Foam concrete compared with the alternatives
Foam concrete competes with four other materials, and it does not beat any of them across the board. Knowing which property decides a given job is more useful than any general ranking, so this page sets out the comparisons one at a time.
Autoclaved aerated concrete (AAC)
AAC is the comparison people mean when they say "aerated concrete", and it is genuinely a different material despite the similar appearance. The distinction is worth getting right, because data sheets and code provisions are not transferable between them.
| Foam concrete | Autoclaved aerated concrete | |
|---|---|---|
| Air introduced by | Pre-formed foam, mixed in physically | Chemical reaction: aluminium powder with alkali releases hydrogen |
| Binder | Portland cement, often with fly ash | Cement, lime and a high proportion of fine silica |
| Curing | Ambient, or low-pressure steam at 50–70 °C | Autoclave at roughly 180–190 °C and 10–12 bar saturated steam |
| Binding phase | Ordinary C-S-H hydrates | Crystalline tobermorite, formed only under autoclaving |
| Void structure | Spherical, mostly closed, 0.1–1 mm | Interconnected, finer, partly open |
| Where it can be made | Factory or in situ on site | Factory only; the autoclave cannot travel |
| Capital cost | Low to moderate | High |
| Strength at equal density | Lower | Higher |
| Drying shrinkage | High: 0.1–0.35 % | Low: autoclaving largely completes shrinkage before delivery |
| Product forms | Cast in situ, poured fill, blocks, panels | Blocks and reinforced panels only |
| Standards | Dedicated guidance and national approvals | Harmonised European standards, e.g. EN 771-4 and EN 12602 |
Where AAC wins: a certified, dimensionally stable masonry unit with higher strength at equal density, low shrinkage, and a mature standards framework. For a masonry wall, AAC is generally the better product.
Where foam concrete wins: anything that has to be poured. AAC cannot fill a trench, a void, an annulus or a screed, and it cannot be produced on site. Foam concrete also reaches far lower densities — down to 75 to 150 kg/m3 — and needs a fraction of the capital investment.
Lightweight aggregate concrete
Lightweight aggregate concrete replaces normal aggregate with a porous one — expanded clay, expanded shale, pumice or sintered fly ash — putting the porosity inside the aggregate particles instead of between them. Typical densities are 800 to 2000 kg/m3.
Where it wins: real structural capability. Lightweight aggregate concrete with a closed structure and an oven-dry density between 800 and 2000 kg/m3 falls within the scope of EN 206 as lightweight concrete, so it can be designed to Eurocode 2 with established provisions for reinforcement, cover and durability. It reaches strength classes foam concrete cannot approach, and its shrinkage and creep behave conventionally because there is an aggregate skeleton.
Where foam concrete wins: everything below about 800 kg/m3, where lightweight aggregate concrete cannot go; self-levelling placement without compaction; and cost, since lightweight aggregate is an expensive manufactured product while foam is compressed air and a litre of concentrate.
What "lightweight slab" can mean
The phrase covers three different things with completely different design consequences:
- A slab cast from lightweight aggregate concrete. The only one of the three that is itself a structural slab, designed to Eurocode 2 with reinforcement.
- A normal-weight slab on a lightweight sub-base or fill. The slab is unchanged; what is light is the layer beneath it, reducing the load on soft ground. See the insulating layer build-up on the applications page.
- A foam concrete layer that is not structural at all. Insulation, void fill or a level-up screed under or over the load-carrying slab.
Foam concrete is not a weaker way of doing what lightweight aggregate concrete does. If the slab has to carry load, specify lightweight aggregate concrete; foam concrete goes under the slab or over it, never as the slab. Where lightweight aggregate concrete does replace normal-weight concrete, the common surprise is stiffness rather than strength: its lower elastic modulus means a section that passes every capacity check can still deflect more than expected, so check deflection explicitly.
| Material | Density | Mass per m2 | Saving |
|---|---|---|---|
| Normal-weight concrete | 2400 kg/m3 | 360 kg | — |
| Lightweight aggregate concrete | 1900 kg/m3 | 285 kg | 21 % |
| Foam concrete, top of range | 1600 kg/m3 | 240 kg | 33 % |
| Foam concrete, insulating | 400 kg/m3 | 60 kg | 83 % |
EPS blocks and geofoam
Expanded polystyrene geofoam is the direct competitor for lightweight fill applications, at densities of 10 to 40 kg/m3 — an order of magnitude lighter again.
Where EPS wins: the lowest achievable weight by a wide margin, dry installation by hand without plant, no curing time, and predictable factory-made properties.
Where foam concrete wins: it is poured, so it fills irregular voids that prefabricated blocks cannot; it is non-combustible where EPS is not; it is not attacked by fuels and solvents, which is a live issue for fill near carriageways; it is not degraded by ultraviolet exposure or by rodents; and it does not require the interface detailing that stacked geofoam blocks need to transfer load.
Compacted granular fill
The default against which foam concrete is usually being justified.
Where granular fill wins: material cost, on its own terms, is far lower. Aggregate is locally available almost everywhere, needs no specialist plant, and has no curing time.
Where foam concrete wins: the comparison is rarely material cost against material cost. Granular fill has to be placed and compacted in layers, which means plant, working room and time; it settles if any layer is compacted poorly, and post-reinstatement settlement is expensive to put right; it cannot be placed under or around obstructions; it weighs roughly four times as much, which matters on soft ground; and it needs excavation support that a self-supporting poured fill can sometimes avoid. On a narrow urban trench with services in it, foam concrete frequently wins on installed cost even at several times the material price.
Cost: material price against installed cost
No price is quoted here; it would be wrong within a year and different in every market. What can be stated is the structure of the cost, per cubic metre placed:
Cost = binder + foaming agent + water + filler + plant and pump hire + labour + haulage
Binder, filler and agent quantities come from the mix design; the rates are local and yours to insert. Plant, pump and labour are priced by time, so they only become a per-cubic-metre figure once divided by the volume placed that day, which is why small jobs price high per unit.
- Binder is the lever. Dry density is roughly 1.2 × binder plus filler, so binder cost scales with the density specified and with whether the mix carries sand. At 800 kg/m3 a neat mix needs about 667 kg of cement; a filled mix reaching the same density with 416 kg of sand needs about 320 kg. Both are valid designs. Compare quotations on the mix design, not on the density alone.
- Foaming agent: small cost, large risk. About one litre of concentrate per cubic metre at low density, a small fraction of the binder line. A cheaper agent that gives an unstable foam costs a remade pour.
- The truck is volume-limited. A 6 m3 drum of normal-weight concrete is about 14 t, at or past a typical payload; the same drum of 400 kg/m3 foam concrete is about 3.3 t wet. Haulage cost per cubic metre therefore does not fall with the mass saved.
Material cost per cubic metre usually favours ready-mix or granular fill. Installed cost is what a project pays, and it moves the other way where foam concrete removes compaction plant and labour, replaces several compacted lifts with one pour, avoids later settlement repairs, shortens time on a live carriageway, or reduces the dead load that foundations and temporary works below must carry. It does not move the other way when there is full access and ground that takes ordinary fill, when the volume is so small that mobilisation exceeds the material cost, or when compressive strength governs the specification.
Summary comparison
| Material | Density | Compressive strength | λ | Placed by |
|---|---|---|---|---|
| Foam concrete | 75–1600 kg/m3 | 0.2–25 MPa | 0.04–0.70 | Pouring or pumping |
| AAC | 300–800 kg/m3 | 2–8 MPa | 0.08–0.20 | Laid as masonry units |
| Lightweight aggregate concrete | 800–2000 kg/m3 | 8–50 MPa | 0.20–1.20 | Placed and compacted |
| EPS geofoam | 10–40 kg/m3 | 0.05–0.30 MPa | 0.03–0.04 | Stacked as blocks |
| Compacted granular fill | 1800–2200 kg/m3 | n/a | 1.0–2.0 | Layered and compacted |
| Normal-weight concrete | 2200–2500 kg/m3 | 20–60 MPa | 1.4–2.0 | Placed and vibrated |
Which property decides
| If the governing requirement is | Then specify |
|---|---|
| Filling something you cannot compact | Foam concrete |
| Absolute minimum weight on very soft ground | EPS geofoam, unless fire or fuel exposure rules it out |
| A certified masonry unit with low shrinkage | AAC |
| Reinforced structural capacity at reduced weight | Lightweight aggregate concrete |
| Thermal resistance in a mineral, non-combustible form | Foam concrete at 150–300 kg/m3 |
| Backfill that must be re-excavatable later | Foam concrete at 400–800 kg/m3 |
| Lowest material cost with room to compact | Compacted granular fill |
| Compressive strength | Not foam concrete |
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