---
title: "Transition phase and its effect"
subject: "Concrete Technology and Masonry Structure"
chapter: 2
key: "semester_5/concrete_technology_and_masonry_structure:transition-zone-effect"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/transition-zone-effect
site: BCT Engineering
---

# Transition phase and its effect

Concrete Technology and Masonry Structure · Chapter 2: Structure of concrete

- Asked in: 2074 Ashwin, 3+3 marks
- Video: [The Impact of the Transition Zone and Cracking on Fluid Penetration in Concrete](https://www.youtube.com/watch?v=iWljfrwAWf8) — 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/transition-zone-effect
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-2

## Question

What do you understand by the transition phase of concrete? Explain the effect of the transition phase on the properties of concrete.

## Short answer

The transition zone (ITZ) is a 10–50 µm porous layer around aggregates, high in oriented CH and microcracks from wall effect and bleeding. It weakens concrete: reduces strength, modulus, durability; causes nonlinear behavior; increases permeability. Improve with low w/c, silica fume, and proper curing.

## Answer

The **transition phase (transition zone, ITZ)** is the thin shell of cement paste, about 10-50 µm thick, immediately surrounding the aggregate particles. Its structure differs from that of the bulk paste.

#### How it forms
1. Because of the "wall effect", cement grains cannot pack closely against the large aggregate surface, so the zone is richer in water and has a higher w/c.
2. A film of bleed water accumulates around the aggregate (especially under flat, large particles) and gets trapped.
3. In this water-rich space, larger crystals of calcium hydroxide (oriented with the c-axis perpendicular to the aggregate) and ettringite grow, and less C-S-H forms. The result is a zone with high porosity, a low density and microcracks (from shrinkage and thermal movement).

```text
 aggregate | ITZ (porous,  | bulk paste
           | CH crystals)  | (dense C-S-H)
```

#### Effect on concrete properties
- **Strength**: concrete is weaker than either aggregate or paste; microcracks start in the ITZ at about 30-40% of ultimate load. Tensile and flexural strengths are very sensitive to it.
- **Stress-strain relationship**: non-linear, due to progressive bond cracking.
- **Elastic modulus**: lower than predicted from the aggregate and paste.
- **Durability and permeability**: the porous, interconnected zone allows ingress of water and aggressive ions; affects frost, chloride and sulphate resistance.
- **Fracture**: governs crack paths and brittleness.

It can be improved by a low w/c ratio, silica fume and fly ash, a superplasticiser, rough clean aggregate, and good compaction and curing.

## Check yourself

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

1. The transition zone is...
   A. A steel-concrete layer
   B. A thin 10-50 µm shell of paste around aggregate with different structure
   C. The air void system
   D. A layer of sand
   Answer: B. It is the paste immediately surrounding aggregate, differing from bulk paste. (Key point: Definition)

2. What is the wall effect?
   A. Cement grains cannot pack closely against aggregate, so the zone has higher w/c
   B. Aggregate absorbs paste
   C. Formwork cracking
   D. Steel bar slipping
   Answer: A. Loose packing and trapped bleed water raise the local w/c. (Key point: Wall effect)

3. What grows in the water-rich ITZ?
   A. Dense C-S-H
   B. Large oriented Ca(OH)2 and ettringite crystals
   C. Only unhydrated cement
   D. Gypsum only
   Answer: B. Large oriented crystals with less C-S-H give porosity and low density. (Key point: CH orientation)

4. Effect of the ITZ on elastic modulus?
   A. Raises it
   B. Lowers it below that predicted from the phases
   C. None
   D. Doubles it
   Answer: B. Bond cracks and porosity reduce stiffness. (Key point: Modulus)

## Related in Concrete Technology and Masonry Structure

- [Three phase system and transition zone](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Athree-phase-transition-zone) — Concrete Technology and Masonry Structure, chapter 2, asked 8×
- [Fatigue effect in concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Afatigue-effect) — Concrete Technology and Masonry Structure, chapter 4
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- [Aggregate properties and their influence on concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aaggregate-properties-influence) — Concrete Technology and Masonry Structure, chapter 1
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