AAC Block Construction: Thin-Bed Jointing & On-Site Techniques
A practical, contractor-focused guide to building walls with Autoclaved Aerated Concrete (AAC) blocks — from surface preparation and thin-bed jointing to curing, plastering and finishing.
Site preparation & tools
Recommended tools
- Notched trowel (3–4 mm) for thin-bed mortar application
- Rubber mallet for aligning blocks without damage
- Hand saw or band saw for on-site cutting
- Spirit level and plumb line
- Wire brush and sponge for surface cleaning
- Electric drill/grinder for chases and openings
Materials
- SSB AAC blocks — dry-stored, sorted by thickness
- Block Jointing Mortar (BJM) — polymer-modified, thin-bed
- Cement-sand levelling mortar for the first course only
- Clean, potable water
Laying AAC block walls
1. Level the first course
Lay the first course on a 12–15 mm bed of cement-sand mortar (1:6) to correct any base unevenness. Check level and plumb — this course dictates the accuracy of the entire wall.
2. Mix Block Jointing Mortar (BJM)
Add BJM powder to clean water (approx. 30–33% water by weight) and mix for 3–4 minutes to a smooth, lump-free paste. Use within 2 hours; do not re-temper with extra water.
3. Apply thin-bed mortar
Use a notched trowel to spread a uniform 2–3 mm layer of BJM on both horizontal and vertical faces. Thin-bed jointing eliminates thermal bridges and speeds construction.
4. Place & align blocks
Place each block firmly and tap with a rubber mallet to achieve a tight joint. Verify level and plumb every 2–3 blocks. Stagger vertical joints by at least 100 mm between courses.
5. Reinforcement & bonding
Provide RCC bands at sill and lintel levels. Use wall-to-column ties every 600 mm. For openings, cast a proper lintel with 150 mm minimum bearing on each side.
6. Curing
AAC walls need minimal curing compared to red brick. Light water spraying on plastered surfaces for 3 days is sufficient. Over-soaking blocks weakens the wall.
Plastering & finishing best practices
Internal plaster
Apply a 10–12 mm cement-sand plaster (1:4) with a chemical bonding agent or gypsum plaster. Lightly dampen the wall — do not saturate.
External plaster
Use a 15–18 mm double-coat cement-sand plaster (1:5) with polymer waterproofing admixture. Fibre-reinforced base coat is recommended at dissimilar-material junctions.
Avoid these on-site errors
- Soaking AAC blocks in water like red bricks — reduces strength.
- Using thick 10–12 mm mortar joints — defeats thermal advantages.
- Skipping the RCC sill/lintel bands on longer walls.
- Cutting chases with a hammer instead of a wall chaser.
- Plastering without a bonding coat, causing hairline cracks.
- Over-watering during curing — surface efflorescence and weakening.
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