​NT Build 492: Non-Steady State Chloride Migration (Diffusion Coefficient)

NT Build 492 comes from the NordTest (Based in Finland) family of test methods and is not yet widely used in the United States. This is an alternative to ASTM C1202, and the result is a diffusion coefficient that can be used to assess the quality of your concrete.

How we conduct NT Build 492

NT Build 492 testing
Photo credit: Mary Vancura, NT Build 492 test cells

This test is an electrical method, very similar to the rapid chloride (RCP) test. The sample for the test is cut from a 4‑inch by 8‑inch cylinder. It does not have to be coated with epoxy, but it goes through 24‑hour conditioning similar to the RCP samples. During the exposure period, the sample is sandwiched between a sodium chloride solution and a sodium hydroxide solution. The test is run for a time and at a voltage determined by the sample’s initial current with an applied 30V. The time of exposure ranges from 24 to 96 hours. After the test is completed, the sample is split in half and a silver nitrate solution is sprayed on the fractured surface. White silver nitrate precipitate forms on the part of the face where chloride ions are present. The depth of chloride penetration is measured as the average of seven measurements across the width of the sample. Diffusion coefficient is calculated from values of chloride penetration, sample thickness, exposure time, voltage, and average sample temperature.

White silver chloride deposits mark where chloride ions have penetrated.
Photo credit: Mary Vancura, NT Build 492 sample evaluation. White silver chloride deposits mark where chloride ions have penetrated.

What works about the NT Build 492 test:

This takes slightly longer than the RCP test, but not nearly as long as ASTM C 1556. The results of ASTM C1556 and NT Build 492 are similar but not identical. The diffusion coefficient it determines can be used to predict the service life of the concrete or help us design a more durable concrete.

What doesn’t work about this test:

This test should not be used with steel fiber reinforced concrete or concrete that contains conductive solutions (i.e. corrosion inhibitor).