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Questions and Answers
What is the purpose of using water-reducing admixtures or plasticizers in concrete?
What is the purpose of using water-reducing admixtures or plasticizers in concrete?
- To increase the water-cement ratio
- To reduce the strength of concrete
- To achieve higher strength by decreasing the water-cement ratio at the same workability (correct)
- To decrease the workability of concrete
What is the function of plasticizers in concrete with respect to the heat of hydration?
What is the function of plasticizers in concrete with respect to the heat of hydration?
- Have no effect on heat of hydration
- Decrease heat of hydration (correct)
- Accelerate heat of hydration
- Increase heat of hydration
Which type of compound are plasticizers based on?
Which type of compound are plasticizers based on?
- Finely ground solid materials
- Water-soluble compounds
- Metallic salts
- Organic polymers (correct)
How do plasticizers affect the workability of concrete?
How do plasticizers affect the workability of concrete?
Which purpose do plasticizers serve in relation to cement content?
Which purpose do plasticizers serve in relation to cement content?
What is the relationship between water reduction and strength of concrete?
What is the relationship between water reduction and strength of concrete?
Which type of polymer are plasticizers typically derived from?
Which type of polymer are plasticizers typically derived from?
What is the impact of using plasticizers on cracks in young concrete?
What is the impact of using plasticizers on cracks in young concrete?
How do plasticizers affect the water-cement ratio and workability simultaneously?
How do plasticizers affect the water-cement ratio and workability simultaneously?
What is one common material that is used as a plasticizer in concrete?
What is one common material that is used as a plasticizer in concrete?
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Study Notes
Super Plasticizers
- Sulphonated melamine formaldehyde condensates (SMF) provide 16-25% water reduction with little or no retardation, giving a good finish and no staining in white concrete.
- Sulphonated naphthalene formaldehyde condensates (SNF) offer 16-25% water reduction, but tend to increase the entrapment of larger, unstable air bubbles, with retardation more than SMF but not exceeding 90 minutes.
- Polycarboxylate ether superplasticizers (PCE) give 20-35% water reduction, are relatively expensive but powerful, and require a lower dose due to high efficiency.
Accelerators
- Accelerators increase the rate of hydration of hydraulic cement, shorten the time of set-in concrete, and increase the rate of hardening or strength development.
- Calcium chloride is the most effective accelerator, but its use is limited due to acceleration of corrosion of steel reinforcement and decrease in resistance of cement paste in a sulfate environment.
- Chloride-free accelerators are based on salts of nitrate, nitrite, formate, and thiocyanate.
- WRA (Workability Retention Agent) increases initial concrete compressive strength by 10% and achieves higher concrete density, making it less permeable and more durable.
Mechanisms of Super Plasticizers
- Dispersion: Surface active agents alter the physical-chemical forces at the interface, adsorbing on cement particles, giving them a negative charge, and causing repulsion and disintegration, making free water available for workability.
- Lubrication: Organic agents lubricate the mix, reducing friction and increasing workability.
- Retardation: A thin layer forms over cement particles, protecting them from hydration and increasing the setting time, with most normal plasticizers giving 30-90 minutes of retardation.
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