(Aktu Btech) Concrete Technology Important Unit-2 Chemical and Mineral Admixtures

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Q1. Explain the accelerators with suitable example. Also give the functions of accelerators.

Ans. Accelerator: An admixture is use to speed up the initial set of concrete is called accelerator. 

Examples: Calcium chloride, sodium nitrate, calcium nitrate, etc. 

Functions: Following are the functions of accelerators: 

• 1. These are added to concrete either:
  • i. To speed up the hydration of hydraulic cement and, as a result, the development of strength.
  • ii. To shorten the setting time.
• 2. An increase in the rate of early strength development may help in:
  • i. Earlier removal of forms,
  • ii. Reduction of required period of curing, and 
  • iii. Earlier placement of structure in service.
• 3. When the concrete is to be set at a low temperature, accelerating admixtures are also employed. placed. 
• 4. The benefits of reduced time of setting may include:  
  • i. Early finishing of surface, 
  • ii. Reducing the amount of pressure applied to forms or the amount of time they are under hydraulic pressure; and 
  • iii. More effectively repairing leaks under hydraulic pressure.
• 5. With the advent of strong accelerators, it is now simple to do operations like concreting, basement waterproofing, and underwater repairs of waterfront structures in tidal zones.

Q2. What is air-entrained concrete ? What are the air-entraining agents ? What are factors affecting the air-entrainment in the concrete ?

Ans. A. Air-Entrained Concrete: 

• 1. The internal formation of microscopic air bubbles in concrete is known as air-entrainment. By incorporating an air entraining chemical into the mixture, a concrete manufacturer creates the bubbles. When plastic concrete is mixed, air bubbles are produced, and the majority of these bubbles survive to become a component of the hardened concrete.
• 2. Each cubic foot of it includes billions of microscopic air cells. These air pockets create microscopic chambers for frozen water to expand into, relieving internal pressure on the concrete.
• 3. It is made with air-entraining Portland cement or by adding air-entraining agents while the concrete is being mixed on the job, both under strict engineering supervision.
• 4. Typically, four to seven percent of the concrete’s volume is made up of entrained air.

B. Air-entraining Agents: Following are the air-entrainment agent used in the concrete: 

• 1. Natural resins from wood.
• 2. Vegetable and animal fats and oils, including stearic and oleic acids found in tallow and olive oil.
• 3. A variety of wetting agents, such as sulfonated organic compounds or alkali salts.
• 4. Acidic resin soaps that are water soluble.
• 5. Other substances, such as hydrogen peroxide, aluminium powder, sodium salts of petroleum sulphonic acids, etc.

C. Factor Affecting Air Entrainment: Following are the factor affect the air entrainment: 

• 1. Type and quantity of air entraining agents used. 
• 2. Water cement ratio of mix.
• 3. Type and grading of aggregates. 
• 4. Mixing time. 
• 5. Temperature. 
• 6. Type of cement. 
• 7. Influence of compaction. 
• 8. Admixtures other than air entraining agents used.

Q3. What are the effects of air entrainment admixture on the properties of concrete ?

Ans. Effect of Air Entrainment on Concrete Properties: Following are the effect of air entrainment on concrete properties: 

• 1. A loss of strength.
• 2. An increase in workability.
• 3. Enhanced ability to withstand freezing and thawing.
• 4. It lessens the propensity for segregation.
• 5. Lessens bleeding and lactation.
• Reduces permeability by six.
• 7. Strengthens defence against chemical assault.
• 8. Allows for a decrease in the cost, heat, and water content of hydration.
• 9. Decreases elastic modulus, alkali aggregate reactivity, and unit weight.
• 10. Increase the resistance of concrete to de-icing salts and climate-related freezing and thawing cycles.

Q4. Describe the effect of superplasticizer on the properties of fresh and hardened concrete. 

Ans. A. Effect on Fresh Concrete: 

• 1. Superplasticizers make materials more workable. The result is influenced by the type, dosage, and timing of addition (best with mixing water). Water usage is decreased by 15 to 30 percent. The resulting concrete is stronger and less permeable.
• 2. Superplasticizers generate workability that is higher than average for 30 to 60 minutes before it rapidly declines.
• 3. Bleeding is typically reduced (less water). When creating flowing concrete, care must be taken to prevent bleeding and segregation.

B. Effect on Hardened Concrete: 

• 1. Water reduction enables the creation of concrete with high strength.
• 2. Superplasticized concrete shrinks as much as or less than regular concrete.
• 3. Superplasticized concrete creeps similarly to reference concrete creep.
• 4. By using superplasticizers, the link between concrete and reinforcing steel can be improved.

C. Effect on Durability: 

• 1. Less permeability and increased strength and durability result from a lower water/cement ratio.
• 2. The necessary distance between air bubbles in typical concrete is 200 m. The value is exceeded in concrete that has been superplasticized. For air entrained superplasticized concrete, better freeze-thaw durability can be anticipated.
• 3. Resistance to sulphate attack relies on the specific mechanism and method of exposure. Better durability for expansion can result from lesser permeability (less surface penetration).
• 4. A lower water/cement ratio results in less permeability and better steel corrosion and penetration resistance to chloride.
• 5. Reduced moisture penetration caused by a lower water/cement ratio could increase the cement’s resistance to alkali expansion.

Q5. Discuss fly ash in concrete. Give the advantages and disadvantages of fly ash.

Ans. A. Fly Ash: One of the byproducts of combustion, fly ash is made up of the tiny particles that rise with the flue gases. Fly ash is typically used in an industrial setting to describe the ash created from coal burning.

B. Advantages of Fly Ash in Concrete: Following are the advantages of fly ash: 

• 1. Less permeability and improved sulphate attack resistance.
• 2. Less porosity and shrinking because to the lower water content.
• 3. Increased performance in terms of long-term strength and durability.
• 4. Workability is improved while the rate of bleeding is decreased.
• 5. Improved workability at the same water content or a lower water content for a given workability.

C. Disadvantages of Fly Ash in Concrete: Following are the disadvantages of fly ash: 

• 1. Fly ash-containing concrete is more difficult to color-control than concrete made solely of Portland cement.
• 2. Concrete mixtures high in fly ash frequently need additional air-entraining additive because fly ash reduces the quantity of air entrainment.
• 3. Fly ash admixtures may prolong the setting time of concrete.
• 4. Concrete hardens a few hours after being poured, although the curing procedure could take considerably longer.

Q6. Explain the effects of silica fume on concrete properties. 

Ans. A. Effect of Silica Fume on the Properties of Fresh Concrete: 

• 1. Workability: 
  • i. Reduced workability. 
  • ii. Water usage rises proportionately to the amount of silica fume injected. For every 1% of cement that is replaced, water is required.
  • iii. A mix with less slump and better balance.
• 2. Bleeding and Segregation: 
  • i. As silica fume particles get trapped between two cement grains, bleeding is decreased.
  • ii. Segregation is lessened since there are more solid-to-solid contact sites in the concrete mix, which makes it more cohesive.
• 3. Time of Setting: There is not much of an influence on the initial and final setting times. The increase might be around 30 minutes.
• 4. Plastic Shrinkage: Fresh concrete is vulnerable to plastic shrinkage because silica fume concrete does not exhibit bleeding.

B. Effects of Silica Fume on the Properties of Hardened Concrete: 

• 1. Drying Shrinkage: The inclusion of silica fume does not appreciably impact the long-term shrinkage of concrete.
• 2. Creep: Silica fume-containing concrete will have less creep than Portland cement-based concrete of the same strength.
• 3. Chemical Resistance: The decrease in the CalOH), content of the cement paste, which decreases linearly with the amount of silica fume added, is a significant factor in the enhanced resistance of concrete to acidic and sulphate waters. 
• 4. Alkali Aggregate Reaction: Compared to fly ash, which needs to be replaced by 30% to 40%, less than 10% of silica fume is found to be sufficient for minimizing alkali aggregate expansion.
• 5. Strength: Strength of 62-80 MPa can be easily achieved. 
• 6. Permeability: When 10% of silica fume by weight of cement is added, it reduces the amount of voids in hydrated cement paste, making them practically impermeable even at young ages.
• 7. Freeze and thaw Effect: The effect of silica fume concrete on freeze thaw affect is not very significant. 

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