Clause 4.4 determines the cement content from the free water content (Clause 4.3) and the selected water-cement ratio (Clause 4.2). The calculated cement content is then checked against the minimum cement content required by IS 456 Table 5 for durability, and the higher value is adopted. An upper limit of 450 kg/m³ is also specified to limit heat of hydration and shrinkage cracking.
Key Requirements
•Cement content = water content / W-C ratio
•Cement content shall not be less than the minimum specified in IS 456 Table 5 for the given exposure condition
•Maximum cement content shall not exceed 450 kg/m³ unless special measures are taken to control shrinkage and cracking
•If minimum cement content governs, the actual W/C ratio will be lower than the selected value — this provides an additional safety margin
•When mineral admixtures are used, refer to Clause 5 for calculating equivalent cement content
Reference Tables
Minimum Cement Content for Durability (from IS 456 Table 5)
Exposure Condition
Min Cement Content (kg/m³)
Max W/C Ratio
Mild
300
0.55
Moderate
300
0.5
Severe
320
0.45
Very Severe
340
0.45
Extreme
360
0.4
Formulas
C = W / (W/C)
Cement content from water content and water-cement ratio
C = Cement content in kg/m³W = Free water content in kg/m³ (from Clause 4.3)W/C = Selected water-cement ratio (from Clause 4.2)
Practical Notes
✓For M25, moderate exposure: W = 186 kg/m³, W/C = 0.50 → Cement = 186/0.50 = 372 kg/m³. Minimum per IS 456 is 300 kg/m³, so 372 kg/m³ governs.
✓For M20, mild exposure: W = 186 kg/m³, W/C = 0.55 → Cement = 186/0.55 = 338 kg/m³. Minimum is 300 kg/m³, so 338 kg/m³ governs.
✓The 450 kg/m³ upper limit is critical for mass concrete (rafts, retaining walls) where thermal cracking is a concern. Use mineral admixtures to reduce OPC content.
✓In practice, most M25-M30 mixes with OPC 53 use 340-400 kg/m³ of cement. Higher grades (M40+) often need 420-450 kg/m³ unless fly ash or GGBS is used to supplement.
Common Mistakes
⚠Forgetting to check minimum cement content per IS 456 Table 5 — a strength-based calculation may give less than the durability minimum.
⚠Exceeding 450 kg/m³ without addressing thermal cracking — this is common in high-strength mixes and must be mitigated with cooling or pozzolans.
⚠Not recalculating the effective W/C ratio when minimum cement content governs — the actual W/C becomes lower than the selected value.
⚠Confusing total cementitious content (cement + fly ash + GGBS) with OPC content when checking limits.