IS 2185 Part 3:1984 (reaffirmed in 2019) is the Indian Standard governing 'Concrete Masonry Units — Autoclaved Cellular (Aerated) Concrete Blocks'. It's a 14-page document, which sounds simple, but each page references three or four further standards. For a project architect writing a wall specification, here's what to actually check.
1. Density grade (Section 4.1)
The standard defines three grades — Grade 1 (501–650 kg/m³), Grade 2 (451–500 kg/m³), and Grade 3 (≤450 kg/m³). Most commercial AAC supplied in India is Grade 1. Specify the grade explicitly — don't leave it open. A wall designed assuming Grade 1 strength but built with Grade 3 will not perform as expected.
2. Compressive strength (Section 4.2)
Minimum 3 N/mm² for Grade 1, 2 N/mm² for Grade 2. VAJRA blocks are independently tested at 4.0 N/mm² average. Critically: the test is done on whole blocks, capped and loaded between rigid platens. If you see a supplier quoting strength from cube-test extracts, it's not a comparable number.
3. Dimensional tolerance (Section 5)
Length ±5 mm, height ±3 mm, thickness ±2 mm. These limits sound generous, but they matter when you're laying with thin-bed adhesive — a thickness variation greater than ±2 mm forces the mason to compensate with adhesive, blowing out the joint thickness budget. Insist on wire-cut blocks (vs. moulded), which hold tolerance much better.
4. Drying shrinkage (Section 4.4)
Maximum 0.05% per IS 6441 Part 2. This is the single most-skipped check. Excess shrinkage causes hairline cracks at junctions with RCC columns 6–9 months after handover. Ask the supplier for the latest shrinkage test report. If they can't produce one less than 12 months old, walk away.
5. Marking and identification (Section 7)
Every block must carry the manufacturer's name/mark, grade, and the standard reference (IS 2185-3). VAJRA blocks have all three stamped on the long face during the casting stage. Site inspectors check this on randomly pulled blocks from arriving pallets.
The free spec template
We maintain a one-page IS 2185 compliance checklist that you can attach to your project specification. It covers all five sections above plus the IS 3346 thermal requirement and IS 3809 fire requirement. WhatsApp us and we'll send it.
