11.3
Effect of Sea Water on Concrete
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Q1: How does seawater damage concrete through chemical dissolution?
Seawater dissolves needle-shaped crystals of ettringite and gypsum in concrete, increasing porosity and reducing strength. Magnesium sulfate in seawater reacts with calcium hydroxide in concrete to form magnesium hydroxide, which can clog pores and mitigate further damage. This chemical degradation is accelerated in tidal zones experiencing constant wet-dry cycles compared to permanently submerged concrete.
Q2: What role does permeability play in seawater damage to concrete?
Concrete's permeability allows salt solutions to ascend through pores via capillary action, enabling seawater penetration and damage. Salt crystallization within pores causes expansion, particularly above the waterline where evaporation occurs. Reducing permeability through low water-cement ratios and proper compaction is critical to prevent seawater infiltration and subsequent deterioration.
Q3: Why do tidal zone concrete structures degrade faster than submerged ones?
Concrete in tidal zones experiences constant wet-dry cycles that accelerate degradation compared to permanently submerged concrete. These cycles promote salt crystallization, capillary rise, and evaporation-driven expansion. Submerged concrete remains saturated, limiting oxygen availability and reducing the rate of chemical reactions and physical damage from salt crystallization.
Q4: How does salt absorption lead to reinforcement damage in seawater-exposed concrete?
Seawater salt absorption causes cracking in concrete around corroded reinforcement through corrosion of reinforcing steel. Salt ions penetrate the concrete cover and initiate electrochemical corrosion of the steel bars. This corrosion of reinforcement generates expansive products that crack the surrounding concrete, compromising structural integrity and durability.
Q5: What concrete specifications prevent seawater damage to marine structures?
Reinforced concrete requires 2 to 3 inches of cover over reinforcement and well-compacted concrete with 600 lbs/yd³ cement above the waterline and 500 lbs/yd³ below. A water-cement ratio not exceeding 0.40 to 0.45 reduces permeability. Precise construction joint craftsmanship and thorough compaction enhance durability against seawater attack.
Q6: How does magnesium hydroxide formation protect concrete from seawater damage?
Magnesium hydroxide forms when magnesium sulfate in seawater reacts with calcium hydroxide in concrete. This precipitate deposits in concrete pores, clogging them and reducing porosity. Pore clogging blocks further seawater entry and slows chemical deterioration, providing a natural protective mechanism against continued seawater damage.
Q7: What physical and chemical factors combine to damage marine concrete?
Seawater damage results from both chemical effects like salt crystallization and dissolution, and physical damage from frost, wave action, and abrasion. Salt absorption worsens corrosion of reinforcing steel, compounding structural degradation. Combined chemical and physical deterioration accelerates concrete failure in marine environments, requiring comprehensive protective measures.