Durability

Question 1

What is durability defined as?

Answer 1

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

Question 2

What is the design life?

Answer 2

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

Question 3

What are the four methods of water ingress

Answer 3

permeation

diffusion

absorption

wick action

Question 4

describe permeation

Answer 4

flow induced by pressure gradient

Question 5

describe diffusion

Answer 5

flow induced by concentration gradient

Question 6

describe absorption

Answer 6

flow induced by capillary action into unsaturated concrete

Question 7

describe wick action

Answer 7

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

Question 8

What do we mean by transport?

Answer 8

The movement of “something” through hardened concrete

Question 9

Why is transport relevant to concrete?

Answer 9

It is a highly porous material

Question 10

What do all deterioration mechanisms involve?

Answer 10

water and the ingress of other aggressive species

Question 11

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

Answer 11

oxide form

Question 12

Why doesn’t steel rebar corrode rapidly?

Answer 12

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

Question 13

When drying, concrete pore solution is:

Answer 13

concrentrated

Question 14

when wetted, concrete pore solution is:

Answer 14

diluted, leaching and carbonation occur

Question 15

The passive iron oxide layer is destroyed either by

Answer 15

carbonation

or chloride attack

Question 16

steel depassivates when

Answer 16

pH of solution is less than 11

and chloride threshold level is reached near the steel surface

Question 17

steel corrodes if what two molecules are present?

Answer 17

O2 and H2O

Question 18

How does carbonation of concrete begin

Answer 18

CO2 from atmosphere dissolves in pore solution to form carbonic acid

Question 19

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

Answer 19

reacts with hydration products

to form calcium carbonate

Question 20

What does carbonation result in?

Answer 20

depletion of pH

Question 21

How is the depth of carbonation measured?

Answer 21

phenolphthalein test.

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

Question 22

How do cracks affect carbonation depth?

Answer 22

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

Question 23

What equation is used to predict carbonation depth

Answer 23

D=K*sqrt(t)

D is depth

K is carbonation coefficient (mm/year^0.5)

t is exposure time in years

Question 24

How does chloride-induced corrosion compare to carbonation?

Answer 24

More severe

Question 25

How does chloride--induced corrosion occur?

Answer 25

chloride attacks passive iron oxide layer activates steel, forms an anode corrosion occurs if water and oxygen are available chloride is not consumed

Question 26

What are two sources of chloride corrosive agents?

Answer 26

sea water de-icing salts

Question 27

reinforcement corrosion is an [] reaction

Answer 27

electro chemical - relies on the movement of e-

Question 28

What happens in reinforcement corrosion? (basically rusting process)

Answer 28

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

Question 29

What are the effects of corrosion damage?

Answer 29

reduces area of steel reduces load carrying capacity (loss of strength) loss of bond between rebar and concrete gateway for aggressive agent ingress

Question 30

Total chloride content required to initiate corrosion is []

Answer 30

0.4% wt cement

Question 31

What is spalling?

Answer 31

When concrete breaks off into smaller peices

Question 32

How does frost damage occur?

Answer 32

water enters concrete voids ice crystals push pore walls, damage cement paste free thaw cycles induce cumulative damage

Question 33

What are the most vulnerable surfaces to frost damage?

Answer 33

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

Question 34

How do you make concrete frost resistant?

Answer 34

Use air-entrainment admixtures

Question 35

How does sulphate enter groundwater?

Answer 35

decay of organic matter or from industrial/agriculture activities

Question 36

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

Answer 36

sulphates + cement paste = ettringite + gypsum

Question 37

How does sulphate attack work?

Answer 37

sulphate in groundwater penetrates concrete, reacts with cement paste.

Question 38

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

Answer 38

4% excessive gypsum and ettringite

Question 39

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

Answer 39

calcium hydroxide - stacked hexagonal shapes ettringite - fibrous, needle like structure calcium silicate hydrate - fine grain sponge looking structure

Question 40

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

Answer 40

strength and density further expansion, leading to cracking

Question 41

What is the Thaumasite form of sulphate attack?

Answer 41

it softens concrete, weakening it

Question 42

How does thaumasite sulphate attack occur?

Answer 42

reaction between CSH and carbonate aggregate and sulphate produces thausamite This replaces CSH, expanding the concrete and softening it

Question 43

What is the basic premise of alkali-aggregate reaction?

Answer 43

Alkalis + reactive silica/carbonate + water = alkali silicate gel (concrete expansion)

Question 44

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

Answer 44

alkali-silica reaction

Question 45

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

Answer 45

0.3 - 0.4 %

Question 46

What are the consequences of ASR?

Answer 46

leaching of colourless gel facilitates ingress of other aggressive agents map-cracking

Question 47

Why is ASR particularly harmful?

Answer 47

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