Lightweight concrete mix quantities
For normal concrete, order a volume and the mass follows automatically. For foam concrete it does not, because density is a free variable you set, not a fixed property of the material. This page works through what that does to bag counts, silo draw and truck loads.
Density is a free variable, mass is not
Order a cubic metre of normal-weight ready-mix and you get roughly 2400 kg, wherever it comes from, because the aggregate grading and the water/cement ratio that produce a workable, durable concrete only vary within a narrow band. Density is not something you specify; it is what you get.
Foam concrete inverts that. Dry density is the number you choose first, and everything else – binder content, filler content, water and, above all, how much air is entrained – is derived from it by the absolute volume method. Two loads described identically as "one cubic metre of foam concrete" can differ enormously in what they actually contain: a lean 300 kg/m3 insulating mix might carry 250 kg of cement and nothing else, while a rich structural-grade mix approaching 1200 kg/m3 carries more cement again plus a substantial filler content, for a combined dry solids mass several times higher. Across the density range used for slab work, binder content alone can run from roughly 170 kg/m3 in a very lean ultra-light mix to 800 kg/m3 in a rich structural-grade one. Ordering "a cubic metre" without stating the density specifies almost nothing.
Cement, sand, water and foam by density
Each row is derived by the same absolute volume method used throughout this site: fix the dry density, split it between cement and filler using ρdry ≈ 1.2 × cement + filler, choose a water/binder ratio, then convert cement, filler and water to absolute volumes (cement 3100 kg/m 3, sand 2650 kg/m3, water 1000 kg/m3) and fill whatever volume is left with foam. Foam is taken at 50 g/L and foaming agent dilution at 1:30, the working figures used in the worked examples on the mix design page, whose 400 and 1000 kg/m3 rows are reproduced here unchanged. Water/binder ratio falls as density rises because richer, filler-bearing mixes need less water to stay workable; the values below are a reasonable working set, not the only valid choice. The same densities can equally be reached with neat, filler-free mixes, which need far more cement and land at a different cost; what drives foam concrete cost tabulates that alternative and compares the two.
| Dry density | Cement | Sand | Water | w/b | Foam volume | Foam mass | Predicted wet density | Concentrate, 1:30 |
|---|---|---|---|---|---|---|---|---|
| 300 kg/m3 | 250 kg | 0 kg | 150 kg | 0.60 | 0.769 m3 | 38.5 kg | 439 kg/m3 | 1.24 L |
| 400 kg/m3 | 335 kg | 0 kg | 184 kg | 0.55 | 0.708 m3 | 35.4 kg | 554 kg/m3 | 1.14 L |
| 600 kg/m3 | 500 kg | 0 kg | 265 kg | 0.53 | 0.574 m3 | 28.7 kg | 794 kg/m3 | 0.93 L |
| 800 kg/m3 | 320 kg | 416 kg | 166 kg | 0.52 | 0.574 m3 | 28.7 kg | 931 kg/m3 | 0.93 L |
| 1000 kg/m3 | 400 kg | 520 kg | 200 kg | 0.50 | 0.475 m3 | 23.8 kg | 1144 kg/m3 | 0.77 L |
| 1200 kg/m3 | 360 kg | 768 kg | 162 kg | 0.45 | 0.432 m3 | 21.6 kg | 1312 kg/m3 | 0.70 L |
Read the 600 and 1200 rows together and the free-variable point becomes concrete: the neat-cement 600 kg/m3 mix uses 500 kg of cement per cubic metre, more than the sand-bearing 1200 kg/m3 mix at 360 kg, despite the finished material being twice as dense. Density does not track cement content on its own; it tracks cement plus filler together, and the two trade off against each other depending on how the mix is formulated.
What this does to bag counts and silo draw
Take a single order of 10 m3 and compare the 400 and 1200 kg/m3 rows above.
| 400 kg/m3, 10 m3 | 1200 kg/m3, 10 m3 | |
|---|---|---|
| Cement | 3350 kg ≈ 67 bags (50 kg) | 3600 kg ≈ 72 bags (50 kg) |
| Sand | 0 kg | 7680 kg |
| Water | 1840 L | 1620 L |
| Total delivered mass | 5540 kg (5.54 t) | 13,120 kg (13.12 t) |
Cement demand barely moves – 67 bags against 72, a 7 % difference – because the extra mass in the denser mix is sand, not cement. Silo draw-down for the cement silo is likewise almost identical between the two jobs. Total delivered mass tells a completely different story: 13.12 tonnes against 5.54, a factor of 2.4, for the identical 10 m3 order. A batching plant or a site logistics plan that reads "10 cubic metres" and assumes a mass from experience with one density will be badly wrong for the other. Bag counts and silo draw have to be checked against the specific mix design, not inferred from the order volume.
Why the truck is volume-limited, not weight-limited
A ready-mix drum has a fixed volumetric capacity – a common size is around 6 m3 – and the chassis has a fixed payload rating, typically in the range of 10 to 14 tonnes for that size of truck. Which limit bites first depends entirely on density.
Fill a 6 m3 drum with normal-weight concrete at 2400 kg/m3: 6 × 2400 = 14,400 kg, 14.4 tonnes – at or beyond a typical payload rating before the drum is even full. Normal-weight concrete trucks are routinely weight-limited: the axle rating, not the drum, decides how much can go on in one load.
Fill the same drum with foam concrete. At the 400 kg/m3 wet density from the table above (554 kg/m3): 6 × 554 = 3324 kg, 3.3 tonnes. At the densest mix in the table, 1200 kg/m 3 (wet 1312 kg/m3): 6 × 1312 = 7872 kg, 7.9 tonnes. Both are well under any payload limit relevant to that size of vehicle. For foam concrete across its normal density range, the drum runs out of volume long before the chassis runs out of payload capacity: the truck is volume-limited, the reverse of the normal-weight case. The practical consequence is that a delivery schedule built around "loads per day" transfers reasonably well between foam concrete mixes of different densities – the drum count does not change – but a schedule built around tonnage delivered does not transfer at all.
Pump-priming and waste allowances
Two allowances apply on top of the nominal geometric volume, and neither is specific to a single density:
- Placing and pump-priming allowance, typically 5 to 8 %. Every pump line has to be primed and flushed before and after the pour, and that material is entirely waste. On a long line the volume is real and should be allowed for up front, not discovered on the day.
- No compaction allowance, ever. Foam concrete is self-compacting and does not reduce in volume on placing. Carrying over a compaction factor from granular-fill or normal-concrete estimating over-orders the job; there is nothing for it to compensate for.
These allowances apply to the ordered volume regardless of the target density; what changes with density is the mass and the foam demand that volume then converts to, using the table above.
Never carry a wet density target across mixes
Wet density is what gets measured on the batch sheet; dry density is what gets specified. The gap between them is the free water – mixing water not chemically bound in the cement paste, plus water carried in by the foam – and the table above shows it is nowhere near constant. Expressed as a percentage of the dry target it runs from 46 % at 300 kg/m3 down to 9 % at 1200 kg/m3, falling steadily as density rises because the water content per cubic metre grows much more slowly than the solids content does.
A wet density figure is therefore only valid for the specific mix it was calculated from. Moving a 300 kg/m3 mix's wet-density check figure onto a 1200 kg/m3 job, or the other way round, will reject good material or pass bad material, because the two mixes carry completely different proportions of free water for the same nominal density gap. Calculate the predicted wet density fresh for every mix design using the method above, write that number on the batch sheet for that job, and treat it as belonging to that mix and no other – the same production-control discipline set out on the mix design page. Where the foaming agent itself changes, recalculate again: consumption and foam stability both shift with concentration and product, covered on the foaming agents page, and a wet density figure calculated for one agent is not safely transferable to another. For the properties each density class is actually chosen for, see the density chart.
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