Lightweight vs normal-weight concrete slab
"Lightweight" is used for three different materials with three different property sets. This page defines them, compares them against normal-weight concrete on the properties that actually govern a specification, and sets out where the weight saving is worth paying for and where it is not.
Three things people mean by "lightweight"
The search term bundles together three materials that behave nothing alike, and mixing them up in a specification is the most common source of trouble on this subject.
- Foam concrete (cellular concrete). Cement paste or mortar with a pre-formed foam mixed in to entrain air as a stable, closed void structure. Density is a design choice, typically 400 to 1600 kg/m3 for slab work, achieved by varying how much air is entrained rather than by changing the aggregate. See density classes and properties for the full range down to 75 kg/m3.
- Lightweight aggregate concrete. Conventional concrete with a porous aggregate — expanded clay, expanded shale, pumice or sintered fly ash — in place of gravel or crushed rock. The porosity sits inside the aggregate particles, not between them, so the material keeps an aggregate skeleton and behaves structurally like normal concrete, only lighter. Density runs 800 to 2000 kg/m3.
- Normal-weight concrete with a lighter finish build-up. Not a different concrete at all: a standard 2400 kg/m3 slab with a thinner screed, a lighter floor finish, or a reduced topping thickness. This route saves some dead load in the build-up above the slab without touching the structural material, and it is frequently the right answer when the actual constraint is finish depth rather than slab weight.
A specification that just says "lightweight slab" has not actually specified anything. The comparisons below assume the first two are in competition, since the third is a detailing choice rather than a material choice.
Density, strength, thermal conductivity and shrinkage compared
Figures for lightweight aggregate concrete at 1900 kg/m3 are interpolated within its published range and labelled indicative; the foam concrete figures are the classed values used throughout this site.
| Property | Normal-weight, 2400 | Lightweight aggregate, ~1900 | Foam concrete, 400–1600 |
|---|---|---|---|
| Density | 2400 kg/m3 | ~1900 kg/m3 | 400–1600 kg/m3 |
| 28-day compressive strength | 20–60 MPa | indicative 25–40 MPa (range 8–50) | 0.8–25 MPa |
| Thermal conductivity, λ | 1.4–2.0 W/(m·K) | indicative 0.70–1.20 (range 0.20–1.20) | 0.09–0.70 W/(m·K) |
| Drying shrinkage | indicative 0.03–0.08 % | indicative 0.04–0.10 % | 0.08–0.30 % |
| Takes embedded reinforcement | Yes, routinely | Yes – within EN 206 scope, Eurocode 2 provisions apply | Poorly, without added corrosion protection |
| Placed by | Poured and vibrated | Poured and compacted | Poured or pumped, self-levelling, no compaction |
The pattern that matters: lightweight aggregate concrete keeps an aggregate skeleton and therefore keeps conventional structural behaviour at reduced weight, while foam concrete trades structural capability for a much wider density range and a placement method that needs no plant at all. Neither is a strictly better version of the other; they solve different problems. See the comparison page for how foam concrete stacks up against these and other alternatives on a wider set of properties, including autoclaved aerated concrete and granular fill.
Specifying a lightweight slab
"Lightweight slab" is a search term more than a specification, and it covers three genuinely different things. Treating them as interchangeable is where the ambiguity in the term actually causes trouble, because the three have completely different design consequences.
- A slab cast from lightweight aggregate concrete. A structural element made with a porous aggregate in place of gravel or crushed rock, 800 to 2000 kg/m3, within EN 206 scope and designed to Eurocode 2. The only one of the three that is itself a structural slab.
- A normal-weight slab on a lightweight sub-base or fill. The slab is ordinary 2400 kg/m3 concrete, unchanged. What is lightweight is what sits beneath it — typically a foam concrete or lightweight granular fill reducing the load reaching soft or compressible ground, or filling a void economically. See foam concrete as a slab-on-grade sub-base.
- A foam concrete layer that is not structural at all. Density 400 to 1600 kg/m3, used as insulation, void fill or a level-up screed under or over the load-carrying slab. See density classes and properties.
Only the first can carry load and take embedded reinforcement as a structural slab needs to. The second carries load by definition, since it is unchanged normal-weight concrete, but the lightweight element beneath it carries nothing. The third carries no structural load at all; foam concrete's low alkaline reserve and fast carbonation make it a poor host for embedded steel regardless.
| What it is | Typical density | Structural | Takes reinforcement | Typical thickness | What it is for |
|---|---|---|---|---|---|
| Lightweight aggregate concrete slab | 800–2000 kg/m3 | Yes | Yes – Eurocode 2 | As designed, like any structural slab | The slab itself: suspended floors, decks, precast units where dead load matters |
| Normal-weight slab on lightweight sub-base or fill | 2400 kg/m3 (slab) | Yes | Yes, routinely | As designed | Reduces load reaching weak ground; the slab itself is unchanged |
| Foam concrete layer | 400–1600 kg/m3 | No | Poorly, without added protection | Typically 50–300 mm | Insulation, void fill or level-up beneath or above a structural slab |
Foam concrete is not a weaker way of doing what lightweight aggregate concrete does; it is not a structural material at any density, and no amount of reinforcement changes that. Its role in a slab build-up is to sit under it or over it, not to be it.
Where lightweight aggregate concrete substitutes for normal-weight concrete in a structural slab, the most common surprise is not strength. It has a lower elastic modulus than normal-weight concrete of the same compressive strength — the porous aggregate itself is less stiff than gravel or crushed rock, which shows up directly in the stiffness of the concrete made with it. A slab designed on strength alone can pass every capacity check and still deflect more than expected, because deflection is governed by stiffness rather than ultimate capacity. Spans are more often governed by deflection than by bending, the opposite of what many designers expect coming from normal-weight practice. Check deflection explicitly rather than assume a strength-adequate section is stiffness-adequate.
The practical rule: if the slab has to carry load, specify lightweight aggregate concrete, not foam concrete. If the goal is reducing dead load in the build-up around a structural slab — the sub-base beneath it, the screed or insulation above it — foam concrete is the right tool, but it goes under the slab or over it, never as the slab. Working out that dead load at any thickness or density is covered on the lightweight concrete slab weight page.
Dead load at a common thickness
Weight saving is only meaningful expressed as an actual number. At a common slab thickness of 150 mm (5.9 in), mass per unit area follows directly from density:
| Material | Density | Mass per m2 | Mass per sq ft | Saving vs normal-weight |
|---|---|---|---|---|
| Normal-weight concrete | 2400 kg/m3 | 360 kg/m2 | 73.7 lb/sq ft | — |
| Lightweight aggregate concrete | 1900 kg/m3 | 285 kg/m2 | 58.4 lb/sq ft | 20.8 % |
| Foam concrete, top of range | 1600 kg/m3 | 240 kg/m2 | 49.2 lb/sq ft | 33.3 % |
| Foam concrete, bottom of range | 400 kg/m3 | 60 kg/m2 | 12.3 lb/sq ft | 83.3 % |
The working is direct: mass per square metre is density multiplied by thickness, so 2400 kg/m 3 × 0.15 m = 360 kg/m2, converted to lb/sq ft at 0.062428 lb/ft3 per kg/m3 and 3.280840 ft per metre. The same calculation at any thickness or density is worked out on the lightweight concrete slab weight.
Note that only foam concrete reaches the low end of this table. Lightweight aggregate concrete's practical floor for a poured, reinforceable slab is around 1600–1800 kg/m3; below that it moves out of structural territory and the comparison with foam concrete stops being apples-to-apples.
Where the weight saving pays
The saving is worth having when something downstream is actually constrained by weight:
- Refurbishment over an existing structure. An existing floor or roof has a fixed residual load capacity. A new topping, screed or infill slab at reduced density can fit inside that budget where a normal-weight equivalent cannot, without strengthening work.
- Soft or compressible ground. Every kilogram removed from the slab is a kilogram the foundations, piles or ground improvement do not have to carry. On genuinely soft ground this can change the foundation solution, not just its size.
- Long spans. Self-weight is often the governing load on a suspended slab. Cut it and the same structural depth spans further, or the same span needs less structural depth — relevant to precast, post-tensioned and composite floor design.
- Transport and handling of precast elements. Element mass is capped by crane capacity and by transport axle loads. A lighter mix allows a larger element within the same lifting and hauling limits, or the same element on lighter plant.
Where it does not pay
Equally often, the honest answer is that lightweight buys nothing:
- A ground-bearing slab on competent ground with no span to speak of. The subgrade already carries normal-weight concrete without difficulty, so there is no downstream cost to offset the change.
- Anywhere compressive strength or wear resistance is the governing requirement. Foam concrete's strength ceiling — 25 MPa at its densest, structural-grade class — sits well below normal-weight concrete's range, and its surface hardness is low enough that an exposed wearing surface needs a topping regardless.
- Reinforced elements where corrosion protection has not been budgeted. Foam concrete's low alkaline reserve and fast carbonation make it a poor host for embedded steel without deliberate additional protection; see density classes and properties for why. Lightweight aggregate concrete does not have this problem and is the better choice for a reinforced lightweight element.
Cost: relative terms, not a price per square foot
Material cost per cubic metre for foam concrete is usually higher than normal-weight ready-mix. It uses a specialist foaming agent and, at the lower densities, a batching process that needs more skill and more quality control than ordinary concrete. That comparison, on its own, makes foam concrete look like the more expensive choice.
It is the wrong comparison to stop at. Installed cost is what a project actually pays, and three things regularly move it the other way:
- No compaction. The material is self-levelling and needs no vibrating plant, no compaction labour and no associated noise or vibration restrictions.
- Less plant on site. Placement is by pour or by pump from a mobile unit, not by a fleet of compaction equipment moving material in layers.
- Less structure downstream. Every kilogram saved per square metre, worked through the reasoning in the section above, can mean smaller foundations, lighter formwork and propping, and a smaller crane — costs that show up nowhere on the concrete supplier's invoice.
None of that can be reduced to a single figure per square foot without knowing the specific job, and this page will not invent one. What to get quoted, specifically: the delivered material cost per cubic metre at the target density; the placement method and labour it implies compared with a compacted alternative; and, separately, whether the reduced dead load changes anything in the structural design below it. Comparing raw material cost alone, without that last item, is the most common way this decision gets made badly.
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