Brick lot for a coastal load-bearing school room
A village panchayat is building a single-storey school room, 12 m × 8 m clear, roofed with a 100 mm RCC slab on 230 mm load-bearing brick walls 3.0 m high. The lowest tender's supplier delivers 15,000 bricks. On a random sample of 20 bricks the mason records: 24-hour water absorption 22% by weight; average compressive strength of five bricks 5.2 N/mm²; two bricks show a white powdery deposit after soaking and drying; the bricks give a clear metallic ring when struck; nominal size 230 × 110 × 70 mm, but three bricks are 5 mm short in length and a few are overburnt with hairline cracks. This lot costs ₹6.20 per brick. A verified first-class lot costs ₹8.00 per brick and only 6,000 pieces are available in time. The mason wants to use the truckload for the whole room and finish in three weeks; the monsoon arrives in six weeks. The roof load works out to about 5 kN/m².
1. Is this lot fit for the 230 mm external load-bearing walls? Show the reasoning that decides the issue, and name the property that actually governs your decision.
Read the worked answer
Step 1 - what the wall actually carries. Roof load 5 kN/m² over a tributary width of about 4 m gives 20 kN per metre of wall; wall self-weight ≈ 0.23 × 3.0 × 19 ≈ 13 kN/m. Total ≈ 33 kN/m. Stress = 33 000 N / (230 × 1000 mm²) ≈ 0.14 N/mm². Permissible stress for a 5.2 N/mm² brick in 1:6 mortar is of the order of 0.5–1.0 N/mm².
Step 2 - conclusion on strength. The bricks are roughly 35 times stronger than the stress the wall will see. Strength is not the governing property, so the mason's argument that the bricks will 'hold the roof' is technically true but irrelevant.
Step 3 - what actually governs: durability.
- 22% absorption is above the accepted ceiling of about 20% and far above the 15% desirable in humid, salt-laden coastal air; saturated bricks soften mortar and cause damp patches.
- Efflorescence proves soluble salts; these will migrate and flake plaster for the life of the building.
- 5 mm dimensional variation prevents a uniform 10 mm joint and good bond.
- Overburnt, cracked bricks have low impact resistance.
Verdict: fit in strength, unfit in durability - reject for the external load-bearing wall.
2. If the panchayat insists on buying both lots, how would you allocate the 15,000 bricks and the ₹ budget? Justify the split with quantities.
Read the worked answer
Quantities. External wall volume = 2(12 + 8) × 3.0 × 0.23 ≈ 27.6 m³, say 500 bricks per m³ → about 13,800 bricks, less openings (≈12%) → roughly 12,000 bricks for the external walls alone, plus internal walls.
Allocation. Use the 6,000 first-class bricks in the most vulnerable zone - the external wall from plinth to about 1.2 m, the splash and damp zone in salt air: 40 m perimeter × 1.2 m × 0.23 m ≈ 11 m³ ≈ 5,500 bricks. That fits the available 6,000 almost exactly. Put the remainder of the truckload only above 1.2 m in external walls and in internal non-load-bearing partitions, after rejecting every cracked or overburnt piece and any brick that fails the ring test.
Cost check. Extra outlay = 6,000 × ₹1.80 = ₹10,800. Against this, one replastering of 40 m × 3 m of wall plus repainting every few years will exceed that figure within a single maintenance cycle, and the damp course of the structure is protected for its whole life.
Contractual step. Record the split on the measurement sheet so the panchayat cannot later claim the whole truckload was accepted as first class; retain 5% of the payment until the tests in the next task are cleared.
3. State what you would test or check before the bricks are accepted, and explain what the white deposit means and how it changes the acceptance decision.
Read the worked answer
Tests to require (on a properly drawn sample, not bricks chosen by the supplier):
- Compressive strength - five bricks, soaked, bed faces ground flat, tested flatwise; the average must suit the class you are paying for, and no single brick should be wildly below it.
- Water absorption - five bricks, 24 h cold soak; 22% will be confirmed or corrected, and either way the result must be on record.
- Efflorescence - five bricks soaked and dried in a shaded tray, graded nil / slight / moderate / heavy.
- Dimensional tolerance and visual check - squareness, straightness, uniform colour, ringing sound, freedom from cracks, stones, and unburnt or overburnt lumps.
Meaning of the white deposit. It is efflorescence: soluble sulphates and chlorides in the clay migrate to the surface with water and crystallise on drying. It is not a surface stain to be brushed off - it proves the brick body contains salts that will keep coming back, and in a coastal building those salts also attack plaster and, eventually, embedded steel.
Effect on the decision. Efflorescence graded moderate or heavy is a rejection criterion on its own, independent of strength. Combined with 22% absorption it converts a 'buy with caution' into 'reject for external work'. Ask for the supplier's test certificate from a recognised laboratory, do not accept a certificate covering a different batch, and keep the test results with the measurement book.