Steel’s Weakness: Corrosion
Many complex chemical reactions occur inside concrete over its useful life. Perhaps the most important reactions are those that occur with the steel reinforcement. These reactions are known as a form of ‘concrete cancer’, which can be detrimental to a structure. Identifying early signs of corrosion are the best chance of maintaining healthy concrete.
The Concrete Revolution
Concrete (in one form or another) has been around since ancient times, but it wasn’t until the late 19th century that concrete was reinforced with steel. Concrete has low tensile capacity, but a high compressive strength. The combination of concrete and steel result in construction benefiting from the high tensile capacity of steel and the low cost of concrete. Steel is an amazing material, but does has one major weakness: rust. Oxidation of iron in steel produces rust, and rust can occupy a volume up to four times that of unaffected steel. Rusting of steel also reduces effective cross-sectional area of the reinforcement, reducing its capacity. If not managed, rusting reinforcement can lead to loss of tensile strength, concrete spalling (when concrete starts to fall away from a building) and other significant structural damage.
How is it Measured?
Three components are necessary for corrosion to occur: metal (in this case iron), oxygen, and an electrolyte (usually water). Metals almost always occur in their oxidised form, as this is what is most stable. The process of refining a mineral to a metal increases the material’s reactivity. Each metal has different relative chemical reactivities (electropotentials), which dictates how dissimilar metals will act when in electrical contact. If two metals are placed in an electrolyte, the more reactive metal will experience galvanic corrosion.
Severe corrosion (or rusting) can often be identified through a visual inspection. Rust-coloured staining may be present on the surface of concrete. As rusted steel occupies more space than non-rusted steel, rusted reinforcement can expand and crack concrete, sometimes causing concrete spalling. In some instances, spalling can be so severe that reinforcement is exposed. If concrete is at this stage, it has likely already lost significant strength. Corrosion engineers can use electrochemistry to infer information about the state of reinforcement in a non-destructive manner.
Using Half-Cell Potential
As steel rusts, iron is oxidised to iron ions. This leaves free electrons in the steel. At the steel’s surface, these electrons are consumed by a cathodic reaction, where oxygen and water are reduced and hydroxyl ions are produced. An indicator of the degree to which this reaction is taking place is the half-cell potential. The greater this potential, the higher the probability is that corrosion is taking place. An electrochemical cell is moved along the concrete to measure half-cell potentials over an area, which is recorded in the corrosion meter. The areas of likely corrosion are found where the currents leave the rebar and enter the concrete (where potentials are most negative). This data is analysed, and a contour map of corrosion likelihood is generated.
What Should You Do?
The significance of corrosion should not be underestimated. It is estimated that approximately 5 % of an industrialised nation’s income is spent on corrosion prevention or related maintenance (Akovali 2005). Rusting steel can go undetected if visual inspection is solely relied upon. Rapid Design Engineering (Rapid) provides half-cell potential and resistivity testing to non-destructively determine the likelihood of corrosion and corrosion rate of steel reinforcement. If it is determined that rusting and corrosion is an issue, Rapid also provide designs for cathodic protection (CP) for our Clients, through galvanic protection or impressed current cathodic protection (ICCP) systems. These systems can halt rusting and corrosion in affected sites and stop new sites from forming.
If you have a project that you think will benefit from corrosion analysis, give Rapid a call and find out what we can do for you. Once testing has been completed, Rapid can design corrosion management for the structure. For more information, contact us today.
Akovali, R 2005. Polymers in Construction. Rapra Press, Shawbury.