Does the water for concrete have to be completely void of salt (from seawater)? What happens if it is not?

Two factors must be considered in answering this question. First, will the concrete include steel reinforcing? If so, any salt in the mix water will contribute to corrosion problems, which in turn can lead to serious deterioration of the concrete. Even brackish water should not be used for reinforced concrete.

The other consideration is concrete strength and performance. If the mix water contains dissolved salts up to about 35,000 parts per million, it may produce high early strength, with a slight reduction in the concrete's long-term strength, as compared to the same mix made with potable water. Additionally, seawater can be problematic for alkali reactive aggregates as well as being more prone to efflorescence issues.

As far as standards go, the question of water quality as it relates to concrete has been covered by several short paragraphs in section 5 of ASTM C 94, "Standard Specification for Ready-Mixed Concrete," and a similar section in ASTM C 685, "Standard Specification for Concrete Made by Volumetric Batching and Continuous Mixing." Based on a vote taken at its June meeting in Denver, the C-94 committee of ASTM is preparing two new standard specifications concerning water for mixing concrete, which will then be referenced in C 94. One will be a standard specification for the water itself, while the other will deal with a test method for determining solids content in the water. The new water specification is expected to require alkalinity testing per ASTM D 4191, "Standard Test Method for Sodium in Water by Atomic Absorption Spectrophotometry,” and ASTM D 4192, “Standard Test Method for Potassium in Water by Atomic Absorption Spectrophotometry."