Clause 4.2 covers the selection of the free water-cement ratio. The W/C ratio is first selected based on the generalized relationship between compressive strength and free W/C ratio for the type of cement used (Figure 1 of IS 10262, or published curves for the specific cement). This value is then checked against the maximum W/C ratio permitted by IS 456 Table 5 for the applicable exposure condition, and the lower of the two values is adopted.
Key Requirements
•Select W/C ratio from the relationship between 28-day compressive strength and free W/C ratio (Figure 1 or cement manufacturer data)
•The selected W/C ratio shall not exceed the maximum value specified in IS 456 Table 5 for the exposure condition
•The lower of the two W/C ratios (strength-based and durability-based) shall be adopted
•For pozzolanic or blended cements, separate strength-vs-W/C curves shall be used
•W/C ratio refers to free water-cement ratio (excluding water absorbed by aggregates)
Reference Tables
Maximum Water-Cement Ratio for Durability (from IS 456 Table 5)
Exposure Condition
Max W/C Ratio
Min Grade
Min Cement (kg/m³)
Mild
0.55
M20
300
Moderate
0.5
M25
300
Severe
0.45
M30
320
Very Severe
0.45
M35
340
Extreme
0.4
M40
360
The W/C ratio from strength consideration may be lower than these maxima. Always adopt the lower value.
Approximate Compressive Strength vs W/C Ratio (OPC 43/53 Grade)
W/C Ratio
28-day Strength — OPC 43 (MPa)
28-day Strength — OPC 53 (MPa)
0.35
43
48
0.4
37
43
0.45
32
38
0.5
27
33
0.55
22
28
0.6
17
23
Indicative values. Actual strength depends on cement brand, aggregate quality, curing, and admixture use. Always verify with trial mixes.
Practical Notes
✓For M25 in moderate exposure: strength-based W/C ≈ 0.50 (OPC 43) and durability max is also 0.50. If using OPC 53, strength-based W/C could be 0.55, but durability limits it to 0.50.
✓Most RMC plants in India use OPC 53 grade or PPC. With PPC, the 28-day strength at a given W/C is typically 5-10% lower than OPC 53, but 56/90-day strength is comparable or higher.
✓Superplasticizers allow workable concrete at low W/C ratios (0.35-0.40) that would otherwise be unplaceable. This is essential for M40+ grades.
✓A W/C reduction of 0.05 typically increases 28-day strength by 4-6 MPa — this is the most effective lever in mix design.
Common Mistakes
⚠Selecting W/C ratio only for strength and ignoring durability limits from IS 456 Table 5.
⚠Using total water (including aggregate absorption) instead of free water in the W/C calculation.
⚠Assuming the same W/C-strength curve applies to all cement types — PPC, PSC, and OPC 43 vs 53 have significantly different curves.
⚠Not accounting for site addition of water — even 10 litres extra per m³ can raise W/C by 0.03 and drop strength by 3-4 MPa.