Name: seawater damage and concrete durability
Uploaded: 2024-11-07T14:48:45.000Z
Duration: 1 min 22 s
Channel: JoVE

海水にさらされたコンクリート構造物は、エトリンガイトと石膏の針状の結晶が溶解し、多孔性が増加し、強度が低下します。
逆に、喫水線の上のコンクリートの細孔内の塩の結晶化による圧力により膨張が発生する可能性があります。
しかし、塩溶液は毛細管現象によってコンクリートを通って上昇する可能性があり、損傷が発生することは、コンクリートの透過性の重要性を示しています。
潮汐帯のコンクリートは、一定の湿潤-乾燥サイクルから大幅に劣化しますが、水中コンクリートは影響が少なくなります。
水酸化マグネシウムで細孔が詰まり、進入を塞ぐと、コンクリートの海水によるダメージが軽減されます。
水酸化マグネシウムは、海水中の硫酸マグネシウムとコンクリート中の水酸化カルシウムとの反応により、石膏で形成されます。
海水は塩分を吸収してコンクリートに損傷を与え、腐食した鉄筋の周りのコンクリートにひび割れを引き起こすことがあります。
海水に抵抗するために、鉄筋コンクリートは補強のためのカバーの2〜3インチと、喫水線の上に600ポンド/yd³セメントと500ポンド/yd³の下に十分に圧縮された平面コンクリートが必要です。

海水がコンクリートに与える影響

Effect of Sea Water on Concrete

海水にさらされたコンクリートは、エトリンガイトや石膏の溶解などの劣化が起こり、材料の空隙率が増して、強度が低下します。対照的に、コンクリートの細孔内の塩の結晶化は、特に蒸発が起こる水面より上で膨張を引き起こす可能性があります。しかし、この膨張は、海水が構造物に浸透した場合にのみ発生しますが、これはコンクリートの透水性によって可能になります。


 干満帯にあるコンクリートは、定期的に湿潤と乾燥を繰り返すため、特に危険にさらされており、恒久的に水没している場所よりも劣化が激しくなる傾向があります。海水による被害は、硫酸マグネシウムと水酸化カルシウムの反応によってコンクリート内に形成される水酸化マグネシウムの堆積によってある程度緩和され、細孔が詰まり、空隙率が低下します。


 海洋環境におけるコンクリートは、化学的影響に加えて、霜、波の作用、摩耗による物理的損傷も受けることがあります。この物理的損傷は、特にコンクリートに吸収された塩分が鉄筋の腐食に寄与する場合、化学的劣化の影響を悪化させる可能性があります。


 海水による被害を防ぐためには、硫酸塩腐食対策と同様の対策が推奨されます。これには、透水性の低いコンクリートの使用、鉄筋の上に少なくとも 5.08 ～ 7.62 cm の適切な被覆を確保することが含まれます。水面上では 1 ㎥ あたり 355.97 kg 、水面下では 1 ㎥ あたり 296.64 kg のセメント含有量、および水セメント比は 0.40 ～ 0.45 以下にすることが推奨されます。さらに、コンクリートの徹底した圧縮と施工ジョイントの正確な職人技は、海水腐食に対するコンクリートの耐久性を高めるために不可欠です。

海水にさらされたコンクリート構造物は、エトリンガイトと石膏の針状の結晶が溶解し、多孔性が増加し、...

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Permeability of Concrete

コンクリートの透水性

KEY TERMS AND CONCEPTS

主要用語と概念

seawater damage and concrete durability

Seawater can damage concrete by changing its chemistry, increasing porosity, and lowering strength. In marine settings, concrete is exposed to dissolved salts and repeated wetting, so its durability depends on how much water can enter the material.

One major form of damage is the breakdown of ettringite and gypsum, two concrete minerals that can dissolve in seawater. As these materials are lost, the concrete becomes more porous. Salt can also crystallize inside pores, especially above the waterline where evaporation is stronger, and that crystallization can force the concrete to expand.

That expansion only happens when seawater gets into the concrete through its pore system. Concrete in the tidal zone, where it is repeatedly wet and dried, is often damaged more than concrete that stays fully submerged. Seawater attack is partly reduced when magnesium hydroxide forms inside the concrete. It forms from the reaction of magnesium sulfate with calcium hydroxide and can clog pores, which lowers porosity.

Marine concrete can also be harmed by physical forces such as frost, wave action, and abrasion. These effects can make chemical damage worse. Salt that enters the concrete may also help corrode reinforcing steel, which weakens the structure even further.

Protection starts with low-permeability concrete, so seawater has a harder time entering the structure. Recommended practice also includes at least 2 to 3 inches of concrete cover over 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 no higher than 0.40 to 0.45, helps improve durability. Good compaction and careful construction joints are also important.

Seawater Damage and Concrete Durability

permeability of concrete

Concrete Durability and Water Penetration

Concrete durability depends partly on how easily water and gases can move through it. This property, called permeability, is important for judging water-tightness and resistance to chemical attack in concrete structures. Permeability is measured with comparative lab tests. In a typical test, the sides of a concrete specimen are sealed, water pressure is applied to the top surface, and the amount of water that passes through is measured. The test is often run for about ten days until a steady...

sulfate attack on concrete

Concrete Damage from Sulfate Exposure

Concrete damage from sulfate exposure starts with a whitish discoloration, often first seen at the edges and corners. As the attack continues, the concrete can crack and spall. This deterioration happens when sulfates react with hardened concrete and create new compounds that take up more space than the materials they replace. Sulfates may enter concrete from soil, groundwater, or industrial effluents. Inside the concrete, they can form calcium sulfate and calcium sulfoaluminate. The reaction...

effect of sea water on concrete

Seawater can damage concrete by changing its chemistry, increasing porosity, and lowering strength. In marine settings, concrete is exposed to dissolved salts and repeated wetting, so its durability depends on how much water can enter the material. One major form of damage is the breakdown of ettringite and gypsum, two concrete minerals that can dissolve in seawater. As these materials are lost, the concrete becomes more porous. Salt can also crystallize inside pores, especially above the...

acid attack on concrete

Concrete Damage from Low pH Water

Concrete damage from low pH water happens when acids react with the hydrated cement paste. This reaction dissolves the paste, so the concrete becomes softer and weaker. The problem is often seen in chimneys, sewers, and industrial settings. The amount of damage depends on how acidic the water is. When pH falls below 6.5, the attack becomes more severe. A pH below 4.5 can cause major concrete damage. The rate of damage also depends on how fast hydrogen ions move through the cement gel. This...

alkali aggregate reaction in concrete

How Moisture Cracks Alkali-Reactive Concrete

Alkali-aggregate reaction in concrete can weaken a structure when siliceous minerals in the aggregate react with alkaline hydroxides from cement. The reaction makes an alkali-silica gel, which pulls in water and swells. As the gel expands inside the rigid cement paste, it creates pressure that can crack and disrupt the concrete. The damage comes from both osmotic hydraulic pressure and the swelling of the solid reaction products. Particle size also matters. Fine siliceous particles can cause...

corrosion of reinforcement

Why Concrete Can Fail Around Rebar

Steel reinforcement in concrete can corrode when its protective environment changes. Fresh concrete is highly alkaline because of calcium hydroxide, with a pH of about 13. That high pH helps form a passive iron oxide layer on the steel surface, which slows corrosion. Over time, carbonation, chloride ingress, and cracking can weaken this protection. Concrete contains hydrated cement paste with salts, and it acts as an electrolyte. Different areas of the steel then behave as anodic and cathodic...

key terms and concepts

Learn how seawater weakens concrete through salt attack, porosity, and steel corrosion, and how low-permeability mixes help protect marine structures.

Seawater Damage and Concrete Durability | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

civil engineering

Seawater Damage and Concrete Durability

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