11.3 : 海水がコンクリートに与える影響

Seawater-exposed concrete structures experience the dissolution of the needle-shaped crystals of ettringite and gypsum, increasing porosity and reducing strength.

Conversely, expansion can occur due to pressure from salt crystallization within the concrete's pores above the waterline.

However, the salt solution can ascend through the concrete via capillary action, and damage occurs, indicating the criticality of the concrete's permeability.

Concrete in tidal zones degrades significantly from constant wet-dry cycles, whereas submerged concrete is less affected.

Sea water damage to concrete is reduced when pores clog with magnesium hydroxide and block the entry.

Magnesium hydroxide is formed with gypsum from the reaction of magnesium sulfate in sea water with calcium hydroxide in concrete.

Sometimes, seawater can cause concrete damage with salt absorption, leading to cracking of concrete around the corroded reinforcement.

To resist seawater, reinforced concrete needs 2 to 3 inches of cover for reinforcement and well-compacted plane concrete with 600 lbs/yd³ cement above and 500 lbs/yd³ below the waterline.