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
