# Ultrasonic evaluation of a steel-concrete structure using embedded Fiber Bragg Gratings (FBG) and piezoelectric sensors

Maximilien Lehujeur, Tom Druet, Cyril Lefeuve, Jean-Baptiste Quelene, Odile Abraham, Guillaume Laffont, Olivier Durand, Thibaud Devie, Gautier Gugole, Bastien Chapuis, et al.

► To cite this version:

Maximilien Lehujeur, Tom Druet, Cyril Lefeuve, Jean-Baptiste Quelene, Odile Abraham, et al. Ul-
trasonic evaluation of a steel-concrete structure using embedded Fiber Bragg Gratings (FBG) and
piezoelectric sensors. NDT-CE 2022: The International Symposium on Nondestructive Testing in
Civil Engineering, Aug 2022, Zurich, Switzerland. 10.58286/27218. hal-04387329

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# Ultrasonic evaluation of a steel-concrete structure using embedded Fiber Bragg Gratings (FBG) and piezoelectric sensors

**Article** in e-Journal of Nondestructive Testing · September 2022

DOI: 10.5828/27218

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<figure><img src="image_2.png" /><figcaption>Jean-Baptiste Quélène</figcaption></figure>

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<figure><img src="image_3.png" /><figcaption>Guillaume Laffont</figcaption></figure>

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# Ultrasonic evaluation of a steel-concrete structure using embedded Bragg gratings and piezoelectric sensors

M. LEHUJEUR<fnref n="1" />*, T. DRUET<fnref n="2" />, C. LEFEUVE<fnref n="2" />, J. QUELENE<fnref n="2" />, O. ABRAHAM<fnref n="1" />, G. LAFFONT<fnref n="2" />, O. DURAND<fnref n="1" />, T. DEVIE<fnref n="1" />, G. GUGOLE<fnref n="1" />, B. CHAPUIS<fnref n="2" />, and N. ROUSSEL<fnref n="2" />

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## Abstract

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We present a Steel-Concrete wall model equipped with several embedded fiber Bragg Gratings (FBG) as well as several embedded and surface piezoelectric (PZT) sensors. We compare the performances of these embedded sensors for several sources of ultrasonic waves applied on the surface of the wall for frequencies ranging from a few tens to a few hundreds of kHz. The signals obtained reveal that a significant part of the wave energy can be transmitted to the concrete through the surface liner, despite the high mechanical contrasts between steel and concrete. Using a collection of random hammer impacts applied on the steel surface, we further investigate the feasibility of reconstructing empirical Green's functions between pairs of sensors buried into the structure using passive seismic methods.

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**Keywords**: Steel-concrete structures, Non-destructive evaluation, Embedded ultrasonic sensors, Optical Fibres, Braggs gratings.

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## 1 Introduction

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Steel-Concrete-Steel (SCS) structures are made of concrete poured between two metal plates (liners), serving both as a form-work and a mechanical reinforcement. Metal spikes anchored to the liner can be used to provide the coupling between the concrete and the steel plates. These structures are used in confinement structures in nuclear power plants.

<footnote n="1">¹Gustave Eiffel University, France</footnote>

<footnote n="2">²CEA, France</footnote>

<footnote>*Corresponding author: maximilien.lehujeur[at]univ-eiffel.fr</footnote>
Because of the liner, visual inspection of the surface of the concrete is not possible. Non Destructive Testing and Evaluation methods (NDT, NDE) are therefore required to probe the concrete. The transmission of ultrasonic waves from the liner to the internal part of the structure is difficult due to the high mechanical contrasts between steel and concrete or when local detachments of the liner occur. This may alter the efficiency of the standard NDE approaches involving ultrasonic waves [3, 7].

In this project, we attempted to overcome this difficulty by using acoustic sensors embedded in the concrete part of a SCS module, which allows us to reduce the distance to the external sources (disposed on the liner). In particular, we focused on the use of Fibre Bragg Gratings (FBGs). This sensing technology based on state-of-the-art optoelectronics allows us to measure the dynamic strain at the location of the Bragg gratings along the fibre axis. These sensors can be used at ultrasonic frequencies over broad frequency ranges, and have proven their potential in Structural Health Monitoring applications based on passive diffuse noise correlation techniques [4, 6]. The deployment of FBGs embedded in concrete provides numerous advantages [5], their footprint on the structure is negligible, one can have several measurement points on the same fibre and the fibre are resistant to challenging environments (temperature or radiations).

We built a reduced scale model of a Steel-Concrete structure, in which we embedded several FBGs together with more conventional piezoelectric (PZT) accelerometers. Our main objectives are (a) to characterize the signals recorded with embedded FBGs, and to improve our understanding of their response in comparison to PZT accelerometers. (b) to evaluate the transmission of ultrasonic waves in the concrete for sources placed on the metal plates, and (c) to test the feasibility of passive correlation methods that aims to approximate Green's functions between pairs of embedded receivers using random sources applied on the liner.

# 2 Methods

## 2.1 Samples

A reduced scale model of a Steel-Concrete confinement wall (25 × 120 × 135 cm³) was designed and built for the project. During the casting, 5 traversing optical fibres with a total of 36 Bragg gratings were buried in the wall, as well as 8 PZT accelerometers (Brüel & Kjær 4393). Only one side of the wall was covered with a steel liner, but studs were inserted on both sides of the wall (Fig. 1). A few additional PZT sensors were glued outside the structure, on both sides of the wall.
## 2.2 Sources

Several piezoelectric sources (Ultran GC30 30 kHz, Panametrics V1011-100 kHz, V1012-250 kHz, V106-2.25 MHz) were applied to the faces of the wall to test the following source functions

* a Ricker function with a center frequency incremented from 10 to 300 kHz,

* a sweep function covering all frequencies between 150 and 350 kHz

Additionally, random impacts made with a hammer or with a robot-controlled impactor able to cover all the surface of the metal plate. The spectrum of the latter sources extended from about 5 to 70 kHz.

## 2.3 Data analysis

The recorded signals were visualized in the temporal and spectral domains. For the random hammer sources applied on the liner face, we implemented a correlation method initially designed to process ambient acoustic or seismic noise to recover the empirical Green's function of a medium between a pair of receivers according to the following relation [1]

$$ \frac{\partial C_{AB}}{\partial t} \approx G_{A \rightarrow B}(-t) - G_{B \rightarrow A}(t) \quad (1) $$

In this method, the signals generally needs to be pre-processed using spectral and/or temporal normalization [2] in order to equilibrate the contribution of every sources and to approach the ideal case of an uniform distribution of the sources or a diffusive wavefield.
<figure><img src="image_7.png" /></figure>

Figure 1: Scheme showing the geometry of the Steel-Concrete wall model built in this project. The front side is equipped with the steel liner. The concrete is bare on the back, lateral and upper faces of the wall. Studs were installed on both sides of the wall. The dashed lines denote the traversing optical fibres.

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# 3 Results

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## 3.1 Comparison of an embedded FBG and PZT accelerometer

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A first series of measurements was carried out using sources glued to the concrete on the upper face of the wall, in order to avoid issues related to the transmission of waves through the liner. The emitted source was a Ricker pulse, whose center frequency was gradually increased from 50 to 300 kHz (Fig. 2, red).

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The longitudinal dynamic strain measured with the embedded FBG named "V2B4" (Fig. 2, blue signals) and the particle acceleration measured with the embedded PZT accelerometer "A2" (Fig. 2, black signals) have a good signal to noise ratio and allow us to identify the first arrivals clearly. The shape of the spectrum obtained with the FBG is in good agreement with the source one, while the PZT accelerometer has a peak near its resonance frequency (at 55 kHz for this sensor).
<figure><img src="image_8.png" /><figcaption>Figure 2: Signals recorded inside the wall by two nearby FBG (V2B4, blue) and PZT accelerometer (A2, black) for several Ricker source functions (SRC, red) centered between 50 and 300 kHz. The signals are shown in time (left) and spectral domains (right).</figcaption></figure>

## 3.2 Transmission of ultrasonic waves to the concrete

A second series of measurement was carried out in order to evaluate the transmission of ultrasonic waves in the concrete despite the high contrasts of impedance at the steel-concrete interface. We deployed a source emitting a sweep from 150 to 350 kHz on either faces of the test wall, covered or not by the liner (Fig. 3, see source points named "B2a" for the concrete side of the wall and "M2a" for the liner side). The input sweep was correlated with the recorded signals in order to remove its temporal signature and to return to an impulsive record. This test showed that acoustic signals can be recorded with embedded receivers (FBG and PZT accelerometers) at frequencies between 150 and 350 kHz. A reduction of the signal to noise ratio was observed when the source is placed on the liner (especially for the PZT accelerometers), but this did not seem to affect the waveform and the shape of the spectrum significantly.

Another potential limitation of ultrasonic methods for this application comes from the presence of local detachments of the steel plate. A possible alternative consists in placing the sources at the emerging end of the studs (Fig. 3, the source point "ST21" corresponds to the source placed on a stud, on the concrete side of the wall and "MST21" corresponds
to the source being placed on a stud on the liner side). Here also, no clear changes in waveform and spectral shape could be observed between the two source locations. A complementary experiment in which the liner would be deliberately decoupled from the concrete would help concluding on the usefulness of placing sources at the extremity of the studs.

<figure><img src="image_9.png" /><figcaption>Figure 3: Signals recorded inside the wall by two nearby sensors: a Bragg grating (H2B5, blue) and a piezoelectric accelerometer (A4, black) for a source placed at various positions on the wall and emitting a sweep between 150-350 kHz (red). The signals are shown in time (left) and spectral (right) domains after correlation with the input sweep (shown in light red). The source is placed on the liner or concrete side (M2a/B2a), at the extremity of a stud on the metal or concrete side (MST21/ST21). The amplitudes are normalized for each trace individually.</figcaption></figure>

## 3.3 Noise source correlation

Finally, we tested the possibility to obtain empirical Green's functions between pairs of embedded acoustic sensors for a collection of hammer impacts randomly distributed at the surface of the liner. Some pre-processing steps were applied to the recorded signals for each sensor individually. These steps implied spectral normalization (whitening) and the clipping of the largest amplitudes of the signal in the time domain. The signals were then correlated between pair of sensors, stacked and derivated temporally following equation 1.
For frequencies between 5 and 15 kHz, we obtain an anti-symmetrical correlation function (as expected from eq. 1), which suggests that both causal and anticausal estimates of the Green's function are in good agreement, and that the wave-field is nearly equipartitionned. For a pair of PZT accelerometers, a good similarity between the correlation functions of each individual shot was obtained (Fig. 4a), which reveals a fast convergence of the correlation function toward the Green's function. For a pair of FBGs, the signal to noise ratio is lower, but a clear pattern emerges by stacking all the source contributions (Fig. 4b).

<figure><img src="image_10.png" /><figcaption>Figure 4: Estimates of the empirical Green's function by correlation of hammer shots, for a pair of PZT accelerometers (a) or FBG sensors (b). The upper part of the figures indicate the correlation functions obtained for each source individually. The stack function (black) and a reversed version of it (red) are shown at the bottom of the figures.</figcaption></figure>

# 4 Conclusions

In this work, we evaluated our ability to acquire signals using acoustic sensors embedded in concrete for the non-destructive evaluation of a Steel-Concrete-Steel module. We tested a large panel of sources with known source functions (Ricker and sweep) emitting at frequencies from 10 to 350 kHz, as well as random hammer sources applied on the liner, whose spectrum extended from 2 to 70 kHz.

Our experiments carried out in a transmission regime revealed that a significant part of the energy emitted by a source positioned on the liner can be recorded inside the concrete with
both PZT and FBG sensors, which suggests that this methodology can be used to evaluate
the health state of a SCS module even though only the liner is accessible. An advantage
of FBG lies in the shape of the spectra, which accurately reproduces the source one in the
frequency range of interest. The signals obtained with embedded FBGs are usually more
noisy than those of embedded PZT accelerometers. When the source term is well known
and sufficiently repeatable, this can be easily addressed by direct summation of the records.
For sources with an un-controlled or weakly repeatable source function, summation of the
source contributions can be achieved through the correlation method usually employed for
the processing of passively acquired noise wave-fields. Using this approach, we were able
to approximate the Green's function of the medium between pairs of PZT or Bragg gratings
at frequencies between 5 and 15 kHz. Future efforts will concentrate on the possibility to
extend this result to ultrasonic frequencies (above 20 kHz) and to use these signals in a
non-destructive evaluation process.

## 5 Acknowledgments

This research is part of the Smart-SCS project, a collaboration between Gustave Eiffel
University and the French Alternative Energies and Atomic Energy Commission (CEA).
The authors thank F. Blaineau from the COSYS/SII laboratory (Gustave Eiffel University,
France) for his numerical plans of the instrumented form-works and D. Hamon from the
MAST/GPEM (Gustave Eiffel University) for his help during the casting of the concrete
in the form-works.

## References

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[2] G. D. Bensen et al. "Processing seismic ambient noise data to obtain reliable broadband surface wave dispersion measurements". en. In: *Geophys. J. Int.* 169.3 (June 2007), pp. 1239–1260. ISSN: 0956-540X, 1365-246X. DOI: 10.1111/j.1365-246X.2007.03374.x.

[3] Herbert Wiggenhauser. "Nondestructive Evaluation of Steel-Concrete Mockups". deu. In: (2013).

[4] Tom Druet et al. "Passive guided waves measurements using fiber Bragg gratings sensors". eng. In: *J Acoust Soc Am* 144.3 (Sept. 2018), p. 1198. ISSN: 1520-8524. DOI: 10.1121/1.5054015.
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[5] Simon Nehr et al. "Towards Embedded Fiber Optics for the Ultrasonic Monitoring of Concrete". en-US. In: *Structural Health Monitoring 2019* 0.0 (2019). Number: 0. DOI: 10.12783/shm2019/32404.

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[6] Arnaud Recoquillay et al. "Guided wave imaging of composite plates using passive acquisitions by fiber Bragg gratings". In: *The Journal of the Acoustical Society of America* 147.5 (May 2020). Publisher: Acoustical Society of America, pp. 3565–3574. ISSN: 0001-4966. DOI: 10.1121/10.0001300.

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[7] Ludovic Fülöp et al. "Assessing the challenges of condition assessment of steel-concrete (SC) composite elements using NDE". en. In: *Case Studies in Construction Materials* 16 (June 2022), e00887. ISSN: 2214-5095. DOI: 10.1016/j.cscm.2022.e00887.