How Heavy Is Brickbat Coba? Dead Load per Square Metre, Worked Out
Structural consultants ask one question before they approve any change to a terrace or sunk fill: how much does it weigh? This article works out the dead load of brickbat coba from standard unit weights, and compares it with perlite lightweight concrete.
Density of brickbat coba
Brickbat coba is broken brick laid in cement mortar. Using the unit weights in IS 875 (Part 1):
Brick masonry: about 1,920 kg/m³
Cement mortar: about 2,080 kg/m³
A typical coba mix lands at roughly 1,970 kg/m³, close to solid brickwork. That is the number to use, not the much lower figure people assume because it is "just broken bricks".
Dead load per square metre
Average thickness | Brickbat coba | Perlite concrete 1,200 grade | Perlite concrete 1,400 grade |
75 mm | 148 kg/m² | 90 kg/m² | 105 kg/m² |
115 mm | 226 kg/m² | 138 kg/m² | 161 kg/m² |
150 mm | 295 kg/m² | 180 kg/m² | 210 kg/m² |
At the same thickness, the 1,200 grade is about 39% lighter than brickbat coba and the 1,400 grade about 29% lighter. Lighter insulating grades (down to about 300 kg/m³) are used where heat insulation, not filling, is the goal.
What it adds up to on a whole building
Terrace. A 600 m² terrace with coba at 115 mm average carries about 136 tonnes of coba. The 1,200 grade at the same thickness weighs about 83 tonnes, a saving of about 53 tonnes on the roof slab.
Sunk fills. Take a sunken toilet filled 300 mm deep:
Brickbat fill: about 590 kg per m²
Perlite concrete 1,200 grade: 360 kg per m²
Saving: about 230 kg per m²
A flat with three toilets of about 4 m² each saves roughly 2.8 tonnes. In a 20-storey tower with four flats per floor, that is about 220 tonnes less dead load, all of it carried by slabs, beams, columns and foundations.
Why dead load matters
Every kilogram of fill has to be carried down through the frame to the foundation.
Lower dead load can mean lighter reinforcement in the slabs that carry it, which your structural consultant can verify for the design.
On existing buildings, the saving can decide whether a terrace re-do is safe at all.
In seismic zones, less mass at the top of the building reduces the forces the frame has to resist.
How to use these numbers
Use the densities above for preliminary comparison only. Final design loads should always come from your structural consultant, using the actual mix density and thickness on your drawings. We can supply the density of the specific perlite concrete grade proposed for your project.
Frequently asked questions
What is the density of brickbat coba?
About 1,970 kg/m³, based on IS 875 (Part 1) unit weights for brick masonry and cement mortar.
How much does 100 mm of brickbat coba weigh per square metre?
About 197 kg/m². The same thickness of perlite concrete at the 1,200 grade weighs 120 kg/m².
Is perlite concrete structural?
No. Perlite lightweight concrete is used for non-structural fills, screeds and insulation. The slab below carries the load.
Related reading: Brickbat coba alternative · Sunken slab filling material
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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