---
title: "Crack control methods as per IS 456"
subject: "Design of RCC Structure"
chapter: 8
key: "semester_7/design_of_rcc_structure:crack-control-methods-as-per-is"
source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/crack-control-methods-as-per-is
site: BCT Engineering
---

# Crack control methods as per IS 456

Design of RCC Structure · Chapter 8: Limit States of Serviceability: Deflection and Cracking

- Asked in: 2073 Shrawan, 5 marks
- Video: [How to calculate Crack width in RCC members as per IS456:2000 #rcc #crack #design #civilengineering](https://www.youtube.com/watch?v=t_f3pwWlimY) — The Civil sensei
- Page: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/crack-control-methods-as-per-is
- Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-8

## Question

Discuss the methods of crack control as per IS 456-2000 in RC structures.

## Short answer

IS 456-2000 controls cracking by limiting bar spacing (slabs: main bars 3d or 300 mm, distribution 5d or 450 mm; beams: Table 15), using small-diameter bars, providing side-face reinforcement for beams >750 mm deep, ensuring minimum steel, limiting cover to 50 mm, and restricting surface crack width to 0.3 mm (0.2 mm in aggressive environments).

## Answer

**Cracking** is a serviceability limit state: cracks spoil the appearance and let moisture reach the steel, causing corrosion. IS 456-2000 does not require a crack width calculation for every member; instead it controls cracking by detailing rules and, where needed, by a check on crack width.

#### Methods of crack control
1. **Limit the bar spacing** (cl. 26.3.3(b), cl. 43 and Table 15).
   - In slabs, the horizontal distance between parallel main bars should not exceed $3d$ or 300 mm, whichever is less; for distribution bars $5d$ or 450 mm.
   - In beams, the clear distance between tension bars is limited by Table 15 (it depends on the service stress, i.e. on $f_y$ and the redistribution of moments), e.g. a maximum clear spacing of about 180 to 300 mm.
2. **Use small-diameter bars** well distributed around the tension zone instead of a few large bars.
3. **Side-face reinforcement**: in beams with overall depth above 750 mm, provide 0.1% of the web area on each face, spaced not more than 300 mm or the web thickness, to control cracks on the sides (cl. 26.5.1.3).
4. **Minimum steel**: provide at least the minimum tension steel (e.g. $0.85bd/f_y$ for beams, 0.12-0.15% for slabs) so that cracking does not occur suddenly.
5. **Limit the cover** to the tension reinforcement to 50 mm; larger cover needs extra mesh to control cracks. Adequate cover is also needed for durability.
6. **Limit the crack width**: the surface width of a crack should not exceed **0.3 mm** in members where cracking is not harmful (0.2 mm in aggressive environment, 0.1 mm in severe) - cl. 35.3.2. It is calculated by the formula of Annex F when required.
7. **Control shrinkage and temperature cracks**: proper curing, low water-cement ratio, contraction/expansion joints and distribution steel.

## Check yourself

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

1. Why is cracking controlled?
   A. They reduce self-weight
   B. They increase the span
   C. Cracks spoil appearance and let moisture reach steel, causing corrosion
   D. They improve bond
   Answer: C. Cracking is a serviceability limit state affecting durability. (Key point: Why control)

2. Maximum spacing of main bars in slabs is
   A. 5d only
   B. 450 mm
   C. Lesser of 3d and 300 mm
   D. Greater of 3d and 300 mm
   Answer: C. Distribution bars use 5d or 450 mm. (Key point: Slab spacing)

3. When is side-face steel required and how much?
   A. Never
   B. Depth above 300 mm; 1%
   C. Depth above 450 mm; 0.5%
   D. Overall depth above 750 mm; 0.1% of web area
   Answer: D. Spaced not more than 300 mm or web thickness, on each face. (Key point: Side-face steel)

4. What is the cover limit to tension bars without extra mesh?
   A. 150 mm
   B. 100 mm
   C. 50 mm
   D. 10 mm
   Answer: C. Larger cover needs extra mesh to control cracks. (Key point: Cover limit)

## Related in Design of RCC Structure

- [Load combinations for column per IS 456](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aload-combinations-for-column-per-is) — Design of RCC Structure, chapter 3
- [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
- [Deflection control, sufficient stiffness method](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adeflection-control-sufficient-stiffness-method) — Design of RCC Structure, chapter 8, asked 3×

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