---
title: "Effect of w/c ratio on durability"
subject: "Concrete Technology and Masonry Structure"
chapter: 6
key: "semester_5/concrete_technology_and_masonry_structure:wc-ratio-durability"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/wc-ratio-durability
site: BCT Engineering
---

# Effect of w/c ratio on durability

Concrete Technology and Masonry Structure · Chapter 6: Concrete durability

- Asked in: 2079 Baisakh, 2 marks
- Video: [what is water cement ratio | wc ratio meaning |water cement ratio concept](https://www.youtube.com/watch?v=w4GqY2BO5Z0) — Civil wale here
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/wc-ratio-durability
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6

## Question

How does water cement ratio affect the durability of concrete?

## Short answer

Higher water-cement ratio leaves more capillary pores, greatly increasing permeability and ingress of water, chlorides, and CO2, thus lowering durability. IS 456 limits maximum w/c from 0.40 to 0.55 depending on exposure, with minimum cement content of 300–360 kg/m³.

## Answer

The water-cement ratio is the main factor controlling the durability of concrete. Water in excess of that needed to hydrate the cement (w/c above about 0.38) leaves **capillary pores**. A higher w/c gives more and larger interconnected pores, so permeability is higher, strength is lower, and water, chlorides, sulphates and CO$_2$ enter faster. This leads to corrosion, carbonation, freeze-thaw damage and chemical attack.

Hence IS 456 (Table 5) limits the maximum w/c for reinforced concrete: 0.55 (mild), 0.50 (moderate), 0.45 (severe and very severe) and 0.40 (extreme exposure), with a minimum cement content of 300 to 360 kg/m$^3$.

## Check yourself

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

1. Why does a higher w/c make concrete less durable?
   A. It increases hydration and so density
   B. It lowers permeability but also strength
   C. It only affects workability
   D. More and larger interconnected capillary pores raise permeability, so faster ingress of water, chlorides, sulphates and CO2
   Answer: D. Water beyond about 0.38 needed for hydration leaves capillary pores. (Key point: w/c and durability)

2. Why does permeability rise rapidly with w/c above about 0.38?
   A. Excess water makes extra gel pores that are sealed
   B. More water increases cement content
   C. Higher w/c reduces air voids
   D. Excess water beyond hydration needs leaves capillary pores that become interconnected
   Answer: D. Between w/c 0.4 and 0.7 permeability may rise several hundred times. (Key point: w/c and capillary pores)

3. What maximum w/c does IS 456 Table 5 give for RC, and the trend?
   A. 0.40 for mild rising to 0.55 for extreme
   B. 0.70 for all exposures
   C. 0.55 (mild) falling to 0.40 (extreme), with min cement 300 to 360 kg/m^3
   D. No limit but only minimum cement
   Answer: C. Harsher exposure needs a lower w/c and more cement. (Key point: IS 456 w/c limits)

## Related in Concrete Technology and Masonry Structure

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