Admixtures Flashcards
What is adm.?
Chemical admx. are ingredients in concrete other than OPC, Water and aggregate that are added to the mix.
Why use adm.?
Modifies the properties of hardened concrete
High Early Strength
Examples of admx. + use
Accelerating Admx. (increase HE;high early strength+saving time+cold weather casting)
Water-reducing admixtures (reduce water content+increase workability)
How does the accelerating adm. accelerates the early strength? Does it impact the setting time?
By accelerating the hydration Rate (high CSH, high heat, high strength). Yes, the higher the strength, the shorter the setting time.
When to accelerating adm?
Used in cold weather to accelerate construction by reducing the curing time and formwork removal time.
What are the type of accelerators (name 3)?
Alkali hydroxydes
Sodium chloride
CALCIUM CHLORIDE
Silicates
Describe the Calcium chloride accelerator
Since 1873
Available
Low $
Predictable performance (exact mechanism of this catalytic effect is not well understood)
Accelerates Cement Hydration
Faster hardening and strength gain
Effect on microstructure by acc. admx.?
Change the fibrous structure of CSH to spherulite
Effect on fresh prop. by acc. admx.?
increases workability, lower initial and finale setting time, lower bleeding rate and bleeding capacity
Effect on mech. prop. by acc. admx.?
increases compressive strength at early stage
lower long term compressive strength
What is the price to pay when adding acc. admx?
LOWER DURABILITY
Increases Corrosion
Increases drying shrinkage
Increases ASR
lower Durability to sulfate attacks
What is the purpose of retarding adm.?
Delay setting/hardening of concrete (when super hot/have difficult placement)
SLOW setting rate (loose workability; reduces the amount of slump in longer time)
Why water reducer adm?
IMPROVE workability all by adding long term strength (low w/c)
How can water red. adm. increase workability by reducing water content?
By dispersing the cement particles entrapping the water.
What are the 4 main compounds of water red. adm?
Lignosulfonates
Hydrocarboxylic acid
Carbohydrates
Other compounds
Description of Lignosulfonates in water red. adm
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
Description of Hydrocarboxylic acid
in water red. adm
Developed in 50ies
Act also as retarding agent
Description of Carbohydrates
in water red. adm
Acts as retarding agents; contains glucose and sucrose
What are the effects on fresh concrete by adding water reducer adm?
Decrease bleeding rate and capacity
INCREASE WORKABILITY
Delay the rapid heat evolution
What are the effects on hardened concrete when adding water reducer adm?
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 )
What are the effects on durabilty when adding water reducer adm?
Increase frost durability
Less expansion during sulfate attacks
More corrosion durability by decreasing chloride diffusion
What are the 4 types of water red adm?
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)
Describe 1st gen. water red. adm.
less than 5% water reduction
retarding effects
Describe 2nd gen water red. adm.
Mid range wat. red. adm.
Water reduction content 6 to 12%
reduce cement content
reduce w/c ratio
No retardation
Improve placability and finishibility
Describe 3rd gen water red. adm.
Reduce water content 12 to 30%
Reduce w/c ratio:
Compressive strength > 70 MPA
Increased early strength gain
Reduced CL ion penetration
Describe 4th gen. water red. adm.
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.
What are the main Cement-superplasticizer compatibility points?
- C3A absorbs sulphonate type superplasticizers; therefore need more superplasticizer to achieve a certain workability
- C3A/SO4 ratio; SO4 reacts with C3A; Sulphonated superplasticizers can retard the hemi-hydrate/gypsum conversion and react with with C3A
- Influence alkali content of cement; water reduction with superplasticizer is more achievable with low alkali content cement
Which admx is necessary for durability of freezing-thawing cycles?
Air entrainer
How is air is generated?
By mixing (entrapped air)
AEA stabilize the air (no generation)
What are the chemical composition of AEA?
Class of chemicals: surfactants –>material that adsorb (ions/atoms gathering to a surface) strongly on air-water or air solid
What is spacing factor? How can it be measured?
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.
For AEA, more SA –>?
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)
As size on aggregates increases, Air requirements _____
decreases
The higher the proportion of Fine agg., the ___ air generated.
more
What does higher temperature leads in air content?
High T— > lower air content
The higher the slump, the ___ air content?
higher the air content
What does air entrainer in concrete causes?
Increase slump and decrease in strength.
Increase workability
lower ASR
lower strength
increase freeze-thaw resistance
Name other chemical admixtures
Colouring admixtures
Waterproofing agents (reduce permeability)
Damp-proofing admx. (water repellency)
Corrosion inhibitors
Alkali- Aggregate Reaction Inhibiting admixtures
LOW SPACING FACTOR –>
MORE FROST RESISTANCE
