Lightweight concrete is concrete in which the usual gravel has been replaced by lightweight aggregates such as expanded clay, pumice, lava or perlite. The pores in those grains make the concrete considerably lighter than ordinary concrete and give it better thermal performance — at the cost of compressive strength.
Under the concrete standard EN 206, concrete counts as lightweight when its oven-dry density lies between roughly 800 and 2000 kg/m³. Ordinary structural concrete sits well above that, at around 2200 to 2400 kg/m³.
Properties
- Lower weight. Less dead load on the foundation, lighter floors, blocks that are easier to handle.
- Better insulating value. The thermal conductivity of lightweight concrete blocks is broadly of the order of 0.7 to 1.0 W/(m·K) — better than ordinary concrete, but far worse than a real insulation material. Lightweight concrete is a structural material that happens to insulate a little; it is not insulation.
- Lower compressive strength. Cube strengths of common lightweight concrete products typically fall in the order of 8 to 25 N/mm², depending on density and mix.
- Porous. Lightweight concrete absorbs water. Used externally as an outer leaf it always needs a watertight mineral finish; unprotected it is not suitable as an external wall.
- Easy to work. Sawing, drilling and routing is easier than with ordinary concrete — handy for service chases.
Applications
- Internal walls, both loadbearing and non-loadbearing, usually as lightweight concrete blocks.
- Separating walls where acoustic mass and fire resistance are wanted without the weight of calcium silicate.
- Floors and roof slabs where dead load is a problem, for example when adding a storey to an existing building.
- Fill and levelling, such as a light infill over an existing floor of limited load capacity.
Blocks and units come in a range of thicknesses, from around 70 mm for light partitions to roughly 240 mm for loadbearing work.
Do not confuse it with aerated concrete
Autoclaved aerated concrete (AAC, the familiar white blocks) is not lightweight concrete in the sense above. There the lightness comes from gas bubbles in the concrete matrix itself rather than from lightweight aggregate, and the density is usually much lower. AAC insulates better but is also weaker and even more absorbent.
Working tips
- Use the right adhesive or mortar. Thin-joint mortars exist for lightweight blocks; ordinary mortar in a thick joint makes the wall heavier and thermally worse.
- Fixing needs care. A plain hammer-in plug holds poorly in porous material. Use plugs rated for hollow or porous substrates, and hang heavy items (kitchen units, wall-hung WC) from a continuous frame where possible.
- Dampen before plastering. Porous lightweight concrete pulls water out of the plaster, causing cracks. Follow the plaster manufacturer’s preparation instructions.
- Always finish it outside. Without a watertight coating the wall saturates and is destroyed by frost.
- Lift sensibly. “Lightweight” is relative: a 240 mm lightweight block is still heavy enough to hurt your back.
Related terms
- Expanded clay aggregate
- Aerated concrete
- Cube compressive strength
- Concrete
- Screed
- Thermal resistance (R-value)
- Reinforcement
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