Density classes and properties
Dry density is the master variable of foam concrete. Fix it and you have fixed the strength, the thermal conductivity, the shrinkage and the cost to within a fairly narrow band. This page sets out the five working classes, what each is good for, and where the boundaries actually lie.
Why density governs everything
In foam concrete the solid phase and the void phase compete for the same cubic metre. Adding air removes load-bearing paste in exactly the proportion it removes weight, so almost every mechanical and thermal property tracks density directly. Compressive strength follows a power law of the form f = a·ρn with the exponent typically between 2 and 3, which is why halving the density costs far more than half the strength.
This makes specification unusually clean. Rather than juggling water/cement ratio, aggregate grading and admixtures, you choose a target dry density and accept the property envelope that comes with it. What the mix design then controls is where within that envelope you land — and the difference between the top and the bottom of an envelope at constant density can be a factor of two in strength, which is entirely down to void structure and therefore to the foaming agent.
The five working classes
| Class | Dry density | fc, 28 d | λ | Drying shrinkage | Typical use |
|---|---|---|---|---|---|
| Ultra-light | 75–150 kg/m3 | < 0.3 MPa | 0.04–0.06 W/(m·K) | very high | Insulating fill, void and annulus filling, thermal breaks |
| Insulating | 200–300 kg/m3 | 0.3–1.0 MPa | 0.06–0.09 W/(m·K) | 0.25–0.35 % | Insulation boards, insulating roof screeds, sub-screed layers |
| Light non-structural | 400–600 kg/m3 | 0.8–3 MPa | 0.09–0.18 W/(m·K) | 0.15–0.30 % | Non-load-bearing blocks, floor screeds, trench reinstatement |
| Semi-structural | 800–1000 kg/m3 | 2–8 MPa | 0.18–0.33 W/(m·K) | 0.10–0.20 % | Load-bearing blocks in low-rise construction, wall panels |
| Structural-grade | 1200–1600 kg/m3 | 5–25 MPa | 0.32–0.70 W/(m·K) | 0.08–0.15 % | Precast elements, load-bearing panels, structural screeds |
Compressive strength
The relationship between density and strength is steep and reasonably predictable within a given production setup, but the constant of proportionality is specific to that setup. Two plants running at 600 kg/m3 can differ by a factor of two in 28-day strength, and the difference is void structure, not cement content.
What raises strength at constant density:
- Finer, more uniform voids. The largest single lever. A narrower void-size distribution with fewer merged voids means fewer stress concentrations. Nambiar and Ramamurthy (2007) quantified this directly through air-void characterisation.
- Fine fillers. Silica fume and fine fly ash densify the paste in the thin films between voids, where failure initiates.
- Extended curing. Foam concrete continues gaining strength well past 28 days, especially with fly ash. A 90-day figure can be 20 to 40 % above the 28-day value.
- Lower effective water content, remembering that the foam brings its own water.
Testing note: foam concrete strength is measured on cubes or cylinders in the usual way, but the material is sensitive to moisture state at test. A specimen tested saturated reads significantly lower than the same specimen tested at equilibrium. ASTM C495 covers the procedure for lightweight insulating concrete and should be followed rather than improvised.
Thermal conductivity
Thermal performance is the reason most people reach for foam concrete, and it is where the low-density classes earn their keep. Normal-weight concrete conducts at roughly 1.4 to 2.0 W/(m·K). A 200 kg/m3 foam concrete at around 0.07 W/(m·K) is in the territory of a light insulating material while remaining a mineral, non-combustible, pumpable, cement-bound product.
Moisture matters more than the datasheet suggests
Published λ values are almost always oven-dry. Water conducts heat about 25 times better than air, so in-service conductivity at equilibrium moisture content is materially higher than the dry figure — commonly 20 to 40 % higher for foam concrete in a normal indoor environment, and more in a wet one. Design with a moisture-corrected value, and never assume a freshly cast element is performing at its dry conductivity: it can take months to reach equilibrium.
Void structure influences conductivity as well as strength: at equal density, smaller and more closed voids conduct less, because they suppress convection within the void and lengthen the conductive path. Othuman Mydin (2022) examined density, porosity and void size as separate influences on the conductivity of lightweight foamed concrete.
Drying shrinkage
This is foam concrete's most serious practical weakness. Drying shrinkage of 0.1 to 0.35 % — 1 to 3.5 mm per metre — is normal, roughly an order of magnitude above normal-weight concrete. The cause is structural rather than chemical: there is no coarse aggregate skeleton to restrain the shrinking paste, and the enormous internal surface area accelerates moisture loss.
Shrinkage rises sharply as density falls, because the paste fraction that shrinks makes up a larger share of what little solid there is, and because there is less to restrain it. Practical mitigation:
- Cure sealed for at least seven days; this is not optional.
- Substitute fly ash for a substantial part of the cement.
- Add polypropylene fibres at 0.1 to 0.5 % by volume for plastic and early drying cracking.
- Allow elements to reach equilibrium moisture before installation, particularly blocks.
- Detail movement joints for the actual shrinkage, not for a conventional-concrete figure.
Water absorption and durability
The intuition that a highly porous material must absorb water freely is only half right. The artificial voids created by foam are mostly closed and are not, by themselves, a transport path. Water moves through the capillary porosity of the paste between them, which is the same mechanism as in any cement-based material.
The consequence is that absorption by volume is not dramatically worse than conventional concrete, while absorption by mass looks alarming simply because the mass is so much lower. Comparisons should always be made on a volume basis. Where the voids do matter is in the response to damage: once a void system is opened up by cracking or by cutting, water fills it and stays.
Carbonation proceeds faster than in normal-weight concrete because of the open capillary structure and the low binder content per cubic metre, which is one reason foam concrete is a poor host for embedded reinforcement without additional protection. Freeze-thaw behaviour is generally good in dry service and poor in saturated service. Sulfate resistance was examined by Indu Siva Ranjani and Ramamurthy (2012); the behaviour depends strongly on binder composition, and fly-ash mixes perform considerably better.
Fire and acoustic behaviour
Foam concrete is mineral and non-combustible, and classifies as Euroclass A1 in the absence of organic additions. Its fire resistance is better than its density suggests, because the low conductivity that makes it a good insulator also slows the passage of heat through an element, and because the free and chemically bound water absorbs energy as it is driven off. Low-density material is the better fire insulator; high-density material retains more residual strength after exposure.
On acoustics, the common claim that foam concrete is a good sound insulator needs qualifying. Airborne sound insulation is governed largely by mass per unit area, so a lightweight wall is a worse sound barrier than a heavy one of the same thickness — this is unavoidable physics. What foam concrete does well is sound absorption, where an open or partially open void structure dissipates energy. Use it to reduce reverberation, not to meet a party-wall sound reduction requirement on mass alone.
Specifying a density
A workable specification for foam concrete states, at minimum:
- Target oven-dry density with a tolerance, conventionally ±50 kg/m3 or ±5 %, whichever is greater.
- Minimum 28-day compressive strength at that density, tested to a named method.
- Test method and moisture state at test — omitting this makes the strength requirement unenforceable.
- Curing regime, since it materially changes the result.
- Where relevant: thermal conductivity with the moisture state it applies to, maximum drying shrinkage, and fire classification.
Specify wet density only as a production control tolerance, never as the acceptance criterion — it is a proxy that depends on the mix. See mix design and calculation for converting between the two, and standards for the documents to cite.
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