Some of the most basic strategies for increasing durability and extending service life of reinforced concrete structures include:
- Providing a thicker concrete cover;
- Using a high density concrete mix; and
- Building with epoxy coated rebar.
Though these techniques can be effective, they can also be cost prohibitive and sometimes impractical. A much easier way for a contractor to add corrosion protection to a building project is to incorporate a migrating corrosion inhibitor admixture at a small fraction of the total construction cost.
Calcium Nitrite vs. Migrating Inhibitor Admixtures
Not all corrosion inhibiting admixtures are equal. Those based on salts of amine carboxylates and known as migrating corrosion inhibitors have many advantages when compared to traditional calcium nitrite admixtures. Calcium nitrite admixtures require a progressively higher dosage rate if more chloride exposure is expected, while migrating corrosion inhibitors rely on a fixed dose rate independent of chloride loading. Calcium nitrite admixtures also accelerate concrete set time and decrease workability; whereas migrating corrosion inhibitors slow set time unless a normal set version is requested for certain applications such as shotcrete.
Migrating corrosion inhibitor admixtures meet and often exceed ASTM C1582 requirements for strength, durability and corrosion protection.
Figure 1: ASTM C1582 Physical Property and Corrosion Results for a normal set migrating corrosion inhibitor admixture. Credit: “2005 NS Admixture to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete (ASTM C 1582),” Prepared for Cortec Corporation by Tourney Consulting Group.
Over time, carbonation causes concrete structures to lose the high alkalinity environment that initially protected the rebar from corrosion. Migrating corrosion inhibitors counteract the effects of this problem by forming a protective molecular layer on the surface of the rebar, shielding it from corrosive electrochemical reactions that would otherwise occur with the ingress of oxygen, moisture and chlorides. While corrosion on steel reinforcement can never be permanently stopped, it can be significantly delayed and corrosion rates reduced once started.
Figure 2: Amine carboxylate migrating corrosion inhibitors form a protective layer against corrosion at the level of the rebar.
When to Choose a Migrating Corrosion Inhibitor Admixture
While migrating corrosion inhibiting admixtures could theoretically be used to enhance durability of any reinforced concrete structures, there are key situations in which they should be considered.
Marine Environments
Bridges and buildings in marine environments are more likely to be exposed to the natural chloride content of sea spray, which can accelerate corrosion. In tropical locations, the risk will be increased because of high humidity. Hotels, bridges and seawalls all stand to benefit from extra corrosion protection.
Severe Winter Climates
The opposite extreme is wintry climates such as those in the upper Midwest. Snow and ice create dangerous road conditions that are fought with deicing salts and liquids. These take a heavy toll on highways, bridges and parking ramps, making them a prime candidate for migrating corrosion inhibitor admixtures.
High Water Tables and Ground Sulfates
Some Middle East regions struggle with corrosive underground environments from high water tables and soil with high chloride content. Foundations or underground parking structures built in these environments require extra protection against corrosion. Migrating corrosion inhibitors have therefore been used to enhance the durability of many new building substructures in the United Arab Emirates.
Figure 3: Many structures in the United Arab Emirates have employed migrating corrosion inhibitors in concrete substructures to slow the combined corrosive forces of a marine environment, high water tables, and soil with high chloride content. Adobe Stock image.
Water reservoirs are at heightened risk for corrosion because of constant moisture exposure. Calcium nitrite admixtures cannot be used in potable water structures because of their tendency to leach into the water. Fortunately, several migrating corrosion inhibitor admixtures have been UL certified to meet ANSI/NSF Standard 61 for use in potable water structures.
Weighing the Cost/Benefits
When considering whether or not to include a migrating corrosion inhibitor in a new structure, the designer must take into account budget constraints, the corrosiveness of the environment and the desired service life. With these details in place, the builder can input variables such as climate, concrete mix and the admixture’s expected chloride threshold/corrosion propagation period to predict the estimated service life of the new structure (Life-365™ is one option that is available for free download at www.life-365.org).
Prediction modeling software was used in the decision to add migrating corrosion inhibitors to the substructure of one of the tallest residential high rises in the world: Princess Tower in Dubai, UAE. The environment struggles with a high water table and high chloride content in the soil. At 0.07 percent of the total construction cost, the builder was able to add an estimated more than 50 years of service life (time to first repair) to the structure according to estimates made by LIFE-365™ prediction modeling (see figure 4).
Figure 4: Princess Tower Cost and Service Life Predictions. Credit: “Organic Corrosion Inhibitors – New Build and Existing Structures Performance,” presented at the Brian Cherry International Concrete Symposium.
In many cases, migrating corrosion inhibitors will be the most cost effective and easy to use option for reducing corrosion and promoting a longer service life of the structure. Builders facing highly corrosive environments should consider the potential benefits of adding migrating corrosion inhibitor protection at a fraction of the building cost.
