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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
Why does failure occur when using crushed rock?
Fracturing of the aggregate High strength
26
Between well graded and gap graded aggregate selections, what is best for concrete?
Well graded Less voids More workability, reduction of cost
27
An excess of large aggregates leads to . . .
Segregation
28
What is the typical porosity of natural aggregates? (3)
Igneous 0.5~2% Sedimentary 5% Porous sandstone and limestones 10~40%
29
What does porosity influence?
Density Water absorption Workability Strength Mix design
30
What is the definition of a deleterious substance?
Any substance that interferes with hydration, setting and hardening of concrete. May cause swelling and disruptions
31
What are some examples of deleterious substances ? (6)
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
What is the interfacial transition zone? (ITZ)
30-50micron thick region that surrounds each aggregate particle
33
The ITZ contains (2)
Lower cement Higher porosity
34
What causes the ITZ?
Inefficient packing of cement grains on large aggregate
35
How is the ITZ in terms of strength
Weak, structurally inferior
36
How is micro cracking at the ITZ caused?
When concrete dries, the matrix shrinks, faults often begin at the ITZ
37
What are concrete admixtures?
Materials other than aggregates that is added to cement paste in the mixing phase to change fresh/hardened state properties
38
What are the four main categories of admixture?
Superplasticizers Air entraining agents Accelerators Retarders
39
What does the superplastisizer do?
Decreases surface tension
40
What is surface tension?
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
Superplasticizers consist of
Anionic polar group joined to long hydrocarbon chain, which is polar and hydrophilic. (lil worm looking structure)
42
How do Superplasticizers work? (5) Think about negative sperm cells charging cement particles
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
What are the advantages of Superplasticizers?
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
What are air-entraining agents?
As they seem Consist of anionic polar group joined by long hydrocarbon chain that is non polar & hydrophobic Non polar Hydrophobic
45
How do air-entraining agents work?
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
What are the applications of air-entrained concrete that would be less strong
Lightweight concrete produced through higher volume of voids Reduces likelihood of freeze thaw weathering
47
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
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
What do accelerators achieve?
Accelerated hardening and setting of concrete Higher rate of heat release
49
Applications of accelerators?
Good for cold weather concreting
50
What do retarders achieve?
Extends setting time, slows hardening
51
What are some applications of retarders?
Allowing further transport of concrete Helps to not set too fast in hot weather concreting
52
Generally admixtures are not essential or even cheap, but have the advantages of:
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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