Detecting corrosion of steel in reinforced concrete structures with ultrasonic guided waves

Most of our buildings are made of reinforced concrete. It is important to monitor the health of these structures periodically.

Yet, every now and then, we hear of building collapses where people lose lives. Many such tragedies are avoidable, and occur because warning signs have not been heeded.

This is what got Rajeshwara Chary, a research scholar with the IITB-Monash Research Academy interested in a project titled, ‘Integrity evaluation of reinforced concrete structures using ultrasonic guided waves’.

“Reinforced concrete is widely used as a building material due to its low cost and flexibility during casting,” explains Rajeshwara. “The steel inside the concrete provides structural rigidity just like our bones. However, this steel is prone to corrosion due to various environmental or structural conditions. Corrosion of reinforcement steel is one of the most studied yet unsolved problems in the field of structural health monitoring.”

The signs of advanced corrosion, as shown in the image, are cracks and bleeding of rust on the surface of the concrete. These are irreversible. It is therefore vital to look for early warning signs. The focus of Rajeshwara’s research is to detect the corrosion at its incipient stage using ultrasonic waves generated by embedded piezoelectric transducers.

“Concrete is a natural protector to rebars as it forms a passive layer and inhibits corrosion,” says Rajeshwara. “But excess exposure to the acidic environment may de-passivate this layer which could lead to rust formation. During this process the effective diameter of the rebar gets reduced. The corrosion product formed during this process is bulkier. It may sometimes be larger than three times the volume of the original steel. Due to this the concrete cracks and the scales formed get isolated from the intact rebar. This reduces the strength of the structural member.”

“The earlier methods to detect corrosion are point-based,” he continues, “which essentially means they rely on the sensor measurements taken at discrete points. A vast number of measurements required and even these cannot estimate the corrosion at inaccessible areas such as joints.

Put simply, the previous approach was to detect the problem when the sign of distress became apparent, rather than conducting periodic tests to monitor the condition of the structure on a more frequent basis.

Rajeshwara explains the benefits of his approach. “Guided waves travel long distances and provide a signature of the damage that is encountered in their propagation path; thus, even inaccessible locations can be monitored. Very few sensors and measurements are needed to evaluate the structural health using ultrasonic guided waves. What’s more, the sensors need to be installed only once and can be used to monitor the structure continuously. The damage can be identified very early and necessary repairs can be undertaken at a relatively low cost.”

The IITB-Monash Research Academy is a collaboration between India and Australia that endeavours to strengthen scientific relationships between the two countries. Graduate research scholars study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience.

Says Prof Murali Sastry, CEO of the Academy, “Corrosion of steel in concrete is like cancer in the human body, the earlier it is detected the easier it is to save the structure. The sensors that researchers like Rajeshwara are working on have the capability of turning inert concrete into a ‘device’ that can identify its own bugs!”