Flashcards in Chemical Admixture Deck (35):
What are admixtures?
The ingredients in concrete other than portland cement, water and aggregate that are added to the mix.
Why use admixtures?
- 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
What are the different types of admixtures?
- Air Entrainment
- Shrinkage Reducers
- Corrosion Inhibitors
- ASR inhibitors
What is CSH?
The binder (glue) responsible for strength
How does the accelerator affect the mix?
- Increase in CSH
- Increase in Heat
- Increase in strength
How does a faster hydration reaction affect the mix?
- Higher strength
- Shorter setting time
When do you use accelerators?
Used in cold weather to accelerate construction:
- Reduces the Curing time
- Reduces formwork removal time
What are the different types of accelerators?
- Alkali Hydroxides
- Organic Compounds
- Sodium Chloride
- Calcium Nitrate
- Calcium Chloride
Which accelerator is the most used?
Calcium Chloride, because of availability, low cost and predictable performance.
What does Calcium Chloride do?
- Accelerates cement hydration
- Faster hardening and strength gain
What is the effect of accelerators in microstructure?
Change from fibrous structure of CSH to spherulite or lace-like structure is reported by researchers
What is the effect of accelerators on Fresh properties?
- Increases workability
- Reduces significantly both initial and final stting time
- Reduces both bleeding rate and bleeding capacity
What is the effect of accelerators on Mechanical Properties?
- Accelerates compressive strength gain at early stage
- Long-term strength is lower than that of a non-accelerated control mixture
What is the effect of accelerators on Durability?
Chlorides in CaCl2 increase corrosion of steel. CaCl2 usually banned in pre-stressed concrete.
What happens when the amount of CaCl2 is increased?
- Drying Shrinkage of concrete
- Alkali Aggregate expansion is increased
- Leads to lower durability under sulfate attack
- Leads to reduced frost resistance
What are retarding admixtures used for?
Delay the setting or hardening rate for:
- Hot weather concreting
- Need longer setting time: difficult placements or special finishing processes.
What can be used as a retarding admixture?
What is the main use of Water reducer admixtures?
- Reduce the mixing water by around 5%
- Reduce the water-cement ratio
- Reduce cement content
What is the chemical composition of Water reducer?
2. Hydrocarboxylic acid
4. other compounds
Lignosulfonate is the..?
- 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
Hydrocarboxylic acid is...?
- Developed as water reducers in the 1950s
- Usually synthesized chemically
- Act also as retarding agents
- Include natural products such as glucose and sucrose
- Act also as retarding agents
What is the effect of water reducers on fresh concrete?
- Workability is improved, less cold joints and air pockets
- Decrease the bleeding rate and capacity
- Heat of Hydration: Delay the rapid heat evolution
What is the effect of water reducers on hardened concrete?
- 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.
What is the effect of water reducers on durability?
- Increase frost durability (by about 39%)
- Less expansion under sulfate attack
- Decrease chloride diffusion and enhance corrosion durability
What are super-plasticizers?
- Essentially High-range Water Reducers
- Produce flowing concrete with high slump
- Reduce bleeding
- Extended-slump-life plasticizer reduces slump loss
Explain Air-entraining admixtures?
The process by which small air bubbles are incorporated into concrete at the mixing stage and remain dispersed in concrete at the hard state.
In which situation is air entraining admixtures essential used?
For the durability of concrete under freezing-thawing cycles.
Chemical compositon of air entraining admixtures?
- 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.
What is spacing factor?
- 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)
What is specific surface?
- Surface area of a quantity of air voids that have a total volume of one cubic inch
- ASTM 457: 24mm2/mm3 or more
Proportioning Air Entrained Concrete
- Air causes increase in slump and decrease in strength
- Less water is required, w/c ratio can be decreased to compensate for strength loss
What are waterproofing admixtures?
- Reduce water permeability (under pressure)
- Those that give water repellency are termed damp-proofing admixtures.
What are corrosion inhibitors?
- Control corrosion of steel reinforcement
- Dosage dependent on anticipated chloride level