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Role of admixtures; superplasticizer

Short answer

Admixtures are materials added during mixing to modify concrete fresh or hardened properties. They can improve workability, control setting, increase strength, reduce permeability, or save cement. Superplasticizers reduce water by 15-30% at equal slump for high-strength concrete, or produce flowing concrete for congested reinforcement and pumping; they are typically dosed at 0.5-2% and added on-site due to slump loss.

Define admixtures. What is the role of admixtures in concrete? Explain the use of superplasticizer in concrete.

Admixture: a material, other than water, aggregate, cement and fibre, added to the concrete in small quantities immediately before or during mixing to modify the properties of fresh or hardened concrete (IS 9103).

Role of admixtures in concrete

  • Improve workability (plasticisers) or reduce water.
  • Control setting (accelerators, retarders) and early strength.
  • Improve strength and durability (superplasticisers, silica fume, fly ash); reduce permeability.
  • Entrain air for frost resistance.
  • Reduce heat of hydration, shrinkage, bleeding and segregation.
  • Give special properties: waterproofing, corrosion inhibition, colour, pumpability, gas formation in grouts.
  • Economy: cement saving.

Use of superplasticiser

Superplasticisers (high-range water reducers, Type F/G) are polymers (naphthalene, melamine, polycarboxylate) that disperse cement flocs and release trapped water.

  1. Reduce water by 15-30 per cent at the same workability, so w/c falls and strength and durability rise (high-strength concrete).
  2. Produce flowing concrete (slump above 150 mm) with no extra water, for heavily reinforced sections, pumping, tremie placement and self-compacting concrete.
  3. Save cement at the same strength.
  4. Dosage is 0.5-2.0% of cement mass; added with the last part of mixing water or on site, since the slump loss occurs within 30-60 minutes. Over-dose can cause segregation and retard setting.

✓ Answer checked by Claude Opus 5.5 (high effort)

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