Technical articles 6 min read
Infill lightweight concrete: which grade, which strength
An infill is not structural concrete. The commonest mistake is treating it as if it were, and the result is more weight rather than more strength.
Filling with lightweight concrete does three things: it cuts dead load, it fills dead space, and it insulates against heat and sound. None of the three is structural strength — which is precisely what gets forgotten on site.
Settle the question first: does this layer carry load? If all that sits on it is a floor finish and ordinary traffic, a non-structural infill will do and the aim is the lowest density you can get. If concentrated loads land on it, or it forms part of a load-bearing section, it is no longer an infill and belongs in a structural lightweight mix design — a different subject.
Grade for pure infill: 10–20 mm. The coarsest fraction gives the lightest result and the lowest cement demand, because there is less surface area to coat. For filling between joists, over old slabs and behind retaining walls, this is the right choice.
Grade for a layer that gets floated: 4–10 mm. Slightly heavier, but the surface closes under a float, which suits falls and levelling.
The 0–3 mm fraction replaces sand, not coarse aggregate. Do not use it to fill volume: it raises density and pulls cement consumption up with it.
Two mistakes that keep recurring:
Too much cement. The instinct is that more cement means a sounder job. In a lightweight infill, more cement means more paste between granules, which means higher density — the very thing you were trying to reduce — along with more shrinkage and cracking. For a non-structural fill, the least cement that binds the granules together is enough.
Too much water. Dry granules take up mixing water, the mix looks stiff, and water gets added. Pre-soaking the aggregate for thirty minutes before mixing removes the temptation and stops the paste separating.
One point of execution: light granules want to rise in the mix. Prolonged vibration drives them to the surface and leaves the section in two layers — aggregate above, paste below. Vibrate for less time than you would normal-weight concrete, and settle for placing it evenly.
Settle the question first: does this layer carry load? If all that sits on it is a floor finish and ordinary traffic, a non-structural infill will do and the aim is the lowest density you can get. If concentrated loads land on it, or it forms part of a load-bearing section, it is no longer an infill and belongs in a structural lightweight mix design — a different subject.
Grade for pure infill: 10–20 mm. The coarsest fraction gives the lightest result and the lowest cement demand, because there is less surface area to coat. For filling between joists, over old slabs and behind retaining walls, this is the right choice.
Grade for a layer that gets floated: 4–10 mm. Slightly heavier, but the surface closes under a float, which suits falls and levelling.
The 0–3 mm fraction replaces sand, not coarse aggregate. Do not use it to fill volume: it raises density and pulls cement consumption up with it.
Two mistakes that keep recurring:
Too much cement. The instinct is that more cement means a sounder job. In a lightweight infill, more cement means more paste between granules, which means higher density — the very thing you were trying to reduce — along with more shrinkage and cracking. For a non-structural fill, the least cement that binds the granules together is enough.
Too much water. Dry granules take up mixing water, the mix looks stiff, and water gets added. Pre-soaking the aggregate for thirty minutes before mixing removes the temptation and stops the paste separating.
One point of execution: light granules want to rise in the mix. Prolonged vibration drives them to the surface and leaves the section in two layers — aggregate above, paste below. Vibrate for less time than you would normal-weight concrete, and settle for placing it evenly.