Question 1

How does concrete protect steel reinforcement from corrosion?

Accepted Answer

Concrete's high alkalinity, around pH 13 from calcium hydroxide, creates a protective iron oxide layer on steel through a process called passivity. This passive layer shields the reinforcement from corrosive agents. However, this protection can be compromised by carbonation, chloride ingress, and concrete cracking over time.

Question 2

What role does the hydrated cement paste play in steel corrosion?

Accepted Answer

Hydrated cement paste containing salts acts as an electrolyte, bridging anodic and cathodic regions on the steel surface. At the anode, iron dissolves as positively charged ions. At the cathode, electrons react with water and oxygen to form hydroxyl ions, which combine with dissolved iron to create ferric hydroxide that oxidizes into rust.

Question 3

What is pitting corrosion and how does it occur in reinforced concrete?

Accepted Answer

Pitting corrosion is localized corrosion caused by chloride ions in cement paste forming hydrochloric acid at anodic spots. This acid breaks down the protective iron oxide layer, creating small pits on the steel surface. Chloride-induced pitting is more aggressive than general corrosion and can rapidly compromise reinforcement integrity.

Question 4

Why does corroded steel cause concrete to crack and spall?

Accepted Answer

When steel rusts, it increases in volume significantly. This expansion exerts pressure on the surrounding concrete, causing internal stress. Eventually, the concrete cracks and spalls as it cannot withstand the outward pressure from the expanding rust layer on the reinforcement.

Question 5

What are the main preventative measures against steel reinforcement corrosion?

Accepted Answer

Protective coatings such as epoxy or zinc applied to steel reduce corrosion by creating a barrier against corrosive agents. Additionally, reducing the permeability of concrete hinders the ingress of chloride ions and oxygen, slowing electrochemical reactions. Both approaches work to maintain the passive layer and extend reinforcement lifespan.

Question 6

What are anodic and cathodic regions on steel reinforcement?

Accepted Answer

Steel has areas with different electrochemical potentials, creating anodic and cathodic regions. At anodic regions, iron oxidizes and dissolves as positively charged ferrous ions into the electrolyte. At cathodic regions, electrons are consumed in reactions with water and oxygen. These regions drive the electrochemical corrosion process.

Question 7

How do chloride ions compromise the protective layer on steel?

Accepted Answer

Chloride ions in cement paste form hydrochloric acid at anodic spots on the steel surface. This acid directly attacks and breaks down the protective iron oxide layer, exposing bare steel to further corrosion. The resulting pitting corrosion is more damaging than uniform corrosion across the reinforcement surface.

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