Durability

Question 1

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

Answer 1

Severe premature deterioration can occur if durability is not adequatly considered

Question 2

What can concrete deterioration lead to?

Answer 2

Safety, financial and environmental concerns

Question 3

What are the two definitions of durability?

Answer 3

“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”

Question 4

What is the definition of design life?

Answer 4

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

Question 5

What are the transport properties (penetrability)?

Answer 5

• Permeation
• Diffussion
• Absorption
• Wick action

Question 6

What is the definition of permeation?

Answer 6

Flow induced by a pressure gradient

Question 7

What is the definition of diffusion?

Answer 7

Flow induced by a concentration gradient

Question 8

What is the definition of absorption?

Answer 8

flow induced by capillary action into unsaturated concrete

Question 9

What is the definition of wick action?

Answer 9

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

Question 10

What chemical is involved in all types of major degredation?

Answer 10

Water

Question 11

What controls he rate of deterioration?

Answer 11

The movement of water and dissolved aggressive agents by transport mechanisms

Question 12

Why is transport particularly relevant to conrete

Answer 12

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

Question 13

What are some deterioration mechanisms?

Answer 13

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

Question 14

What causes corrosion in metals?

Answer 14

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

Question 15

Why is metal corrosion such an important issue?

Answer 15

It presents a serious safety concern

It can generate a huge cost to fix

Question 16

Do acids or alklais protect or corrode metals?

Answer 16

Alkalis = protection
Acids = corrosion

Question 17

Why does concrete help to reduce corrosion of metal reinforcements?

Answer 17

It is strongly alkalinic

Provides protection and regenerates as long as alkaline environment is maintained

Question 18

What helps protect reinforcements in concrete?

Answer 18

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

Question 19

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

Answer 19

Drying

Question 20

What decreases the concentration of alkalis in concrete

Answer 20

Wetting, leaching and carbonation

Question 21

What is the pH in concrete?

Answer 21

13.5

Question 22

What breaks down the passive layer?

Answer 22

Carbonation and chloride ingress

Question 23

At what pH does steel start to depassivate?

Answer 23

<11

Question 24

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

Answer 24

The presence of oxygen and water

Question 25

What causes carbonation?

Answer 25

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

Question 26

What is the product of carbonation in concrete?

Answer 26

Carbonic acid

Question 27

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

Answer 27

Calcium Hydroxide - CH which precipitates calcium carbonate

Question 28

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

Answer 28

CaCO3 and silica gel

Question 29

How do we detect and measure carbonation depth?

Answer 29

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

Question 30

What is the equation for the depth of carbonation?

Answer 30

``` D = K*sqrt(t) K = mm/sqrt(year) ```

Question 31

Where does carbonation occur from?

Answer 31

The exposed concrete surface

Question 32

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

Answer 32

Slows down as CO2 has to move through the carbonated zone

Question 33

What slows down the rate of corrosion?

Answer 33

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

Question 34

At what point does corrosion start?

Answer 34

when the carbonation front reaches the steel surface

Question 35

What is the pH reduced to during carbonation?

Answer 35

~8.5

Question 36

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

Answer 36

Sea water or de-icing salts

Question 37

Why is chloride-induced corrosion particularly problematic?

Answer 37

It doesn't require a drop in pH

Question 38

What happens during chloride-induced corrosion?

Answer 38

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

Question 39

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

Answer 39

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

Question 40

What kind of chemical reaction is chloride-induced corrosion?

Answer 40

Electro-chemical

Question 41

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

Answer 41

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

Question 42

What are types of critical limits in concrete?

Answer 42

- cracking - spalling - delamination of the cover - strucutral collapse

Question 43

What are some of the consequences of corrosion damage?

Answer 43

- reduced area of steel - reduced load carrying capacity - cracking and loss of bond between rebar and concrete - increases risk of ingress of aggressive agents

Question 44

What is frost action?

Answer 44

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

Question 45

What are some damage types from frost action?

Answer 45

- Progressive damage - spalling - disintegration

Question 46

How much does capillary pore water expand on freezing?

Answer 46

~9%

Question 47

What surfaces are most vulnerable to frost action?

Answer 47

Horizontal surfaces, roads, roof slabs. | Especially prone in places where de-icing salts are used

Question 48

How do you make concrete more frost resistant?

Answer 48

- air entrainment - using dense concrete and protecting it from water ingress - increase amount of aggregate

Question 49

What is conventional sulphate attack?

Answer 49

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

Question 50

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

Answer 50

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

Question 51

What type of sulphate is particularly aggressive and why?

Answer 51

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

Question 52

What is Thaumasite?

Answer 52

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

Question 53

Why is thaumasite degredation so dangerous?

Answer 53

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

Question 54

What is an alkali-aggregate reaction?

Answer 54

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

Question 55

Why is an alkali aggregate reaction dangerous?

Answer 55

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

Question 56

What is the most common form of alkali-aggregate reaction

Answer 56

Alkali silica reaction | the other common form is an alkali carbonate reaction

Question 57

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

Answer 57

>0.6% will cause significant expansion and then damage

Question 58

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

Answer 58

Use low alkali cement

Question 59

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

Answer 59

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

Question 60

What is the identifying feauture of alkali aggregate reactions?

Answer 60

Colourless jelly like exudation on the cracks