The North Carolina Department of Transportation (NCDOT) has recently developed Chemical Procedure #C-20.0 to determine nitrite levels in hardened concrete. Structure owners use this test to find out if enough corrosion inhibitors are present in a concrete mix.

The steel reinforcement in concrete can corrode when deicing agents, oxidation from marine environments, and other oxidizing agents penetrate the concrete. This is a significant and costly problem. However, corrosion inhibitors can make reinforced concrete more durable by protecting the steel reinforcement, increasing the structure’s life span and resulting in significant cost savings.

Corrosion inhibitors for reinforced concrete typically contain a high concentration of nitrite. Nitrite-based corrosion inhibitors in concrete form a film on the steel reinforcement that resists corrosion. This film suppresses the flow of electrons from the anodic reaction at the bar surface, which is the basis for nitrite-type corrosion inhibitors in a concrete mix.

Determining how much nitrite is in hardened concrete can verify an appropriate amount of corrosion inhibitor. In other words, if you can determine the level of nitrite, you can determine the level of protection against corrosion. This test therefore assures owners that the structure will be durable and meet their life span requirements.

EBA Engineering, Inc., can now perform this test in our laboratory using a powder sample extracted from the hardened concrete that is filtered and diluted. An aliquot of the filtrate is treated with sulfanilic acid and N-(1-naptthyl) ethylenediamine. The aliquot absorbance is measured in a spectrophotometer and compared against a calibration curve generated for the laboratory-specific equipment and reagents. The concentration of the nitrite in the hardened concrete is calculated by comparing the absorbance at 540 nm to the calibration curve. A typical requirement for the concentration of nitrite in hardened concrete containing a nitrite-based corrosion inhibitor is 5.1 lbs of nitrite per cubic yard of concrete. We recommend that our clients perform this test regularly during a project to ensure that the content of nitrite-based corrosion inhibitor meets their requirements.

Materials testing is vital for ensuring quality construction and building projects that stand the test of time. To learn more about EBA’s materials testing laboratory capabilities and pricing, please visit our website.