Concrete Corrosion Inhibitors

The Bond between Steel and Concrete is Important to the Development of Strength.

Corrosion under a Coating of Paint Produces Acid, which further Dissolves Iron.

The Corrosion Triangle

Much like the fire triangle, the corrosion triangle depends on three ingredients: moisture, salt, and air. If any of these three is completely absent, corrosion is unlikely to gain a foothold.

It is almost impossible to exclude air and moisture. The salt is where we might be able to exert some control. Salt is frequently encountered in cold climates and near the seacoast. However, it is present in the parts per million range in the cement itself and in the mixing water. Normally, the reserve alkalinity of concrete is enough to inhibit corrosion from these small amounts of salt. But we get much larger amounts of salt if we have exposure to seawater, deicing salts or some salt containing aggregates. When the amount of salt at the level of the rebar reaches about 1 pound per cubic yard of concrete, not even the high pH (alkalinity) of concrete is adequate to inhibit corrosion.

One of the best ways to resist the catalytic action of salt is to use a corrosion inhibitor. The inhibitor counteracts the action of the salt by causing the iron to form a protective oxide layer on the steel itself. The transport of soluble iron away from the rebars is stopped. This action prevents the spread of corrosion and actually closes over the corrosion site. Addition of a corrosion inhibitor can raise the threshhold for corrosion to 5 or even 10 pounds per cubic yard. At the same time, the compressive strength may be increased. The bond strength between the steel and the concrete is not lost.

Slow Down Salt Ingress To Slow Down Corrosion

Another way to hold the line on corrosion is to reduce the permeability of the concrete. Air (oxygen) and moisture will still be available, but since salt is almost exclusively from external sources, reducing the permeability of the concrete will reduce the amount of salt reaching the rebars and extend the time until the threshold is reached. This also increases the life of the concrete structure.

The most effective ways to reduce permeability are to: 1) reduce water by use of admixtures such as superplasticizers, and 2) fill pore spaces with mineral admixtures like fly ash and slag.

Symposium

The Seventh International Symposium on Utilization of High Strength/High Performance Concrete will be held in Washington, DC, June 20-24, 2005. Engineers and materials scientists from around the world will gather to discuss topics ranging from the latest applications to the most recent research on high strength concrete.

For further information on this conference, contact William Tolley at ACI, (248) 848-3700 or by e-mail bill.tolley@concrete.org

CCIA Assistance Available

The members of the Concrete Corrosion Inhibitors (CCIA) are Axim Concrete Technologies, Grace Construction Products and Master Builders. These major suppliers produce corrosion inhibitors that have been tested and sold for use in severe conditions for many years. For information, see our web page www.corrosioninhibitors.org or contact us at 301/340-7368.