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.

corrosion of reinforcement

Corrosion of Reinforcement

The corrosion of steel reinforcement within concrete is a process influenced by the material&#39;s inherent properties and external factors. The high pH level of around 13, provided by calcium hydroxide present in concrete, initially protects the steel reinforcement by promoting the formation of a passive iron oxide layer on its surface.
However, over time and under certain conditions like carbonation, chloride ingress, and cracking this protective state can be compromised. Steel has areas with different electrochemical potentials, known as anodic and cathodic regions. The hydrated cement paste, containing salts, acts as an electrolyte that connects these regions. At the anodic regions, iron from the steel dissolves as positively charged ions into the electrolyte, while at the cathodic regions, electrons are consumed in reactions that involve water and oxygen from the surrounding concrete to produce hydroxyl ions.
These hydroxyl ions combine with the dissolved iron to create ferric hydroxide, which further oxidizes to become rust. The presence of chloride ions exacerbates this process, leading to localized pitting corrosion by forming hydrochloric acid that attacks the protective iron oxide layer. This corrosion can expand the steel, causing the concrete to crack and spall.
Preventative measures against the corrosion of steel reinforcement include using protective coatings such as epoxy or zinc on the steel itself or improving the concrete&#39;s quality to reduce its permeability, thereby hindering the ingress of corrosive agents.

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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Civil Engineering

Unit 1

Permeability of Concrete

KEY TERMS AND CONCEPTS

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

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Concrete Damage from Sulfate Exposure

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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...

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 offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

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Why Concrete Can Fail Around Rebar

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