---
title: "Retrofitting need; repairing cracks of different width"
subject: "Building Technology"
chapter: 10
key: "semester_6/building_technology:retrofit-repair-crack-width"
source: https://www.bctengineer.com/q/semester_6/building_technology/retrofit-repair-crack-width
site: BCT Engineering
---

# Retrofitting need; repairing cracks of different width

Building Technology · Chapter 10: Causes and prevention of cracks in buildings

- Asked in: 2081 Chaitra, 6 marks
- Video: [The Types of Concrete Cracks and how to prevent the cracks](https://www.youtube.com/watch?v=3XVKlcdCZzs) — Brendan Hasty
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/retrofit-repair-crack-width
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-10

## Question

Why retrofitting is necessary? Explain methods of repairing crack of different width.

## Short answer

Retrofitting is necessary to strengthen existing structures against earthquakes, damage, increased loads, or updated codes. Crack repair methods: hairline cracks (<0.3 mm) are surface-sealed; 0.3-1 mm cracks are raked and filled with mortar; 1-3 mm cracks are injected with epoxy/cement grout; 3-10 mm cracks are V-grooved and stitched; cracks >10 mm require rebuilding and jacketing.

## Answer

#### Need for retrofitting
**Retrofitting** is the strengthening of an existing structure to make it safer and serviceable. It is necessary because:

- Many old buildings were built without earthquake-resistant design and may collapse in future earthquakes (as in Nepal 2015).
- Damage from earthquake, fire, corrosion and ageing reduces strength.
- Loads or building use change (extra storeys).
- Design or construction errors are found.
- Codes (NBC 105) have been revised, and retrofitting is cheaper than demolishing.

#### Repair of cracks by width
| Width | Method |
|---|---|
| Hairline, < 0.3 mm | Surface painting with cement slurry or sealant |
| 0.3 to 1 mm | Rake and fill with cement-sand mortar 1:3, or polymer mortar |
| 1 to 3 mm | Epoxy or cement grout injection through packers at 150-300 mm spacing |
| 3 to 10 mm | Rake into V groove, epoxy mortar, with stitching by clamps |
| > 10 mm | Remove and rebuild the portion; add jackets, bands or mesh; stitching |

**General steps:** remove the cause (settlement, leakage), clean the crack with compressed air, seal the surface, inject or fill, then cure and finish.

## Check yourself

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

1. Why is retrofitting necessary?
   A. Codes never change
   B. Buildings never need strengthening
   C. Demolition is cheaper
   D. Old buildings lack earthquake-resistant design and may collapse
   Answer: D. Retrofitting is cheaper than rebuilding and makes buildings safer. (Key point: Need for retrofitting)

2. Cracks 1 to 3 mm wide are repaired by:
   A. Epoxy or cement grout injection through packers at 150-300 mm
   B. Surface paint
   C. Rebuilding
   D. Raking with 1:3 mortar
   Answer: A. Injection through packers is used. (Key point: 1-3 mm cracks)

3. Cracks wider than 10 mm need:
   A. Slurry
   B. Remove and rebuild, with jackets, bands or mesh
   C. Paint
   D. Nothing
   Answer: B. Large cracks require rebuilding or strengthening. (Key point: Cracks above 10 mm)

## Related in Building Technology

- [Methods of repairing cracks](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acrack-repair-methods) — Building Technology, chapter 10, asked 3×
- [What is retrofitting; why it is necessary](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-why-necessary) — Building Technology, chapter 11
- [Crazing and cracks on roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acrazing-roof-cracks) — Building Technology, chapter 10

## Related in other subjects

- [Deflection, crack width control, crack formula](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adeflection-crack-width-control-crack-formula) — Design of RCC Structure, chapter 8
- [Built-in potential and depletion width derivation](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Apn-builtin-depletion-derivation) — Electrical Engineering Material, chapter 6, asked 4×
- [Built-in potential and depletion width defined](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Abuiltin-depletion-definition) — Electrical Engineering Material, chapter 6, asked 4×