical façade plates,
• a compensation device for the geophone allowing to incline a geophone to different angles for tunnel voussoirs and large pipes.

Do not hesitate to get in touch with C.E.B.T.P. for any additional information :

M. Ludovic DENIS ou M. Frédéric BOURDET

For they have a constant concern to improve their products, C.E.B.T.P. indulge themselves to modify a characteristic or a presentation of the hereabove described material without any previous notice.
The texts, schemes and photographs represented in the present pages are an exclusive property of C.E.B.T.P. and they cannot be reproduced under any form without a previous written authorization of C.E.B.T.P.
Copyright © 1999, C.E.B.T.P. All rights reserved.