---
title: "Retempering of concrete; shape index"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:retempering-shape-index"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/retempering-shape-index
site: BCT Engineering
---

# Retempering of concrete; shape index

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

- Asked in: 2082 Bhadra, 4 marks
- 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/retempering-shape-index
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

Explain retempering of concrete and shape index of aggregate.

## Short answer

Retempering is adding water to stiffened concrete to restore workability, permitted only before initial set with limited water to avoid exceeding w/c ratio; it lowers strength and raises permeability. Shape index quantifies deviation from cube, e.g. flakiness/elongation indices or length/thickness ratio; a value near 1 indicates cubical, good shape.

## Answer

#### Retempering of concrete
**Retempering** is the addition of water and remixing of concrete that has started to stiffen (partially set, with a loss of slump) so as to restore its workability to a level that allows placing.
- It is allowed only if the concrete has not reached its initial set; concrete must be placed and compacted before initial setting begins, and good practice is to place it within about 30 minutes of mixing.
- Only a limited amount of water may be added (for example, water held back from the original batch), so that the specified maximum w/c ratio is not exceeded; mixing must then be continued. If more water is needed, cement must be added with it to keep the w/c ratio unchanged.
- Effects of adding extra water: lower strength (about 5-10 per cent), more shrinkage and higher permeability. Concrete that has set should never be retempered. Re-dosing with a superplasticiser is the better way to restore workability.

#### Shape index of aggregate
Shape index is a quantitative measure of how far the shape of the aggregate deviates from a cube. In IS practice, shape is expressed by the **flakiness index** (percentage by weight of particles whose thickness is below 0.6 times the mean size), the **elongation index** (length above 1.8 times the mean size) and the angularity number (IS 2386 Part 1). In BS EN 933-4, the shape index is the percentage by mass of particles whose length-to-thickness ratio exceeds 3. For a single particle, it can be expressed in terms of its length : width : thickness, or by its **sphericity**:

$$
\text{Shape index} = \frac{\text{length}}{\text{thickness}} \quad(\text{or } \frac{L + W}{2T})
$$

A value close to 1 means a cubical particle (good shape). Higher values show flaky or elongated aggregate, which gives poor workability and strength.

## Check yourself

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

1. Retempering is:
   A. Adding water and remixing concrete that has started to stiffen to restore workability
   B. Curing concrete
   C. Adding cement to hardened concrete
   D. Reheating aggregate
   Answer: A. It restores workability of stiffening concrete by water and remixing. (Key point: Retempering definition)

2. When may concrete be retempered?
   A. Even after final set
   B. Only if it has not reached initial set, with limited water and w/c not exceeded
   C. Always, with unlimited water
   D. Only after 24 hours
   Answer: B. Set concrete must never be retempered; added water should not exceed the original w/c. (Key point: Retempering limits)

3. Added water in retempering causes:
   A. Strength gain
   B. Lower strength (about 5-10%), more shrinkage and permeability
   C. No change
   D. Faster hardening
   Answer: B. Extra water reduces strength and increases shrinkage and permeability. (Key point: Effect of retempering)

4. A shape index close to 1 means the particle is:
   A. Very flaky
   B. Very elongated
   C. Cubical (good shape)
   D. Needle-like
   Answer: C. Value near 1 is cubical; higher values indicate flaky or elongated. (Key point: Shape index)

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