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Materials > Durability > Flashcards

Durability Flashcards

1
Q

Why is it very important to consider deterioration when designing structures?

A

Severe premature deterioration can occur if durability is not adequatly considered

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2
Q

What can concrete deterioration lead to?

A

Safety, financial and environmental concerns

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3
Q

What are the two definitions of durability?

A

“ability to resist weathering, chemical attack, abrasion or any forms of deterioration, when exposed to its intended environment”

“ability to perform its intended funtion, retain its form, required strength and servicibility, throughout its expected design life”

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4
Q

What is the definition of design life?

A

The minimum period a structure is expected to perform without significant loss of utility and without excessive maintenance

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5
Q

What are the transport properties (penetrability)?

A
  • Permeation
  • Diffussion
  • Absorption
  • Wick action
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6
Q

What is the definition of permeation?

A

Flow induced by a pressure gradient

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7
Q

What is the definition of diffusion?

A

Flow induced by a concentration gradient

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8
Q

What is the definition of absorption?

A

flow induced by capillary action into unsaturated concrete

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9
Q

What is the definition of wick action?

A

Flow induced by combination of permeability, diffusion and absorption in structures exposed to water on one side, and drying on the other

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10
Q

What chemical is involved in all types of major degredation?

A

Water

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11
Q

What controls he rate of deterioration?

A

The movement of water and dissolved aggressive agents by transport mechanisms

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12
Q

Why is transport particularly relevant to conrete

A

It is highly porous and contains many cracks and porous interfaces
Contains steel, which can be corroded from inside the concrete
Long service lives are expected

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13
Q

What are some deterioration mechanisms?

A
  • Reinfocement corrosion: carbonation-induced, chloride-induced
  • Freeze/ thaw
  • Sulphate attack
  • Alkali aggregate reaction
  • salt scaling
  • leaching
  • acid attack
  • delayed ettringite formation
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14
Q

What causes corrosion in metals?

A

Reacting with oxygen and moisture to go back to their orginal oxide form

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15
Q

Why is metal corrosion such an important issue?

A

It presents a serious safety concern

It can generate a huge cost to fix

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16
Q

Do acids or alklais protect or corrode metals?

A 
Alkalis = protection
Acids = corrosion
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17
Q

Why does concrete help to reduce corrosion of metal reinforcements?

A

It is strongly alkalinic

Provides protection and regenerates as long as alkaline environment is maintained

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18
Q

What helps protect reinforcements in concrete?

A

An iron oxide layer formed by the alkali (passive layer)

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19
Q

How do you increase the concentration of the alkalis in conrete?

A

Drying

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20
Q

What decreases the concentration of alkalis in concrete

A

Wetting, leaching and carbonation

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21
Q

What is the pH in concrete?

A

13.5

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22
Q

What breaks down the passive layer?

A

Carbonation and chloride ingress

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23
Q

At what pH does steel start to depassivate?

A

<11

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24
Q

What factors other than a reduced pH are needed for corrosion?

A

The presence of oxygen and water

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25
Q

What causes carbonation?

A

The diffusion of CO2 from atmosphere and dissolves into the pore solution

26
Q

What is the product of carbonation in concrete?

A

Carbonic acid

27
Q

What is the produce of the reaction between carbonic acid and the hydration products?

A

Calcium Hydroxide - CH which precipitates calcium carbonate

28
Q

What is formed when C-S-H is carbonated?

A

CaCO3 and silica gel

29
Q

How do we detect and measure carbonation depth?

A

Phenolphthtalein solution - pH indicator goes from colourless to purple @pH<9.5
Colourless indicates the area of the carbonated concrete

30
Q

What is the equation for the depth of carbonation?

A 
D = K*sqrt(t)
K = mm/sqrt(year)
31
Q

Where does carbonation occur from?

A

The exposed concrete surface

32
Q

Does what happens to the rate of carbonation over time and why?

A

Slows down as CO2 has to move through the carbonated zone

33
Q

What slows down the rate of corrosion?

A

Dense, higher alkalinity, dry (no reaction), fully saturated

34
Q

At what point does corrosion start?

A

when the carbonation front reaches the steel surface

35
Q

What is the pH reduced to during carbonation?

A

~8.5

36
Q

Where does chlorine tend to come from during chloride induced corrosion?

A

Sea water or de-icing salts

37
Q

Why is chloride-induced corrosion particularly problematic?

A

It doesn’t require a drop in pH

38
Q

What happens during chloride-induced corrosion?

A

Chloride destroys the passive layer.
Forms ferrous chloride at the intermediate phase
CHloride is regenerated to form deep pitting corrosion

39
Q

What role do anodes and cathodes play in chloride-induced corrosion?

A

Fe2+ ions at the anode dissolve into solution
Electrons flow through rebar into the cathod and combine with water oxygen to form OH- ions
Pore solution acts as an electrolyte

40
Q

What kind of chemical reaction is chloride-induced corrosion?

A

Electro-chemical

41
Q

What is the service life of a concrete structure defined as?

A

The time taken for corrosion to initate plus the time taken to reach a critical limit state

42
Q

What are types of critical limits in concrete?

A
  • cracking
  • spalling
  • delamination of the cover
  • strucutral collapse
43
Q

What are some of the consequences of corrosion damage?

A
  • reduced area of steel
  • reduced load carrying capacity
  • cracking and loss of bond between rebar and concrete
  • increases risk of ingress of aggressive agents
44
Q

What is frost action?

A

Water expanding on freezing causes hydraulic pressure on pore walls, repetitive freezing and thrawing damage

45
Q

What are some damage types from frost action?

A
  • Progressive damage
  • spalling
  • disintegration
46
Q

How much does capillary pore water expand on freezing?

A

~9%

47
Q

What surfaces are most vulnerable to frost action?

A

Horizontal surfaces, roads, roof slabs.

Especially prone in places where de-icing salts are used

48
Q

How do you make concrete more frost resistant?

A
  • air entrainment
  • using dense concrete and protecting it from water ingress
  • increase amount of aggregate
49
Q

What is conventional sulphate attack?

A

Sulphate ions are transported in water and react with cement paste to form ettringite and gypsum

50
Q

Sulphate attack initially increases strength, so why is it dangerous?

A

The more product that is created the more expansion, the higher the risk of severe cracking

51
Q

What type of sulphate is particularly aggressive and why?

A

Magnesium sulphate - targets C-S-H and softens cement paste

52
Q

What is Thaumasite?

A

When carbonates from aggregate particles changes the reaction between C-S-H and sulphate to rpoduce thaumasite, replacing C-S-H and causing expansion

53
Q

Why is thaumasite degredation so dangerous?

A

It casues the bond between aggregate and cement to decrease and the concrete to soften and eventually disintegrate

54
Q

What is an alkali-aggregate reaction?

A

When alklais from the pore solution and reactive aggregates form a gel that can absorb water

55
Q

Why is an alkali aggregate reaction dangerous?

A

Gel expands as it takes on water - gel is confined and internal pressure leads to cracking in the aggregate and paste

56
Q

What is the most common form of alkali-aggregate reaction

A

Alkali silica reaction

the other common form is an alkali carbonate reaction

57
Q

What % of Na2O should be avoided when used with reactive aggregates?

A

> 0.6% will cause significant expansion and then damage

58
Q

What can be done to reduce the chances of alkali-aggregate reactions?

A

Use low alkali cement

59
Q

What is the range of crack widths from alkali aggregate reactions?

A

0.1-10mm (similair to sulphate attack, freezing/thaw or plastic shrinkage)

60
Q

What is the identifying feauture of alkali aggregate reactions?

A

Colourless jelly like exudation on the cracks

Materials (13 decks)

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