You may think that corrosion in structures is a just a durability-related issue in the first glance. Well, that is far from the truth. In fact, corrosion can directly affect mechanical and structural properties of components. Corrosion can affect the global response of the structure. In this article, we will review some of the main structural effects of corrosion on steel structures and reinforced concrete structures.
Before we begin, let’s review the basics in corrosion science. Corrosion is a chemical process, in which refined metals – steel in our case – revert back to their lower energy, more natural and stable state of ore (iron oxides in our case). The phenomena is scientifically explained with the Law of Entropy. The reaction happens with losing steel material and producing red rust, which is generally 4 to 7 times larger in volume.
Structural Effects of Corrosion
1. Loss of Strength
Corrosion reduces the effective cross section of structural components. This will reduce the axial, and flexural strength of elements, and makes them structurally weak. Even if corroded elements look stable, it does not mean they are safe; in fact, the corroded structures become vulnerable for design loads (ultimate loads), i.e. a strong ground motion can increase the stress actions beyond the capacity of the sections. Loss of strength can happen in steel and reinforced concrete structures. Corrosion under insulation -CUI- is a frequent observation in refineries, and oil and gas industry. Steel sections covered under fireproofing insulation experience corrosion over their service life. Another famous example is the reduced flexural, and shear capacity of the RC element. Du et al (2005) developed a mathematical model for the residual area and strength parameters (such as yield strength). This formula described the residual area based on the corrosion rate.
Another structural effect of corrosion is on the fatigue strength of steel elements, connections, and RC elements. Corrosion may accelerate fatigue crack propagation in structural steels. Development of pitting corrosion introduces additional points of stress concentration at which cracking may develop, which will reduce the fatigue strength. Apostolopoulos (2006) studied the effect of corrosion on high and low cycle fatigue of reinforcing steel.
3. Reduced Bond Strength
The capacity of composite elements such as RC elements depends on the characteristics of concrete-rebar interface. When steel corrodes, the products of corrosion expand. This will leave a poor quality steel layer over the surface of the reinforcement. This layer has a poor bond with surrounding concrete; therefore, it will reduce the capacity of the section. In case of lap splices or anchorage, this may reduce the effective length of anchorage, and resulting in premature failure of sections. Stanish (1997) studied the effect of corrosion on the bond strength in RC elements.
4. Limitted Ductility
Corrosion can significantly reduce the ductility of RC sections. This is critical in seismic design and evaluation. Corroded sections have lower ductility, which means their plastic deformation is limited. This will affect the seismic response of the elements. Corrosion of reinforcement in the lap splices will affect the load transfer in the laps, preventing the to develop yield stress. Asri and Ou (2011) studied the seismic response of corroded bridge columns by nonlinear pushover analysis.
5. Reduced Shear Capacity
Corrosion can reduce the effective cross sectional area of transverse reinforcement in beams and columns, and reducing the shear capacity of the section. In concrete slabs, this can reduce the shear strength of the slab close of the columns, and increasing the chance of punching shear failure. In footings, the corrosion can result in shear failure of the footing, anchorage failure, or flexural yielding of steel reinforcement.