Water-cement ratio
The water-cement ratio is the mass of free water divided by the mass of cement (or total cementitious material) in a mix. It is the single most important number in concrete. Duff Abrams showed a century ago that, for fully compacted concrete made with given materials, strength falls steadily as the water-cement ratio rises. Lower ratios also mean lower permeability, which is the root of durability.
Why? Cement needs only about a quarter of its weight in water to hydrate fully. Extra water is added to make the mix workable. When that surplus water dries out, it leaves pores. More pores mean lower strength and easier paths for chlorides, carbon dioxide and sulphates to reach the steel or attack the paste. Above a ratio of about 0.4, capillary pores remain connected even after long curing.
On site the temptation is to add water to make placing easier, and it is the most common cause of weak, permeable concrete. The correct way to get workability without extra water is to use a plasticiser or superplasticiser, or to improve aggregate grading. Codes cap the maximum water-cement ratio for each exposure condition and specify minimum cement content. Free water is what counts: water absorbed by dry aggregate must be added, and surface moisture on wet sand must be deducted from batch water.
Keep a moisture meter or a quick frying-pan test for sand at the batching point during monsoon.
w/c = free water mass / cement massAbrams' law: f = A / B^(w/c)Water for given w/c: W = (w/c) × CMoisture correction: batch water = design water − surface moisture + absorption deficit
Adding water at the site to restore slump lost in transit.
Problem. A mix needs 380 kg cement and w/c = 0.45. Sand (720 kg) has 2 percent surface moisture; coarse aggregate (1 150 kg) is dry and absorbs 0.5 percent. Find the batch water.
- Free water = 0.45 × 380 = 171 kg.
- Surface water on sand = 0.02 × 720 = 14.4 kg; deduct it.
- Coarse aggregate will absorb 0.005 × 1 150 = 5.75 kg; add it.
- Batch water = 171 − 14.4 + 5.75 = 162.4 kg.
Answer. About 162 kg of water at the batching plant
Quick quiz
1What happens to strength as w/c rises?
It falls.
2How should workability be increased without raising w/c?
Use a plasticiser or superplasticiser, or improve grading.
3Why must surface moisture on sand be deducted?
It adds to free water and would raise the effective w/c.
- IS 456 (India): Plain and reinforced concrete: general rules for materials, durability, design and construction.
- IS 10262 (India): Guidelines for proportioning concrete mixes.
- IS 383 (India): Coarse and fine aggregate for concrete: requirements.
- IS 269 (India): Ordinary Portland cement: requirements.
Clauses and documents to read
- IS 456:2000 cl. 8.2.4.1 and Table 5 (maximum w/c and minimum cement content, cited by number)
- IS 10262:2019 cl. 4.3 (selection of water-cement ratio)
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- M. S. Shetty, Concrete Technology: Theory and Practice, S. Chand, latest edition.
- A. M. Neville, Properties of Concrete, Pearson, 5th edition.
- P. K. Mehta, P. J. M. Monteiro, Concrete: Microstructure, Properties, and Materials, McGraw Hill, latest edition.
Free and official sources
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