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Materials > Aggregates And Admxitures > Flashcards

Aggregates And Admxitures Flashcards

1
Q

What is an aggregate?

A

A granular material used in construction

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2
Q

How much volume does aggregates of concrete take up?

A

60~80%

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3
Q

What do aggregates within concrete influence?

A

Workability
Density
Strength
Dimensional stability
Durability
Mix design
Cost

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4
Q

What are the three sources of aggregates?

A

Natural

Manufactured

Recycled

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5
Q

What are some examples of natural aggregate sources? (Primary aggregates)

A

Crushed rock

Sand & gravel

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6
Q

What are some examples of manufactured aggregates?

A

Industrial by-products such as

blast furnace slag
Fly ash

Expanded clay

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7
Q

What is an example of recycled aggregates?

A

Construction & demolition waste

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8
Q

What goes into the production of crushed rock aggregates? (4)

A

Removal of material overlying useful aggregates

Drilling and blasting

Processing

Quality control

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9
Q

What goes into sand and gravel aggregate production? (4)

A

Removal of overburden

Extraction dry/wet or dredging

Processing

Quality control

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10
Q

What is the main difference about marine aggregates?

A

Chloride & shell content

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11
Q

What is shell marine aggregate mostly made of and why is it important?

A

Mostly calcium carbonate

Stable in concrete

Strong, but affects workability

Max is 10% of concrete volume

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12
Q

How are manufactured aggregates . . . Manufactured?

A

Raw materials are pelletised then sintered at 1000-1200°C

Gases generated make pellets expand

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13
Q

The interior of a manufactured aggregate looks like black pudding, what am I talking about?

A

Spherical particles with porous centre and low density

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14
Q

What are the advantages of manufactured aggregates

A

Porous, lightweight, you can decide density

More voids, more air trapped, better insulation

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15
Q

Why not choose manufactured aggregates?

A

Weak aggregate

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16
Q

Why do we add aggregates to concrete at all?

A

Increase strength

Increase volume

Increase thermal insulation

Increase dimensional stability

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17
Q

How does primary aggregate strength compare to concrete

A

Aggregate - 200MPa

Cement paste - 30~50MPa

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18
Q

Why would we care about the strength and elastic modulus of our concrete aggregates?

A

They influence the overall mechanical property of concrete

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19
Q

Angular, rough particles require more [] & [] to produce a workable mix, increasing []

A

Water and cement

Cost

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20
Q

What kind of aggregates should be avoided?

A

Flaky and elongated

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21
Q

What is special about gravels?

A

They have rounded edges, smooth surfaces due to attrition from water or glacial transportation

22
Q

What is special about crushed rock?

A

They are angular, have rough surfaces

23
Q

Between gravels and crushed rock, what makes a better aggregate?

A

Crushed rock

24
Q

Why does failure occur when using gravels?

A

Debonding at the concrete-aggregate interface

Low strength

25
Q

Why does failure occur when using crushed rock?

A

Fracturing of the aggregate

High strength

26
Q

Between well graded and gap graded aggregate selections, what is best for concrete?

A

Well graded

Less voids

More workability, reduction of cost

27
Q

An excess of large aggregates leads to . . .

A

Segregation

28
Q

What is the typical porosity of natural aggregates? (3)

A

Igneous 0.5~2%

Sedimentary 5%

Porous sandstone and limestones 10~40%

29
Q

What does porosity influence?

A

Density

Water absorption

Workability

Strength

Mix design

30
Q

What is the definition of a deleterious substance?

A

Any substance that interferes with hydration, setting and hardening of concrete.

May cause swelling and disruptions

31
Q

What are some examples of deleterious substances ? (6)

A

Fine materials - clay, silt,dust

Weak or unsound particles - disintegrate

Gypsum or other sulphites

Chlorides - accelerate steel corrosion

Reactive silica and carbonate - react with alkalis in concrete pores to produce expansive gel

32
Q

What is the interfacial transition zone? (ITZ)

A

30-50micron thick region that surrounds each aggregate particle

33
Q

The ITZ contains (2)

A

Lower cement
Higher porosity

34
Q

What causes the ITZ?

A

Inefficient packing of cement grains on large aggregate

35
Q

How is the ITZ in terms of strength

A

Weak, structurally inferior

36
Q

How is micro cracking at the ITZ caused?

A

When concrete dries, the matrix shrinks, faults often begin at the ITZ

37
Q

What are concrete admixtures?

A

Materials other than aggregates that is added to cement paste in the mixing phase to change fresh/hardened state properties

38
Q

What are the four main categories of admixture?

A

Superplasticizers

Air entraining agents

Accelerators

Retarders

39
Q

What does the superplastisizer do?

A

Decreases surface tension

40
Q

What is surface tension?

A

Tension on the surface of a liquid caused by the attraction of the surface particles to the bulk of the liquid.

This tends to minimise surface area.

41
Q

Superplasticizers consist of

A

Anionic polar group joined to long hydrocarbon chain, which is polar and hydrophilic. (lil worm looking structure)

42
Q

How do Superplasticizers work? (5)

Think about negative sperm cells charging cement particles

A

Absorbed on cement particles, negatively charging them.

cement particles repel each other.

Polar chain reduces surface tension of water, makes cement hydrophilic

Increases mobility of cement particles

Increased cement grain surface area exposure to improve hydration

43
Q

What are the advantages of Superplasticizers?

A

Improves workability without increasing w/c ratio

Consequently allows reduction of w/c ratio whilst keeping workability

More uniform dispersion of cement particles

Improves hydration, increases strength, particularly at early stage

(Think of the solid slumped Superplasticized concrete sample)

44
Q

What are air-entraining agents?

A

As they seem

Consist of anionic polar group joined by long hydrocarbon chain that is non polar & hydrophobic

Non polar
Hydrophobic

45
Q

How do air-entraining agents work?

A

Polar group oriented towards water or cement particles

Non polar & hydrophobic group is directed to air

Covers air bubbles formed during mixing with a sheath of air-entraining molecules which repel each other. Tensile bubble shield

(polar to water and cement, non polar to air)
polar to pool (of water)

46
Q

What are the applications of air-entrained concrete that would be less strong

A

Lightweight concrete produced through higher volume of voids

Reduces likelihood of freeze thaw weathering

47
Q

What is the difference between entrapped and entrained air?

One is trapped, the other is trained, one we don’t want, the other was manufactured

A

Entrapped:

Irregular, non-uniform, closer to top surface.

Cause ugly honeycomb voids

Entrained:

Spherical, uniformly distributed all throughout concrete with 10 micron diameter and spacing factor of 200-250 microns

48
Q

What do accelerators achieve?

A

Accelerated hardening and setting of concrete

Higher rate of heat release

49
Q

Applications of accelerators?

A

Good for cold weather concreting

50
Q

What do retarders achieve?

A

Extends setting time, slows hardening

51
Q

What are some applications of retarders?

A

Allowing further transport of concrete

Helps to not set too fast in hot weather concreting

52
Q

Generally admixtures are not essential or even cheap, but have the advantages of:

A

Improving workability

Accelerating/retarding setting

Control strength development

Achieve higher strength

Enhance durability to:

Frost action
Thermal cracking
Reinforcement corrosion
Sulphate attack

Materials (13 decks)

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