Clause 4.3 provides the estimated water content required per cubic metre of concrete for the desired workability. Table 2 gives the maximum water content for nominal maximum sizes of aggregate (10 mm, 20 mm, and 40 mm) at a reference slump of 25-50 mm. Adjustments are specified for higher slump ranges, and for the use of superplasticizers which can reduce water content by 20% or more while maintaining the same workability.
Key Requirements
•Select water content from Table 2 based on nominal maximum size of aggregate for 25-50 mm slump
•For higher workability (slump > 50 mm), increase water content by 3% for every 25 mm increase in slump up to 100 mm
•For slump above 100 mm, use superplasticizer — do not increase water content beyond 100 mm slump level
•When superplasticizers are used, water content can be reduced by 20% or more (based on manufacturer data and trials)
•For pumpable concrete, increase fine aggregate proportion rather than water content
•Water content refers to the total mixing water including free moisture on aggregates
Reference Tables
Table 2 — Maximum Water Content per Cubic Metre of Concrete for Nominal Maximum Size of Aggregate (Clause 4.3)
Nominal Max Aggregate Size (mm)
Maximum Water Content (kg/m³)
10
208
20
186
40
165
Values are for angular coarse aggregate, 25-50 mm slump, and without superplasticizer. For sub-angular aggregate, reduce by approximately 10 kg. For gravel (rounded), reduce by approximately 20 kg.
Water Content Adjustment for Slump (Clause 4.3)
Slump Range (mm)
Adjustment
25-50
As per Table 2 (no adjustment)
50-75
Increase by 3%
75-100
Increase by 6%
100-125
Increase by 8%
125-150
Increase by 10%
> 150
Use superplasticizer
For slump above 100 mm, it is recommended to use chemical admixtures (superplasticizers) rather than additional water.
Formulas
W_adjusted = W_table × (1 + adjustment%/100)
Water content adjusted for slump higher than 25-50 mm
W_adjusted = Adjusted water content in kg/m³W_table = Water content from Table 2 in kg/m³adjustment% = Percentage increase for target slump range
W_sp = W_adjusted × (1 - reduction%/100)
Water content with superplasticizer
W_sp = Reduced water content with superplasticizer in kg/m³W_adjusted = Water content without superplasticizer in kg/m³reduction% = Water reduction percentage from superplasticizer (typically 20-30%)
Practical Notes
✓For 20 mm MSA at 50 mm slump: 186 kg/m³ is the starting point for most structural concrete in India (20 mm is the most common MSA).
✓With a good superplasticizer (PCE-based), water can be reduced to 148-155 kg/m³ for 20 mm MSA while achieving 100-150 mm slump — this is standard RMC practice.
✓The 2019 revision removed the earlier water content values for 12.5 mm and 25 mm MSA, simplifying to just 10/20/40 mm.
✓Aggregate shape matters significantly: crushed angular aggregates need 20 kg/m³ more water than rounded gravel for the same workability.
Common Mistakes
⚠Using Table 2 values directly for 100+ mm slump — high slump requires superplasticizer, not extra water.
⚠Ignoring moisture corrections: if sand has 5% free moisture, 800 kg of sand carries 40 kg of water that must be subtracted from mixing water.
⚠Not adjusting for aggregate shape — using crushed aggregate values for river gravel or vice versa.
⚠Adding water on site to increase slump — this raises W/C ratio and can drop strength by 10-15%.