Durability of materials

Question 1

How does the corrosion of steel occur?

Answer 1

Ferrous ions react with water to form an electrolyte.
Electrons are released and move through the metallic structure to the cathode, where they react with water and oxygen to form OH-.
These ions react with iron to form ferrous hydroxide.

Question 2

What does iron in steel oxidise to ?

Answer 2

Rust

Question 3

What are the 3 most common methods to protect steel from electrochemical corrosion?

Answer 3

Paint systems, metallic coating, concrete encasement.

Question 4

How do pain systems protect steel from corrosion?

Answer 4

Consists of 3 layers, provides a barrier between steel and corrosive environment.

Question 5

How do metallic coatings protect steel from corrosion?

Answer 5

Steel can be ‘galvanised’ by being dipped in a pool of hot zinc. Zinc is non reactive.

Question 6

How does concrete encasement protect steel from corrosion?

Answer 6

Good quality concrete acts as a layer between steel and corrosive environment.

Question 7

What is stainless steel?

Answer 7

A non-corrosive steel and require little to no maintenance. Additions of up to 20% and 12% of chromium and nickel create a layer which is resistant to oxidation.

Question 8

What is weathering steel?

Answer 8

A ferrous alloy with a number of metallic inclusions, which forms an oxide on the surface, lower the rate of corrosion.

Question 9

How does carbonation cause concrete to deterioirate?

Answer 9

Calcium hydroxide is present in cement paste, which can react with carbon dioxide in the air to form calcium carbonate
Calcium carbonate lowers the alkalinity around the area and the protective oxide layer breaks down, so corrosion becomes possible.

Question 10

When can carbonation occur?

Answer 10

When the solution is wet, and steel is present.

Question 11

How can a chloride attack cause concrete to deteriorate?

Answer 11

When the chloride content in the concrete exceeds a threshold limit, the protective oxide layer can no longer be maintained, leading to corrosion.
Chlorides can be present as accidental contaminants of the aggregates.

Question 12

How can a sulphate attack cause concrete to deterioirate?

Answer 12

Salts can react with concrete to form gypsum, resulting in cracking.

Question 13

How can Alkali-Silica Reaction (ASR)
cause concrete to deteriorate?

Answer 13

Some aggregates react with the calcium hydroxide in the cement, resulting in expansion and cracking over years.H

Question 14

How can freeze thaw concrete result in the deterioration of concrete?

Answer 14

Moisture in concrete can freeze at low temps and expand, causing cracking.

Question 15

What 2 factors is the durability of concrete affected by?

Answer 15

A concrete mix is designed based on the environmental conditions, and sufficient cover is used to reduce corrosion.

Question 16

How does sulphate attack affect the durability of masonry?

Answer 16

Clay contains sulphate particles, which react slowly with the portland cement, causing it to expand.

Question 17

How can frost attack effect the durability of masonry?

Answer 17

Expansion, frost-resistant bricks should be specified.

Question 18

How does Salt Effloresence effect the durability of masonry?

Answer 18

The crystallisation of soluble salts in bricks often causes a white deposit,
known as efflorescence, to appear on the surface of brickwork. While not
being particularly harmful in general, efflorescence is unsightly.

Question 19

In damp and humid conditions, what can happen to timber?

Answer 19

Degration caused by wood-destroying organisms, which require moisture, oxygen and food to survive.

Question 20

What does the majority of wood-destroying fungi prefer a sustained moisture content of?

Answer 20

20%

Question 21

When is termite attack a risk in wood?

Answer 21

If the moisture content is less than 20%

Question 22

Why are fibre-reinforced polymers very durable?

Answer 22

They don’t rust and provide excellent fatigue resistance.