Coarse aggregate content: a packing and placement decision, not just a table lookup
Coarse aggregate occupies much of a normal concrete volume. A good proportion reduces unnecessary paste while still allowing the concrete to flow, pump, pass reinforcement and consolidate. The correct fraction depends on aggregate size, combined grading, shape, sand characteristics, workability and the actual placement method.
What increasing coarse aggregate can do
Within a stable, workable grading system, more coarse aggregate can reduce paste demand, shrinkage and cost because aggregate is usually dimensionally more stable and less binder-intensive than paste. But pushing the fraction too far can make concrete harsh, difficult to pump, prone to rock pockets and unable to pass congested reinforcement.
Dmax is constrained by the member—not chosen for economy alone
Larger nominal maximum aggregate can reduce surface area and paste/water demand, but the aggregate still has to pass through reinforcement and fit the member/placement system. Check the project limits for bar spacing, cover, section dimensions, pump line and finishing requirements before selecting Dmax.
Combined grading matters more than one sieve label
A nominal 20 mm aggregate does not tell you how efficiently the entire coarse/fine skeleton packs. Flaky/elongated particles, gap grading and an incompatible sand can increase voids and paste demand. Use actual sieve data and trials rather than assuming all “20 mm aggregate” behaves the same.
Illustrative absolute-volume example
Suppose the calculated total aggregate volume after subtracting binder, water, admixture and air is 0.68 m³. A trial combined grading uses 60% of that aggregate volume as coarse aggregate. The coarse volume is therefore 0.408 m³. With coarse-aggregate specific gravity 2.70, its SSD mass is approximately:
0.408 × 2.70 × 1000 ≈ 1,102 kg/m³.
The 60% is an illustrative trial fraction, not a copied IS table value. The standard-based calculator provides a starting framework; actual grading and pumpability trials should confirm the split.
Pumpability usually needs a different balance
Pumped concrete needs enough mortar/fines to lubricate the line and prevent blockage. A coarse-rich mix that places acceptably by chute may pump poorly. Instead of applying one universal “reduce coarse by X%” shortcut, assess line diameter/length, Dmax, sand grading, paste volume, slump/flow and actual pump trial where the project warrants it.
Split between 20 mm and 10 mm fractions
When more than one coarse size is used, the total coarse mass must be divided into a combined grading that satisfies the aggregate specification and mixture performance. The best split is not necessarily the one that maximizes the larger fraction; it is the split that provides dense packing, workability and stable production with the available materials.
Moisture and absorption change the batch mass
Design aggregate masses are commonly calculated on an SSD basis, while the plant weighs actual wet stockpile material. Correct both the aggregate batch weight and the water balance. A coarse aggregate that is below SSD can absorb water; a wet stockpile can contribute free water.
Trial checklist
- Actual combined grading and particle shape.
- Fresh cohesion and segregation resistance.
- Pumpability/flow through the real placement route where relevant.
- Yield/density versus the absolute-volume model.
- Moisture correction and absorption.
- Finishability and surface aggregate distribution.
Last reviewed: 15 September 2026. Use the current IS/project aggregate and mix-design requirements for binding grading/proportioning rules.