Concrete exposed to seawater can undergo degradation like the dissolution of ettringite and gypsum, increasing the material's porosity and decreasing its strength. In contrast, the crystallization of salts within the concrete's pores can cause expansion, particularly above the waterline where evaporation occurs. Nonetheless, this expansion only happens when seawater, enabled by the concrete's permeability, manages to infiltrate the structure.
Concrete in areas between tide marks, which undergo regular cycles of wetting and drying, are particularly at risk and tend to degrade more severely than areas that remain permanently submerged. The damage from seawater is somewhat mitigated by the deposition of magnesium hydroxide, which forms in the concrete by the reaction of magnesium sulfate with calcium hydroxide, leading to clogged pores and reduced porosity.
In addition to chemical effects, concrete in marine environments can also suffer from physical damage due to frost, wave action, and abrasion. This physical damage can worsen the effects of chemical deterioration, particularly when salts absorbed by the concrete contribute to the corrosion of reinforcing steel.
To prevent damage from seawater, measures similar to those used against sulfate attack are recommended. These include using concrete with low permeability and ensuring proper coverage of at least 2 to 3 inches over the reinforcement bars. A cement content of 600 pounds per cubic yard above the waterline and 500 pounds per cubic yard below, along with a water-cement ratio of not more than 0.40 to 0.45, is advised. Additionally, achieving thorough compaction of the concrete and exercising precise craftsmanship in construction joints are critical to enhance the durability of concrete against seawater attack.
