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CIVE40007 – Materials > Aggregates & Admixtures > Flashcards

Aggregates & Admixtures Flashcards

1
Q

What is concrete?

A

Concrete is a composite material that consists essentially of
aggregate particles embedded in a binding medium.

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2
Q

What is an aggregate?

A

A granular material used in construction. It can be natural, manufactured, or recycled.

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3
Q

What is fine aggregate and what size does it come in?

A
  • It is usually sand
  • 62 micrometers - 4mm
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4
Q

What is coarse aggregate usually and what sizes does it come in?

A
  • Crushed gravel or rocks
  • > 4mm
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5
Q

What do the materials of concrete influence?

A
  • Workability
  • Density
  • Strength
  • Dimension stability
  • Durability
  • Mix design
  • Cost
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6
Q

What are primary/ natural aggregates?

A

Primary aggregates are naturally occurring deposits of minerals that come in the form of crushed rocks, sand, and gravel which can be found along river beds, on the ocean floor, and in most natural environments.

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7
Q

What type of crush rock can you get?

A

Igneous
Sedimentary

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8
Q

What is good about igneous rocks?

A

Tough, strong and excellent

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9
Q

What’s good about sedimentary rocks?

A

They are cost effective and can be extracted near the surface

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10
Q

What are the textures of crushed rock usually like?

A

Rough and angular

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11
Q

Where does gravel and sand originate from?

A

From weathering and erosion of hard rocks by river, glacier, and wind action

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12
Q

What is gravel texture like?

A

Gravel is smooth and rounded from attrition

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13
Q

What are manufactured/secondary aggregates?

A
  • Industrial by products from quarrying or mining
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14
Q

What are recycled aggregates?

A

Construction or demolition waste

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15
Q

What are the stages in the production of crushed rock?

A
  • Removal of overburden (material overlying useful aggregates)
  • Drilling & blasting
  • Processing: crushing, screening, washing, blending & stockpiling
  • Quality control test
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16
Q

What are the stages of sand and gravel production?

A
  • Removal of overburden
  • Extraction (dry or wet working/dredging)
  • Processing: screening, washing, blending, stockpiling
  • Quality control testing
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17
Q

Where is there an increasing demand for sand?

A

In Bangladesh

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18
Q

Where did marine aggregates originate from?

A

Deposited by ‘ancient’ rivers, then submerged due to rising sea levels after the most recent Ice Age

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19
Q

What is the composition of marine aggregates like?

A
  • Similar composition, strength, and other physical properties to land-based aggregates
  • Chloride and shell content
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20
Q

What does shell content affect?

A

Workability

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21
Q

What is shell predominantly made from?

A

CaCO3

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22
Q

What is involved in the production of manufactured aggregates?

A
  • Raw materials pelletised, then sintered at 1000 - 1200 degrees
  • Generation of gases → Expansion (clay)
  • Spherical particles with porous cellular interior, with low density
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23
Q

What influences workability, water demand and mix design?

A
  • Shape, surface texture, size & grading
  • Porosity, water absorption
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24
Q

What influences the overall mechanical properties of concrete?

A
  • Strength
  • Elastic modulus
  • Dimension stability
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25
Q

What influences mix design and density?

A

Specific gravity

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26
Q

What influences workability, bond with cement paste and long term durability?

A

Presence of deleterious substances, cleanliness

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27
Q

What are deleterious substances?

A

A deleterious substance is any substance that, if added to water or is present in the water, would adversely affect the water quality and potentially harm aquatic organisms

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28
Q

What types of shapes can you have?

A
  1. Irregular
  2. Angular
  3. Rounded
  4. Flaky
  5. Elongated
29
Q

What particle shapes should be avoided?

A
  • Flaky
  • Elongated
30
Q

What types of surface are there?

A
  1. Glassy
  2. Smooth
  3. Rough
  4. Crystalline
  5. Honeycombed/ porous
31
Q

What is not good about rough and honeycombed surfaces?

A

Requires more water and cement to produce a workable mixture, increasing the cost

32
Q

What does the splitting tension test show?

A
  • Failure occurs due to de-bonding at the concrete-aggregate interface - low strength
  • Failure occurs due to fracturing of the aggregate - higher strength
33
Q

Why is well graded rock the best?

A

Well graded the best as it will result in a dense structure and good load-bearing capacity.

34
Q

What is the Sieve analysis?

A

The sieve analysis determines the gradation (the distribution of aggregate particles, by size, within a given sample).

35
Q

What is segregation in concrete?

A

Segregation of concrete refers to the separation of the constituent materials in freshly mixed concrete.

36
Q

When does segregation occur?

A

This occurs when the heavier aggregates settle down due to gravity, leaving behind the lighter cement and water mixture on top.

37
Q

What does porosity influence?

A

density, water absorption, workability, strength, mix design, durability

38
Q

What is the porosity of natural aggregates?

A

0.5% - 2%

39
Q

What are the different moisture states?

A
  • Oven-dry
  • Air-dry
  • Saturated & surface dry
  • Wet
40
Q

What are examples of deleterious substances?

A
  • Organic impurities
  • Fine materials
  • Weak or unsound particles
  • Gypsum
  • Chlorides
41
Q

What is the interfacial zone?

A

The interface between aggregates and cement paste.

42
Q

What are properties of the interfacial zone?

A
  • Lower cement content and higher porosity
  • Weak region
43
Q

How thick is the ITZ?

A

~ 30-50µm

44
Q

What are examples of micro-cracking?

A
  • Bond cracks
  • Matrix cracks
45
Q

What are matrix cracks?

A

Cracks within cement

46
Q

What are bond cracks?

A

Cracks on the boundary of aggregate and cement

47
Q

What are potential effects of micro-cracking?

A
  • Reduce overall strength
  • Reduce overall stiffness
  • Permits increased fluid ingress
  • Reduce long-term durability
48
Q

What are concrete admixtures?

A

Material (other than cement, additions, water, and aggregates) that is added to a paste, mortar, or concrete, during the mixing process to modify the fresh and/or hardened state properties.

49
Q

How much admixture should you add to mix?

A

Dosage ≤ 5% by weight of cement

50
Q

What are the two types of admixtures?

A

Chemical and mineral

51
Q

What do chemical admixtures do?

A

Chemical admixtures are used to improve the quality of concrete during mixing, transporting, placement, and curing.

52
Q

What do mineral admixtures do?

A

Mineral admixtures make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties.

53
Q

What are the few functions of admixtures?

A
  • Improve workability
  • Accelerate or retard setting
  • Control strength development
  • Achieve higher strength
54
Q

What can admixtures enhance durability against?

A
  • Frost action
  • Thermal cracking
  • Reinforcement corrosion
  • Sulphate attack
55
Q

What are surfactants?

A

Surface-active agent – compounds that lower surface tension

56
Q

What is surface tension?

A

Tension of the surface of a liquid is caused by the attraction of the surface particles to the bulk of the liquid, which tends to minimize surface area

57
Q

What do super plasticisers consist of?

A

Consists of anionic polar group joined to a long hydrocarbon chain that is polar & hydrophilic

58
Q

How does the super-plasticiser works?

A

Absorbed on cement particles, giving them a negative charge - the polar chain lowers the surface tension of water and makes cement hydrophilic

59
Q

What’s the effect of super-plasticisers?

A
  • Increases mobility of cement particles, water freed from a flocculated cluster is available to lubricate and improve workability
60
Q

What are the advantages of super-plasticisers?

A
  • Improves workability without increasing w/c ratio
  • Allows reduction in w/c ratio without affecting workability
  • Improve hydration
61
Q

What are air-retaining agents?

A

Can reduce the surface tension of fresh concrete to generate uniform, stable, and optimally packed microbubbles during the mixing of concrete

62
Q

What happens to the air entrained voids when the air retaining agent is added?

A

They remain empty and are not filled with hydration products

63
Q

What do air-retaining agents improve?

A

Improve the workability, frost resistance, carbonization resistance, and other durability parameters

64
Q

What do accelerator admixtures do?

A
  • Accelerate setting and hardening
  • Increase rate of early strength development
65
Q

What are examples of accelerators?

A

Calcium nitrite, calcium nitrate, calcium formate, sodium formate

66
Q

What are retarders?

A
  • Used to prevent premature hardening of the cement slurry before it reaches the area to be cemented.
  • Extends setting time
67
Q

What’s the most commonly used retarder?

A

Gypsum

68
Q

How do retarders work?

A

Impedes the dissolution of cement compound

CIVE40007 – Materials (16 decks)

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