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

ASTM C1556
Photo credit: Mary Vancura, 1556 sample milling

We use 4 inch x 8 inch concrete cylinders that are 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 (usually 2 to 3 days). Following, the sample is immersed in a sodium chloride solution (concentration prescribed by the ASTM) for at least 35 days. The sample is then allowed to air dry for 24 hours, and 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:

bulk chloride diffusion testing
Photo credit: Tim Davis, 1556 milled samples

It is a good method for measuring the bulk diffusion coefficient of a concrete mixture.

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.