Metals

Question 1

What are the 3 main behaviours of metals?

Answer 1

Yield, Fracture and Fatigue

Question 2

What are the 4 main metals used in construction?

Answer 2

Wrought Iron
Cast Iron
Steel
Prestressed Steel

Question 3

What is Mild Steel?

Answer 3

Steel with low carbon content. It is quite ductile but has a low elastic limit so is rarely used in construction now

Question 4

How do Mild Steel and High Strength Steel differ in their stress/strain graphs?

Answer 4

Mild: Short period of elastic behaviour up to yield stress, followed by work hardening and a small increase in stress before reaching UTS
High Strength: Long period of elastic behaviour which gradually decreases in gradient up to UTS - proof strength estimated using 0.2% strain

Question 5

In what direction does steel crack under tension?

Answer 5

Diagonally

Question 6

Define fatigue

Answer 6

Failure of a material due to repeated loading

Question 7

How does increasing the carbon content affect a steel’s properties?

Answer 7

Increases strength but lowers ductility

Question 8

What is wrought iron often used for and why?

Answer 8

Suspension bridges, because of its high ductility

Question 9

What are the two primary non-ferrous metals used in construction and what are their properties?

Answer 9

Aluminium: Low density, high resistance to corrosion, high ductility
Copper: High electrical conductivity, resistance to chemical weathering via green patina

Question 10

Why is concrete almost always reinforced with steel?

Answer 10

It allows for cheap yet strong constructions which can take on a greater variety of shapes.
Without reinforcement, concrete cracks under tension and hence fails immediately via brittle failure

Question 11

How can reinforced concrete still crack?

Answer 11

If the ratio of the cross-sectional area of the steel and concrete is too low (i.e. there is not enough steel), hence the concrete takes most of the tension

Question 12

How does reinforcement change the failure behaviour of concrete?

Answer 12

Instead of a single large crack, many smaller cracks form along the length proportional to the amount of tension in that part of the beam

Question 13

What is a cheap alternative to reinforced concrete in areas with small stresses?

Answer 13

Leaving the area devoid of structural material

Question 14

What is prestressing and why is it used?

Answer 14

Where the steel is stressed in tension before the structure is assembled such that the concrete around it is in compression and hence better resists tensile loading

Question 15

Name and describe the two methods of prestressing

Answer 15

Pre-tensioning: Using external jacks to tense the steel before casting the concrete. After setting, the jacks are removed. Grooves in the steel grab and compress the concrete.
Post-tensioning: Casting the concrete with a space for the steel to be fed into. The steel is tensed as it is put in and then anchored to the ends of the beam, compressing the concrete.

Question 16

What is important to consider when prestressing?

Answer 16

The length of the concrete beam will be decreased

Question 17

Define concrete creep

Answer 17

The process where, under constant loading, water migrates out of the concrete such that its height decreases and width increases plastically.
The rate of creep increases over time

Question 18

What other mechanical process can negate the effect of prestressing?

Answer 18

Steel relaxation

Question 19

How can creep be accounted for?

Answer 19

Decreasing the water in the concrete mix design

Adding admixtures that oppose creep

Question 20

How can internal metal tendons be used to resist bridge collapse?

Answer 20

Putting the tendons in an “M” configuration means that tensile forces from loading will lift the centre of the bridge

Question 21

What is wet corrosion?

Answer 21

When metal is in contact with an electrolyte which ionises the metal and can form voluminous products which reduce structural integrity by creating internal stresses. This can lead to complete failure

Question 22

How is reinforced concrete designed to oppose corrosion?

Answer 22

Concrete is designed to have a high pH, creating a passive layer around the steel which prevents corrosion

Question 23

What are 3 types of corrosion?

Answer 23

Pitting corrosion: Corrosion localised on a specific point
Stress corrosion: Corrosion creates a crack which can then propagate due to applied stresses
Corrosion fatigue: Reduced fatigue life as a result of minor corrosions

Question 24

What are 3 ways corrosion can be stayed?

Answer 24

Protective Coatings
Cathodic Protection: Creating an anode which has no structural purpose but is corroded instead of the structural members
Passive Coatings: Creating a thin, chemically resistant film across the metal

Question 25

How is fire resistance defined as a metal property?

Answer 25

The time taken for the deflection of a metal member to reach a specific limit under high heat

Question 26

At 400C and 700C how are the metal properties changed?

Answer 26

400C - Young’s Modulus and Yield Stress are greatly reduced

700C - Both the above properties have decreased by 90% each

Question 27

What is intumescent paint?

Answer 27

Paint that expands into a low-density foam when heated, creating a heatproof barrier around the metal