James M. Gaidis and Arnold M. Rosenberg, Concrete Corrosion Inhibitors Association
Although corrosion inhibitors are considered to be relatively new materials, some inhibitors have been employed successfully for over 20 years. This article briefly explains corrosion mechanisms and how corrosion inhibitors are used to extend the life of concrete bridges.
Corrosion Mechanism
When chloride ions from deicing salts or marine environments enter concrete from the surface, they can diffuse through the concrete to the steel reinforcement. The rate of migration depends on the quality of the concrete and can be reduced through the use of high performance concrete (HPC). If the chloride salt reaches the steel reinforcement, a soluble iron complex is formed and the original protective passive iron oxide layer at the reinforcement is destroyed. This soluble complex carries iron away from the reinforcement into the concrete where it oxidizes further producing a larger volume than the original iron. This causes the concrete to crack and spall. The amount of chloride necessary to initiate corrosion is generally considered to be in the range of 0.03 to 0.07 percent by mass of concrete.
Corrosion Inhibitors
Corrosion inhibitors are various admixtures consisting of chemicals chosen to interfere with the corrosion process without affecting concrete quality. In general, the effect of an inhibitor is as follows:
- To raise the chloride threshold at which corrosion starts,
- To slow the rate of corrosion after it begins.
Corrosion of steel in concrete is an electrochemical process that requires an anode and a cathode. Corrosion inhibitors may operate on the anode, on the cathode, or on both. The chief anodic inhibitor in commercial use today is calcium nitrite. It acts by oxidizing ferrous ions so quickly to ferric ions that they precipitate on the steel and stifle corrosion. Salt cannot attack the ferric oxide coating.
Cathodic inhibitors are usually based on amine chemistry, and adsorb tightly to the iron oxide film, interfering with the corrosion process. Mixed inhibitors operate at both the anode and cathode. Mineral or chemical admixtures that decrease the permeability of concrete against chloride or oxygen penetration also help reduce corrosion. Some of the newer inhibitors also decrease the permeability of concrete to chloride penetration. When corrosion inhibitors are used, the threshold level for initiation of corrosion is increased to three to nine times the threshold level without an inhibitor, thereby extending the time before corrosion begins.
Mix Proportions
Corrosion inhibitors are supplied as liquids and are added separately to the concrete mix in the same manner as other chemical admixtures. The quantity of corrosion inhibitor is different for each brand but generally ranges from 1 to 6 gal/cu yd (5 to 30 L/cu m). For each product, the level of corrosion protection increases with the dosage. With some corrosion inhibitors, it is necessary to adjust the quantity of mix water to compensate for water in the inhibitor. The manufacturer should be consulted to determine if adjustments are necessary.
Corrosion inhibitors are compatible with all cements and other admixtures from the same manufacturer. Since the use of a corrosion inhibitor may influence the effectiveness of other admixtures, trial mixes should be made prior to construction to verify that the proposed mix will conform with project requirements. Since some inhibitors may accelerate concrete setting times, it is important to check for slump retention and adequate setting time. Some inhibitors may require more or less airentraining admixture, so it is wise to check with the manufacturer.
Concrete Properties
Finishing and curing concrete containing a corrosion inhibitor is usually the same as that for conventional concrete. In some cases, corrosion inhibitors significantly increase the strength at 28 days, others may slightly reduce the strength. The manufacturer’s literature should be consulted prior to using an inhibitor to ensure the optimum concrete mix.
Service Life and Life Cycle Cost
The benefits of using a corrosion inhibitor can be evaluated using software known as Life-365. Life-365 is a model that computes the chloride threshold value for a given addition rate of inhibitor. Using Fick’s law and the mix proportions, Life 365 predicts the time before onset of corrosion; this is the initiation period. Corrosion then takes place for a time called the propagation period, which is taken as six years, but can be changed by the user. The initiation period plus the propagation period is the service life or the time to first repair.
The model estimates costs over the whole life cycle of the project. Initial construction costs include costs of the concrete, corrosion inhibitor, steel reinforcement, and any surface protection such as a membrane or sealer. For one scenario, with a design life of 75 years, initial construction cost for the reference concrete was $3.05/sq ft ($32.84/sq m) of deck. The initial cost for HPC with an inhibitor at a dosage rate of 4 gal/cu yd (20 L/cu m) was $3.52/sq ft ($37.84/sq m). For about 18 years, the reference concrete is least expensive, after which costly repairs are needed. After 50 years without repairs, the total cost of the concrete deck with the corrosion inhibitor is only half the cost of the deck with the reference concrete.
The FHWA has long recommended a multi-pronged strategy in which several protection strategies are employed to reduce corrosion. Water reduction, mineral admixtures, adequate cover, and a corrosion inhibitor are all part of a total plan—together, these modifications add up to high performance concrete.
Further Information
Further information on the use of corrosion inhibitors in high performance concrete can be obtained from the CCIA at email: [email protected] or web site: www.corrosioninhibitors.org.
Editor’s Note
This article is the sixth in a series that addresses the benefits of specific materials used in HPC. The benefits of silica fume, lightweight aggregate, different cements, slag cement, and fly ash were discussed in previous issues of HPC Bridge Views.