Rebar Corrosion – Induced Damage Detection in Concrete through Ultrasonic Imaging

Eva Schelze, Winter Semester 2022/23

Corrosion on rebars poses a major risk to concrete structures. Cracks develop internally and are only visible from the outside when the material is already severely damaged. Therefore, the application of ultrasound for early detection is presented.

The Problem of Corrosion

Reinforcing bars (rebars) are important for the stability of concrete. They need to be reliable as the service life of bridges and other structures depend on it but are prone to corrosion.

Corrosion on rebars is critical for the surrounding concrete. With the chemical reaction, a layer of oxide is generated and settles on the metal. This leads to an increased diameter and by surpassing the concrete’s tensile strength, cracks occur in the surrounding material. These anomalies expand further until the surface is reached. ^[4]

Prestressed concrete structures are used under all kind of weather conditions. In addition, harmful substances can easily enter the cracks and thus penetrate deep into the material. Consequently, the destruction progresses rapidly, and the material strength deteriorates strongly.

The biggest problem with rebar corrosion is that the destruction goes unnoticed in an early stage. However, once it becomes visible, it progresses rapidly, and action must be taken quickly to prevent greater damage. To avoid this harmful situation, the non-destructive testing method of ultrasound imaging can be used. Currently, Gosh et al ^[1][2] have focused on this topic, but there is still a large research gap. Their experimental setup is shown in the following section.

Experimental Setup

A reinforced concrete specimen with a rebar at 45 mm depth is prepared as shown in Figure 1. A NaCl solution is placed on the surface above the rebar in an open-bottomed container. Then the positive side of a current source is connected to the rebar. A copper plate is placed in the liquid and connected to the negative side. In this way, anodic oxidation of the rebar and cathodic reduction of the oxygen in the liquid occur simultaneously. In alkaline environments, the rebars are usually protected by a thin layer of iron oxide. However, with the oncoming chloride, the alkalinity decreases and the rebars begin to rust. A detailed formula-based derivation can be found in this wiki article. Next, the sample is analyzed with the Pulse-Echo-Method and a C-Scan is derived by using the SAFT algorithm. ^[1][2]

Ultrasound imaging for corrosion detection

For the SAFT analysis, a grid is drawn on the sample as shown in Figure 2. The lines are spaced 10 mm apart in both directions and the two ultrasonic heads are moved along the grid points. They travel over the entire specimen and create a complete image. The transmitter emits a signal with a frequency of 250kHz and thus a compressible wave with an estimated wavelength of 15 mm is created in the material. This value is in the same order of magnitude as the rebar, which has an approximate diameter of 20 mm. Consequently, the wave is either reflected from the bottom of the material or from the rebar. The receiver captures this reflected signal and an oscillator records it with a sampling frequency of 5 MHz so that it can be digitized. ^[1]

By examining reinforced concrete with ultrasound, the rebars reflect the compressional waves and thus get visible as in figure 2a. Trough experiments, the steel was caused to rust and on the 8^th day, with an intermediate corrosion of the rebar, the ultrasound image shows a different picture. The rebar’s echo disappears partly and with ongoing rusting process, the visibility degrades more. Severe corrosion occurs only three days later as the inner cracks reach the surface. The rebar becomes completely invisible on the scan as depicted in figure 2c and thus, not detectable anymore with the compressional wave ultrasound technique. However, the rebar is still present and not completely rusted. The disappearance is caused partly directly by the corroded material which attenuates the signal. The compressional waves also scatter because of the resulted cracks as the difference in impedance between concrete and air is large. This monitoring technique is very sensitive and can be applied easily and fast. However, the examiner has usually no reference medium and therefore, a missing signal is not always helpful. Consequently, a complementary method can be used. ^[1][2]

Complementary use of Rayleigh waves

For further investigation Rayleigh waves were chosen instead of bulk waves. Figure 3 shows the experimental setup with a wedged transducer on the left. It is used to generate the surface waves. The optimal wedge angle$//<![CDATA[ \begin{array}{l}\alpha\end{array} //]]>$ is calculated with Snell’s law ^[2] where$//<![CDATA[ \begin{array}{l}C_{LW}\end{array} //]]>$ is the compressional wave velocity in the wedge material and $//<![CDATA[ \begin{array}{l}C_{R}\end{array} //]]>$ is the Rayleigh wave velocity in the concrete.

$//<![CDATA[ \begin{array}{l}\alpha = \sin^{-1}(\frac{C_{LW}}{C_R}})\end{array} //]]>$

The Rayleigh wave is emitted at a frequency of 100 kHz and propagates along the surface. This low frequency results in a large wavelength of 24.2 mm and the signal can consequently penetrate deeper into the material. In case of a crack near the surface, part of the wave is reflected and part is transmitted. The reflected wave is detected and thus records the interference pattern. ^[2][3]

In contrast to the first method, the fully functional rebar is not visible in the scan. As the steel is in intermediate corrosion state, the observation method shows cracks along the rebar. On the last day, the number of visible cracks on the scan increased massively which can be directly attributed to a high corrosion rate. The evaluated images are available in ^[2].

In summary, Rayleigh waves are surface waves and are therefore more suitable for detecting near-surface damage such as cracks. However, the structure inside the material as well as the rust itself cannot be visualized with this method. Traditional compressional waves, on the other hand, are bulk waves and propagate deeper in the material. They result in better visualization of functioning rebars but are scattered as cracks progress. Therefore, to detect corroded rebars, the ultrasonic method using Rayleigh waves can first detect cracks in the concrete sample. Once the position is known, compressional bulk waves indicate the exact corrosion rate. ^[2][3]

Conclusion

Finally, corrosion on rebars can be detected by the ultrasonic echo method. Compressional waves identify metal in good condition, but corrosion scatters the signal. Rayleigh waves detect corrosion introduced cracks near the surface, but not the rebar itself. Therefore, a combination of both methods is best for detecting corrosion.

Literature

1. Ghosh, D., Rahul, K., Ganguli, A., Mukherjee, A.: Ultrasonic Imaging as a Diagnostic Tool for Detection of Rebar Corrosion. 7th Asia-Pacific Workshop on Structural Health Monitoring (2018), p.1-9
2. Ghosh, D., Rahul, K., Ganguli, A., Mukherjee, A.: Nondestructive Evaluation of Rebar Corrosion–Induced Damage in Concrete through Ultrasonic Imaging. Journal of Materials in Civil Engineering (2020) 32:10, p.1-13
3. Ghosh, D., Beniwal, S., Ganguli, A., Mukherjee, A.: Reference free imaging of subsurface cracks in concrete using Rayleigh waves. Structural Control and Health Monitoring (2018) 25:10, p.1-16
4. Andrade, C., Alonso, C., Molina, F.J.: Cover cracking as a function of bar corrosion: Part i-Experimental test. Materials and Structures (1993) 26:8, p.453-464

All pictures taken from ^[1] are licensed under Creative Commons CC-BY-NC license https://creativecommons.org/licenses/by/4.0/