Mix design and calculation
Foam concrete mix design runs backwards from the density target. You fix the dry density, derive the solids that produce it, calculate the volume left over, and fill that volume with foam. The arithmetic is simple enough to do on site, and doing it properly is the difference between a repeatable product and a series of experiments.
The absolute volume method
Every cubic metre of foam concrete is made of solids, water and air, and the three volumes sum to one. Take one cubic metre of finished material as the basis and work in absolute volumes:
Vcement + Vfiller + Vwater + Vfoam = 1.000 m3
where each solid volume is its mass divided by its particle density: cement ≈ 3100 kg/m3, siliceous sand ≈ 2650 kg/m3, fly ash ≈ 2200 kg/m3, GGBS ≈ 2900 kg/m3, water = 1000 kg/m3.
The procedure is four steps:
- Fix the target oven-dry density.
- Choose binder and filler contents that produce that dry density.
- Choose a water/binder ratio and compute the mixing water.
- Sum the three absolute volumes and subtract from 1.000 — the remainder is the foam volume.
The foam volume, not the foam mass, is what gets dosed. Foam mass is then simply the volume multiplied by the measured foam density, and it is small: at 50 g/L, even 700 litres of foam weighs only 35 kg.
Predicting dry density from the solids
Oven-dry density is not the sum of the solids alone, because cement chemically binds part of its mixing water and retains it through oven drying. The working relationship used throughout the foam concrete literature is:
ρdry ≈ 1.2 × (cement + supplementary binder) + filler [kg/m3]
The factor of 1.2 accounts for roughly 20 % of the binder mass being retained as chemically bound water in a fully hydrated paste. It is an estimate: mixes with high fly ash content or incomplete hydration sit slightly below it, very rich mixes slightly above. Every plant should calibrate its own factor against measured oven-dry densities within the first few production runs, after which the prediction is generally good to within 20 to 30 kg/m3.
Worked example: 1000 kg/m3 block mix
Target: 1000 kg/m3 oven-dry, sand-containing mix for load-bearing blocks.
Step 1 — solids
Choose a binder content of 400 kg/m3 CEM I. Rearranging the dry density relationship:
sand = 1000 − 1.2 × 400 = 1000 − 480 = 520 kg/m3
Step 2 — water
At w/b = 0.50: water = 0.50 × 400 = 200 kg/m3
Step 3 — absolute volumes
| Component | Mass | Particle density | Volume |
|---|---|---|---|
| CEM I | 400 kg | 3100 kg/m3 | 0.129 m3 |
| Sand, 0–2 mm | 520 kg | 2650 kg/m3 | 0.196 m3 |
| Water | 200 kg | 1000 kg/m3 | 0.200 m3 |
| Slurry total | 1120 kg | — | 0.525 m3 |
Step 4 — foam
Foam volume = 1.000 − 0.525 = 0.475 m3 = 475 litres
At a measured foam density of 50 g/L, foam mass = 0.475 × 50 = 23.8 kg
Result
Predicted wet density = 400 + 520 + 200 + 23.8 = 1144 kg/m3, against a target oven-dry density of 1000 kg/m3. The 144 kg/m3 difference is the free water that will leave during drying. 1144 kg/m3 ± 3 % is the number the batching operator checks.
Worked example: 400 kg/m3 insulating screed
Target: 400 kg/m3 oven-dry, neat cement mix for an insulating roof screed.
Step 1 — solids
No filler at this density; the mix is cement paste and foam. Binder = 400 / 1.2 = 333 kg/m3, rounded to 335 kg/m3.
Step 2 — water
At w/b = 0.55, higher because the slurry must stay fluid without a superplasticiser: water = 0.55 × 335 = 184 kg/m3
Step 3 — absolute volumes
| Component | Mass | Particle density | Volume |
|---|---|---|---|
| CEM I | 335 kg | 3100 kg/m3 | 0.108 m3 |
| Water | 184 kg | 1000 kg/m3 | 0.184 m3 |
| Slurry total | 519 kg | — | 0.292 m3 |
Step 4 — foam
Foam volume = 1.000 − 0.292 = 0.708 m3 = 708 litres, so the material is just under 71 % air by volume.
At 50 g/L, foam mass = 0.708 × 50 = 35.4 kg
Result
Predicted wet density = 335 + 184 + 35.4 = 554 kg/m3 against a 400 kg/m3 dry target. Note how much larger the wet-to-dry gap is in proportion at low density: 154 kg/m3 here is 38 % of the dry density, against 14 % in the first example. Anyone who takes a wet density reading as if it were the dry density will reject good material at 1000 kg/m3 and accept bad material at 400.
Foaming agent consumption
Foam is generated from a diluted solution, so concentrate consumption follows from the foam volume, the foam density and the dilution ratio:
concentrate [L] = foam volume [m3] × foam density [kg/m3] ÷ (1 + dilution ratio), taking the solution density as approximately 1 kg/L.
For the 400 kg/m3 screed above, at 1:30 dilution:
foaming solution = 0.708 × 50 = 35.4 kg ≈ 35.4 L → concentrate = 35.4 ÷ 31 = 1.14 litres per m3
For the 1000 kg/m3 block mix: 0.475 × 50 = 23.8 L of solution, so 0.77 litres per m3.
Roughly one litre of concentrate per cubic metre is a serviceable planning figure across the common density range, falling as density rises. It makes the agent a small share of material cost and a large share of technical risk — which is the right way round to think about selecting one.
Wet density as a production control
Wet density is measured, dry density is specified, and they are not the same number. The gap is the free water: mixing water that is neither chemically bound nor retained after oven drying, plus the water carried in by the foam.
| Dry density | Predicted wet density | Difference | As % of dry |
|---|---|---|---|
| 400 kg/m3 | 554 kg/m3 | +154 | 38 % |
| 1000 kg/m3 | 1144 kg/m3 | +144 | 14 % |
Calculate the wet density target for each mix, write it on the batch sheet, and check it on every batch. Do not carry a wet density target across from one mix design to another.
Job quantities and waste allowance
Volume calculation is the ordinary geometry, with two foam-concrete-specific adjustments.
Worked example. An insulating roof screed, 250 m2 at 80 mm nominal thickness:
- Nominal volume = 250 × 0.08 = 20.0 m3
- Add a falls and substrate-tolerance allowance. On a roof deck with falls, average thickness is typically 10 to 20 % above nominal: say 12 % → 22.4 m3
- Add a placing and pump-priming allowance of 5 to 8 % → 23.8 m3, order 24 m3
Two points that catch people out:
- There is no compaction allowance. Foam concrete is self-compacting and does not reduce in volume on placing, unlike a compacted granular fill. Adding a compaction factor over-orders the job.
- There is a priming loss. Every pump line has to be primed and flushed. On a long line this is a real volume, and it is entirely waste.
Then, for costing, convert to materials with the mix design: 24 m3 of the 400 kg/m3 screed above needs 24 × 335 = 8040 kg of cement — about 161 bags at 50 kg, or a part silo load — 4.4 m3 of water, 27 litres of foaming agent concentrate, and 17 m3 of generated foam.
Sizing the plant for the job
Foam demand is the number that decides whether the plant can keep up. Continuing the same job:
- 24 m3 of material × 0.708 m3 foam per m3 = 17 m3 of foam
- Placed over six productive hours: 2.8 m3 of foam per hour
Commercial foam generators are rated from roughly 10 to over 100 m3/h, so the generator is almost never the bottleneck. In practice the limits are, in order: mixer batch cycle time, pump throughput on the discharge line, and — on any job with lift-height constraints — the time each lift needs to stiffen before the next can go on. Size against those, and check the generator only to confirm it is not undersized for a very low-density mix, where foam demand per cubic metre is at its highest.
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