Inspection of Steel Bars Corrosion in Reinforced Concrete Structures by Nondestructive Ground Penetrating Radar

Abstract

In this study, the degree of corrosion of steel reinforcement is compared to the reflected voltage of electromagnetic waves and the ASTM C876 specification. This study conducted some tests on steel bars with different degrees of corrosion by transmitting electromagnetic waves in the reinforced concrete. In the test, a corrosion potentiometer was used to analyze and compare the experimental results. The electromagnetic wave reflection signals from the image generated by the ground penetrating radar were used to capture the different media in the concrete components. The reflection coefficient method was used to analyze the calculated relative dielectric constant of the media and to obtain the reflection energy and phase changes on the medium interface. We compared and discussed the corrosion potential and the reflected voltage from the corroded steel bars in the reinforced concrete. The reflected voltage analysis of the ground penetrating radar showed that the average corrosion potential on the corroded plate on the 3rd floor was −280 m V (Area C). According to ASTM C876, the steel bars in the concrete were moderately corroded. The reflected voltage analysis of the electromagnetic waves concluded that areas A and B were moderately corroded, which is consistent with the conclusion from ASTM C876. Using the innovative method, this study has established a reference standard for the degree of corrosion of steel bars based on ASTM C876 and has calculated the quantitative state of corroded steel bars. The variations of the electromagnetic wave reflected voltage of the ground penetrating radar can mark the changes in the corrosion interface of concrete and steel reinforcement with different degrees of corrosion and different thicknesses of the protective layer.

1. Introduction

In the past, the corrosion detection of steel bars in reinforced concrete structures used the chemical potential method as the measurement method, which could only measure the difference in the chemical potential of the deep corrosion of steel bars from the concrete surface, and it was necessary to destroy the drilled holes to the surface of the steel bars. Using the GPR electromagnetic wave reflection voltage method, the most important thing is that there is no need to damage the reinforced concrete structure. Secondly, GPR can increase the detection of a large area of reinforced concrete structures, and the corrosion status of each steel bar inside the structure can be measured. The traditional chemical potential method is used to predict the corrosion probability of steel bars, which is relatively inaccurate. This study uses the voltage reflected by the electromagnetic waves of ground-penetrating radar to present the degree of corrosion of steel bars; this method can detect the corrosion state of individual steel bars, and there is no need to perform destructive opening detection on the concrete surface.

Effective and comprehensive condition assessment can reduce the maintenance costs of concrete structures. The ground penetrating radar (GPR) can provide completely nondestructive results in real time and has been widely used in the condition assessment of reinforced concrete structures [1]. During the corrosion process, the GPR data were recorded. The results show that the relative amplitude between the reflection signals in both the parallel and the perpendicular polarization is decreasing as the corrosion degrades [2]. In Taiwan, the high humidity and corrosive factors caused by the extensive coastal environment or oceanic climate have caused serious corrosion to the structure of reinforced concrete bridges [3]. Most of the methods for detecting the degree of corrosion of the bars are based on corrosion potential or corrosion current [4]. However, these methods based on corrosion potential and corrosion current must establish electrochemical pathways by destructing the surface of the concrete components. Therefore, it is necessary to develop a detection technology that can improve detection efficiency and does not generate damage to concrete structures [5]. Corrosion is one of the complex mechanisms that most affects the resistance of reinforced concrete. A combined methodology using ground penetrating radar (GPR) and infrared inspection techniques for the detection and evaluation of corrosion is proposed [6]. This study proposes an improved repair and retrofit technique that includes the removal of rust from oxidized rebar parts by applying low viscosity epoxy resin to the slab base and allowing it to fully penetrate the concrete cracks and surface of the rebar’s inside, thus producing a protective layer and repairing the bond [7].

The current research of ground penetrating radars detects the degree of corrosion of steel bars by using the methods of ground resistance and corrosion potential [8]. A technique for accelerating rebar corrosion using direct current with sodium chloride solution was used to induce corrosion into embedded rebars. A GPR was used to assess the corrosion of the rebar. The results show that corrosion of the rebar could be inspected [9]. Non-destructive testing, quasi-continuous results, and efficient data acquisition are the main advantages of mobile GPR systems. In this paper, the benefits and limitations of mobile GPR systems are discussed using examples from EMPA’s recent work. The emphasis is on the investigation of pavement thicknesses and depths of rebar on concrete bridges [10]. This article proposes a deep learning-based autonomous concrete crack detection technique using hybrid images. The hybrid images combining vision and infrared thermography images are able to improve crack detectability while minimizing false alarms [11]. Recently, scholars have applied the wave propagation of reflection signals transmitted by the ground penetrating radar to examine the corrosion phenomenon of steel bars with different thicknesses of protective layers in concrete components at different times (different status of corrosion) and have compared and analyzed the corrosion potential [12]. In recent years, the use of ground penetrating radar (GPR) at frequencies from 500 MHz to 2.5 GHz has yielded very good results for the inspection of concrete structures. The possibility of performing nondestructive measurements quickly and with convenient recording of the measurement results is particularly beneficial [13]. Two experimental cases are realized: in the first one, GPR and ERT were performed in full scale laboratory conditions where a road segment was investigated in different conditions; in the second test, a radiant floor was investigated with a comparison of the results obtained with GPR and IR. The study demonstrates the feasibility of integrating the collected data obtained with non-destructive testing to enhance the knowledge of engineering issues [14]. This study will derive and integrate the digital coded signal of ground penetrating radar electromagnetic wave scanning and electromagnetic wave transmission theory to explore the application of the reflected voltage of the corrosion product interface of steel reinforcement to the detection of corroded steel bars in concrete components and compare it with the corrosion chemical potential of corroded steel bars [15]. The use of ground penetrating radar (GPR) to find bridge corrosion was discussed. A simple pull cart was constructed with mounts for the antennas to keep the antennas an inch above the concrete surface. A threshold level was determined to differentiate between the good and the corroded rebars. The variance simulations helped to determine the reflectivity of the corroded rebar [16]. A physical model is presented for the electromagnetic signature of a buried reinforcing steel bar, which takes into account the radius of the rebar. This is achieved by subjecting GPR radar grams to a series of digital image processing stages, followed by different power reflectivity within the energy zone during the motion of the GPR antenna along the reinforced concrete surface [17]. Studies in other countries using the ground penetrating radar to detect and evaluate the corrosion of bridges found out that the corrosion products of steel bars were correlated with the electromagnetic wave signal transmitted by the ground penetrating radar. The electromagnetic wave reflection of ground penetrating radar can effectively examine the deterioration and defects of reinforced concrete. The reflected waves show the different characteristics in sound and defective reinforced concrete [18].

The comparison analysis of these two different waves can identify the defects and the degree of corrosion. Meanwhile, the comparison between the ground penetrating radar method and the corrosion potential method showed a correlation. These qualitative detections are discussed more frequently than quantitative tests. This study also explores the application of the ground penetrating radar in detecting and analyzing the interface of corroded steel bars by propagating electromagnetic waves. The contents of this research include:

A.

Electromagnetic wave propagation behavior and the principle of the reflected voltage.

B.

Accelerated corrosion test of steel bars with different thicknesses of the protective layer.

C.

The reflected voltage of electromagnetic waves from different corroded steel bars.

D.

The detection of the electrochemical corrosion potential of different corroded steel bars.

E.

Establishing a reference standard for detecting the degree of corrosion of the reflected voltage of electromagnetic waves.

2. Electromagnetic Wave Propagation Behavior and the Principle of the Reflected Voltage

2.1. Fundamental Theory of Electromagnetic Waves

Electromagnetic waves are an illustration of the transmission phenomenon of the interaction between electrons and the magnetic field. It means that the electric field at any point in space will generate a magnetic field that varies over time. The changing magnetic field and the electric field are reciprocally transmitted forward to each other, as shown in Figure 1.

2.2. Resolution of Electromagnetic Waves

As electromagnetic waves are propagated in a medium, the frequency of the main reflected wave is slightly lower than the frequency of the center. Therefore, during the detection process of the ground penetrating radar, the limit of resolution is 1/2 of the wavelength of the central frequency. The higher the frequency of the electromagnetic wave, the greater the attenuation coefficient and the shallower the detection thickness. The resolution of the electromagnetic wave can be illustrated as follows.

A.

Vertical resolution

The minimum resolution length of the ground penetrating radar is 1/4 of the wavelength of the electromagnetic wave in the medium. The vertical resolution is correlated to the thickness detected by the ground penetrating radar.

B.

Horizontal resolution

The horizontal resolution of the electromagnetic wave of the ground penetrating radar is related to the size of the calculated first Fresnel zone. If the first Fresnel zone cannot provide enough reflected energy, the resolution of the object cannot be measured, as is shown in Figure 2.

2.3. Relative Dielectric Constant

The relative dielectric constant of a material is the ratio of its (absolute) permittivity to the vacuum permittivity. The formula of the relative dielectric constant is expressed as follows:

$${\epsilon }_r=\frac{\epsilon }{{\epsilon }_0}$$

(1)

Different media have different dielectric constants, which will generate different reflected energies, as shown in

Table 1. Dielectric Constant.

Medium	Conductivity σ (MS/m)	Relative Dielectric Constant (εr)	Velocity ν (m/ns)
Air	0	1	0.3
Concrete	1	4~10	0.09~0.15
Iron	1010	300	0.017
Steel Bar	0.6 × 107	2600	-
Iron oxide (Corrosion products)	-	14.2	-

. The dielectric constants are public information.

2.4. Reflection Coefficient and Power Reflection Coefficient

We used the reflection coefficient method to calculate the relative dielectric constants of the concrete surface and concrete materials and obtained the changes of the reflection coefficient and the power reflection coefficient of electromagnetic waves in different media. We calculated the relative dielectric constant by propagating the electromagnetic waves from the ground penetrating radar into the air, as shown in Equation (2):

$${\epsilon }_{air}=\left(\frac{1-\left[A_{air}/A_{max}\right]}{1+\left[A_{air}/A_{max}\right]}\right)$$

(2)

In Equation (2), ε_air is the relative dielectric constant, A_air is the reflected voltage in the air interface, and A_max is the maximum reflected voltage.

Because the reflection coefficient is the ratio of reflection in different medium interfaces, and due to the cumulative characteristics of the reflection signal, the relative dielectric constant of the second layer of air to concrete can be calculated by Equation (3):

$${\epsilon }_{con}={\epsilon }_{air}\left(\frac{1-{\left[A_{air}/A_{max}\right]}^2-{\left[A_{con}/A_{max}\right]}^2}{1-{\left[A_{air}/A_{max}\right]}^2+{\left[A_{con}/A_{max}\right]}^2}\right)$$

(3)

where ε_air is the relative dielectric constant of the concrete layer, and A_con is the reflected voltage of the concrete interface.

The relative dielectric constant analysis of the third concrete layer to the corroded products can be calculated by Equation (4):

$${\epsilon }_{ferro}={\epsilon }_{con}\left(\frac{1-{\left[A_{air}/A_{max}\right]}^2-R_1\left[A_{con}/A_{max}\right]-{\left[A_{ferro}/A_{max}\right]}^2}{1-{\left[A_{air}/A_{max}\right]}^2-R_1\left[A_{con}/A_{max}\right]+{\left[A_{con}/A_{max}\right]}^2}\right)$$

(4)

In Equation (4), ε_ferro is the relative dielectric constant of the corroded products, and A_ferro is the reflected voltage in the interface of the corroded products.

The electromagnetic wave reflection signals from the image generated by the ground penetrating radar can capture different media in the concrete components, as shown in Figure 3.

The content of the ground penetrating radar technology includes the detection of medium position, thickness, shape, size, etc., which are all analyzed according to the waveform variations of the electromagnetic reflection signals transmitted by the ground penetrating radar. Based on the analysis of the changes of signal intensity using the reflection coefficient method, we can determine the relative dielectric constant of the medium interface. The calculated relative dielectric constant of the medium represents the reflection energy and the phase changes in the medium interface in order to analyze the reflection coefficient in the medium interface, as shown in Equation (5):

$$R_{mn}=\frac{\sqrt{{\epsilon }_m}-\sqrt{{\epsilon }_n}}{\sqrt{{\epsilon }_m}+\sqrt{{\epsilon }_n}}$$

(5)

where R_mn is the reflection coefficient in the interface of the medium, ε_m is the relative dielectric constant of medium 1, and ε_n is the relative dielectric constant of medium 2.

The power reflection coefficient, P_r, is the square of the reflection coefficient. Its value represents the intensity of electromagnetic wave reflection in the medium interface, as shown in Equation (6):

$$P_r=R_{mn}{}^2={\left|\frac{\sqrt{{\epsilon }_m}-\sqrt{{\epsilon }_n}}{\sqrt{{\epsilon }_m}+\sqrt{{\epsilon }_n}}\right|}^2$$

(6)

2.5. Reflected Voltage Mode of Electromagnetic Waves

During the propagation of the electromagnetic waves of the ground penetrating radar in the medium, the reflected voltage power of the object can be used as an indicator to determine whether there is a sufficient reflection signal, and its value depends on the characteristic impedance and the resistance of the object and medium. When the electromagnetic wave propagates in the medium interface A, the reflected coefficient between the air and the medium interface is the ratio of the reflected voltage to the incident voltage, as shown in Equation (7):

$$R_I=\frac{r_I\left(t\right)}{s\left(t\right)}$$

(7)

where r_I(t) is the reflected voltage from the air to the concrete medium, and s(t) is the incident voltage at interface I. Therefore, the reflected voltage of the interface I between the air and the medium can be calculated by Equation (8):

$$r_I\left(t\right)=R_I·s\left(t\right)$$

(8)

The reflection behavior of the interface between the air and the concrete is shown in Figure 4.

In a double-layered medium, the reflected wave A in medium II will generate another reflected wave, as shown in Figure 5.

The reflection coefficient of the interface can be calculated by Equation (9):

$$R_{II}=\frac{r_{II}\left(t\right)}{s\left(t\right)·w_i}$$

(9)

The reflected voltage of the interface II can be calculated by Equation (10):

$$r_{II}\left(t\right)=R_{II}·s\left(t\right)·w_i$$

(10)

where r_II(t) is the reflected voltage of the interface between medium A and medium B, s(t) is the incident voltage at interface II, w_i is the reflected power, calculated by (w_i = 1 − R_II²), and R_II² is the reflected power of the interface I.

The total reflected voltage of the electromagnetic wave of the ground penetrating radar can be expressed by Equation (11):

$$r\left(t\right)=r_I\left(t\right)+r_{II}\left(t\right)$$

(11)

Then, Equation (12) can be derived from Equation (11):

$$\sum r\left(t\right)=R_{I}s\left(t-t_A\right)+R_{II}\left(1-R_I{}^2\right)s\left(t-t_A-t_B\right)$$

(12)

This equation is the function of the incident signal voltage, the reflection coefficient of the medium I, and the reflection coefficient of the interface II in the double-layer medium consisting of medium A and medium B.

3. Accelerated Corrosion Test of Steel Bars in Different Corrosion Degree

3.1. Test Content

This study conducted some tests on the steel bars with different degrees of corrosion by propagating the electromagnetic wave of the ground penetrating radar through the reinforced concrete. The experimental results were compared and analyzed using a corrosion potentiometer.

3.2. Accelerated Corrosion Test on Steel Bars

We accelerated the corrosion rate of the steel bars by adjusting the impressed current of the direct current (DC) power supply. In this test, the object other than the steel bars was immersed in water. The anode of the power supply was connected to the steel bars, and the cathode was connected to the titanium mesh. Then, we adjusted the impressed current of the DC power supply, as shown in Figure 6.

3.3. Corrosion Test on Steel Bars Using the Ground Penetrating Radar

The steel bars with different protective layer thicknesses (4 cm, 6 cm, 7 cm, 9 cm) and different accelerated corrosion durations (0 to 408 h) were placed in 165 cm (length) × 15 cm (width) × 60 cm (height) concrete, and the #6 steel bar was scanned by the ground penetrating radar, as shown in Figure 7.

3.4. Corrosion Test on Steel Bars by Using a Half-Cell Potentiometer

We used a half-cell potentiometer as a reference electrode (copper/copper sulfate) to scan the corrosion potential of the steel bars, as shown in Figure 8.

3.5. The Scanning Profiles of Different Corroded Steel Bars Using the Ground Penetrating Radar

We conducted the accelerated corrosion test on the #6 steel bar placed in the concrete. The ground penetrating radar was also used to detect the protective layers (with the thicknesses of 4 cm, 6 cm, 7 cm, and 9 cm) in different accelerated corrosion durations (0~408 h) under different corrosion statuses, i.e., slight, moderate, and serious corrosion. The radar scanning profiles are shown in Figure 9, Figure 10, Figure 11 and Figure 12.

Figure 9a is a non-corrosive cross-sectional view, and Figure 9b is a mildly corroded cross-sectional view. Figure 9c is moderate corrosion; Figure 9d is severe corrosion. Because the ground penetrating radar profile is a simulated signal graphic, its difference cannot be visually distinguished, and this needs to be determined through the coding analysis of this study for corrosion determination.

Figure 9, Figure 10, Figure 11 and Figure 12 are calculated and analyzed by transverse radar analog signal coding to present the longitudinal axis electromagnetic wave reflected voltage values of Figure 13, Figure 14 and Figure 15.

Figure 9, Figure 10, Figure 11 and Figure 12 are a transparent radar cross-sectional view of different degrees of corrosion; because the profile map simulates the signal graphic, the difference in corrosion degree cannot be directly visually judged from the image, and this needs to be determined by the coding analysis method of this study.

3.6. Electromagnetic Wave Reflected Voltage of Steel Bars with Different Degrees of Corrosion

The electromagnetic wave reflected voltage of steel bars in concrete with different thicknesses of the protective layer (4 cm, 6 cm, 7 cm, and 9 cm) was analyzed at different durations of accelerated corrosion (0~408 h):

A.

Analysis of the reflected voltage for the 4 cm protective layer

The results of the ground penetrating radar showed that in the three different acceleration periods of 72 h, 192 h, and 288 h the reflected voltage with different degrees of corrosion had different changes, as shown in Figure 13. From the results, the degree of corrosion increases with the increasing accelerated duration.

The changes of the reflected voltage of the ground penetrating radar became apparent at 72 h, which demonstrated that the steel bars began to corrode after 72 h of electrification. When the duration of the accelerated corrosion was 288 h, the reflected voltage only had small changes.

B.

Analysis of the reflected voltage for the 6 cm protective layer

From the detection results of the ground penetrating radar, the reflected voltages had different changes with the degree of corrosion at the three different accelerated corrosion durations of 72 h, 192 h, and 312 h, as shown in Figure 14. At 72 h, the reflected voltage of the electromagnetic wave propagated by the ground penetrating radar for the 6 cm protective layer had significant changes. The steel bars with 4 cm and 6 cm protective layers began to corrode at 72 h. At 312 h, the change of the reflected voltage slowly increased.

C.

Analysis of the reflected voltage for the 7 cm protective layer

From the detection results of the ground penetrating radar at the three different accelerated corrosion durations of 48 h, 168 h, and 288 h, the reflected voltage had different changes with different degrees of corrosion, as shown in Figure 15.

From the figure, the reflected voltage of the electromagnetic waves propagated by the ground penetrating radar for the 7 cm protective layer changed significantly at 48 h, which was 24 h earlier than that for the 4 cm and 6 cm protective layers. After the current passing through the steel bars for 48 h, they began to corrode. When the duration of the accelerated corrosion was 288 h, the variations of the reflected voltage slowly increased. For the moderate and serious corrosion, there is a 24 h gap between the corrosion potential and the reflected voltage of the ground penetrating radar. The reason for this phenomenon is that the protective layer of the steel bars has a soaking level of 7 cm, which makes the corrosion faster than the other protective layers. Thus, the corrosion potential cannot explain whether the steel bars have uniform corrosion. However, the reflected voltage of electromagnetic waves propagated by the ground penetrating radar can explain the uneven corrosion in the accelerated corrosion process.

D.

Analysis of the reflected voltage for the 9 cm protective layer

From the detection results of the ground penetrating radar, there were three different accelerated corrosion durations, i.e., 72 h, 192 h, and 312 h, as shown in Figure 16. Furthermore, the changes of the reflected voltage of the electromagnetic wave propagated by the ground penetrating radar for the 4 cm and 6 cm protective layers became significant at 72 h, which indicates that the steel bars with 4 cm and 6 cm protective layers began to corrode after the current passing through for 72 h. At the accelerated corrosion duration of 312 h, the variations of the reflected voltage gradually decreased.

E.

Figure 13, Figure 14, Figure 15 and Figure 16 are the reflected voltage measurement results of steel reinforcement corrosion at different protective layer depths, and the images tend to be similar, indicating that with the increase in accelerated corrosion time, the more serious the corrosion degree of the steel reinforcement.

3.7. Detection of Electrochemical Corrosion Potential of Different Corroded Steel Bars

The potential of steel reinforcement with different thicknesses of the protective layer (4 cm, 6 cm, 7 cm, and 9 cm) was detected at different durations of accelerated corrosion (0~408 h):

A.

Corrosion potential analysis for 4 cm protective layer

Based on the reference standard of ASTM C876 for the corrosion potential of steel bars, we obtained the critical levels of the corrosion potential of the 4 cm protective layer for slight, moderate, and serious corrosion at the accelerated durations of 192 h and 312 h, as shown in Figure 17. The slight corrosion of the reflected voltage of the electromagnetic wave means that the reflected voltage of the ground penetrating radar in its early stage changed significantly at 72 h, which was 120 h earlier than the corrosion potential. The results demonstrated that the reflected voltage of the ground penetrating radar can detect the corrosion of steel bars more precisely. When the corrosion product (iron oxide) is generated, the reflected voltage of the ground penetrating radar can detect the corrosion more precisely than the corrosion potential.

B.

Corrosion potential analysis for 6 cm protective layer

Based on the ASTM C876 reference standard for the corrosion potential of steel bars, we obtained the critical levels of the corrosion potential of the 6 cm protective layer for slight, moderate, and serious corrosion during the accelerated corrosion periods of 192 h and 312 h, as shown in Figure 18. The test result of the reflected voltage of the ground penetrating radar in the early stage was the same as the result for the 4 cm protective layer. The accelerated corrosion changed significantly at 72 h, which was 120 h earlier than the corrosion potential. The test result indicated that the reflected voltage of the electromagnetic wave of the ground penetrating radar can precisely detect the corroded steel bars with 4 cm and 6 cm protective layers.

C.

Corrosion potential analysis for 7 cm protective layer

Based on the ASTM C876 reference standard for the corrosion potential of steel bars, we obtained the critical levels of the corrosion potential of the 7 cm protective layer for slight, moderate, and serious corrosion during the accelerated corrosion periods of 144 h and 312 h, as shown in Figure 19. The test result of the reflected voltage of the ground penetrating radar for the 7 cm protective layer in the early stage was different from the results for the 4 cm and 6 cm protective layers. The accelerated corrosion was significant at 48 h, which was 96 h earlier than the corrosion potential. The reason for this result is that the steel bars with a 7 cm protective layer that were immersed in water generated corrosion faster than those with other protective layers. The corrosion potential, however, cannot reveal whether the steel bar is uniformly corroded. Through the reflected voltage of the electromagnetic wave propagated by the ground penetrating radar, we can find the uneven corrosion caused by the accelerated corrosion.

D.

Corrosion potential analysis for 9 cm protective layer

Based on the ASTM C876 reference standard for the corrosion potential of steel bars, we obtained the critical levels of the corrosion potential of the 9 cm protective layer for slight, moderate, and serious corrosion during the accelerated corrosion periods of 192 h and 312 h, as shown in Figure 20. The test result of the reflected voltage of the ground penetrating radar in the early stage was the same as that for the 4 cm and 6 cm protective layers. The accelerated corrosion was significant at 72 h, which was 120 h earlier than the corrosion potential. The test result indicated that the reflected voltage of the electromagnetic wave of the ground penetrating radar can precisely detect the corroded steel bars on the 4 cm, 6 cm, and 9 cm protective layers.

The protective layer is 4, 6, 7, and 9 cm. By means of the reflected voltage of the GPR electromagnetic wave, the corrosion acceleration products on the surface of the steel bar of the reinforced concrete structure are measured. Through analog signal coding and formulas 9, 10, 11, and 12, the reflected voltage of the various steel corrosion conditions is calculated and re-calculated. In different protective layers and different current-passing times, the greater the reflected voltage, the more serious the steel corrosion.

3.8. Establishment of a Reference Standard for Determining the Degree of Corrosion of the Reflected Voltage of Electromagnetic Waves

The image generated by the reflection signal of the electromagnetic wave was digitalized, and the changes of the reflected voltage were calculated by the relative dielectric constant, the reflection coefficient, and the reflection coefficient of reverse power. Then, the correlation between the degree of corrosion and the corrosion potential of the corroded steel bars was established. We explored the changes of reflected voltage by the ground penetrating radar detected in different protective layers and compared them with the STM C876 reference standard. The details are shown in

Table 2. Comparison of the Degree of Corrosion by Potential and Voltage.

Corrosion Status	Electrochemical Corrosion Potential	Electromagnetic Wave Reflected Voltage
Degree of corrosion: less than 10% (slight corrosion)	>−200 mV	4 cm	1~64 mV
		6 cm	23~69 mV
		7 cm	8~87 mV (Soaked/severe corrosion)
		9 cm	0~12 mV
Degree of corrosion: between 10% and 90% (moderate corrosion)	−200~−350 mV	4 cm	64~107 mV
		6 cm	69~118 mV
		7 cm	87~169 mV (Soaked/severe corrosion)
		9 cm	12~71 mV
Degree of corrosion: greater than 90% (severe corrosion	<−350 mV	4 cm	>107 mV
		6 cm	>118 mV
		7 cm	>169 mV (Soaked/severe corrosion)
		9 cm	>71 mV

.

In this study, the corrosion degree of the reflected voltage value of the electromagnetic wave of 4~9cm is defined by the corrosion potential ASTM C876, which is, for example, mild corrosion before 192 h, moderate corrosion between 192 and 312 h, and severe corrosion after 312 h, corresponding to the reflected voltage values measured by the transmitter radar at present, as shown in Table 2.

4. Case Study-Accelerated Corrosion Test of Bars in Concrete Cover with Different Thicknesses

4.1. Corrosion Detection of Bars in Reinforced Concrete Columns

Four corroded steel bars in a building in northern Taiwan (on 2F, 4F, and 8F) were detected by the ground penetrating radar, and the schematic diagram is shown in Figure 21. Based on the analysis of the reflected voltage of the ground penetrating radar, the distribution of the corroded steel bars was obtained by the reflected voltage of the electromagnetic wave and compared to ASTM C876, as shown in Figure 22.

4.2. Detection of Corroded Bars in Reinforced Concrete Floor

We compared and discussed the corrosion potential and the reflected voltage of the corroded bars in the reinforced concrete, which was scanned by the ground penetrating radar at the frequency of 1 GHz. Based on the analysis of the reflected voltage of the ground penetrating radar, the average corrosion potential on the corroded plate on the 3rd floor was −280 m V (Area C). Based on ASTM C876, the steel bars in the concrete had moderate corrosion. The reflected voltage analysis of the electromagnetic waves concluded that areas A and B had moderate corrosion, which is consistent with the results from ASTM C876, as shown in Figure 23.

5. Conclusions

In this study, we comprehensively developed the digitally encoded signals for the scanning electromagnetic waves of the ground penetrating radar and the electromagnetic wave transmission theory. First, the reflected voltage at the corrosion interface was applied to detect the corroded steel bars in concrete structures. Second, the result was compared with the corrosion potential of the corroded steel bars. Finally, the reflected voltage characteristics of electromagnetic waves between media with different dielectric constants were applied to identify the degree of corrosion of the steel bars. The results also showed that the ground penetrating radar method can directly detect the degree of corrosion of steel bars in concrete without the requirement of damaging the concrete surface and forming a pathway to the steel bars. The key findings of the study are as follows:

A.

The changes of the reflected voltage of the electromagnetic wave of the ground penetrating radar can indicate the changes of the corrosion interface of the concrete and steel reinforcement with different corrosion degrees and different thicknesses of the protective layer.

B.

The changes in the reflected voltage of the propagated electromagnetic wave to the corrosion of the steel bar from the ground penetrating radar have greater sensitivity than the detection of the electrochemical corrosion potential. The ground penetrating radar method can detect the degree of corrosion of steel bars in the early stage.

C.

The reference standard and reflected voltage analysis technology for determining the degree of corrosion by the reflected voltage of electromagnetic waves can effectively solve the limitation of destroying the concrete surface by electrochemical corrosion potential detection and can determine the degree of corrosion of steel bars more quickly and effectively.