None of these mistakes are dramatic on the day they happen. All of them are cheap to avoid and expensive to fix once the plaster is on.
1. Skipping curing
Concrete gains strength only while it stays moist. IS 456 calls for at least 7 days of curing for OPC and 10 days for PPC or blended cement. Slabs left dry after two days can lose a large part of their design strength and crack at the surface. Ponding on slabs and wet hessian on columns cost almost nothing.
2. Missing cover blocks
Steel needs a protective layer of concrete: about 20 mm in slabs, 25 mm in beams, 40 mm in columns and 50 mm in footings. Without cover blocks the bars sit on the shuttering, rust appears within a few monsoons, and the concrete spalls.
3. Substituting bar diameters
"12 mm is not available, use 10 mm" silently removes almost a third of the steel area. Any change must go back to the structural engineer.
4. Too much water in the mix
Extra water makes concrete easier to place and permanently weaker. Use the right slump, a vibrator, and admixtures if needed. Do not reach for the hose.
5. No vibration
Honeycombing at column bases and beam junctions is almost always a vibration problem. Keep a working needle vibrator and a spare on site for every pour.
6. No damp-proof course
A DPC at plinth level stops ground moisture rising into walls. Skip it and you get peeling paint and salt marks along the bottom of every wall, which no amount of repainting fixes.
7. Services cut in after the fact
Electrical conduits and plumbing sleeves should be placed before the slab is cast. Cutting chases through fresh beams and slabs later damages the structure and the waterproofing. We issue electrical and plumbing drawings along with the structural set for this reason.
