Infrastructure and Corrosion – Part 3 – An Overview of Control Methods

Posted on May 1, 2014

There are several approaches to controlling corrosion of rebar in concrete and the associated damage. This is a summary of some of the more important techniques that are used. NOTE: Much of this brief overview was obtained from information in the valuable reference (*) cited below.

Control of infrastructure damage due to rebar corrosion can be logically divided into three categories:  Modifying the concrete, the rebar itself or using cathodic protection (CP).

Finished, intact concrete can be made more resistant to diffusion of damaging ions and oxygen (and thus more resistant to rebar corrosion) if the composition of the initial, mix includes a lower value of the ratio of water- to-cement, i.e., w/c. Transport of ions and oxygen occur by capillary action through pores in the harden material. A lower initial w/c ratio produces desirable, smaller pores so that permeation is retarded. However, a w/c ratio that is too low makes the concrete less fluid and more difficult to place correctly. An alternative to the problem is to add what are called superplasticizer compounds to the wet mix. These additives permit a lower w/c ratio in the concrete while maintaining sufficient fluidity. This discussion of the effects of w/c ratio assumes no cracks in the finished concrete. If present, cracks in finished concrete allow much more rapid transport of ions and oxygen to the rebar compared to the rate via pores.

Nonmetallic fibers or microfibers may be added to a concrete mix to reduce potential cracking especially for very low w/c ratio mixes that may be more susceptible to cracking. Chemical corrosion inhibitor compounds such as calcium nitrite may be added to the water used for the concrete mix to minimize rebar corrosion. Increasing the natural pH of finished concrete by using high alkalinity cement or adding sodium or potassium hydroxide to the pore water used for the mix are beneficial actions. Raising the concrete’s pH increases the minimum concentration of chlorides required to breakdown the passivity on bare rebar. This minimum ion concentration is known as the chloride threshold for the initiation of corrosion.

The type of rebar used (or its coating) has a significant effect on the rate of corrosion and thus the service life of a reinforced structure. Some of the common options include bare carbon steel; carbon steel with an epoxy coating (ECR); galvanized carbon steel or one of the stainless steel alloys. There are advantages and disadvantages of each option. Making selections based on life-cycle cost analyses – especially important for desired long-life infrastructure applications – are desirable but often are not followed. Initial cost comparisons frequently override life-cycle cost results. Bare steel has the least initial cost but usually the shortest service life (without the use of CP). ECR is very resistant to corrosion and is widely used. When the epoxy coating is fully intact the ECR is fine BUT, even though special handling instructions are usually in force, it is subject to localized epoxy damage (and accelerated corrosion) due to mishandling during transport or at the construction site. Galvanized steel may be a good option because the coating combines a barrier and a cathodic protection effect – the zinc is anodic to and protects the steel substrate. Solid stainless steels have the highest first cost of these options but offer the best, long-term resistance to corrosion especially in higher chloride ion environments such as a marine application. Stainless steel cladding on carbon steel at less initial cost has been less successful than solid stainless.

Cathodic protection (CP) is a valuable electrochemical technology for corrosion control. There are many factors important to its effectiveness. Typically it is used in combination with bare steel. The several considerations in using CP are beyond the scope of this overview. These are covered in some detail in the (*) cited reference.

Very briefly – CP functions by supplying current at a specific potential, i.e., electrical voltage, to the rebar so that its surface condition is maintained (or reestablished) in a passive, corrosion-resistant state. CP has been widely used to protect metals from corrosion in soils and in full or partial liquid immersion applications for approximately 70 years but the theory of its use has been known for about twice as long. However CP for rebar in concrete has seen widespread use for only about twenty-five years. In addition successful application of CP for reinforced concrete differs in several ways from its traditional use in soils or for full submersion. Specialized knowledge is required to correctly design and then monitor and maintain a given CP system in each concrete application. Perhaps the most significant reason for unsuccessful use of CP for rebar protection is lack of consistent and competent monitoring of properly designed and installed systems. This method cannot be a one-time, “install and forget” approach to corrosion control.

(*) “Corrosion in Bridges and Highways”, an article in the ASM Handbook, Volume 13C, Corrosion: Environments and Industries, pages 559-597, ASM International, 2006.           

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