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Materials > Durability > Flashcards

Durability Flashcards

1
Q

What is durability defined as?

A

The ability to resist weathering, chemical attack, abrasion or any forms of deterioration when exposed to the environment

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2
Q

What is the 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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3
Q

What are the four methods of water ingress

A

permeation

diffusion

absorption

wick action

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4
Q

describe permeation

A

flow induced by pressure gradient

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5
Q

describe diffusion

A

flow induced by concentration gradient

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6
Q

describe absorption

A

flow induced by capillary action into unsaturated concrete

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7
Q

describe wick action

A

flow induced by a combination of permeation, diffusion, absorption. Occurs when one side of concrete is dry while the other is wet

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8
Q

What do we mean by transport?

A

The movement of “something” through hardened concrete

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9
Q

Why is transport relevant to concrete?

A

It is a highly porous material

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10
Q

What do all deterioration mechanisms involve?

A

water and the ingress of other aggressive species

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11
Q

Metals originate in an [] form, corrosion is merely nature returning refined metals to their natural state

A

oxide form

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12
Q

Why doesn’t steel rebar corrode rapidly?

A

metals corrode in acid

concrete is an alkaline solution, so is therefore protective

A passive iron oxide layer forms around rebar, and protects so long as the solution is alkaline

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13
Q

When drying, concrete pore solution is:

A

concrentrated

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14
Q

when wetted, concrete pore solution is:

A

diluted, leaching and carbonation occur

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15
Q

The passive iron oxide layer is destroyed either by

A

carbonation

or chloride attack

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16
Q

steel depassivates when

A

pH of solution is less than 11

and chloride threshold level is reached near the steel surface

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17
Q

steel corrodes if what two molecules are present?

A

O2 and H2O

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18
Q

How does carbonation of concrete begin

A

CO2 from atmosphere dissolves in pore solution to form carbonic acid

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19
Q

Carbonic acid reacts with [] to form [] during carbonation

A

reacts with hydration products

to form calcium carbonate

20
Q

What does carbonation result in?

A

depletion of pH

21
Q

How is the depth of carbonation measured?

A

phenolphthalein test.

Involves spraying a purple solution into a concrete cross section that loses colour with acidity.

22
Q

How do cracks affect carbonation depth?

A

cracks become a site of carbonation, allowing it to continue deeper into the concrete

23
Q

What equation is used to predict carbonation depth

A

D=K*sqrt(t)

D is depth

K is carbonation coefficient (mm/year^0.5)

t is exposure time in years

24
Q

How does chloride-induced corrosion compare to carbonation?

A

More severe

25
Q

How does chloride–induced corrosion occur?

A

chloride attacks passive iron oxide layer

activates steel, forms an anode
corrosion occurs if water and oxygen are available

chloride is not consumed

26
Q

What are two sources of chloride corrosive agents?

A

sea water

de-icing salts

27
Q

reinforcement corrosion is an [] reaction

A

electro chemical - relies on the movement of e-

28
Q

What happens in reinforcement corrosion? (basically rusting process)

A

Iron ions at anode dissolve into solution

Electrons through to cathode, combine with H2O and O2 to form OH- ions

pore solution acts as an electrolyte to complete circuit

29
Q

What are the effects of corrosion damage?

A

reduces area of steel

reduces load carrying capacity (loss of strength)

loss of bond between rebar and concrete

gateway for aggressive agent ingress

30
Q

Total chloride content required to initiate corrosion is []

A

0.4% wt cement

31
Q

What is spalling?

A

When concrete breaks off into smaller peices

32
Q

How does frost damage occur?

A

water enters concrete voids

ice crystals push pore walls, damage cement paste

free thaw cycles induce cumulative damage

33
Q

What are the most vulnerable surfaces to frost damage?

A

horizontal surfaces

road, roof, slabs subjected to wetting and wherever de-ccing is used

34
Q

How do you make concrete frost resistant?

A

Use air-entrainment admixtures

35
Q

How does sulphate enter groundwater?

A

decay of organic matter or from industrial/agriculture activities

36
Q

In simple terms, what is the reaction that governs sulphate attack of concrete?

A

sulphates + cement paste = ettringite + gypsum

37
Q

How does sulphate attack work?

A

sulphate in groundwater penetrates concrete, reacts with cement paste.

38
Q

If the sulphate content exceeds []% weight of cement, it forms excessive [] and []

A

4%

excessive gypsum and ettringite

39
Q

What are the big three hydration products and what are their characteristics?

A

calcium hydroxide - stacked hexagonal shapes

ettringite - fibrous, needle like structure

calcium silicate hydrate - fine grain sponge looking structure

40
Q

Seeing as sulphate attack leads to the formation of new hydration products, it initially leads to an increase in . . . but soon results in . . .

A

strength and density

further expansion, leading to cracking

41
Q

What is the Thaumasite form of sulphate attack?

A

it softens concrete, weakening it

42
Q

How does thaumasite sulphate attack occur?

A

reaction between CSH and carbonate aggregate and sulphate produces thausamite

This replaces CSH, expanding the concrete and softening it

43
Q

What is the basic premise of alkali-aggregate reaction?

A

Alkalis + reactive silica/carbonate + water

=

alkali silicate gel (concrete expansion)

44
Q

what is the most common form of alkali-aggregate reaction?

A

alkali-silica reaction

45
Q

what expansion due to ASR % is concrete expected to fail at?

A

0.3 - 0.4 %

46
Q

What are the consequences of ASR?

A

leaching of colourless gel

facilitates ingress of other aggressive agents

map-cracking

47
Q

Why is ASR particularly harmful?

A

There is no reversing the propagation, you may need to remove and replace parts of the structure once it has started

Materials (13 decks)

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