Infrastructure (in its multiple forms) may fail in different ways. However, the most common failure mechanism is likely the breakage of concrete that contains corroded steel, reinforcing bars (rebar). How does this occur? This blog post provides a brief summary of the process.

Reinforcement is generally necessary in all concrete structures because concrete alone is excellent for resisting compressive (pushing together) stresses but very poor in resisting tensile (pulling apart) stresses. Practical structures must resist both types of forces. Embedded rebar provides resistance to tensile stresses acting on the concrete and thus a practical construction material is created.  Some common applications for reinforced concrete include the decks of bridges, vertical pillars under various types of structures and concrete pipe.

Hardened concrete is semi-solid  with microscopic pores. In addition it is an alkaline material with a relatively high pH. The high pH is an environment favorable for embedded, bare steel rebar to resist corrosion. In alkaline conditions steel is normally passive, i.e., a protective oxide film forms on its surface, and essentially no corrosion will occur. However, when passive steel is exposed to certain chemical ions its passive film breaks down. Chloride ions that occur in deicing road salts or in most waters are the most aggressive of these ions. Water, as rain (or splashed fresh or seawater) and oxygen from the air slowly diffuse by capillary action through the pores in concrete to the rebar. Corrosion initiates when sufficient levels of chlorides plus oxygen and water reach the rebar. During the corrosion process solid iron on the steel surface is converted to hydrated iron oxides. This corrosion product is rust and it includes significant attached water. It has a volume that is 4 to 6 times larger than the original volume of iron from which it formed.

Wedging forces in concrete are created by the volume expansion of corroding rebar. Often this increases the number and sizes of pores. Concrete is brittle and as corrosion continues small cracks form and grow from around attacked rebar. This leads to more rapid diffusion of chloride ions; water and oxygen so that more corrosion occurs. The process is self-accelerating. Eventually growing sub-surface cracks reach the outer surface of the concrete and sections of concrete break away until significant amounts of rebar are exposed. This final result is often depicted in photographs of deteriorating infrastructure. Part 3 will survey some actions to combat this damage process.

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