Autoclaved aerated concrete (AAC) is not a difficult material to lay. That is the problem. A crew with 20 years of dense-block habit picks it up on Monday, lays a clean-looking wall by Wednesday, and the defects surface months later at fit-out, when somebody drills into it.
As a supplier and installer, I have watched the same three things break on almost every crew's first project. They break in a predictable order: the mortar, the fixings, and the cutting.
The block we install is a Class G2/05 unit to BS EN 771-4. (In the US, AAC is specified under ASTM C1386.) Dry density is 500 kg/m³, about a quarter of dense concrete masonry. The compressive strength is 3.2 N/mm², roughly 460 psi. Its thermal conductivity is 0.13 W/m·K, with a four-hour fire rating. A standard unit is 600 mm long and 250 mm high, in thicknesses from 100 to 300 mm. AAC is covered within ACI PRC-526-19 and TMS 402/602.
Everything a crew has to unlearn comes from those numbers. It is light. It is soft. It is thermally efficient only if the joints do not undo it.
First course levelling on the semi-dry cement-sand bed. This is the only course where slab tolerance can be corrected.ASLAN Contracting & Transportation
The First Course Decides the Wall
Dense block is laid on a 10 mm bed that absorbs error at every course. AAC is laid in a thin-bed adhesive mortar with an application thickness of 2 to 6 mm. A joint that thin cannot correct anything. Whatever error is in the course below is inherited by every course above, with no opportunity to take it up later.
Nobody lays course two until course one has been checked for line and level along its full length
That is why the first course is laid differently from the rest. It goes onto a semi-dry cement-sand levelling bed, minimum 10 mm and maximum 30 mm thick, and it is the only place in the wall where the mason can correct for slab tolerance. This course must be laid correctly for efficient progress. On-site, that becomes a rule: "nobody lays course two until course one has been checked for line and level along its full length." Where the first course may be the fastest with dense block, it is the slowest on AAC, and it should be.
From course two and upward, the adhesive is applied with a notched trowel the width of the block, and the consistency has to be right: the strands left by the trowel teeth should stay visible and not flow together. If they merge, the mix is too wet. The mortar is a thin-bed adhesive - in the US, thin-bed mortar for AAC is specified under ASTM C1660 (or BS EN 196), mixed at roughly 1.3 to 1.5 US gallons of water per 55 lb. bag (or 5 to 5.5 liters per 25 kg bag). Mix the powder into water — never the reverse, and never with sand added.
Pot life is 65 minutes and open time is 30. A crew that mixes a wheelbarrow for the morning will throw most of it away. Quantities get prepared for an hour of work, no more.
Two details that dense-block crews miss are that 1) vertical joints must overlap by at least 100 mm between courses, and 2) the top of the wall is not mortared tight to the slab or beam: a gap of about 20 mm is left and filled with compressible material, or the top course cracks. Both are on every drawing. Both get skipped when nobody explains why.
Thin-bed adhesive applied with a notched trowel. The strands left by the teeth should stay visible; if they merge, the mix is too wet.ASLAN Contracting & Transportation
Fixings: Where the Callbacks Come From
This is the expensive one. An expansion anchor sized for dense concrete crushes its own seating in a cellular block instead of gripping it. It appears to hold on the day and fails later under load when the trade drilling into it was never told what it was drilling into. This is also usually after the wall has been plastered, painted, and signed off.
AAC has its own fixing system, and the permissible loads are published rather than guessed.
ASLAN Contracting & Transportation
The part crews get wrong is that the fixing is a system, not a screw. The hole is drilled, the plug is then tapped home with a hammer so that it seats tight in the cell structure instead of being pushed in by hand, and only after that is the screw driven. Our method statement goes a step further for plain cellular block and calls for the hole itself to be formed by hammering a steel bar of the plug diameter, on the basis that compacting the material around the hole grips better than drilling it out. Either way the point is the same: the loads in Table 1 belong to an AAC-rated plug, properly seated, and not to whatever anchor happens to be in the fixer's pouch.
Surprisingly, AAC is generous with fixings if you respect the numbers. A single-leaf door frame is fixed directly to the block with six plugs per side at 350 to 400 mm centers, using 100 mm screw nails for timber frames and 75 mm for metal, and no stiffeners are needed. The problem is never the material. It is that the fixing table stays in the submittal instead of going up on the wall where the second-fix trades can see it.
An AAC block cut with a hand saw. No wet-cutting station and no diamond blade, which is the advantage and the discipline problem in one.ASLAN Contracting & Transportation
The Advantage That Becomes a Problem
AAC cuts with a hand saw. A band saw is faster for repetitive cuts. Chases for conduit can be run with a chasing tool or a router. Use a power drill for back boxes. No wet-cutting station, no diamond blade, no waiting. Crews love this, and it is exactly where discipline quietly leaks out of the job.
On dense block, every cut is a small event. It goes to the saw bench. Somebody measures it. It comes back. With an AAC block, a cut takes 30 seconds at the wall, so adjustments start happening informally instead of being set out. Closers get sawn by eye; a chase gets run where the electrician is standing rather than where the drawing puts it. Each one is trivial. Together they are how a wall ends up with a 60 mm closer where the bond needs 100 mm of overlap.
The fix is procedural, not technical. Cuts are set out and marked like any other masonry, and chases and damage are made good with the AAC repair mortar, which is manufactured from the same base material as the block. The patch then shares the wall's movement and absorption behaviour instead of fighting it.
Two Things a Foreman Needs Before Pricing
Tying to the frame. AAC infill is tied to concrete columns every third or fourth course with a galvanized angle or a bent stainless connector. Between AAC units, a perforated stainless plate 300 mm long, 30 mm wide and 1 mm thick is fixed every 500 mm of wall height with two 100 mm AAC nails. Lintels bear a minimum of 150 to 200 mm at each end. All of it is quick to fit and impossible to retrofit.
Wall length limits. Non-bearing AAC walls have published size limits that depend on thickness, height and how many edges are restrained. A 200 mm external infill wall 3 m high, restrained on four edges, is limited to 6.4 m in length; at 250 mm it can run to 10 m. Internal partitions are tighter again — 3.8 m at 200 mm, 5.25 m at 250 mm. A crew that has never had to think about wall length on dense block needs that table before setting out, not after.
Blockwork meeting a concrete column. Infill is tied to the frame every third or fourth course.ASLAN Contracting & Transportation
Where AAC is the Wrong Choice
An article that only lists advantages is a brochure, so here is the other side.
At 3.2 N/mm² (about 460 psi), G2/05 is not a material for concentrated loads. Heavy bracketry, plant, or anything that wants a 10 mm bolt at close centers belongs on a concrete element or on the denser G4/06 class, at 600 kg/m³ and 5.0 N/mm², which costs you thermal performance: conductivity rises from 0.13 to 0.16 W/m·K.
Plaster has to be matched to the block. Any material applied to an AAC wall should be no stronger than 3.5 N/mm², or it will crack off. Plaster goes on over a rush coat formulated for AAC, at a finished thickness of 8 to 15 mm, and conventional mixes are deliberately weakened to get there. And the base course is not the place to economise on damp-proofing: a wet AAC block is a poor insulator, and it stays wet longer than dense block does.
The Crew Checklist
Before the first pallet arrives, remember the following steps.
- Print the fixing table and post it where the second-fix trades will work.
- Write the levelling-bed thickness and the first-course check into the sequence.
- Mix thin-bed mortar by the hour, not by the morning.
- Set out every cut like any other masonry, and make chases good with AAC repair mortar.
- Check the maximum permitted wall length for the thickness and height you are building, before you set out.
Do those five, and AAC is faster than dense block. Skip them, and it is slower because you will be back.
