---
title: "Effect of water cement ratio on strength"
subject: "Concrete Technology and Masonry Structure"
chapter: 3
key: "semester_5/concrete_technology_and_masonry_structure:wc-ratio-effect-strength"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/wc-ratio-effect-strength
site: BCT Engineering
---

# Effect of water cement ratio on strength

Concrete Technology and Masonry Structure · Chapter 3: Mix design of concrete and property of green concrete

- Asked in: 2081 Bhadra, 2 marks; 2068 Baisakh (old course), 2.5 marks; 2066 Bhadra (old course), 2.5 marks; 2065 Shrawan (old course), 2 marks (asked 4 times)
- Video: [Effect of Water Cement ratio on Concrete properties](https://www.youtube.com/watch?v=n1p94fhjG0A) — Civil Engineering
- 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/wc-ratio-effect-strength
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-3

## Question

Explain the effect of water cement ratio on the strength of concrete.

## Short answer

Higher water-cement ratio reduces concrete strength because excess water beyond hydration requirements (about 0.23–0.38 of cement mass) leaves capillary pores upon evaporation. According to Abrams' law, strength is inversely related to w/c; for example, w/c 0.4 yields about 45–50 MPa and w/c 0.6 about 28 MPa at 28 days.

## Answer

The **water-cement ratio (w/c)** is the ratio of the mass of water to the mass of cement in the mix. It is the most important factor controlling the strength of workable, fully compacted concrete.

#### Abrams' law
Strength is inversely related to the w/c ratio:

$$
f_c = \frac{A}{B^{\,w/c}}
$$

where $A$ and $B$ are constants that depend on the cement, age and curing ($A = 96.5$ MPa and $B = 7$ in Abrams' original work).

```text
 Strength
  |\
  | \
  |  `.
  |    `-.
  |        `--.___
  +---------------------> w/c
  0.3  0.4  0.5  0.6  0.7
```

#### Why strength falls as w/c rises
- The water needed for hydration is only about 0.23-0.38 of the cement mass; the extra water stays in the paste and leaves **capillary pores** when it evaporates. Strength depends on the gel/space ratio (Powers), so more pores mean lower strength.
- A higher w/c gives a weaker bond between the paste and the aggregate, a weaker transition zone, and more bleeding.
- A lower w/c increases strength, density, durability and impermeability, but reduces workability. Below about 0.35, full compaction becomes difficult and some cement stays unhydrated, so superplasticisers are used.

Typical values: w/c 0.4 gives about 45-50 MPa, 0.5 about 35 MPa, 0.6 about 28 MPa, and 0.7 about 20 MPa at 28 days (OPC, well compacted).

## Check yourself

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

1. According to Abrams' law, strength is...
   A. Directly proportional to w/c
   B. Inversely related to w/c
   C. Independent of w/c
   D. Related to slump only
   Answer: B. Strength falls as w/c rises. (Key point: Abrams' law)

2. Water needed for hydration is about...
   A. 0.23-0.38 of cement mass
   B. 1.0
   C. 0.8
   D. 0.05
   Answer: A. Excess water leaves capillary pores. (Key point: Hydration water)

3. Below about 0.35 w/c...
   A. Compaction is difficult unless superplasticiser is used
   B. Strength falls
   C. Bleeding rises
   D. Cement not needed
   Answer: A. Low workability prevents full compaction. (Key point: Low w/c limit)

## Related in Concrete Technology and Masonry Structure

- [Gel/space ratio and strength (550 gm cement)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Agel-space-ratio-550g) — Concrete Technology and Masonry Structure, chapter 4, asked 3×
- [Gel/space ratio and strength (600 gm cement)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Agel-space-ratio-600g) — Concrete Technology and Masonry Structure, chapter 4, asked 3×
- [Theoretical strength of moist cured concrete (1 kg cement)](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Atheoretical-strength-1kg-cement) — Concrete Technology and Masonry Structure, chapter 4
- [Effect of gel/space ratio on strength](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Agel-space-ratio-effect) — Concrete Technology and Masonry Structure, chapter 4
- [Strength of paste: 300 gm cement](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acement-strength-300g-150g) — Concrete Technology and Masonry Structure, chapter 4
- [Effect of w/c ratio on durability](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awc-ratio-durability) — Concrete Technology and Masonry Structure, chapter 6

## Related in other subjects

- [Compressive strength test of cement](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acement-compressive-strength-test) — Civil Engineering Materials, chapter 5, asked 3×
- [Types of stresses in cement concrete pavement](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Arigid-pavement-stresses) — Transportation Engineering II, chapter 2, asked 2×
- [Rate analysis: plain cement concrete (1:3:4)](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arate-pcc-1-3-4) — Estimating and Costing, chapter 4
- [Construction steps of cement concrete pavement](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Acc-pavement-construction-steps) — Transportation Engineering II, chapter 3