Historical overview of pile integrity control by the reflection and impedance methods at CEBTP :

Three men, Jean PAQUET, from CEBTP, along with his team, Jean-Pierre CHIPART, Michel BRIARD and André CABÉ are the authors of pile controlling by the echo, vibratory and pulse method.
Using a method of echo detection for pile control was first contemplated at CEBTP in 1964.

1965 : The first trials to control the continuity of piles by the echo method took place. The prompting to the pile was provided by four magnetostrictive hammers. The signals were collected by five acceleration sensors. Measurements were made on real piles located at Porcheville, in France.
Trials were also made on replicas of piles made of AFCODUR polymer material, with a diameter of 50 mm, in order to refine the method.

1966 :By the time, the impedance method was called vibratory method or mechanical admittance method. It was already used in aeronautical structures, but the difficulty lied in applying this method to piles in buildings. A more important energy was indeed required, whereas lower frequencies were also to use.

The first technique consisted in fitting the top of the pile with a prompter using a vibrating cylinder. One therefore had to bind this cylinder to the pile, and be certain the cylinder was aligned with the axis of the foundation. To achieve this, a crown shaped rubber mould was set onto the top of the pile, and a resin was cast inside. After drying, a perfectly plane and horizontal surface was available. A plate with a force sensor was placed between the vibrating cylinder and the resin. Oil enabled to have a good contact between the different elements. A geophone (a HALL SEARS H.S.J. minute seismograph) fastened between the whole set collected the response of the pile.

The pile was prompted by the vibrating cylinder by means of a frequency ramp went from 3 Hz up to 1000 Hz.
The velocity collected by the geophone and the force collected by the force sensor were recorded, and the velocity to force ratio allowed to plot the curve of mechanical admittance versus frequency.

One could then get to know the length of the pile, but also its suppleness and its characteristic admittance.

1967 : The prompting of the pile by means of a magnetostrictive hammer was left away and replaced by a simple hammer fitted by a trigger. This allowed to apply the echo method. The velocity signal was collected by a single sensor of the HALL SEARS type.

1972 : The prompting with a vibrating cylinder was abandoned and replaced by a hammer which strikes the pile through a KISLER quartz force sensor.
The vibratory speed was sensed by a HALL-SEARS 4.5 Hz geophone.

This technique allowed to obtain the harmonic and the pulse responses at the same time (echo method).

1977 : A HEWLETT-PACKARD 21 MX computer was first used, whose programme, developed on purpose, was stored on a punched paper tape (the memory capacity was 16 Kbites). The force signal was measured by a SEDEME CC 10 force sensor.

1979 : Birth of the MIMP 11 equipment, based on a LS 11 computer of DIGITAL EQUIPMENT.
For the first models, the result was plotted by a recorded with a step by step engine ; it was plotted by an X-Y analogical plotter for the further models.

The MIMP 11was the equipment on which a hammer directly fitted with PCB brand force sensor was used.

1986 : The MIMP 12 equipment was issued, based on a GRID CASE brand computer. The data were stored on a diskette.

1989 : The MIMP 14 equipment was developed, not to test piles, but to detect voids behind the voussoirs of the tunnel under the English Channel. The MIMP 14 was based on a PC 286 microprocessor, and on a DATA TRANSLATION brand acquisition card.

1990 : Issue of the MIMP 15 material, first based on a PC 286, then on a PC 486. The acquisition card was especially designed by CEBTP.

1991 : Creation of the IMPRO software, allowing to build up the impedance profile of a pile.

1998 : The MIMP 16 material, which is the last generation of a long series of MIMPs, is a large step forward.

It displays advanced performances, it is small, it has a light weight, and it can store as many as 1000 signals in its internal memory.

The MIMP 16 material allows to operate the reflection or impedance pile control method as well as to apply the void detection method under pavings, voussoirs, cemented façade panels, or large pipes.

Moreover, the MIMP 16 material offers an outstanding pile integrity data processing software called IMPRO2 (under WINDOWS 95 or 98).