After a long winter, it is important to check the condition of leftover admixtures before starting production. Admixtures that have frozen then thawed can have varying effects on fresh and hardened concrete. Many admixtures separate into two or more phases during feezing. Water separates from the active ingredients creating different concentrations within the storage container. After thawing, the phases tend to remain dispersed, and only after thorough remixing do they distribute uniformly. Any thawing and remixing that doesn't produce a uniform admixture creates problems.
Most admixtures are aqueous solutions that freeze at about 28 Farenheit degrees. It is better to store admixtures in a heated room than in a heated tank. A heated tank can cause them to overheat. High temperatures make some admixtures less effective. Heating the room keeps pumps, meters, valves, and hoses from freezing.
To thaw an admixture, bring it to at least 50 Farenheit degrees and wait until it thaws. Remix by mechanical agitation (hand stirring is possible) or by agitation with low-pressure air. The agitation should recirculate the mixture from the top to the bottom.
After thawing and agitating, run tests on the admixture to ensure it's homogenous. Take two 1-pint samples, one from the top and one from the bottom, from the storage container. Then have a laboratory conduct the following tests: pH (for air entraining agents only), specific gravity, residue by oven drying, and infrared analysis.