Understanding the Process of Corrosion in Concrete
What is Concrete Corrosion and why is it a Problem?
According to Ueli Angst, a professor at the Institute for Building Materials, corrosion is responsible for up to 90 percent of damage to reinforced concrete structures. Corrosion is defined as the “destruction of metal by chemical, electrochemical, and electrolytic reaction within its environment.” Corrosion forms in reinforced concrete as it ages.
Corrosion is a natural process that occurs when rebar rusts. Since rust has a larger volume than steel, the rust exerts internal pressure on the concrete causing it to form cracks and damage surrounding concrete. Concrete corrosion is initiated when materials that are harmful to steel such as CO2 and chloride from de-icing salt start to penetrate concrete and reach the steel reinforcement. Recently, scientists have discovered that previous concrete samples were too small and therefore not able to deliver adequate results when testing for concrete corrosion.
New NDT methods and technologies such as the iCOR™ can help deliver more accurate results as well as cut down on costs associated with other corrosion detection methods such as linear polarization resistance measurement (LPR) and the galvanostatic pulse technique.
The Cost of Concrete Corrosion
Reinforced steel structures must be tested regularly to detect and prevent corrosion, which is a costly process, especially when taking into account the number of bridges, tunnels, and buildings built using reinforced concrete between the 1950s and 1970s. As a structure gets older, the risk of corrosion in the reinforcing steel gets higher and requires more frequent testing as well as repairs to eliminate damage and slow down the corrosion process.
Extraction of concrete samples is a key process in examining the condition of reinforced concrete structures. The typical sample size taken from concrete structures for laboratory testing are of about 5 to 20 centimeters. Recent studies have shown that although these sample sizes are ideal for handling in the laboratory, they often show higher concentrations of corrosive chloride than larger sample specimens and may provide inaccurate data. According to Angst, only larger specimen of a meter long can present an accurate assessment of the condition of the reinforced concrete. These larger samples are much less practical to work with, which adds difficulty to proper testing and increases the costs associated with testing. Not to mention the level of destruction to the body of concrete.
Since concrete is the most-used manufactured material worldwide, the industrialized world could be facing billions of dollars in testing and repair. In Switzerland alone the annual cost of repairs could amount to between $6.6 billion and $26.3 billion CAD. When taking this information into consideration, it is extremely important to accurately assess the condition of reinforced concrete structures, whether or not repairs are immediately required.
Scientists believe that the use of connection-less corrosion rate measurement and new technology can help decrease the costs of necessary testing and help gather more precise data. They also suggest that switching to expensive high-alloy steel is the only way to prevent corrosion damage entirely. Although high-alloy steel costs nearly ten times more than traditional reinforcing steel and would increase initial production costs of a project, it would reduce the costs associated with regular inspection and repairs in the long run, making it a cheaper and more durable alternative.
New Corrosion Detection Technology
Although high-alloy steel is a great way to prevent corrosion, normal reinforcing steel has been the norm and is present in the majority of reinforced concrete today. As buildings age and corrosion causes damage to concrete, engineers are looking for more effective ways to test for corrosion and cut costs associated with testing.
Among newer systems, the iCOR ™ mentioned above measures the electrical response of the rebar inside the concrete. One advantage to this testing method is that it does not require a connection to the reinforcement, which places it among the most convenient corrosion rate measurement devices in the field and offers an innovative research tool for laboratory studies. It allows engineers to have a very comprehensive understanding of the quality and corrosion of the concrete and can provide them with information necessary to have decisions regarding rehabilitation and repair of concrete structures.
In 2017 alone, two new systems have been introduced, one of which is mounted on a small skid-steer robot, the other of which is mounted in a cart that can be towed along a roadway. Both of these corrosion detection systems make use of machine learning technology and don’t require any sort of destructive intervention to gather results. The robot-mounted system utilizes ground penetrating radar and electrical resistivity sensors to locate any corrosion of steel or deteriorating concrete in bridges and structures. It is also fully autonomous and has proved to be faster and more accurate than human inspectors.
Jinying Zhu, assistant professor of civil engineering from the university of Nebraska-Lincoln, has designed a system to detect defects in concrete bridge decks. Her approach is an early-warning system for bridges based on acoustics. It has proven to be a more accurate alternative to other methods of identifying delamination, a gradual separation of concrete layers that can affect the structural integrity of a bridge or structure and can be caused by rebar corrosion. Her system also delivers much faster results than conventional testing methods allowing people to find delamination in a timelier manner and make the necessary repairs before the damage becomes too significant.
As technology advances and new methods are developed, methods for testing corrosion rate in concrete become more advanced, efficient, and cost effective.