Corrosion of steel reinforcement is often investigated in the context of the durability performance of the structure; however, corrosion can equally impact the structural performance. Steel corrosion may result in reduced flexural capacity (loss of rebar cross section), reduced bond, and limited ductility. In this article, we will review some of the most important effects of steel corrosion on structural performance of reinforced concrete elements.
What is Steel Corrosion?
Corrosion is a chemical process, in which refined metals such as steel revert back to their lower energy, more natural and stable state of ore (i.e. iron oxides). 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. The increase in steel volume increases stresses in concrete to an extent that it can crack, resulting in delamination in concrete bridge decks or parking garage slabs, and loss of concrete cover in beams, girders, and columns.
Structural Effects of Corrosion
1. Loss of Strength
Steel corrosion reduces the effective cross section of structural components. This reduced cross section will reduce the capacity of concrete elements, such as slabs, columns, and beams. Loss of section strength can be crucial in bridge decks, and parking garages slabs.
A major concern is that corrosion of steel components (bars, beams, strands) does not always have visual sign, which makes the inspection process very difficult. The collapse of Algo Centre Mall is a great example of losing capacity and integrity in structural elements that remained unnoticed during the visual inspections. The corrosion of steel elements covered by insulations and fireproofing materials is a major concern in many refineries, industrial plants, and pipelines.
Steel elements 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 of steel bars.
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.
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.
Weijian Yi et al (2010) studied the effect of corrosion on the fatigue behaviour of reinforced concrete beams. They concluded that “The corrosion of steel markedly affects the fatigue performance of RC beams. As the mass loss percentage of corroded steel reinforcement increases, the fatigue life of the beams decreased.”
3. Reduced Steel-Concrete Bond
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. Limited 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.
How to Evaluate Corrosion in Concrete?
While there is no way to prevent corrosion from happening, there are certain methods that have been developed to protect and maintain reinforced concrete elements. Using corrosion inhibitors, cathodic protection, and epoxy coatings has proven to be effective in managing the rate of corrosion. However, proper inspection and monitoring of RC elements is key in preserving their durability and structural performance over time.
1. Half-Cell Corrosion Potential Mapping
Half-Cell Corrosion Potential Mapping is an effective procedure in evaluating the likelihood of active corrosion in RC elements. The test is the most widely used procedure for the inspection of concrete bridge decks, and parking garage structures.
2. Surface Electrical Resistivity
Surface Electrical Resistivity is a cost-effective and rapid nondestructive test that can be used to evaluate the durability performance of concrete materials. The test can be used to assess the permeability (conductivity) of concrete against the ingress of aggressive agents such as chloride. Electrical resistivity test has been adapted by the AASHTO TP95, and several DOTs and MTO.
3. Ground Penetrating Radar
Ground Penetrating Radar (GPR) has proven to be an effective nondestructive test method for rapid screening of large concrete components, such as bridge decks, and parking garage slabs. The test can be to identify the locations that require further testing and inspection. While GPR cannot show corrosion directly, it can help experienced users with identifying areas with higher likelihood of corrosion.