AAC Block Coba vs Perlite Concrete: Which Is Better for Terrace Slope and Sunk Fill?
Once builders saw how heavy brickbat coba is, many switched to AAC block coba: broken or cut AAC blocks laid in mortar for terrace slopes and sunk slabs. It is lighter than brickbat. But it is still a fill made of pieces, bedded in heavy mortar, and that is where the problems start.
Perlite concrete is a poured lightweight concrete. It fills the same space in one pour, with no pieces and no mortar bed.
AAC block coba vs perlite concrete
| AAC block coba | Perlite concrete fill |
How it is laid | Block pieces bedded in mortar, by hand | Poured or pumped, levelled to slope |
Density in place | AAC block itself typically 550 to 700 kg/m³, but the mortar bed and joints add a lot | Chosen grade from 400 to 1,200 kg/m³, same everywhere |
Gaps and joints | Joints between pieces; quality depends on the mason | One continuous fill |
Slope | Built up in steps, then plastered | Formed in the same pour |
Takes tiles directly | Needs a screed on top | Yes |
Water absorption | AAC is porous and soaks up water if the membrane fails | Inorganic, does not rot or decay |
Heat insulation | Moderate | Lab-tested k from 0.08 W/m·K (400 grade) |
Fire | Non-combustible | Non-combustible |
The hidden weight in AAC coba
An AAC block on its own is light. But a coba is not just blocks. Each piece sits in a mortar bed, joints are filled with mortar, and a plaster or screed goes on top to make the slope true. Sand-cement mortar weighs about 2,000 kg/m³, so the more joints and bedding, the heavier the coba gets. On site, AAC coba often ends up far heavier than the block density suggests.
A poured perlite concrete fill has no joints and no bed. What you pour is what the slab carries.
Slope and finish in one pour
Terrace slopes need a smooth fall to the outlets. With AAC coba, the slope is built up in pieces and then plastered true. Perlite concrete is poured and levelled to the slope in one go, and gives a hard surface that takes the waterproofing membrane or tiles directly.
Sunken slabs: why pieces are a risk
In toilet and bath sunk slabs, the fill sits around pipes and fittings. Packing block pieces neatly around them is slow, fiddly work. A poured fill is placed around the pipes in one operation and levelled to the floor slope. If you are filling sunk slabs today, read our guide to sunken slab filling material.
When AAC coba still makes sense
If the work is very small and there is no access for pumping, AAC pieces can be practical. For anything above a few hundred square feet, a poured fill is faster, cleaner and more even.
Frequently asked questions
Is AAC block coba better than brickbat coba?
It is lighter than brickbat coba, but it is still pieces in mortar. Perlite concrete removes the mortar bed and joints altogether.
Can perlite concrete go on a terrace that already has AAC coba?
Yes, over a sound base. For waterproofing failures we usually recommend removing the old coba so the new fill sits on the slab.
What grade of perlite concrete replaces AAC coba?
For terrace slopes we usually use the 1,200 grade. For roofs where heat matters most, a lighter insulating grade is laid under a protective screed.
Compare your own terrace on the project calculator.
Related reading: Brickbat coba alternative · Brickbat coba weight and dead load
About Perlite India. Perlite India designs and lays perlite lightweight insulating concrete for roofs, floors, sunk fills and podiums, with 3M+ sq.ft laid across India. Supported by the Perlite Construction Division of Keltech Energies Ltd, a member of the Perlite Institute (USA).
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