Clause 4.5 determines the proportion of coarse aggregate (CA) and fine aggregate (FA) in the mix. Table 3 provides the volume of coarse aggregate per unit volume of total aggregate for different nominal maximum sizes and for Zone II fine aggregate at a W/C ratio of 0.50. Adjustments are made for other grading zones of fine aggregate, and for W/C ratios different from 0.50. The fine aggregate volume is obtained by difference (1 minus CA fraction).
Key Requirements
•Select volume of coarse aggregate per unit volume of total aggregate from Table 3 for the applicable MSA and Zone II FA
•For Zone I (coarser) FA, increase CA volume by 0.01 (i.e., reduce FA proportion)
•For Zone III (finer) FA, decrease CA volume by 0.01 (i.e., increase FA proportion)
•For Zone IV (finest) FA, decrease CA volume by 0.02
•For every increase of 0.05 in W/C ratio above 0.50, decrease CA volume by 0.01; for every decrease of 0.05, increase by 0.01
•Volume of fine aggregate per unit volume of total aggregate = 1 - volume of CA
Reference Tables
Table 3 — Volume of Coarse Aggregate per Unit Volume of Total Aggregate for Different MSA (Clause 4.5)
Nominal Max Aggregate Size (mm)
Volume of CA per Unit Volume of Total Aggregate (Zone II FA, W/C = 0.50)
10
0.5
20
0.62
40
0.71
Values are for Zone II (medium) fine aggregate and W/C ratio of 0.50. Adjust for other zones and W/C ratios as specified.
Adjustment to CA Volume for Fine Aggregate Grading Zone
FA Grading Zone
Adjustment to CA Volume
Zone I (coarsest)
+0.01
Zone II (medium)
No adjustment (base value)
Zone III (finer)
-0.01
Zone IV (finest)
-0.02
Grading zones as per IS 383. Zone II is most commonly available in India. Positive adjustment means more CA (less FA).
V_CA (adjusted) = Final volume fraction of CA per unit volume of total aggregateV_CA (Table 3) = Base value from Table 3 for the MSAzone adjustment = +0.01 for Zone I, 0 for Zone II, -0.01 for Zone III, -0.02 for Zone IVW/C adjustment = -0.01 for each +0.05 increase in W/C above 0.50; +0.01 for each -0.05 decrease
V_FA = 1 - V_CA
Volume fraction of fine aggregate per unit volume of total aggregate
V_FA = Volume fraction of fine aggregateV_CA = Adjusted volume fraction of coarse aggregate
Practical Notes
✓For 20 mm MSA, Zone II FA, W/C = 0.50: CA fraction = 0.62, FA fraction = 0.38. This translates to roughly 60:40 CA:FA by weight for typical specific gravities.
✓The zone adjustment is small (0.01-0.02) but significant — Zone IV sand is much finer and needs more paste, so CA is reduced to accommodate more FA.
✓For pumpable concrete, FA proportion is often increased by 2-5% beyond the calculated value to improve cohesion and pumpability.
✓Manufactured sand (M-sand) grading often falls in Zone II but has more angular particles — consider increasing FA by 1-2% for workability.
Common Mistakes
⚠Not adjusting CA volume for the actual grading zone of the fine aggregate being used on site.
⚠Forgetting the W/C ratio adjustment — a mix at W/C 0.40 needs more CA (+0.02) than one at W/C 0.50.
⚠Applying CA:FA ratio by weight directly as volume ratio — specific gravities of CA and FA differ, so weight ratio and volume ratio are not the same.
⚠Using the same aggregate proportions for angular crushed stone and rounded river gravel — angular aggregates need more paste (less CA) for the same workability.