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Materials > Aggregates and Admixtures > Flashcards

Aggregates and Admixtures Flashcards

1
Q

What are natural - primary aggregates made from?

A 
Crushed rock (from hard rock formations, then physically processed, igneous makes better aggregate - sedimnetary is cheaper)
Sand and gravel (comes from hard rocks by river, or eroded by glacier and wind action, they can be found on land or dredged)
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2
Q

What are Manufactured - secondary aggregated made from?

A

Industrial by-products (blastfurnace slag, fly ash, expanded clay, require thermal or other processing)

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3
Q

What is the definition of an aggregate?

A

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

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4
Q

How large is the average coarse aggregate?

A

> 4mm in the EU

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5
Q

How large is the average fine aggregate

A

62um - 4mm

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6
Q

What are the particle sizes of silt and clay

A

silt >60um
Clay >2um
Silt and clay are deterious to the properties of concrete

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7
Q

How much of a concrete mix do aggregates make up?

A

70-75% - therefore aggregates should be chosen to enhance the properties

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8
Q

How much of gravel and sand in England and Wales comes from Marine sources

A

~22%

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9
Q

What is the key difference between marine and land aggregates?

A

Chloride and shell content - contributes to corrosion of steel

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10
Q

What are the key aspects of the production of marine aggregates

A

Surveying, dredging, screening, washing, blending, stockpiling and quality control

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11
Q

What is the maximum limit of shell content in concrete

A

<10%

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12
Q

What are shells made of and how do they affect concrete?

A

They are made of CaCO3 and affect workability

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13
Q

How are manufactured aggregates created?

A

They are pelletized then sintered at between 1000-1200C
Gases generated cause them to expand
Spherical particles with porous cellular interior with low density

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14
Q

Why do we add aggregates to concrete?

A

To reduce cost, increase compressive strength, stiffness hardness and abrasion resistance compared to cement paste.

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15
Q

What aggregate properties are important to concrete?

A

Shape, surface texture, size and grading.

We need specific gravity (2.6-2.8)
Strength (200MPa)
Elastic Modulus (55-70 GPa)
and good dimensional stability

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16
Q

What do aggregate properties influence in mixes?

A

angular, rough and porous aggregates require more cement and water. Elongated and flaky mixes can be detremental to stength and durability, as they orientate on one plane and they trap bleed water and air voids
Angular and rough aggregates bond better with cement paste

17
Q

What is the ideal range of porosity in aggregates?

A

< 3-10%

18
Q

What is the effect of a wet porous aggregate?

A

It introduces free moisture to the mix (increases w/c ratio)

19
Q

What is the effect of a dry porous aggregate

A

It can reduce the workability of the mix

20
Q

What should you checn for when using aggregates?

A

Should be relatively clain, low number of impurities, fines, weak or unsound particles

21
Q

Name some Deletrious substances in aggregates

A

Organic: Interfere with hydration, setting and hardening
Fine: (<63um) coat aggregate surface and weakens past aggregate bond
Weak or unsound patricles: disintegrate and develop harmful reactions
Gypsum: Iron pyrites (FeS2) and other sulphates
Chlorides: corrosion
Reactive silica and carbonate: react with alkalis in concrete pore solution to produce expansive gel

22
Q

What causes the ITZ (Interfacial transition zone)?

A

Inefficeit packing of cement grains on large aggregate - wall effect

23
Q

How thick is the ITZ and what is the w/c ratio like?

A

~30-50 um thick, has a higher water content and porosity.

24
Q

What is the effect of the ITZ?

A

Creates a weaker, structurally inferior region

25
Q

Where do microcracks propogate from?

A

The ITZ - known as bond cracks

26
Q

How to microcracks grow?

A

They propogate towards neighboring aggregates as matrix cracks

27
Q

What causes microcracks?

A

When concrete undergoes drying and therefore shrinkage, the shrinkage is restrained by the dense aggregates and causes high stress at the ITZ

28
Q

What is a admixture?

A

Water-based liquid that we add into the concrete mix in small quantities to alter the properties of fresh and hardened concrete.

29
Q

What are the advantages of admixtures?

A

They can bring economic and performance advantages, such as:
Wokability
Accelerate or retard setting
contorl strenght development
Achieve higher strenght
Enhance resistance to temperature, reinforcement corrosion and sulphate attack

30
Q

What are superplasticizers?

A

A type of sulfactant that consist of an ionic polar group joined to a large hydrocarbon chain that is polar and hydrophillic

31
Q

What is the benefit of a superplasticizer?

A

They help to reduce the surface tension of water, dispersing the cmeent particles and increase the mobility and workability of fresh concrete

32
Q

What is an air-entrainer?

A

A type of sulfactant that consists of an ionic polar group joined to a long hydrocarbon chain

33
Q

What is the effect of an air entrainer?

A

They help to separate and disperse microscopic air bubbles

34
Q

What is the difference between entrained and entrapped air?

A

Entrapped air is cause by improper compaction, they are larger and non-uniformly disrtibuted, with more occuring near the top suface. Entrained air is added during the concrete mixing and are closely spaced and uniformly distributed

35
Q

What is an accelerator?

A

Helps cement pahses dissolve in water

36
Q

What is the effect of an accelerator?

A

Causes cocnrete to set and harden quicker by accelerating the hydration process (releases heat)

37
Q

What is the effect of a retarder>

A

Retarders slow down the process of cement dissolution

Materials (13 decks)

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