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CIVE40007 – Materials > Durability of concrete > Flashcards

Durability of concrete Flashcards

1
Q

What is durability?

A
  • Ability to resist weathering, chemical attack, abrasion or any forms of deterioration (chemical, physical or mechanical)
  • Ability to perform its intended function and retain its form
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2
Q

What is the design life of bridges?

A
  • 120 yrs
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3
Q

What is the design life of halls of residence?

A
  • 50 yrs
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4
Q

What is involved in all major degradation?

A

Water

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5
Q

What are the drivers for durability?

A
  • Safety
  • Socio-economy
  • Sustainability
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6
Q

What does deterioration involve?

A
  • Deterioration involves physical, chemical or mechanical actions
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7
Q

What can degradation be caused by?

A

Degradation can be caused by external (environment) or internal agents

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8
Q

What are examples of transport properties?

A
  • Absorption
  • Permeation
  • Diffusion
  • Wick action
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9
Q

What do the transport properties determine?

A

Determine the durability of concrete and of steel embedded within it, as well as the effectiveness of structures

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10
Q

What can be transported in hardened concrete?

A
  • Water
  • Vapour
  • Liquids
  • Dissolved ions
  • Gases
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11
Q

What are some deterioration mechanisms?

A
  • Reinforcement corrosion
  • Freeze/thaw
  • Sulphate attack
  • Alkali-aggregate reaction
  • Salt scaling
  • Leaching
  • Acid attack
  • Delayed ettringite formation
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12
Q

What do metals have a tendency to do?

A

Natural tendency to revert (corrode) to original state when exposed to environment

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13
Q

When does corrosion occur?

A

Corrosion occurs when pH is low

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14
Q

Why does steel in concrete not corrode rapidly?

A
  • Metals protected from corrosion in an alkaline solution
  • Concrete pore solution is highly alkaline
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15
Q

What is the dense and impenetrable film called which protects steel from corrosion

A

Passive iron oxide layer

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16
Q

How can pore solution change?

A
  • Concentrated by drying
  • Diluted by wetting, leaching and carbonation
17
Q

How can passive layer break down?

A

By carbonation or chloride attack

18
Q

When does steel corrode?

A

Steel corrodes if O2 and H2O are present

19
Q

At what condition does steel depassivates?

A
  • pH of pore solution < 11
  • Critical chloride threshold level is reached near the steel surface
20
Q

How is calcium carbonate formed from concrete?

A

Carbonic acid reacts with hydration products

21
Q

How can you check for carbonation depth?

A
  • Phenolphthalein test
22
Q

What happens when ph>10 in concrete?

A

Solution turns purple

23
Q

How can you calculate depth of carbonation?

A

D=k√t

24
Q

What is the carbonation coefficient k influenced by?

A
  • Influenced by properties of concrete
  • Increases with temp
25
Q

What are the negative effects of carbonation?

A
  • Reduces pH to ~ 8.5
  • Depassivates steel, corrodes if moisture &oxygen is present
26
Q

How does chloride induced corrosion work?

A
  • Chloride attacks passive layer, acts as catalyst and activates steel to form anode
  • Corrosion occurs if water and oxygen are available
  • Chloride is not consumed
27
Q

What are the sources of chloride?

A
  • Internal - cement
  • External - diffusion, sea water, de-icing salts
28
Q

What damage can corrosion cause?

A
  • Reduces area of steel
  • Reduces load carrying capacity
  • Cracking & loss of bond between rebar and concrete
  • Increases ingress of aggressive agents
  • Spalling and delamination
29
Q

What is an effective way from protecting concrete against freeze-thawing?

A

Air entrainment is an effective way of protecting concrete from freezing and thawing damage. It is achieved by adding a surface active agent in very small dosages to the concrete mixture.

30
Q

What are the effects of freeze-thawing?

A

expansion, cracking, spalling and disintegration

31
Q

How can you make concrete more frost resistant?

A
  • Protect concrete from moisture
  • Use dense cement paste
  • Use frost resistant concrete
  • Increase aggregate fraction
32
Q
A

CIVE40007 – Materials (16 decks)

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