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CIVI 321 > Admixtures > Flashcards

Admixtures Flashcards

1
Q

What is adm.?

A

Chemical admx. are ingredients in concrete other than OPC, Water and aggregate that are added to the mix.

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2
Q

Why use adm.?

A

Modifies the properties of hardened concrete
High Early Strength

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3
Q

Examples of admx. + use

A

Accelerating Admx. (increase HE;high early strength+saving time+cold weather casting)
Water-reducing admixtures (reduce water content+increase workability)

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4
Q

How does the accelerating adm. accelerates the early strength? Does it impact the setting time?

A

By accelerating the hydration Rate (high CSH, high heat, high strength). Yes, the higher the strength, the shorter the setting time.

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5
Q

When to accelerating adm?

A

Used in cold weather to accelerate construction by reducing the curing time and formwork removal time.

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6
Q

What are the type of accelerators (name 3)?

A

Alkali hydroxydes
Sodium chloride
CALCIUM CHLORIDE
Silicates

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7
Q

Describe the Calcium chloride accelerator

A

Since 1873
Available
Low $
Predictable performance (exact mechanism of this catalytic effect is not well understood)
Accelerates Cement Hydration
Faster hardening and strength gain

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8
Q

Effect on microstructure by acc. admx.?

A

Change the fibrous structure of CSH to spherulite

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9
Q

Effect on fresh prop. by acc. admx.?

A

increases workability, lower initial and finale setting time, lower bleeding rate and bleeding capacity

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10
Q

Effect on mech. prop. by acc. admx.?

A

increases compressive strength at early stage
lower long term compressive strength

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11
Q

What is the price to pay when adding acc. admx?

A

LOWER DURABILITY
Increases Corrosion
Increases drying shrinkage
Increases ASR
lower Durability to sulfate attacks

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12
Q

What is the purpose of retarding adm.?

A

Delay setting/hardening of concrete (when super hot/have difficult placement)
SLOW setting rate (loose workability; reduces the amount of slump in longer time)

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13
Q

Why water reducer adm?

A

IMPROVE workability all by adding long term strength (low w/c)

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14
Q

How can water red. adm. increase workability by reducing water content?

A

By dispersing the cement particles entrapping the water.

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15
Q

What are the 4 main compounds of water red. adm?

A

Lignosulfonates
Hydrocarboxylic acid
Carbohydrates
Other compounds

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16
Q

Description of Lignosulfonates in water red. adm

A

The most used raw material in the manufacture of water reducers;
Waste liquor in production of paper making
30% of sugars
entrain air in concrete

17
Q

Description of Hydrocarboxylic acid
in water red. adm

A

Developed in 50ies
Act also as retarding agent

18
Q

Description of Carbohydrates
in water red. adm

A

Acts as retarding agents; contains glucose and sucrose

19
Q

What are the effects on fresh concrete by adding water reducer adm?

A

Decrease bleeding rate and capacity
INCREASE WORKABILITY
Delay the rapid heat evolution

20
Q

What are the effects on hardened concrete when adding water reducer adm?

A

Low early strength
Higher long term strength than ordinary concrete
(better dispersion of cement grains leads to better particle packing and higher degree of hydration later ages )

21
Q

What are the effects on durabilty when adding water reducer adm?

A

Increase frost durability
Less expansion during sulfate attacks
More corrosion durability by decreasing chloride diffusion

22
Q

What are the 4 types of water red adm?

A

1st gen
2nd gen: mid range wat. red. adm.
3rd gen: high range wat. red. adm.
4th gen: Super-plasticizers (type1: plasticizing; type 2: plasticizing and retarding)

23
Q

Describe 1st gen. water red. adm.

A

less than 5% water reduction
retarding effects

24
Q

Describe 2nd gen water red. adm.

A

Mid range wat. red. adm.
Water reduction content 6 to 12%
reduce cement content
reduce w/c ratio
No retardation
Improve placability and finishibility

25
Q

Describe 3rd gen water red. adm.

A

Reduce water content 12 to 30%
Reduce w/c ratio:
Compressive strength > 70 MPA
Increased early strength gain
Reduced CL ion penetration

26
Q

Describe 4th gen. water red. adm.

A

Super-plasticizers (type1: plasticizing; type 2: plasticizing and retarding)
high slump (>190mm compared to 50-100 mm to regular concrete)
Reduce bleeding
Extended-slump-life plasticizers reduces slump loss.

27
Q

What are the main Cement-superplasticizer compatibility points?

A
  1. C3A absorbs sulphonate type superplasticizers; therefore need more superplasticizer to achieve a certain workability
  2. C3A/SO4 ratio; SO4 reacts with C3A; Sulphonated superplasticizers can retard the hemi-hydrate/gypsum conversion and react with with C3A
  3. Influence alkali content of cement; water reduction with superplasticizer is more achievable with low alkali content cement
28
Q

Which admx is necessary for durability of freezing-thawing cycles?

A

Air entrainer

29
Q

How is air is generated?

A

By mixing (entrapped air)
AEA stabilize the air (no generation)

30
Q

What are the chemical composition of AEA?

A

Class of chemicals: surfactants –>material that adsorb (ions/atoms gathering to a surface) strongly on air-water or air solid

31
Q

What is spacing factor? How can it be measured?

A

For any concrete subjected to freezing-thawing sequence, there exists critical air spacing called spacing factor beyond which concrete deteriorates rapidly; measured using automated image analysis
maximum distance for water to travel the cement paste to reach an air void.

32
Q

For AEA, more SA –>?

A

More bubbles contact
Note: Finely divided materials cause a drop in the EA: less water available for bubble formation + AEA is adsorbed on high surface area (type 10 & type 30)

33
Q

As size on aggregates increases, Air requirements _____

A

decreases

34
Q

The higher the proportion of Fine agg., the ___ air generated.

A

more

35
Q

What does higher temperature leads in air content?

A

High T— > lower air content

36
Q

The higher the slump, the ___ air content?

A

higher the air content

37
Q

What does air entrainer in concrete causes?

A

Increase slump and decrease in strength.
Increase workability
lower ASR
lower strength
increase freeze-thaw resistance

38
Q

Name other chemical admixtures

A

Colouring admixtures
Waterproofing agents (reduce permeability)
Damp-proofing admx. (water repellency)
Corrosion inhibitors
Alkali- Aggregate Reaction Inhibiting admixtures

39
Q

LOW SPACING FACTOR –>

A

MORE FROST RESISTANCE

CIVI 321 (13 decks)

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