---
title: "Role of admixtures; superplasticizer"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:admixture-role-superplasticizer-use"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/admixture-role-superplasticizer-use
site: BCT Engineering
---

# Role of admixtures; superplasticizer

Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials

- Asked in: 2071 Shrawan, 2+4 marks
- Video: [Admixtures for Concrete – What is a Water Reducer?](https://www.youtube.com/watch?v=q_PwZE57Slc) — Tyler Ley
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/admixture-role-superplasticizer-use
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

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

## 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.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. An admixture is:
   A. Water, cement or aggregate
   B. A material other than water, aggregate, cement and fibre added in small quantities before or during mixing
   C. A cement replacement of 40%
   D. Added after hardening
   Answer: B. IS 9103 defines it as a small-dose addition to modify properties. (Key point: Admixture definition)

2. Superplasticisers can:
   A. Produce flowing concrete (slump above 150 mm) without extra water
   B. Only raise w/c
   C. Delay hydration only
   D. Increase voids
   Answer: A. They give flowing concrete for congested reinforcement, pumping and SCC. (Key point: Superplasticiser uses)

3. Dosage is 0.5-2.0% of cement mass; overdose may cause:
   A. Higher strength
   B. Segregation and retarded setting
   C. Faster setting
   D. Lower slump
   Answer: B. Overdose can segregate and retard setting. (Key point: Dosage and overdose)

4. Which is NOT a role of admixtures?
   A. Control setting
   B. Entrain air
   C. Reduce heat of hydration
   D. Increase w/c ratio for strength
   Answer: D. Admixtures aim to reduce water or w/c, not raise it. (Key point: Role of admixtures)

## Related in Concrete Technology and Masonry Structure

- [Role of superplasticizer](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Asuperplasticizer-role) — Concrete Technology and Masonry Structure, chapter 1
- [Role and quality of water in concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awater-role-quality-impurities) — Concrete Technology and Masonry Structure, chapter 1, asked 3×
- [Types and purpose of admixtures](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aadmixture-types-purpose) — Concrete Technology and Masonry Structure, chapter 1, asked 7×
- [Common admixtures; accelerating admixture](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aaccelerating-admixture) — Concrete Technology and Masonry Structure, chapter 1
- [Chemical and mineral admixtures; plasticizers](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aplasticizer-water-reduction) — Concrete Technology and Masonry Structure, chapter 1
- [Chemical admixtures per ASTM; recommendations](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aadmixture-recommendation-weather) — Concrete Technology and Masonry Structure, chapter 1

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- [Site engineer uses weaker mortar and concrete mix](https://www.bctengineer.com/question.md?key=semester_8%2Fengineering_professional_practice%3Amortar-ratio-friend-silence) — Engineering Professional Practice, chapter 7