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CIVI 321 > Chemical Admixture > Flashcards

Chemical Admixture Flashcards

1
Q

What are admixtures?

A

The ingredients in concrete other than portland cement, water and aggregate that are added to the mix.

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2
Q

Why use admixtures?

A
  • Modify the properties of hardened concrete: Increase early strength and reduce water content
  • Reduce concrete construction cost: Saving time and increase workability
  • Overcome emergencies during concrete operations: Cold weather casting and Narrow cross-sections
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3
Q

What are the different types of admixtures?

A
  • Air Entrainment
  • Shrinkage Reducers
  • Corrosion Inhibitors
  • ASR inhibitors
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4
Q

What is CSH?

A

The binder (glue) responsible for strength

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5
Q

How does the accelerator affect the mix?

A
  • Increase in CSH
  • Increase in Heat
  • Increase in strength
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6
Q

How does a faster hydration reaction affect the mix?

A
  • Higher strength

- Shorter setting time

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7
Q

When do you use accelerators?

A

Used in cold weather to accelerate construction:

  • Reduces the Curing time
  • Reduces formwork removal time
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8
Q

What are the different types of accelerators?

A
  • Alkali Hydroxides
  • Silicates
  • Organic Compounds
  • Sodium Chloride
  • Calcium Nitrate
  • Calcium Chloride
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9
Q

Which accelerator is the most used?

A

Calcium Chloride, because of availability, low cost and predictable performance.

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10
Q

What does Calcium Chloride do?

A
  • Accelerates cement hydration

- Faster hardening and strength gain

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11
Q

What is the effect of accelerators in microstructure?

A

Change from fibrous structure of CSH to spherulite or lace-like structure is reported by researchers

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12
Q

What is the effect of accelerators on Fresh properties?

A
  • Increases workability
  • Reduces significantly both initial and final stting time
  • Reduces both bleeding rate and bleeding capacity
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13
Q

What is the effect of accelerators on Mechanical Properties?

A
  • Accelerates compressive strength gain at early stage

- Long-term strength is lower than that of a non-accelerated control mixture

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14
Q

What is the effect of accelerators on Durability?

A

Chlorides in CaCl2 increase corrosion of steel. CaCl2 usually banned in pre-stressed concrete.

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15
Q

What happens when the amount of CaCl2 is increased?

A
  • Drying Shrinkage of concrete
  • Alkali Aggregate expansion is increased
  • Leads to lower durability under sulfate attack
  • Leads to reduced frost resistance
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16
Q

What are retarding admixtures used for?

A

Delay the setting or hardening rate for:

  • Hot weather concreting
  • Need longer setting time: difficult placements or special finishing processes.
17
Q

What can be used as a retarding admixture?

A
  • Sugar

- Salt

18
Q

What is the main use of Water reducer admixtures?

A
  • Reduce the mixing water by around 5%
  • Reduce the water-cement ratio
  • Reduce cement content
19
Q

What is the chemical composition of Water reducer?

A
  1. Lignosulfonates
  2. Hydrocarboxylic acid
  3. Carbohydrates
  4. other compounds
20
Q

Lignosulfonate is the..?

A
  • Most used raw material in the manufacture of water reducers
  • Waste liquor in the production of paper making
  • Contain about 30% reducing sugars
  • They tend to entrain air in concrete
21
Q

Hydrocarboxylic acid is…?

A
  • Developed as water reducers in the 1950s
  • Usually synthesized chemically
  • Act also as retarding agents
22
Q

Carbohydrates…?

A
  • Include natural products such as glucose and sucrose

- Act also as retarding agents

23
Q

What is the effect of water reducers on fresh concrete?

A
  • Workability is improved, less cold joints and air pockets
  • Decrease the bleeding rate and capacity
  • Heat of Hydration: Delay the rapid heat evolution
24
Q

What is the effect of water reducers on hardened concrete?

A
  • Early strength can be decreased due to retarding effect, but 28 days strength is increased beyond what could be expected from reduction of w/c ratio.
  • Better dispersion of cement grains leads to better particle packing and higher degree of hydration at later ages.
25
Q

What is the effect of water reducers on durability?

A
  • Increase frost durability (by about 39%)
  • Less expansion under sulfate attack
  • Decrease chloride diffusion and enhance corrosion durability
26
Q

What are super-plasticizers?

A
  • Essentially High-range Water Reducers
  • Produce flowing concrete with high slump
  • Reduce bleeding
  • Extended-slump-life plasticizer reduces slump loss
27
Q

Explain Air-entraining admixtures?

A

The process by which small air bubbles are incorporated into concrete at the mixing stage and remain dispersed in concrete at the hard state.

28
Q

In which situation is air entraining admixtures essential used?

A

For the durability of concrete under freezing-thawing cycles.

29
Q

Chemical compositon of air entraining admixtures?

A
  • Belong to a class of chemicals called surfactants (short ffor surface active substances)
  • Surfactant is material whose molecules absorb strongly on air-water or air solid interfaces.
30
Q

What is spacing factor?

A
  • An index related to the maximum distance of any point in the cement paste from the periphery of an air void.
  • ASTM 457: Less than 0.2mm (or 0.008in)
31
Q

What is specific surface?

A
  • Surface area of a quantity of air voids that have a total volume of one cubic inch
  • ASTM 457: 24mm2/mm3 or more
32
Q

Proportioning Air Entrained Concrete

A
  • Air causes increase in slump and decrease in strength

- Less water is required, w/c ratio can be decreased to compensate for strength loss

33
Q

What are waterproofing admixtures?

A
  • Reduce water permeability (under pressure)

- Those that give water repellency are termed damp-proofing admixtures.

34
Q

What are corrosion inhibitors?

A
  • Control corrosion of steel reinforcement

- Dosage dependent on anticipated chloride level

35
Q

AAR inhibiting admixtures?

A
  • Soluble salts of lithium, barium and sodium reduce expansion due to alkali-silica reaction
  • Lithium salts and ferric chloride have been reported to reduce expansion due to alkali-carbonate reaction.

CIVI 321 (7 decks)

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