Detection and identification of concrete cracking during corrosion of reinforced concrete by acoustic emission coupled to the electrochemical techniques

NDT&E International 38 (2005) 682–689 www.elsevier.com/locate/ndteint

Detection and identification of concrete cracking during corrosion of reinforced concrete by acoustic emission coupled to the electrochemical techniques B. Assoulia, F. Simescua, G. Debickib, H. Idrissia,* a

Laboratoire de Physico-Chimie Industrielle (INSA Lyon), INSA Lyon-20, Av. Albert Einstein, 69621 Villeurbanne-France b Unite´ de Recherche Ge´nie Civil (INSA Lyon), INSA Lyon-20, Av. Albert Einstein, 69621 Villeurbanne-France Received 27 September 2004; revised 15 April 2005; accepted 17 April 2005 Available online 27 June 2005

Abstract The tenacious oxide passive film, which is formed on the surface of embedded reinforcing steel under high alkaline condition of concrete, protects the steel against corrosion. However, the condition of passivity may be destroyed, due to processes such as leaking out of fluids from concrete, atmospheric carbonation or through the uptake of chloride ions. Passive steel reinforcing corrosion induced by chloride is a wellknown problem, especially where chloride-containing admixtures or chloride contaminated aggregate are incorporated into the concrete. The objective of this work is on one hand to study the effect of chloride ions on passivity breakdown of steel, respectively, in simulated concrete pore solution (SCP) and in concrete reinforcement, and on the other hand to reproduce the carbonation phenomena by applying to the concrete samples a heating–cooling cycles. In this context, the acoustic emission coupled to the electrochemical techniques (potentiodynamic and electrochemical impedance spectroscopy (EIS)) are used. The results show clearly that [ClK]/[OHK] ratio of 0.6 is the critical threshold where the depassivation set-up can be initiated. In addition, the carbonation process is very aggressive with chloride ions and shows a perfect correlation with acoustic emission evolution. A physical model of the reinforcement/electrolyte interface is proposed to describe the behavior of the reinforcement against corrosion in chloride solution. q 2005 Elsevier Ltd. All rights reserved. Keywords: Corrosion; Chloride threshold; Simulated concrete pore; Acoustic emission

1. Introduction Reinforcing steel bars embedded in concrete depassivate when a certain amount of chlorides build up in their surrounding. The ClK/OHK ratio seems to be the most accurate parameter to take into account when testing the corrosion onset in reinforced concrete [1]. Hausmann [2] and Gouda [3] were the first in identifying the mean value of [ClK]/[OHK] ratio, which is around 0.6 in solutions simulating the concrete pore solution. Numerous authors [2–10] have reported different chloride thresholds to depassivate the reinforcing steel, as can be seen in * Corresponding author. Tel.: C33 04 72 43 89 20; fax: C33 04 72 43 87 15. E-mail address: [email protected] (H. Idrissi).

0963-8695/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2005.04.007

Table 1. However, in spite of this extensive research, no agreement among the values obtained is found. This lack of accordance is due to the existence of several parameters influencing the process, for instance, concrete mix proportions, moisture content in the concrete, temperature, C3A content of cement, blended materials, which may affect, in different manners, the cement binding ability and therefore, the amount of free chlorides able to depassivate the steel, as well as the pH of pore solution. Another reason for this lack of accordance could be related to the definition of the threshold itself. That is, how depassivation is identified. Some authors consider that depassivation is produced when a certain shift in the corrosion potential is produced [2,3]. Other authors use the visual inspection and identify depassivation with the appearance of rust spots on the steel surface [2,3,11,12]. Finally, others relate depassivation with a certain level in the corrosion current [4–6,8–10]. Among these, some use

B. Assouli et al. / NDT&E International 38 (2005) 682–689

683

Table 1 Threshold of [ClK]/[OHK] ratio to initiate the corrosion of the reinforcing steel Electrolyte

Polarized potential (mV/SCE)

[ClK]/[OHK]

Testing method

References

Ca(OH)2 (pH 12.5), NaCl, NaOH (pH 13.2) NaOH (pH 11.9) Ca(OH)2 (pH 12.1) NaOH (pH 12.6), NaCl Ca(OH)2CKOH (pH 12–13.2) NaCl, CaCl2

0.5–1.08 0.83 4, 4.1–8.33, 2.5, 3.8, 5

Measurement of Ecorr

[2]

Galvanostatic

[3]

1.8, 1.2, 4.5, 2.9, 8, 12.3, 13.5, 19.2, 12.6 33, 8.3, 3.3

Potentiostatic

[4]

Potentiodynamic

[16]

0.83, 0.33,

Potentiostatic

NaOH, KOH, (pH 13.1) CaCl2 Ca(OH)2 (pH 12–12.6) NaCl

K50 to K230 K185 K200, K300 to K450 K100, K500, K600 C500, C100, C150, 0, K100, K200 to K300 K300 to K400 K200 to K375 K100 to K375 C0 to K275, C100 to K200, K300 to K150 K450 0, K100, K150, K200, K250, K300

Galvanostatic Potentiostatic Potentiostatic

[17] [18]

Ca(OH)2 NaCl Ca(OH)2 (pH 12.5) NaCl Ca(OH)2 (pH 12.3) NaCl

C350, C100, K100 C450 to K250, C500 to K100 K600 to 600

0.033 2 0.03, 0.3, 0.02, 0.06, 0.23, 1.37, 0.12, 0.4, 1.6, 2, 0.05, 0.42, 0.54 – –

Potentiodynamic Potentiodynamic

[19] [20]

0.6

Potentiodynamic

[21]

Ca(OH)2 (pH 12.5) NaCl

the detection of the increase in the galvanic current as the indicator of depassivation [13] and others use the direct measurement of the corrosion rate to indicate the loss of passivity [4–6,8,9,12]. In addition, in some cases, the tests carried out to determine the threshold value have been made not in a ‘free’ corrosion potential, but in potentiostatic conditions [14] or by anodic polarization [3]. The detection technique used in each case in determining the moment of the depassivation is also given in Table 1 together with the critical chloride level found. The data of several authors taken from field and laboratory studies in mortars and concrete indicate that total chloride thresholds may vary by more than one order of magnitude (0.15–2.5% by weight of cement) as pointed out by Glass and Buenfeld [15] who made a review of data from several authors. The objectives of the present work are: 1. To study the stability of passive film on steel in simulated concrete pore (SCP) solution and identify a chloride threshold by potentiodynamic technique, 2. To reproduce the carbonation of the concrete in order to give a mechanism of corrosion of the reinforcement and to quantify its severity by acoustical emission technique, 3. To compare the steel comportment against corrosion in simulated concrete pore solution and in concrete.

2. Procedure 2.1. Electrochemical apparatus The electrochemical measurements were carried out on E24 steels. Chemical composition is presented in Table 2. The specimens were polished using a series of silicon carbide emery papers of grades 400, 800 and 1200. The SCP solution was prepared with KOH and NaOH (pHw13.1) at room temperature with various [ClK]/[OHK] ratios. It was prepared using double distilled water and analytical pure reagents. A standard mortar was used in this work with high porosity rate [22–25]. The composition of this mortar is given in Table 3. The drying time of mortar being a parameter influencing the characteristics of the samples, was selected to be approximately 500 h. The dimensions of these samples were defined in a way representative of actual structures. They were cylinders of 30 mm diameter and 90 mm height (Fig. 1). Table 2 Composition of mild steel (wt%)

E24 (%)

C

S

P

%0.17

%0.045

%0.045

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B. Assouli et al. / NDT&E International 38 (2005) 682–689

Table 3 Composition of mortar Component

2.2. Acoustic emission monitoring

Standard mix Water/Cement Water Portland cement CPA 54-5 Normal sand ISO 679

AE instrumentation consisted of an acquisition card (MISTRAS), a preamplifier (EPA 1220A—gain 60 dB) and a large band piezoelectric transducer (WD) which has a frequency range from 100 to 1000 kHz (Fig. 2). The threshold applied for all AE measurements is above 30 dB and the sample rate is 4 MHz. The WD was shielded with work sample in order to reduce electric noise and was attached to the bottom of cell with a smear of thin layer of white petroleum jelly in order to obtain a good acoustic coupling. AE features such as hit, count, energy and amplitude were extracted and waveform analyses as well as frequency analysis were performed in necessary cases [26–29].

Weight (g) 0.5 225 450 1350

Porous mix 0.75 338 450 1350

3. Results and discussion 3.1. In simulated concrete solution Fig. 1. Test specimen.

Open circuit potential (OCP) of the specimens were measured after 60 min of immersion in solution, using a saturated calomel electrode (SCE) and a platinum wire was used as a counter electrode. The experimental parameters of potentiodynamic polarization were carried out at a constant scan rate of 0.5 mV sK1. Electrochemical impedance spectroscopy (EIS) was performed, made with a RADIOMETER potentiostat/galvanostat using the VOLTALAB version 3.10 software. A sinusoidal voltage signal of 10 (mV) was applied over a wide frequency range (105–10K3 Hz).

3.1.1. Chloride thresholds Fig. 3 shows a typical schematic diagram of cyclic potentiodynamic polarization curve of mild steel in SCP solution and Fig. 4 shows the polarization curves of steel specimens in SCP solution containing chloride ions with [ClK]/[OHK] ratios from 0 to 5. It is shown that an extended passive region exists on the anodic polarization part for uncontaminated chloride solution. This region was started immediately after E corr and continued to Epit at 560 mV/SCE (curve 1 of Fig. 4). Some fluctuations were observed on anodic part of the curve, especially at the passive region, which might be due to the dynamic breakdown/repair process of the passive film or change in the film thickness [30,31]. Fortunately, these fluctuations

Fig. 2. Experimental device.

B. Assouli et al. / NDT&E International 38 (2005) 682–689 -2

685

pH = 13,1

10

-0.2

1 -

1x10

I pass

E depass E corr

-8

10

-0.8

-0.4

Potential (V/SCE)

log I (A.cm-2)

-5

-0.3 3

5

0.0 0.4 Potential (V/SCE)

7

0.8 0

will not have any influence on the principal trend of the polarization curve. Chloride addition to SCP solution below the [ClK] /[OHK] ratio of 0.6 does not affect the polarization curve (curve 2 of Fig. 4). However, the first alteration was observed in [ClK]/[OHK]Z0.8 as an increase in passive current density without modified pitting potential, but in [ClK]/[OHK]Z2 a decrease in pitting potential from 560 to K200 mV/SCE was observed (curve 6 of Fig. 4). This is the initiation of pitting because of a breakdown of passive film as a result of the presence of chloride ion in the threshold concentration ([ClK]/[OHK]O0.6). These sites are commonly recognized as the incipient anodes and would be stabilized at pitting potential [15,21,32]. The influence of ClK ions on depassivation of steel, even at high pH levels, can be observed as a function of net balance between two competing processes on the metal surface, i.e. stabilization (and repair) of the film by OHK ions and disruption of the film by ClK ions [21]. When the activity of OHK ion overcomes that of ClK ion, the pitting growth would be halted [21,33]. The indication of depassivation form can be confirmed by the corrosion potential (Fig. 5) and the polarization

16

20

resistance (Fig. 6). These curves show clearly that [ClK] /[OHK] ratio of 0.6 was the critical threshold where the depassivation set-up can be initiated. 3.1.2. Electrochemical spectroscopy impendence measurements The study of Nyquist plots resulting from impedance measurements of mild steel in SCP solution with and without ClK ion, demonstrated the presence of two capacitive arcs in the impedance spectra (Fig. 7(a)). These arcs are due to two-separated time constants of the reactions taking place on steel surface (Fig. 7(b)). The first arc is probably related to the adsorption of OHK ions on steel surface and not due to the passive film formation [21]. Moreover, its estimated capacitance with a mean value of 0.1!C!0.2 mF cmK2 is too low to be ascribed to double layer capacitance which falls in the range of 10–100 mF cmK2 [34]. By adding chloride ions to SCP solution the high-frequency arc is reduced and, consequently, adsorption capacitance is enhanced (Fig. 7(a)). This trend is continued until high concentration of ClK, i.e. [ClK]/[OHK]Z3 at which the capacitive behavior is

6

10

-4

5 3 1

-6

10

4

2

-

-

1: [Cl ]/[OH ]=0 2: [Cl ]/[OH ]=0,6 3: [Cl ]/[OH ]=0,8 4: [Cl ]/[OH ]=1 5: [Cl ]/[OH ]=1,5 6: [Cl ]/[OH ]=2 7: [Cl ]/[OH ]=3 8: [Cl ]/[OH ]=3,5 9: [Cl ]/[OH ]=5

Rp (KΩ.cm2)

7

8

-3

-5

1 :[Cl-]/[OH-]=0 2 :[Cl-]/[OH-]=0,6 3 :[Cl-]/[OH-]=1,5 4 :[Cl-]/[OH-]=3

9

10

log I (A.cm-2)

8 12 Time (hours)

40 pH 13,1

-2

1x10

4

Fig. 5. Open circuit curves of mild steel in SCP solution with various concentrations of chloride ions.

-1

1x10

6

-0.5

Fig. 3. Schematic diagram of cyclic potentiodynamic polarization for mild steel in SCP solution [21].

10

4

-0.4

-

1: [Cl ]/[OH ] = 0 2: [Cl ]/[OH ] = 0,6 3: [Cl ]/[OH ] = 0,8 4: [Cl ]/[OH ] = 1 5: [Cl ]/[OH ] = 2 6: [Cl ]/[OH ] = 3 7: [Cl ]/[OH ] = 3,5

2

30

1 2

20

3

10

4

-7

10

-0.8

-0.4

0.0

0.4

0.8

1.2

Potential (V/ESC) Fig. 4. Polarisation curves of mild steel in SCP solution with various concentrations of chloride ions.

0

5

10 Time (hours)

15

20

Fig. 6. Polarisation resistance measurements of mild steel in SCP solution with various concentrations of chloride ions.

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- Z'' (kΩ.cm-2)

75 [Cl-]/[OH-] = 3 [Cl-]/[OH-] = 0,6 [Cl-]/[OH-] = 0

50

25

10 Hz 1Hz

0

0,1 kHz

0

25

50

75

100

- Z'' (kΩ.cm-2)

0.2

0.1 100 Hz

3.2. In concrete 3.2.1. Durability and accelerated carbonation Previously to the durability test by electrochemical spectroscopy impendence, the concrete has undergone a heat treatment, which reproduces the carbonation and consists in applying to the concrete samples a heating– cooling cycles: for one cycle of heat treatment, the concrete is maintained 24 h with 100 8C in the furnace, followed of a cooling down to room tempereture. In order to characterize the concrete durability, acoustic emission measurments were realized after each cycle coupled to the electrochemical measurment.

1 kHz

0.0

100 kHz

0.0

10 kHz

0.1

0.2

0.3

9 [Cl-]/[OH-] = 3

1 KHz

6

- Z'' (Ω.cm-2)

performed via dislocation, grain boundary and other imperfections [22].

3.2.2. Acoustic emission measurments Several different AE parameters will be used to describe the behavior of the mortars [22,23]. To better understand these parameters, a typical waveform is presented in Fig. 8. Several features of this waveform (hit) will be discussed in this paper including the amplitude, duration, and energy. The term ‘acoustic hit’ (or acoustic event) describes a burst

3 10 KHz

0 100 KHz

-3

52

56

60

64

68

Z' (kΩ.cm-2) Fig. 7. Nyquist plots of mild steel in SCP solution with various [ClK] /[OHK] ratio.

changed into an inductive one. This inductive response may be due to the formation of an intermediate compound on the surface, resulting from corrosion induced by chloride ions (Fig. 7(c)). This is in agreement with the adsorption theory in which Uhlig and Bohni [35] described the localized breakdown of passive film as a result of the competitive adsorption of ClK and OHK ions. According to this theory, OHK ions adsorbed on the metal surface can be dislodged and displaced by ClK ions. However, coverage of metal surface by ClK ion is not uniform, but ions are preferentially adsorbed on more positive local sites [21,24]. In addition, the effect of various concentrations of chloride ions on the steel EIS spectra in SCP solution is illustrated as a decrease in the second arc (low frequencies), which is due to the breakdown of passive oxide film on steel surface (Fig. 7(a)). The chloride ions are not consumed in the process, but lead to the breakdown of the passive layer and allow the corrosion process to proceed in higher rate [21]. It is known that the resistivity of the original passive film is high, therefore, diffusion of ClK ions through it is

Fig. 8. Waveform recorded for corrosion rebar in concrete [22].

B. Assouli et al. / NDT&E International 38 (2005) 682–689

687

Fig. 10. Cumulated events curves of concrete versus number of heat treatment cycles, pH and Open circuit potential.

chloride ions present in the solution. In fact, this result is confirmed by the reduction of the pH of the solution. In parallel, an increase in the acoustic activity is observed. According to later work [22], this activity is representative of several events: Fig. 9. Cumulated events curves of concrete versus number of heat treatment cycles: with chloride (a) without chloride (b).

of acoustic activity once by recognizing the beginning and end of each discrete burst. The beginning of each acoustic hit (or event) is indicated when the amplitude of the signal from the transducer exceeds some threshold value. The end of each hit is determined as the time the signal amplitude decreases below the threshold limit for the last time. The amplitude of the signal refers to the maximum absolute value of the signal recorded while duration refers to the time during which the signal remains above the threshold. This paper will use these acoustic parameters to better describe the durability of concrete. The curves representative of the acoustic emission (AE) events number versus time (Fig. 9) are plotted with a given threshold of 30 dB and without filtering after acquisition. The shapes of the curves are quite similar irrespective of each heat treatment of the concrete samples. In chloride solutions, a weak activity was detected for the first cycles and increases with heat treatment cycle. Without chloride ions, the acoustic activity remains very weak in comparison with that in the presence of the chlorides ions. These results show that carbonation process is very aggressive with chloride ions. After undergoing the twelve cycles, the concrete displayed large carbonation fronts, since these concrete are characterised by their high porosity [22,23]. Fig. 10 shows, respectively, the open-circuit potential EN, pH and acoustic emission events versus heat treatment cycle for concrete reinforcement in chloride solution. The evolution of the open circuit potential shows a decrease of the potential values in the cathodic direction versus cycle, starting of K200 mV/SCE for the first cycle and attaining K640 mV/SCE for the cycle number 12. This decay of potential suggests loss of passivation induced by the

1. Cracking of concrete, 2. Friction of corrosion products at the inner sides of the pores, 3. Penetration of the solution contaminated in the concrete. In fact, three acoustic signals, respectively A, B and C are detected (Fig. 8), which corresponds to the same form as the one recorded in a later work [22]. Under these conditions, the degradation of the concrete reinforcement follows the three following stages: 1. The first stage can be associated with the infiltration of the medium through porosity and with the modifications of the medium in the vicinity of the steel by diffusion of ions ClK (Wave type A), 2. The second stage can be associated with the accumulation of corrosion products at the interface reinforcement/concrete exerts compressive forces on the internal surface of the mortar. This leads to the initiation of micro cracks which are sources of AE (Wave type B), 3. The third stage corresponding to the microscopic cracks propagation or to evacuation corrosion products (Wave type C).

3.2.3. Electrochemical spectroscopy impendence measurements The impedance diagrams of concrete reinforcement are realized before and after the heat treatment of twelve cycles. Indeed, they are recorded at Ecorr after 1 h of immersion in solution with ClK ion (thresholdZ1) (Fig. 11(a)). Before the heat treatment, one single time constant is observed and the slope of the logjZj versus log f curve is slightly lower than one (in absolute value). Moreover, the low-frequency (LF) limit of the impedance is high, which indicates the presence of a resistive layer at the surface of the electrode in spite of the high threshold of chloride. In

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B. Assouli et al. / NDT&E International 38 (2005) 682–689

Fig. 11. Electrochemical impedance spectroscopy diagrams (Nyquist and Bode plots) of reinforcement’s electrode, plotted at Ecor after 1 h of immersion in solution: without chloride (a) with chloride (b).

fact, without accelerating the corosion test, the chloride cannot infiltrate the medium through porosity during the experiment (a few hours). In these conditions, no localized corrosion is observed. The physical model of the reinforcement/concrete interface at high pH is presented in Fig. 12(a) . From this model, an equivalent circuit for the total impedance can be given (Fig. 12(a)). The steel is covered by a protective passive film (constant phase element relative to the film CPEf and intrinsic film resistance Rf) in contact with the electrolyte (electrolyte resistance RU). The introduction of CPEf in the equivalent circuit is justified by the slope of the logjZj versus log f curve that is systematically lower than one (in absolute value). CPE f illustrates a surface heterogeneity that could be induced by: 1. A distribution of the passive film thickness, 2. A distribution of the defects (vacancies) density in the passive layer.

After the heat treatment of twelve cycles, the impedance diagrams obtained exhibit at least two time constants: one in the medium-frequency (MF) and the low-frequency (LF) range, that corresponds to the same phenomenon as the one observed (before the heat treatment), and a new one in the high-frequency (HF) range. In these conditions, visual observation of the steel surface shows pitting corrosion. The physical model of steel/concrete interface as well as the corresponding equivalent circuit for the total impedance are presented in Fig. 12(b). When the pH decreases, the passive film breakdown leads to a direct contact between the steel and the electrolyte, and therefore to pitting. These pits get deeper and more numerous with decreasing pH. This localized corrosion phenomenon is translated in the equivalent circuit by the introduction of an electrolyte resistance through the pits (Rel), a charge transfer resistance (Rt) and a pseudodouble-layer capacitance (CPEdl). Thus, for the interpretation of impedance data, pits are treated as pores. The value

Fig. 12. Physical models of the E24 steel/solution interfaceCequivalent circuits for the total impedance.

B. Assouli et al. / NDT&E International 38 (2005) 682–689

of Rel varies with the pit geometry and the nature of the electrolyte inside the pits. Rt illustrates a charge transfer (iron dissolution and oxygen reduction) at the pits walls. CPEdl is a measure for a ‘porosity’ induced by the pits.

4. Conclusions The conclusions following from the here presented results are: 1. [ClK]/[OHK] ratio of 0.6 was the critical threshold where the depassivation set-up can be initiated, 2. The carbonation process is very aggressive with chloride ions, 3. Loss of passivation is observed at high carbonation and at strong acoustic activity, 4. Three acoustic signals A, B and C are detected and attributed, respectively, to the infiltration of the medium through porosity, compressive forces and micro cracks, 5. A physical model of the reinforcement/electrolyte interface is proposed to describe the behavior of the reinforcement against corrosion in chloride solution before and after carbonation.

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