Every change of building material carries a risk that rarely shows up in the bid: the crew. Contractors weigh price, lead time and performance when a new product is specified. Far less attention is paid to what happens when experienced tradespeople pick up a material that looks familiar but behaves differently in a few important ways.
In Qatar, where autoclaved aerated concrete block has been used widely for non-load-bearing infill walls, contractors have learned that most wall problems come from installation rather than from the material itself. The lessons apply well beyond one product or one region.
The Experienced Crew Paradox
Skilled masons are confident, and that confidence is usually an asset. With a new material, it can cut both ways. Some crews assume the new block needs an entirely different approach and overcomplicate the job. Others assume nothing has changed and carry every habit from the old material onto the new one.
AAC illustrates the point. It is lighter than conventional concrete block by more than half, which makes it easy to assume it is a specialty product with its own rules. In practice, AAC is a cementitious product, and much of what applies to normal hollow block applies to it as well. New masons often think it needs special treatment when most of the requirements for normal block still apply. The real risk sits in the small number of details where it does differ.
The Few Differences That Decide the Wall
On AAC walls, a handful of details decide whether the finished wall performs as specified. Each one is simple. Each one is also easy to get wrong when a crew relies on habit.
- The first course: The first course is set on conventional mortar and leveled carefully before the courses above are laid in thin-bed adhesive. Every course above inherits the accuracy of the first. A poorly leveled start produces uneven joints and extra work all the way up the wall.
- Thin-bed joints: Above the first course, the block is laid with thin-bed adhesive applied with a notched trowel rather than conventional thick joints. Crews used to building up mortar to correct irregularities need to rely on an accurate first course instead.
- The base course: The detail at the bottom of the wall, where it meets the slab, is often treated as a finishing item. It is one of the places where long-term performance is actually decided.
- Junctions with the concrete frame: Where an AAC wall meets a concrete column or beam, two different materials meet, and cracks tend to appear in the plaster along that line. The common practice in the region is to fix mesh over the junction before plastering, extending about 10 centimeters (roughly 4 inches) on each side, using galvanized metal or fiberglass mesh depending on the plaster type.
- Fixings: Anchors designed for dense concrete do not hold properly in cellular block. Shelving, cabinets and equipment need fixings rated for AAC. When the wrong anchors are used, the problem usually surfaces during fit-out or after the owner moves in.
None of these details requires new skills. They require crews to know which of their existing habits to keep and which to change.
What the First Day Reveals
One practice has proven its value repeatedly: supervising every new crew on its first day on site. At ASLAN Contracting & Transportation, a Doha-based contractor that has supplied and installed AAC since 2007, this is standard practice rather than an occasional check.
The first day is when habits show. Within a few hours, a supervisor can see whether the first course is being set and leveled properly, whether the adhesive is being applied correctly and whether the crew understands where the new material behaves like the old one and where it does not. Correcting a habit on day one costs minutes. Correcting it after a floor of walls has been built costs days, materials and, often, a strained relationship with the finishing trades who inherit the problem.
Building Material-Change Risk Into Planning
Contractors can manage this risk the same way they manage any other: by planning for it rather than reacting to it. Several steps apply regardless of the material involved.
- Treat a material change as a training item, not only a procurement item. When a new product enters the job, schedule the orientation alongside the delivery.
- Put a supervisor onsite for the first day of every new crew. The cost is small compared with the cost of rework.
- Get the critical details onto the drawings. Base course, frame junctions and fixing requirements should be specified, not left to the crew’s judgment.
- Brief the crew on what does not change as well as what does. Knowing which habits still apply keeps crews confident and avoids overcomplicating the work.
- Coordinate with the trades that follow. Plasterers and fit-out crews should know where mesh and AAC-rated fixings are required before they start.
Knowing When the Material Is Not the Answer
Managing workforce risk also means being clear about where a material does not belong. The AAC walls described here are non-load-bearing infill walls within a structural frame. Where a wall must carry structural load, or will stay in persistently wet conditions, a different solution or specific engineering is required. Specification limits on the length and area of non-load-bearing infill panels also apply and should be settled at design stage.
The Bottom Line
A material change is rarely a technical problem alone. It is a workforce problem that shows up first in the details and last in the budget. Contractors that plan for the first day, rather than for the first invoice, avoid most of the rework that material changes are known for.
SEE ALSO: WHAT TESTING DELAYS ACTUALLY COST A PROJECT AND HOW TO AVOID THEM
