A research team at the Indian Institute of Technology Bombay (IIT Bombay) (Mumbai, India) recently developed a new surface probe to measure the extent of corrosion in concrete rebar, which they say is more sensitive and accurate than conventional measurement techniques.
Concrete structures such as bridges, buildings, and roads use embedded steel reinforcement bars—called rebars—to increase structural strength and durability. Despite using preventive measures such as anticorrosive coatings, these rebars are prone to rust, which can lead to corrosion. Contributing factors include water seepage, exposure to chemicals, and incorrect salinity and acidity of concrete.
Current Methodology Limitations
Timely corrosion detection can warn about structural degradation and significantly lower repair and maintenance costs. However, the researchers contend that many of today’s commonly used methods to examine corrosion do not accurately estimate the extent of corrosion. As such, these methods often struggle to estimate the structure’s remaining service life, according to the researchers.
As a result, Siddharth Tallur and Sauvik Banerjee—professors at IIT Bombay—worked to develop a probe to measure the extent of rebar corrosion. According to the research team, the probe can be placed on the surface of a concrete structure to make the measurement, with probe and sensor design improvements and novel signal processing techniques that are making it more sensitive and accurate than many conventional techniques.
The research was partially funded by an Indian government arm and structural engineering firm Sanrachana Structural Strengthening Pvt. Ltd. (Thane, India).
One of the currently used methods, the half-cell potential measurement, involves drilling a hole in the structure to expose a portion of the rebar. This portion is connected to the measuring instrument. The method uses the voltage difference created due to different electrochemical properties of corroded and non-corroded regions.
In addition to being an invasive testing method, this technique only provides a probabilistic estimate of corrosion rate, rather than the true extent of corrosion.
According to the researchers, nondestructive testing (NDT) methods that do not disturb the structure use changes in the material’s electrical, magnetic, or acoustic properties to detect corrosion. An electrical resistance probe uses a sensor embedded in concrete during casting to measure the electric current through the rebar. Corrosion results in a change in the probe’s resistance, which is detected as a change in the electrical signal. However, this technique has low sensitivity, the researchers explain.
Another popular NDT method is acoustic emission sensing, in which a sensor “hears” sound waves formed by stress created in the concrete. This stress often surrounds the rebar when cracks appear in the concrete due to corrosion. However, the researchers say this method cannot directly estimate the extent of corrosion and is not sensitive to early-stage corrosion that occurs prior to crack formation.
Pulsed Eddy Current Measurements
In response to those limitations, IIT Bombay’s current study uses the pulsed eddy current measurement method to directly measure the extent of corrosion. In this approach, a current pulse passed through a coil creates a magnetic field. When such a coil is placed near a metallic sample, this field induces eddy currents in the sample. The eddy currents generate a secondary magnetic field that opposes the original magnetic field produced by the coil.
A magnetic field sensor inside the probe detects the secondary magnetic field and produces a voltage in response to the magnetic field strength. Since corrosion creates an insulating rust layer on the surface of the conductive sample, the eddy current strength reduces. The sensor detects this, seeing it as a signature of reduced secondary magnetic field strength.
The pulsed eddy current method is used commercially to inspect oil, gas, and water pipelines. In these applications, a probe can be placed very close to or directly on the surface of the pipe. However, the steel rebars are buried deep inside concrete, making it challenging to use this method on rebar. Moreover, rebars are thinner than pipes and offer much less surface area. As a result, they require high-sensitivity and expensive probes.
However, the researchers at IIT Bombay say they have designed a high-performance probe using inexpensive electronic components, making it a lower-cost solution. In their work, the researchers improved the probe’s sensitivity by using a highly sensitive magnetic field sensor, which is known as an anisotropic magnetoresistive (AMR) sensor.
They have invented a novel signal processing method that uses a known mathematical technique called principal component analysis to compute features that can be used to calculate the extent of corrosion from the AMR sensor output signal. The team has filed an Indian patent application for this invention.
The researchers tested their probe on rebars corroded artificially in the lab, since natural corrosion takes years to develop. They observed that the probe could detect rebar corrosion at up to 5.5 cm from the probe, making it among the highest depth reported for any NDT methods to detect corrosion.
“This is particularly impressive, considering that the instrument we developed uses a low-amplitude current pulse and compact probe design,” Tallur says. “Therefore this work is a key breakthrough for developing handheld inspection tools based on pulsed eddy current technique.”
Ready for Field Deployment
Tallur suggests that such laboratory-based test samples faithfully mimic the type of specimens that would be encountered in the field after undergoing years of corrosion. Thus, the researchers believe their technology is ready for field deployment in its present form. The researchers are currently working on handheld packaging for the probe to undertake pilot field studies.
According to the team, manufacturers, building owners, service companies, pipeline and storage tank owners, maintenance professionals, and contractors can potentially use this device to directly measure the extent of corrosion of their infrastructure. Based on the type of infrastructure and severity of corrosion, they can then take possible remedial actions. Those may include retrofitting and repairs, or identifying weak spots requiring closer monitoring and inspection to determine necessary repairs.
The team says it is currently creating a handheld scanner product based on this innovation that operators could use for structural inspection or safety audits—all with minimal training. They have developed algorithms that can be implemented in a smartphone app connected to the instrument, which can seamlessly provide information to users on the rebar’s health.
“Our goal is to create a product that is so easy to use that anyone who knows how to use a smartphone should be able to use it without having to consult any user manual,” Tallur concludes.
Source: IIT Bombay, www.iitb.ac.in.