Name: why concrete can fail around rebar
Uploaded: 2024-11-07T14:48:46.000Z
Duration: 1 min 27 s
Channel: JoVE

コンクリート塊の内部では、pHが約13の高アルカリ性の水酸化カルシウムが、不動態性として知られる保護酸化鉄層を作成することにより、鋼の補強を腐食から保護します。
電気化学ポテンシャルが変化するため、鋼の表面には陽極領域と陰極領域が形成されます。
これらの領域は、水和セメント中の塩溶液によって架橋され、電解質として機能します。
アノードからの正に帯電した鉄イオンは、塩溶液に溶解します。
これに対し、負に帯電した電子は陰極に移動し、電解質に吸収され、水や酸素と反応して水酸イオンを形成します。
水酸化第二鉄は、水酸イオンが鉄イオンと結合するときに形成されます。その後の水酸化第二鉄の酸化は、錆の形成につながります。
さらに、周囲のセメントペースト中の塩化物イオンは、陽極スポットで塩酸を形成し、鋼の保護層を破壊し、孔食を引き起こします。
鋼が錆びると、体積が増し、圧力がかかり、コンクリートのひび割れや剥がれにつながる可能性があります。
エポキシ被覆鋼や亜鉛被覆鋼を使用するか、コンクリートの透磁率を下げることで、鋼の腐食を減らすことができます。

鉄筋の腐食

Corrosion of Reinforcement

コンクリート内の鉄筋の腐食は、材料の固有の特性と外部要因によって影響を受けるプロセスです。コンクリートに含まれる水酸化カルシウムによってもたらされる約 13 の高い pH レベルは、鉄筋の表面に不活性な酸化鉄層の形成を促進し、鉄筋を最初に保護します。


 しかし、時間が経つと、炭酸化、塩化物侵入、亀裂などの特定の状況下では、この保護状態が損なわれる可能性があります。鋼鉄には、陽極領域と陰極領域と呼ばれる異なる電気化学電位を持つ領域があります。塩を含む水和セメントペーストは、これらの領域を接続する電解質として機能します。陽極領域では、鋼鉄の鉄が正に帯電したイオンとして電解質に溶解し、陰極領域では、周囲のコンクリートからの水と酸素を含む反応で電子が消費され、ヒドロキシルイオンが生成されます。


 これらの水酸化イオンは溶解した鉄と結合して水酸化鉄（III）を生成し、これがさらに酸化されて錆になります。塩化物イオンが存在すると、このプロセスが悪化し、保護酸化鉄層を攻撃する塩酸を形成して局部的な孔食が発生します。この腐食により鋼が膨張し、コンクリートに亀裂や剥離が生じる可能性があります。


 鉄筋の腐食を防ぐ対策としては、鉄筋自体にエポキシや亜鉛などの保護コーティングを施したり、コンクリートの品質を改善して浸水性を低減し、腐食剤の侵入を防ぐことなどが挙げられます。

コンクリート塊の内部では、pHが約13の水酸化カルシウムの高アルカリ度が、保護...

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

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 regions, which creates a small electrochemical cell.

At the anodic regions, iron dissolves into the electrolyte as positively charged ions. At the cathodic regions, electrons are used in reactions with water and oxygen in the surrounding concrete to form hydroxyl ions. These hydroxyl ions combine with the dissolved iron to make ferric hydroxide, which then oxidizes into rust.

Chloride ions make the problem worse. They can cause localized pitting corrosion by forming hydrochloric acid that attacks the protective iron oxide layer. As rust builds up, the steel expands and can crack the concrete, leading to spalling.

Corrosion can be reduced in a few ways. Protective coatings such as epoxy or zinc can be applied to the steel. Another option is to improve concrete quality so it is less permeable and slows the entry of corrosive agents.

Why Concrete Can Fail Around Rebar

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 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...

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

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 concrete’s high pH protects steel rebar, why carbonation and chlorides trigger rust, and how coatings and dense concrete help prevent cracking.

Why Concrete Can Fail Around Rebar | 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

Why Concrete Can Fail Around Rebar

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