ASTM C1556: Bulk Chloride Diffusion (Diffusion Coefficient)​

ASTM C1556 is a respected method for determining the bulk diffusion coefficient of concrete. We’ve performed this test hundreds of times, both for research and for developing concrete that is resistant to chloride ion penetration.

How we conduct the ASTM C1556 test

milling of ASTM C1556 test specimen
Photo credit: Mary Vancura, milling of ASTM C1556 test specimen.

We use 4 inch x 8 inch concrete cylinders that have cured for 28 days. After 28 days, the top 3 inches are cut from the cylinder and all but the finished surface is coated with epoxy. Once the epoxy is dry, the sample soaks in lime water until it reaches a constant weight. Then the sample soaks in a sodium chloride solution for at least 35 days. The sample dries in air for 24 hours. Then 0.039 inch horizons are milled off the exposed face. The milled powder from each horizon is analyzed for chloride content. The diffusion coefficient of the mixture is determined by fitting an equation to the measured chloride-ion contents using the method of least squares regression analysis.

What works about the ASTM C1556 test:

milled test specimens
Photo credit: Tim Davis, ASTM C1556 milled specimens

It is a good method for measuring the bulk diffusion coefficient of a concrete mixture. ASTM C1556 is also useful in predicting the service life of a concrete structure, whether an existing structure or new construction.

What doesn’t work about this test:

At a minimum, this test takes three months to complete. For high performance concrete, it is recommended that soaking time in the sodium chloride solution be extended beyond 35 days. In some cases NT Build 492, which gives similar results, may be more appropriate.