Most multi-storey buildings today are RCC frames: columns, beams and slabs carry the load, and the walls fill the spaces between them. That changes the question. The wall does not hold the building up, so choosing between AAC blocks and burnt clay bricks is about weight, heat, workmanship and how the wall behaves where it meets the frame.
Two different materials
A burnt clay brick is fired earth: small, dense, familiar to every mason, and made in kilns whose quality varies from one to the next. An AAC block - autoclaved aerated concrete - is a factory product made from cement, lime, fly ash or sand and an aerating agent, cured under steam pressure. It is full of small closed air cells, and nearly all of its properties come from them.
An AAC block is several times the size of a brick and weighs roughly a third as much for the same volume. Both are covered by Indian Standards - IS 1077 for common burnt clay bricks and IS 2185 (Part 3) for autoclaved cellular concrete blocks - and both make sound walls when they are made and laid properly.
Weight
This is the clearest difference. A lighter wall puts less load on the beams, columns and foundations under it, and on a multi-storey frame that saving can be carried into the structural design. It only counts if the structural engineer designs for it: swapping bricks for AAC after the frame was designed for brick saves the weight and nothing else.
If AAC is the plan, tell the structural designer before the frame is designed, not after.
Heat
The air cells make AAC a much better insulator than brick. In a hot climate like Surat's that matters most on west-facing walls and on the top floor, where a better-insulated wall keeps rooms cooler through the afternoon and takes load off the air conditioning.
Brick has the opposite strength: thermal mass. A thick brick wall takes in heat slowly and gives it back slowly, which evens out the swing of the day. For a building that is air-conditioned most of the time, insulation usually counts for more; for one that relies on natural ventilation, the difference is smaller than the brochures suggest.
Cracks at the frame
The most common complaint about AAC walls is cracking, and it is almost always at the junction with the concrete frame or along a lintel - not through the blocks themselves.
- Why it happens Concrete and AAC move differently as they dry and as the temperature changes. Where the two meet, the plaster is asked to bridge two materials moving in different directions, and it cracks along the line.
- Dry blocks AAC shrinks as it loses moisture. Blocks laid wet from the yard or the rain keep shrinking after the plaster is on. They should be stacked off the ground, covered, and allowed to dry before use.
- The right mortar Thin-bed adhesive or a mortar suited to AAC, not the rich mix used for brick. A joint much stiffer than the block concentrates the movement into cracks.
- Mesh at the junctions A strip of mesh fixed across every joint between block and concrete before plastering spreads the movement over a width the plaster can take.
- A gap under the beam The top course is left short and filled later, so the wall is not wedged hard against a beam that will deflect.
Brick walls crack at the frame too, for the same reason, but less often and less visibly. Most AAC cracking is a workmanship problem that is cheap to prevent and expensive to chase afterwards.
Plaster, water and fixings
- Bricks have to be soaked before laying, or they draw the water out of the mortar. AAC blocks must not be soaked - at most the surface is dampened.
- AAC takes in water slowly but deeply. External walls need a plaster and finish meant for it, and the side of the building that takes the monsoon rain deserves the most attention.
- AAC is soft enough to cut and chase easily, which makes conduits simple and careless chasing just as easy. Deep chases weaken a block wall more than a brick one.
- Heavy fixings - kitchen cabinets, water heaters, television brackets, grilles - need anchors made for aerated concrete, or a planned insert. An ordinary plug in AAC can pull out under load.
Which one fits
Neither material is better in general. They suit different situations.
- For a multi-storey frame where the load matters and the structure is designed for it, AAC usually makes sense.
- For walls that face the afternoon sun or sit under the roof, the insulation of AAC is a real advantage.
- For a load-bearing building without a frame, brick is the conventional choice, and AAC belongs there only where the structural design is done for it.
- Where the masons on the job know brick well and AAC less, the junction details need to be written down and checked, not left to habit.
- For boundary walls, plinth masonry and anything that sits in wet ground, brick or solid concrete blocks are the safer choice.
Whichever is chosen, the decision belongs at design stage, alongside the structure and the plaster specification - not on the day the first truck arrives.