# Survey of concrete blocks submitted to artificial sea water tide during one year by electrical resistivity tomography

Géraldine Villain, Marie-Ange Eid, Sergio Palma Lopes, Stéphanie Bonnet

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## To cite this version:

Géraldine Villain, Marie-Ange Eid, Sergio Palma Lopes, Stéphanie Bonnet. Survey of concrete blocks submitted to artificial sea water tide during one year by electrical resistivity tomography. e-Journal of Nondestructive Testing, 2025, Special Issue: International Symposium on Non-Destructive Testing in Civil Engineering (NDT-CE 2025), 24-26 September 2025, Izmir, Turkey (NDTCE 2025), 30 (10), 8p. 10.58286/31674. hal-05444898v2

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# Survey of concrete blocks submitted to artificial sea water tide during one year by electrical resistivity tomography

Géraldine VILLAIN<fnref n="1" />, Marie-Ange EID<fnref n="1" />,<fnref n="2" />, Sérgio PALMA LOPES<fnref n="3" />, Stéphanie BONNET<fnref n="2" />

## Abstract

Electrical resistivity methods are very sensitive to the evolution of water and chloride contents of concrete. The multi-electrode device, used in this study, is applied on the concrete surface to measure the apparent resistivities. After data inversion, we get ‘true’ resistivity profiles function of depth.

In this study, we present the survey methodology based on this electrical resistivity tomography method. It was implemented to monitor the penetration of salt water in concrete blocks placed in a tidal basin. Three concrete formulations are studied: the first named C100-0 mixed with ordinary Portland cement CEM I, the second, named C70-30, containing 30% of replacement blast furnace slag and the third named C50-50, containing 50% of replacement slag. The artificial tidal cycles reproduce natural tides and the salt water contains 35 g/L of sodium chloride like seawater. The blocks were inspected at short intervals at the start of exposure and then at more spaced monthly intervals. The method for inversion of apparent resistivities is briefly summarized.

The results corresponding to the three concretes are presented for one year, firstly as apparent resistivity values and then as ‘true’ resistivity profiles with respect to depth. The influence of slag is discussed: it induces a higher resistivity but the results do not show a mitigation of the penetration kinetics of the salted water in the actual studied conditions.

**Keywords:** ERT, resistivity profiles, chloride ingress, blast furnace slag, durability of concrete.

## 1. Introduction

many reinforced concrete (RC) structures are built along the seaside, making up the port's civil engineering heritage. In order to preserve this heritage, we intend to develop quantitative non-destructive evaluation (NDE) methods useful for durability diagnosis of inspectors, experts in RC structures in the maritime environment. For preventive maintenance, the chloride ingress must be evaluated before corrosion appearance. Electrical resistivity methods are very sensitive to the evolution of water and chloride contents of concrete [1-4].

In this study, we use a multi-electrode sensor developed by Du Plooy et al. [5]. Based on the Wenner four-point method, this sensor enables the measurement of the apparent resistivity in depth but need inversion to get the ‘true’ resistivity vs depth [6-10]. The algorithm implemented by Alhajj et al. [11] is summarized herein.

To improve the RC structure durability in marine environment, blast furnace slag is added in concrete mis design due to his capacity to fix chloride and delay corrosion appearance [12]. Slab addition not only modifies the transport coefficient of chloride ions in concrete [13-14] but it also greatly increases its resistivity [15-16]. Thus, in the research project ANR-DEMCOM [17], it was decided to determine the transport coefficient by means of resistivity profiles obtained by embedded sensors [18-19] of surface survey as in the study presented herein.

<footnote n="1">¹ Univ Gustave Eiffel, MAST-LAMES, Campus de Nantes, F-44344 Bouguenais, France,
[geraldine.villain@univ-eiffel.fr](mailto:geraldine.villain@univ-eiffel.fr) 0000-0002-4478-034X</footnote>

<footnote n="2">² Nantes Université, Ecole Centrale Nantes, CNRS, GeM, UMR 6183, F-44600 Saint Nazaire, France,
[stephanie.bonnet@univ-nantes.fr](mailto:stephanie.bonnet@univ-nantes.fr) 0000-0003-0027-344X</footnote>

<footnote n="3">³ Univ Gustave Eiffel, GERS-GEOEND, Campus de Nantes, F-44344 Bouguenais, France,
[sergio.palma-lopes@univ-eiffel.fr](mailto:sergio.palma-lopes@univ-eiffel.fr) 0000-0001-7022-389X</footnote>
The aim of this study is to survey the evolution of resistivity profiles during the salted water penetration in three concrete mixed with slag in marine environment. The experiment program consists in submitting concrete blocks to artificial tide in a pool during more than one year and performing regularly surface multi-electrode resistivity measurement. The device and the algorithm necessary to get 'true' resistivity profile as function of depth are presented. The results are analysed in terms of experimental conditions and slag content in the concrete mix designs.

## 2. Experimental program

Firstly, in this section, we present the measurement device, the principle and the inversion process. Secondly, we give the concrete mix formulations, characteristics, and the studied specimens. Thirdly we detail the conditions of exposure to tide as well as the measurement schedule to survey the salted water penetration during 1 year.

### 2.1 Electrical Resistivity Tomography

#### 2.1.1 Measurement device and principle

The multi-electrode probe (Fig. 1.a) used in this study was developed by [5]. It comprises 14 electrodes spaced by 2 cm, which can be selected to form Wenner quadrupoles with 4 spacings equal to 2, 4, 6 and 8 cm. The current of intensity $I$ (A) is injected by the outer quadrupole electrodes and the potential drop $\Delta V$ (V) by the inner ones. The geometrical factor $G$ is calculated by finite element method [20]. Thus, the apparent resistivity $\rho_{app}$ (Ω.m) obtained by equation (1), for each quadrupole, each spacing, is corrected from geometrical bias due to the geometry of the blocks (side effects) as well as the geometry of the probe. Each apparent resistivity value corresponds to the mean of four measurements, 2 along diagonal 1 and 2 along diagonal 2 (Fig. 1.b).

$$ \rho_{app} = G \frac{\Delta V}{I} \qquad (1) $$

The resistivity meter is a Syscal Junior Switch 72 (Iris Instruments).

<figure><img src="image_4.png" /><figcaption>Figure 1. Multi-electrode measurement probe (a) and measurement position on the concrete block face (b)</figcaption></figure>

#### 2.1.2 Inversion process
The surface measurements correspond to apparent resistivity integrating the electrical intrinsic resistivity of concrete from the surface to a depth linked to the electrode spacing. The longer is the spacing, the greater is the subsurface volume. So, in order to assess the ‘true’ resistivity as function of depth, an inversion is required [9, 10]. In this study, the inversion process developed by Alhajj et al. [11] was used (Fig. 2). It is constrained by a continuous monotonic Weibull function with 4 parameters corresponding to the surface resistivity, the core resistivity, the depth of the curve inflection point and the slope at this depth. This function describes well purely drying or imbibition profiles in concretes [11].

<figure><img src="image_5.png" /><figcaption>Figure 2. Flowchart of the inversion process [11]</figcaption></figure>

## 2.2 The studied concretes

Three concrete mixes with identical water-to-binder ratio equal to 0.5 are studied. The concrete C100-0 contains CEM I Portland cement, whereas in the concrete C70-30 30% of the Portland cement was replaced by slag and in the concrete C50-50 50% of cement was replaced by slag as presented in Table 1.

<table><caption>Table 1. Concrete mix design and properties</caption><thead><tr><th>Components (kg/m³)</th><th>C100-0</th><th>C70-30</th><th>C50-50</th></tr></thead><tbody><tr><td>Gravel, 4–12 mm (Boulonnais)</td><td>1041.7</td><td>1040.7</td><td>1038.8</td></tr><tr><td>Sand, 0–4 mm (Boulonnais)</td><td>910.3</td><td>909.3</td><td>907.6</td></tr><tr><td>CEM I 52.5 N (Le Teil, Lafarge)</td><td>302.5</td><td>211.8</td><td>151.3</td></tr><tr><td>Slag (Ecocem)</td><td>0.0</td><td>90.8</td><td>151.3</td></tr><tr><td>Superplasticizer MasterGlenium 201 (BASF)</td><td>2.86</td><td>2.94</td><td>2.86</td></tr><tr><td>Efficient water (W<sub>eff</sub>)</td><td>151.7</td><td>151.7</td><td>150.7</td></tr><tr><td>Water-to-binder ratio (W<sub>eff</sub>/B)</td><td>0.5</td><td>0.5</td><td>0.5</td></tr></tbody></table>

### 2.2.2 Concrete specimens

For each concrete, two slabs were casted and cured under water during 3 months, then kept in the laboratory during roughly 10 months. The slab dimensions are 20x17x20 cm. At the age of 3 months, four slab faces were covered with epoxy resin to water tight these faces and allow the exchanges by only one 20x20 cm face (config. 1).

The exposure conditions of the two slabs were slightly different but did not conduct to different see water penetration nor to different resistivity results. For these reasons only the results corresponding to one slab per concrete are presented here.
## 2.3 Experimental program

### 2.3.3 Exposure conditions

The slabs were installed in a pool equipped with pumps to simulated the natural tide cycles with salted water with a [NaCl] concentration equal to 35 g/L. The slabs are submitted to 6-hour high tide and 6-hour low tide. The under face of the slab is exposed to artificial sea water, as shown by Fig. 2, so, for each NDE, the slab is turned and turned back after the NDE. The ND resistivity measurements take place every month.

This protocol was applied during one month (first phase) then the pumps broke. All along the 6-month reparation of the pumps, the slabs remain in the pool at low tide so the exposed surface dried. Afterwards, the regular exposure protocol was applied during one year (second phase).

<figure><img src="image_6.png" /><figcaption>Figure 2. Scheme of the slab exposure conditions in high tide [Palma...25]</figcaption></figure>

### 2.2.3 Complementary tests

The resistivity measurements are sensitive to temperature. The laboratory of the tide pool is not equipped with air conditioning. For these reasons, thermohydrometric probes were put in a specific sample (same cure, same resin water-tight, same exposure conditions). These probes enable to monitor the temperature *T* and the relative humidity *RH* at three different heights equal to 5, 9 and 13±0.5 cm [19].

## 3. Results and discussion

The results of resistivity evolution are presented in this section. Due to the interruption of pumping, the initial profiles of the 1st and the 2nd phases are compared. Then, the evolutions of the resistivity profiles during the 2nd phase are shown and analysed in terms of water and chloride ingress and of the role of binders.

### 3.1 ERT results at the beginning of the first and the second phase

At the end of the drying phase, before to expose the slabs to tide cycles, the resistivity profiles (at T0) are presented in Figure 3 with plain lines. These profiles correspond only to a decrease of the saturation degree in the first centimetres. Let us note that the higher is the slag replacement ratio, the higher is the resistivity, in accordance with literature [16]. In Fig. 3-a, the effect of the salted water penetration (during 98 days of tide) on the resistivity profile can be observed: the resistivity drops near the unprotected surface.
During 6 months, the blocks were expose to drying. The resistivity profiles at T0', beginning of the second phase can be compared to the profile at T0. For the 3 concretes (Fig. 3-b), the profiles at T0 are not so sharp: the resistivity of the sub-surface is lower and the influence of salted water penetration followed by drying is felt more deeply. It seems that the second drying reaches a deeper zone in the C50-50 than in the C100-0 concrete. This effect of the slag on the drying kinetics has to be confirmed.

<figure><img src="image_7.png" /><figcaption>Figure 3. Comparison of initial profiles of 1st phase (T0) and 2nd phase (T0') – a) Effect of drying and chloride penetration during the 1st phase for C100-0 concrete – b) Comparison of the 3 concretes</figcaption></figure>

## 3.2 ERT results for monitoring the sea water ingress along one year

Figure 4 shows the evolution of resistivity profiles during the second phase at several selected test-times: at T0', T0' + 7 days, T0' + 90 days and T0' + 231 days. The difference between the profiles at T0' and T0' + 7 days is very significant because the salted water penetrates by imbibition in the unsaturated layer. Imbibition phenomenon has a stronger effect than the short 6-hour wetting and drying cycles of tide. The tide cycles are still in progress, a later test-time will be soon available.

<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /><figcaption>Figure 4. Monitoring of the sea water ingress during the second phase – a) In concrete C100-0 – b) In concrete C70-30 – c) In concrete C50-50</figcaption></figure>

The evolution of the resistivity profiles vs time is due to salted water penetration, thus both to the increase of the saturation degree and to the chloride ingress. Unfortunately, at this stage the results do not allow to conclude to a positive effect of slag to mitigate the chloride penetration, as would be expected. Indeed, the penetration occurs within the 6 cm thickness of concrete, which corresponds to the recommended depth of investigation for this technique. Between 6 and 8 cm, the reliability of the results decreases due to increased uncertainty.

### 3.4 Discussion and potential improvements

In further works, the resistivity profiles will be compared to relative humidity profiles in other samples of C100-0 and C50-50, when to destructive chloride content after long term penetration in the 3 studied concrete mixes.

Besides, a multi-electrode sensor was embedded in additional concrete slabs of the same concrete mixes and submitted to the same exposure conditions [18-20]. It will allow us to get profiles as function of a greater depth up to 15 cm with a better accuracy. Moreover, the inversion process is influenced by the initial model, chosen after a first inversion based on the RES1D free software [9]. The inversion process used in this study imposes a monotonic continuous profile (with Weibull function). Thus, it can lead to great uncertainties in short wetting and drying cycles during tide phenomena. The locally embedded sensor could avoid a time-consuming and restrictive (monotonic) inversion process.

## 4. Conclusions

Finally, the results highlighted in this paper show the possibility to survey the evolution of resistivity profiles during dying and during the salted water penetration in three concrete mixed with slag in marine environment, by means of electrical resistivity tomography (ERT). The evolution of resistivity profiles vs depth is significant.
The higher is the slag replacement ratio, the greater seams the drying kinetics. Nevertheless, it is not possible to observe if the presence of slab offers a greater fixation capacity of chloride ions with the ERT measurement technique.

## Acknowledgements

The authors acknowledge the French National Research Agency (ANR) for the financial support of the DEMCOM research project ANR-20-CE22-0008-01. The authors also gratefully thank Jean-Luc Geffard, Alexis Cothenet and Gautier Gugole from Université Gustave Eiffel as well as Denis Fournol from Nantes Université for their contribution to the experimental program.

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