Understanding how seawater deteriorates concrete and carefully selecting a mix design can help ensure the maximum service life of the concrete surface. Concrete exposed to seawater may deteriorate from the combined effects of chemical and physical processes. Attack is slowed by reducing concrete permeability. Low permeability helps keep aggressive chemicals out of the concrete, slows leaching of soluble materials such as lime, and limits the depth of carbonation, thus better protecting reinforcing steel from corrosion.
For corrosion protection, ACI requires the maximum water-cement ratio to be 0.40 for concrete exposed to seawater. ACI 318 requires a Type II cement or a Type I plus a pozzolan to resist the moderate sulfate attack from seawater. When freeze-thaw durability is required, both ACI 318 and ACI 357 recommend air entrainment. ACI 318 and ACI 357 also allow the use of chemical admixtures. Water-reducing and high-range water-reducing admixtures are commonly used to enhance uniform cement distribution and to provide workable mixes at low water-cement ratios. Aggregates conforming to ASTM C 33 are acceptable for use, as are marine aggregates that have been washed with fresh water to reduce the chloride ion content. But don't forget that other design and construction practices can affect strength, permeability, and durability.