13.2.1 Chloride induced corrosion

Chloride ions can be present in the concrete due to the use of chloride-contaminated components or the use of CaCl₂ as an accelerator when mixing the concrete, or by diffusion into the concrete from the outside environment (Thuresson, 1996). A localized breakdown of the passive layer occurs when sufficient amounts of chlorides reach reinforcing bars, and the corrosion process is then initiated. Chlorides in concrete can be either dissolved in the pore solution (free chlorides) or chemically and physically bound to the cement hydrates and their surfaces (bound chlorides). Only the free chlorides dissolved in the pore solution are responsible for initiating the process of corrosion (Martin-Perez et al., 2000).

There are three theories about the chloride attack (ACI Committee 222, 1996):

13.2.2 Carbonation induced corrosion

When concrete is exposed to air, the calcium hydroxide reacts with water and carbon dioxide in the air:

$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$ [13.1]

[13.1]

The effect of carbonation is to reduce the pH value of the surface layer of the concrete to less than 8.3. This pH is sufficient to make the passive layer on the reinforcement rebar unstable (Allen and Forrester, 1983). The process of carbonation can be summarized in the following steps:

13.2.3 Mechanism of corrosion in reinforced concrete

Corrosion is an electrochemical reaction that consists of anodic and cathodic half-cell reactions. Micro-cell corrosion is the term given to the situation where active dissolution and the corresponding cathodic half-cell reaction take place at adjacent parts of the same metal part. For a steel reinforcing bar (rebar) in concrete, this process always occurs in practice. The surface of the corroding steel can act as a mixed electrode containing both anode and cathode regions that are connected by the bulk steel. Macro-cell corrosion can also form on a single bar exposed to different environments within the concrete, or where part of the bar extends outside the concrete. In both cases, concrete pore solution functions as an electrolyte.

For steel embedded in concrete, based on the pH of the concrete (electrolyte) and the presence of aggressive ions, the following would be the possible anodic reactions (Hansson, 1984; Ahmed, 2003):

$3\mathrm{Fe}+4{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}_3{\mathrm{O}}_4+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}$ [13.2]

[13.2]

$2\mathrm{Fe}+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}_3{\mathrm{O}}_3+6{\mathrm{H}}^{+}+6\mathrm{e}$ [13.3]

[13.3]

$\mathrm{Fe}+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HFeO}}_2^{-}+3{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}$ [13.4]

[13.4]

$\mathrm{Fe}\to {\mathrm{Fe}}^{2+}+2{\mathrm{e}}^{-}$ [13.5]

[13.5]

The possible cathodic reactions depend on the availability of O₂ and on the pH near the steel surface. The most likely reactions are as follows (Hansson, 1984; Ahmed, 2003):

$2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2+4{\mathrm{e}}^{-}\to 4{\mathrm{OH}}^{-}$ [13.6]

[13.6]

$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{OH}}^{-}$ [13.7]

[13.7]

13.3.1 Half-cell potential technique

The half-cell potential technique is the most widely used technique to evaluate the corrosion of the steel rebars in concrete. It was introduced in the 1970s by Richard F. Stratfull in North America and by the Danish Corrosion Center in Europe (Stratfull 1968,1972; FORCE Technology, 2004). In 1980, the test was approved as a standard by the ASTM (ASTM, 2009). This technique is based on measuring the electrochemical potential of the steel rebar with respect to a standard reference electrode (copper/copper sulfate electrode (CSE) is suggested by the ASTM-C876) placed on the surface of the concrete, and can provide an indication of the corrosion risk of the steel. Figure 13.3 shows the basics of half-cell potential measurement.

According to the ASTM Standard recommended guidelines for interpretation of the results, if the measured potential is more negative than − 350 mV versus CSE, the probability of active corrosion is more than 90%. If the measured potential is more positive than − 200 mV versus the CSE, the probability of not having active corrosion is 90%, and between − 200 and − 350 mV is the uncertainty region. The most common way of presenting the half-cell potential data is plotting the potential distribution or potential mapping contour. It should be emphasized that, since half-cell potential value is defined as the thermodynamic measure of the ease of removing electrons from the metal in steady state condition, it cannot be used as direct measurement of corrosion rate. It should be noted that half-cell potential is the probability of corrosion activity, while I_corr is the direct measurement of corrosion rate. A simple comparison of the half-cell potential data with the ASTM guidelines on steel reinforcement corrosion probability could cause mistakes in the evaluation of the structure. It has been accepted by those who work in the field that a more negative reading of potential means a higher probability of corrosion. However, this general rule may not always be correct. Some precautions are necessary in interpreting the data from half-cell potential measurements, because there are many factors that may affect the magnitude of the potentials. Details about difficulties with the half-cell potential method are discussed in Poursaee and Hansson (2009).

13.3.2 Linear polarization resistance (LPR)

Figure 13.4 shows a schematic plot of the relationship between potential and current in the region of the open circuit potential. The curve plots the applied potential versus measured current, and vice versa. As shown in Fig. 13.4, there is an approximately linear region around the open circuit potential. The LPR measurements are performed by applying a potential in the range of ± 10 mV about the E_corr, either as a constant pulse (potentiostatic) or a potential sweep (potentiodynamic) and measuring the current response. Alternatively, a current pulse (galvanostatic) or a current sweep (galvano-dynamic) can be applied, and potential response is measured. Polarization resistance (R_p) is the resistance of the specimen to oxidation while an external potential is applied, and the corrosion rate, which is inversely related to the R_p, can be calculated from it.

R_p is determined by calculating the slope of this linear region:

${\mathit{R}}_{\mathit{p}}=\frac{\mathrm{\Delta }\mathit{E}}{\mathrm{\Delta }\mathit{I}}$ [13.8]

[13.8]

where ΔE = change in potential and ΔI = change in current. The Stern-Geary equation relates corrosion current to R_p (Stern and Geary, 1957):

${\mathit{I}}_{\mathrm{corr}}=\frac{\mathit{B}}{{\mathit{R}}_{\mathit{p}}}$ [13.9]

[13.9]

$\mathit{B}=\frac{{\mathit{\beta }}_{\mathit{a}}{\mathit{\beta }}_{\mathit{c}}}{2.3\left({\mathit{\beta }}_{\mathit{a}}+{\mathit{\beta }}_{\mathit{c}}\right)}$ [13.10]

[13.10]

The corrosion current density, i_corr, can be calculated by dividing the corrosion current (I_corr) by the surface area of the polarized area (A):

${\mathit{i}}_{\mathrm{corr}}=\frac{\mathit{B}}{{\mathit{R}}_{\mathit{p}}\mathit{A}}$ [13.11]

[13.11]

B is the Stern-Geary constant, and β_a and β_c are anodic and cathodic Tafel constants, respectively. The value of B should be determined empirically. However, for most cases, it can be assumed to be 0.026 V for active and 0.052 V for passive corrosion of steel in concrete (Andrade and González, 1978; Andrade et al., 1990).

The resistance measured by the LPR is actually the sum of the polarization resistance, R_p, and the electrolyte resistance, R_Ω. If R_p >> R_Ω, the resistance that is measured by the LPR is close enough to the polarization resistance, which can be used as the actual value. However, in some environments with low conductivity, and/or high corrosion rates, the R_Ω is significant and should be considered (Jones, 1992).

According to some researchers, corrosion current densities over ~ 1 μA/cm² have been identified as the level of high corrosion risk, and corrosion current density below 0.1 μA/cm² are characterized as passive corrosion in the system (Gonzalez et al., 1995; Alonso et al., 2000; Polder and Peelen, 2002). However, it seems that the equipment used by these researchers generally gives lower values than does other commercial equipment (Gepraegs, 2002). Therefore, applying such definitions may over- or under-estimate the corrosion rate and cause errors in evaluation and life prediction. Interpreting the corrosion current density values of embedded steel bars in concrete, obtained from the LPR technique, is difficult in large part because determining the actual corroding area of steel is almost impossible and usually causes underestimation of the actual corrosion current density in the areas of active corrosion. The LPR has some advantages over the other measurement techniques, which makes it popular in the evaluation of the corrosion rate in reinforced concrete: it is a non-destructive technique; it is a simple method and it usually needs only a few minutes for corrosion rate determination. Because of its rapidity, it can be effectively used in kinetic studies of corrosion monitoring (Jones, 1992).

13.3.3 Potentiostatic LPR

In the potentiostatic LPR technique, a constant potential signal (usually ± 10 mV) is applied for a certain period of time, which is determined by the time for the current to reach steady state, in the form of square wave between the working electrode (steel bar in concrete) and reference electrode, and the response current is measured. By using Equation [13.8], the R_P. and consequently the corrosion current density and corrosion rate, can be calculated.

13.3.4 Galvanostatic pulse technique

The galvanostatic pulse technique was introduced for field application in 1988 (Newton and Sykes, 1988). This method is a rapid non-destructive polarization technique. A short-time anodic current pulse is applied galvanostatically between a counter electrode placed on the concrete surface and the rebar. The applied current is usually in the range of 10–100 μA, and the typical pulse duration is between 5 and 30 s. The reinforcement is anodically polarized and the resulting change of the electrochemical potential of the reinforcement is measured with a reference electrode, which is usually in the center of the counter electrode and recorded as a function of polarization time (Klinghoffer, 1995; Elsener et al., 1997). A typical potential response for a corroding reinforcement is shown in the Fig. 13.5.

When the constant current, I_app, is applied to the rebar, the polarization of the rebar, η_t, at given time t can be expressed as (Jones and Greene, 1966):

${\mathit{\eta }}_{\mathit{t}}={\mathit{I}}_{\mathrm{app}}\times \left[{\mathit{R}}_{\mathit{p}}\times \left[1-{\mathit{e}}^{\left(\frac{-\mathit{t}}{{\mathit{R}}_{\mathit{p}}{\mathit{C}}_{\mathit{dl}}}\right)}\right]+{\mathit{R}}_{\mathrm{\Omega }}\right]$ [13.12]

[13.12]

where R_p = polarization resistance, C_dl = double-layer capacitance, and R_Ω = ohmic resistance of the concrete cover.

By transferring Equation [13.12] to logarithmic form, the values of R_p and C_dl, can be calculated as following (Newton and Sykes, 1988):

$\mathrm{In}\left({\mathit{\eta }}_{\text{max}}-{\mathit{\eta }}_{\mathit{t}}\right)-\mathrm{In}\left({\mathit{I}}_{\mathrm{app}}\times {\mathit{R}}_{\mathit{p}}\right)-\left(\frac{\mathit{t}}{{\mathit{R}}_{\mathit{p}}{\mathit{C}}_{\mathit{dl}}}\right)$ [13.13]

[13.13]

where η_max is the final steady state potential value. Figure 13.6 plots Equation [13.13].

If this straight line is extrapolated to t = 0, it will give an intercept of I_app × R_p and the slope of the line in 1/R_pC_dl. The remaining over-potential corresponds to I_app × R_Ω, which is the ohmic voltage drop across the concrete cover. After determining the polarization resistance (R_p) by the above method, the corrosion current I_corr can be calculated from the Stern-Geary equation (Stern and Geary, 1957; Newton and Sykes, 1988).

13.3.5 Equipment with the guard ring

An auxiliary counter electrode, referred to as a guard ring electrode, can be used to confine the polarization to a known length of reinforcing bar. The counter electrode and the guard ring are typically arranged as annular metal rings with a reference electrode in the center. The potential or current applied from the guard ring tends to repel the signals from the central counter electrode, confining them to an area of the structure located approximately under the counter electrode, as schematically shown in Plate X in the color section between pages 294 and 295.

There are two well-known equipment works based on the galvanostatic pulse technique:

13.3.6 Electrochemical impedance spectroscopy (EIS)

The EIS studies the system response to the application of a small amplitude alternating potential or current signal at different frequencies. The popularity of the EIS methods for reinforced concrete has increased remarkably in recent years, because analysis of the system response provides information about the double-layer capacitance, interface, structure, reactions which are taking place, corrosion rate, and electrolyte (environment) resistance (Silverman, 1990; Jones, 1992; Lasia, 1999). An electrochemical process can be considered as an electrical circuit with basic elements such as resistors, capacitors, and inductors. Therefore, in interpreting the response to an alternating current (AC), the AC circuit theory can be used successfully to demonstrate a corrosion process, and it may also be used to understand the behavior of the corrosion process and predict the corrosion rates.

In direct current, Ohm’s law is as follows:

$\mathit{V}=\mathit{IR}$ [13.14]

[13.14]

(V = Potential, I = Direct current, R = Actual resistor)

In the AC, Ohm’s law becomes:

$\mathit{V}=\mathit{IZ}$ [13.15]

[13.15]

(V = Potential, I = Alternative current, Z = Impedance)

Direct current can be viewed as alternating current at zero frequency. In this case, the resistance is composed of only one or more actual resistors. When the frequency is not zero, all circuit elements that can affect the flow of current, e.g., resistors, capacitors, and inductors, cause the resistance. The resistance created by capacitors and inductors depends on the frequency, while that created by a resistor is not dependent on frequency (Silverman, 1986). A sinusoidal current or voltage can be represented as a rotating vector, as shown in Fig. 13.7. In this figure, the x-component shows the observed current so it becomes the real component of the rotating vector, while the contribution of the y-component is not observed; there- fore, it is called the imaginary component of the rotating vector.

The mathematical descriptions of the two components are as follows:

$\mathrm{Real}\mathrm{current}={\mathit{I}}_{\mathit{x}}=\left|\mathit{I}\right|cos\left(\mathit{\omega t}\right)$ [13.16]

[13.16]

$\mathrm{Imaginary}\mathrm{current}={\mathit{I}}_{\mathit{y}}=\left|\mathit{I}\right|sin\left(\mathit{\omega t}\right)$ [13.17]

[13.17]

where t = time and ω = frequency in radians per second = 2πf (f = frequency in Hertz).

To separate the real (x) and imaginary (y) components, the magnitude of the imaginary part should be multiplied by $\mathit{j}=\sqrt{-1}$ ¹ and then the real and imaginary values can be reported separately. The equations for AC impedance become:

${\mathit{E}}_{\mathrm{total}}={\mathit{E}}_{\mathrm{real}}+{\mathit{E}}_{\mathrm{imaginary}}+{\mathit{E}}^{\prime }+\mathit{j}{\mathit{E}}^{″}$ [13.18]

[13.18]

${\mathit{I}}_{\mathrm{total}}={\mathit{I}}_{\mathrm{real}}+{\mathit{I}}_{\mathrm{imaginary}}={\mathit{I}}^{\prime }+\mathit{j}{\mathit{I}}^{″}$ [13.19]

[13.19]

${\mathit{Z}}_{\mathrm{total}}={\mathit{Z}}^{\prime }+{\mathit{Z}}^{″}=\frac{{\mathit{E}}^{\prime }+\mathit{j}{\mathit{E}}^{″}}{{\mathit{I}}^{\prime }+\mathit{j}{\mathit{I}}^{″}}$ [13.20]

[13.20]

The absolute amplitude of the impedance (that is the length of the vector) and the phase angle are defined by (Princeton Applied Research, 2006):

$\left|\mathit{Z}\right|=\sqrt{{\mathit{Z}}^{\prime }+{{\mathit{Z}}^{″}}^2}$ [13.21]

[13.21]

$tan\mathit{\theta }=\frac{{\mathit{Z}}^{″}}{{\mathit{Z}}^{\prime }}$ [13.22]

[13.22]

The goal of AC impedance is to measure the impedance Z as Z′ and Z″, and then model the response by using an equivalent simple circuit (Silverman, 1986).

Data presentation

There are different ways to illustrate the response of an electrochemical system to an applied AC potential or current. The most common plots are the Nyquist plot and Bode plot. If, at each excitation frequency, the real part is plotted on the x-axis and the imaginary part is plotted on the y-axis of a chart, a ‘Nyquist plot’ is formed. A simple corroding system can be assumed as: solution resistance, in series with a combination of a resistor and a capacitor, which represent the polarization resistance and double-layer capacitance, respectively. This simple representation is called a Randles cell and is shown in Fig. 13.8.

Figure 13.9 schematically illustrates the Nyquist plot for a simple electro-chemical system corresponding to the analog circuit in Fig. 13.8. It should be noted that each point on the Nyquist plot is the impedance at one frequency. On the Nyquist plot, the impedance can be represented as a vector of length |Z|, and the angle between this vector and the x-axis is the phase angle ‘θ’ (Gamry Instruments, 2006; Princeton Applied Research, 2006). At high frequencies, at the leftmost end of the semicircle, where the semicircle touches the x-axis, the impedance of the Randles cell is entirely produced by the ohmic resistance, R_Ω. The frequency reaches its lower limit at the rightmost end of the semicircle. At this frequency, the Randles cell also approximates a pure resistance, but now the value is (R_Ω + R_p) (Princeton Applied Research).

The Nyquist plot has some limitations (Princeton Applied Research):

1. The frequency is not clearly shown on the plot and it is not possible to determine, for a specific point, the frequency used to record that point;

2. The ohmic and polarization resistances can be directly determined from the plot but the electrode capacitance can be only calculated if the frequency information is known, using Equation [13.23]:

$\mathit{C}=\frac{1}{{\mathit{\omega }}_{\text{max}}\times {\mathit{R}}_{\mathit{p}}}$ [13 23]

[13 23]

3. If there are high and low impedance components in the circuit, the larger impedance controls plot scaling, and distinguishing the low impedance semicircle would probably be impossible.

A Bode plot is another popular presentation method for the impedance data. In the Bode plot, the data are plotted with log of frequency on the abscissa and both the log of absolute value of the impedance (|Z|) and phase-shift (θ) on the ordinate (Gamry Instruments, 2005). Figure 13.10 schematically shows a Bode plot for the same system shown in Fig. 13.8. Since the frequency appears as one of the axes in the Bode plot, it is easy to understand the dependence of impedance on the frequency from the plot. The log |Z| versus log ω curve can be used to determine the values of R_p and R_Ω. At very high and very low frequencies, |Z| becomes independent of frequency. At the highest frequencies the ohmic resistance controls the impedance and log (R_Ω) can be read from the high frequency horizontal level. On the other hand, at the lowest frequencies, polarization resistance contributes, and log (R_p + R_Ω) can be read from the low frequency horizontal portion.

The Bode format is advantageous when data scatter prevents satisfactory fitting of the Nyquist semicircle. In general, the Bode plot provides a more understandable description of the frequency-dependent behavior of the electrochemical system than does the Nyquist plot, in which frequency values are not clear (Princeton Applied Research, 2006).

13.3.7 Cyclic polarization

Cyclic potentiodynamic polarization technique is a relatively non-destructive measurement that can provide information about the corrosion rate, corrosion potential, susceptibility to pitting corrosion of the metal, and concentration limitation of the electrolyte in the system. The technique is built on the idea that prediction of the behavior of a metal in an environment can be made by forcing the material from its steady state condition and monitoring how it responds to the force as the force is removed at a constant rate and the system is reversed to its steady state condition. Applied potential is the force and is raised at a continuous, often slow, rate by using a potentiostat (Silverman, 1998). This rate is called the polarization scan rate, and is an experimental parameter. The potential of the specimen is changed continuously while the resulting current is monitored, and then the applied potential is plotted versus the logarithm of the resulting current density. The conductivity of the electrolyte (environment) is a very important factor that should be considered in all electrochemical experiments, especially in the cyclic polarization technique. The electrolyte resistance causes a potential drop between the working electrode and reference electrode and can cause errors. This effect has important impacts on the interpretation, and should be compensated.

13.4.1 Electrochemical sensors

The sensor most used for this purpose is basically an embeddable inert and stable reference electrode in high alkaline environment in concrete such as: Mn/MnO₂ or Ti/TiO₂ (Muralidharan et al., 2007; Duffó and Farina, 2009; Dong et al., 2011), to perform half-cell potential measurement. The other type of sensor is designed to measure the half-cell potential and the corrosion current density, and consequently corrosion rate. These sensors are based on utilizing three electrodes (reference, counter or auxiliary, and working electrodes), which are usually used in conventional electrochemical measuring systems. Most of the sensors in this category use the metal–metal oxide (MMO) as reference electrodes, such as Mn/MnO₂ and Ti/TiO₂, stainless steel as counter electrode, and the reinforcing bar as the working electrode (Anderade and Martinez, 2009; Duffó and Farina, 2009; Dong et al., 2011). In the sensor developed by Poursaee, graphite rods were used as both reference and counter electrodes. The stability of graphite in high pH was evaluated, and it was concluded that the potential of the graphite is stable enough during the period of measurement, and the potentiostatic LPR can be performed successfully, using this configuration (Poursaee, 2009). However, graphite cannot be used as the permanent reference electrode, unless it undergoes special treatment (Swette et al., 1999). The sensors with the three electrochemical components can be used to perform all the techniques, such as the LPR, galvanostatic pulse, cyclic polarization, and the EIS.

13.4.2 Other sensors

The onset of corrosion in steel-reinforced concrete can also be detected by different methods. The inductively coupled magnetic fields, with the core typically made of a simple LC resonator, have been used by researchers to sense the initiation of corrosion (Carkhuff and Cain, 2003; Andringa et al., 2005; Bhadra et al., 2010). Change of the inductance or the capacitance of the LC circuit will change the resonant frequency that is the basis of such sensors. The resonant frequency of the whole LC circuit, schematically shown in Fig. 13.11, is given by the Thompson formula:

$\mathit{f}=\frac{1}{2\mathit{\pi }\sqrt{\mathit{LC}}}$ [13.24]

[13.24]

where L is the total inductance of the circuit and C is the total capacitance of the circuit.

According to the Equation [13.24], the change of the inductance or the capacitance of the circuit will change the resonant frequency. As can be seen in Fig. 13.11, the resonant frequency of the circuit depends on the state of the steel wire. When the steel wire is not broken, the total capacitance of the circuit is equal to the summation of the two capacitances (C = C1 + C2). However, when the steel wire is broken due to corrosion, the total capacitance of the circuit becomes C1. The diameter of the steel wire is usually much smaller than that for the reinforcing steel, and the steel wire will corrode before considerable corrosion damage has occurred in the reinforcement.

Eddy currents have also been used to detect the corrosion on the embedded steel bars in concrete (Gaydecki and Burdekin, 1994; Miller et al., 2003; Kumar et al., 2006). Eddy currents are generated in a conductive material by a changing magnetic field. In the eddy current technique, the magnetic field in a coil induces eddy currents in the rebar. This eddy current generates a magnetic field of its own that interferes with the main magnetic field. The change in inductance of the coil is then measured using the meter.

Sensors based on the application of fiber optics to detect corrosion have also been developed. An optical fiber consists of a glass core surrounded by a glass cladding that differs in index of refraction. The glass fiber is then coated during the manufacturing process with a protective polymer layer (Merzbachery et al., 1996). The direct spectroscopy of corroded versus un-corroded materials is the basis of this method. Spectrally broadband light is coupled into an optical fiber and then illuminates the region under measurement. The presence of corrosion is determined using color modulation of the broadband input signal (Fuhr and Huston, 1998).

The principle of light reflection has also been used to develop fiber optic sensors to detect corrosion of steel in concrete. The sensor consists of an optical fiber reflection sensor, a sacrificial metallic film, joined to a steel tube. One side of the film is finely polished and is isolated from the environment, while the other side is exposed to the corrosive environment. The corrosion pits initiated at the exposed film surface slowly penetrate the sacrificial film as the exposure time increases. The corrosion pits that reach the polished surface reduce the surface reflectivity of the polished surface. This decrease in reflectivity can be detected by the optical fiber reflectivity sensor (Wang and Huang, 2011).

The fiber Bragg grating (FBG) is used for the corrosion detection purpose as well (Lo and Xiao, 1998; Yang et al., 2006; Gaon et al., 2011). Normal optical fibers are uniform along their lengths. In a simple FBG, the refractive index of the fiber core varies periodically along the length of the fiber. The FBG reflects particular wavelengths of light and transmits all others, and therefore can be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector. The difference between the reflection of corrosion products and that of the steel rebar can be used to detect the corrosion activity on the surface of the rebar.

Acoustic emission (AE) monitoring of concrete is the other technique that has been used to detect rebar corrosion (Zdunek et al., 1995; Yoon et al., 2000; Golaski et al., 2002; Idrissia and Limam, 2003; Reis et al., 2003; Assouli et al., 2005; Fariduddin et al., 2007). AE energy is released when a crack propagates. Since corrosion products are expansive (Lide, 1999), they cause multiple micro-fractures, which ultimately lead to macro-cracks. This activity can be detected by AE sensors. As a result, the initiation of corrosion, location of the zone containing intense corrosion products, crack formation mechanism, and the loss of bond strength due to corrosion between reinforcing steel and the surrounding concrete, can be investigated using this method. However, at this time, expensive equipment and the need for highly trained personnel limit the application of AE technique mainly to laboratory investigations.