Reinforcing Steel Corrosion

Martin Tullmin

Tullmin Consulting Kingston,

Ontario, Canada

Concrete is the most widely utilized engineering material, on a tonnage basis (1). The widespread use can be attributed to the inherent durability and cost-effectiveness of concrete. Such durability is amply demonstrated by existing historical structures such as the Pantheon and Colosseum dating back to ancient Rome (2). Steel reinforced concrete represents a significant part of the modern infrastructure, with the steel reinforcement incorporated into concrete to enhance the tensile strength of this composite material. Corrosion damage to the reinforcing steel has been identified as the main mechanism of degradation of steel-reinforced concrete. Further comprehensive information on the extent of this problem and associated costs may be found at the following links: www.tfhrc.gov/structur/corros/corros.htm and www.corrosioncost.com.

The complexity of concrete on a microstructural scale and the number of variables controlling its properties are easily underestimated. Not only is the microstructure (and the resulting properties) influenced during production but it also tends to undergo changes afterwards in service. For example, permeability, a fundamental property relevant to reinforcing steel corrosion, has to be managed through the concrete mix design and during production. It is also subject to changes throughout the service life. Reinforcing steel corrosion problems resulting from the failure to duly distinguish between "new" and "old" concrete in the approach to repair operations have indeed been documented. Further details on the nature of concrete are available at: www.portcement.org.

Considering concrete's microstructural complexity, coupled to time-dependence , it should not be surprising that there are many manufacturing and in-service variables affecting the durability of steel reinforced concrete (some would say too many !). Some industry insiders have been known to comment, jokingly, that the actions of the concrete mix truck driver could be a major variable. The unsatisfactory approach of simply "equating" durability of reinforced concrete to the strength of concrete has been highlighted by Mehta and Burrows (3).

"To build environmentally sustainable concrete structures, it is clear that instead of strength, the 21st century concrete practicemust be driven by considerations of durability."
P.K. Mehta and R.W. Burrows: "Building Durable Structures in the 21st Century", Concrete International, March 2001, pp.57-63. 

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