Understanding the Process of Corrosion in Concrete
Corrosion is a natural process that occurs when the steel rebar within reinforced concrete structures rusts. In scientific terms, concrete corrosion is defined as the “destruction of metal by chemical, electrochemical, and electrolytic reactions within its environment.” It typically forms as the concrete ages.
Why Is Concrete Corrosion a Problem?
Since rust occupies a larger volume than steel, it exerts internal pressure on the concrete causing it to crack 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 discovered that samples collected from traditional concrete testing methods were too small and therefore not able to deliver adequate results when testing for concrete corrosion.
Corrosion is responsible for up to 90% of damage to reinforced concrete structures.
-Ueli Angst, Professor, Institute for Building Materials
The Cost of Concrete Corrosion
Reinforced concrete structures must be tested regularly to detect and prevent corrosion. However, this is a costly process, especially considering 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 the structure 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 the condition assessment of reinforced concrete structures. The typical sample size taken from concrete structures for laboratory testing are 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 specimens and may provide inaccurate results.
According to Angst, only larger specimens, of about one meter in length, present an accurate assessment of the condition of the reinforced concrete. However, these larger samples are much less practical to work with, which makes proper testing more difficult and more expensive. Not to mention the level of destruction to the body of concrete.
Since concrete is the most widely used manufactured material globally, the industrialized world faces billions of dollars in concrete testing and repair costs. In Switzerland alone, the annual cost of repairs can amount to between 6.6-26.3 billion CAD. When taking this information into consideration, it is extremely important to accurately assess the condition of reinforced concrete structures in order to decide if repairs are required immediately.
Scientists believe that wireless corrosion rate measurement and other recent IoT technologies can help decrease testing costs and 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 today's reinforced concrete structures. As buildings age and concrete corrodes, engineers keep searching for cheaper, more effective ways to test for corrosion.
New non-destructive testing methods and technologies such as Giatec's 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.
Among newer systems, the iCOR® (mentioned above) measures the electrical response of the rebar inside the concrete without a physical connection to the rebar. For this reason, it is among the most convenient corrosion rate measurement devices in the field and offers an innovative research tool for laboratory studies as well. It gives engineers a comprehensive understanding of concrete quality and the level of corrosion; ultimately, it allows them to make faster, more informed decisions when it comes to rehabilitation and repair.
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 at 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 more and more NDT methods are developed, concrete testing will become more advanced, efficient, and cost effective.
Photo credit: University of Nebraska-Lincoln College of Engineering