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

Inside a concrete mass, high alkalinity of calcium hydroxide with a pH of around 13 shields steel reinforcement from corrosion by creating a protective iron oxide layer, a state known as passivity.
Due to varying electrochemical potentials, anodic and cathodic regions are formed on the steel surface.
These regions are bridged by the salt solution in hydrated cement, acting as an electrolyte.
Positively charged ferrous ions from the anode dissolve into the salt solution.
In contrast, negatively charged electrons move to the cathode, are absorbed by the electrolyte, and react with water and oxygen to form hydroxyl ions.
Ferric hydroxide is formed when the hydroxyl ions bond with the ferrous ions. Subsequent oxidation of ferric hydroxide leads to the formation of rust.
Further, chloride ions in the surrounding cement paste form hydrochloric acid at anodic spots, breaking down the protective layer of steel and causing pitting corrosion.
After the steel rusts, it increases in volume and exerts pressure that can lead to concrete cracking and spalling.
Using epoxy-coated or zinc-coated steel or reducing the permeability of concrete can reduce steel corrosion.

corrosie van wapening

Corrosion of Reinforcement

De corrosie van stalen wapening in beton is een proces dat wordt beïnvloed door de inherente eigenschappen van het materiaal en externe factoren. De hoge pH-waarde van ongeveer 13, die wordt geleverd door calciumhydroxide in beton, beschermt de stalen wapening in eerste instantie door de vorming van een passieve ijzer oxidelaag op het oppervlak te bevorderen.

Echter, na verloop van tijd en onder bepaalde omstandigheden zoals carbonatatie, chloride-infiltratie en barsten kan deze beschermende toestand in gevaar komen. Staal heeft gebieden met verschillende elektrochemische potentialen, bekend als anodische en kathodische gebieden. De gehydrateerde cementpasta, die zouten bevat, fungeert als een elektrolyt die deze gebieden verbindt. In de anodische gebieden lost ijzer uit het staal op als positief geladen ionen in de elektrolyt, terwijl in de kathodische gebieden elektronen worden verbruikt in reacties waarbij water en zuurstof uit het omringende beton betrokken zijn om hydroxyl-ionen te produceren.

Deze hydroxyl-ionen combineren met het opgeloste ijzer om ferri hydroxide te creëren, dat verder oxideert tot roest. De aanwezigheid van chloride-ionen verergert dit proces, wat leidt tot plaatselijke putcorrosie door de vorming van zoutzuur dat de beschermende ijzeroxide laag aantast. Deze corrosie kan het staal doen uitzetten, waardoor het beton kan barsten en afbrokkelen.

Preventieve maatregelen tegen corrosie van stalen wapening zijn onder andere het aanbrengen van beschermende coatings zoals epoxy of zink op het staal zelf of het verbeteren van de kwaliteit van het beton om de permeabiliteit ervan te verminderen en zo het binnendringen van corrosieve stoffen te belemmeren.

Corrosie van wapening

Inside a concrete mass, high alkalinity of calcium hydroxide with a pH of around 13 shields steel reinforcement from corrosion by creating a protective ...

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Permeability of Concrete

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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

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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.

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Why Concrete Can Fail Around Rebar

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