Coastal School Block: Separating Structural Cracking from Material Decay
A two-storey school block built in the mid-1980s in Balasore, about 1.2 km from the sea, has 230 mm external brick walls in 1:8 cement mortar carrying RCC roof slabs. After every monsoon the ground-floor classrooms smell of damp and white salt deposits reappear up to roughly 900 mm above plinth level. Your inspection of the south wall records: a 0.4 mm wide stepped crack running diagonally from the corners of a 1.5 m × 1.2 m window opening; mortar joints raked out about 15 mm deep by wind-driven rain; bricks that ring hollow when tapped and crumble at arrises; a sample brick showing 22% water absorption after a 24-hour soak. The wall is plumb within 8 mm over its 4.5 m height. There is no damp-proof course, no plinth protection, and the roof rainwater downpipe discharges directly against the wall base.
1. Sort these observations into structural symptoms and material-decay symptoms, and explain the likely mechanism of the diagonal crack. What further information decides whether the wall is repaired or rebuilt?
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Material decay (durability problem): efflorescence, 22% absorption, hollow and crumbling bricks, 15 mm raked joints, lean 1:8 mortar. In a coastal site the chlorides and sulphates plus wetting–drying cycles have penetrated a porous, under-fired brick and a weak mortar; the salts crystallise just below the surface and push the brick face off. This is a fabric problem, not a load problem.
Structural symptom: the 0.4 mm diagonal crack stepping down from the window corners. A crack radiating at about 45° from an opening corner is a classic diagonal-tension pattern - differential settlement of one part of the wall, or slab movement dragging the wall, or a lintel with inadequate bearing allowing the masonry over the opening to sag. At 0.4 mm it is wider than a hairline and must be treated as potentially active.
Before deciding repair vs rebuild I would gather: (a) crack activity - glass tell-tales or a DEMEC/feeler gauge read monthly for 8–12 weeks, including one monsoon; (b) whether the lintel exists and its bearing length (an opening of 1.5 m needs a lintel with at least 150 mm bearing each side); (c) foundation depth and soil, and any nearby excavation or tree; (d) corrosion staining or spalling on the slab soffit, which points to carbonation or chloride attack of reinforcement.
If the crack is dormant and the lintel is sound, local rebuilding plus damp control is adequate. If it is still moving, cosmetic repair is wasted money - the cause must be arrested first.
2. Write the repair specification for the south wall: sequence, materials and mortar mix.
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1. Treat the water source first - re-route the rainwater pipe to a plinth-level channel or soak pit, and provide 600 mm wide, 1:40 sloped plinth protection in 75 mm PCC over 100 mm sand. Without this, no masonry repair survives.
2. Cut out and rebuild the hollow, crumbling bricks in panels not exceeding 1 m² at a time, toothed into sound masonry. Use second-class bricks with absorption not more than 20% and crushing strength not less than 7.5 N/mm². Soak the bricks in clean water before laying until air bubbles stop (roughly 10–15 minutes' immersion, then drain) so they do not suck water out of the mortar.
3. Mortar: 1:6 cement–sand by volume for the rebuild; the existing 1:8 is too lean for a load-bearing external wall in this exposure. Joints 10 mm nominal, never more than 12 mm or less than 8 mm. Fill vertical joints fully - raked joints are the entry path for rain. Frog upward. Raise masonry in lifts not exceeding 1.0 m per day and cure for 7 days.
4. Crack treatment: if monitoring shows the crack is dormant, cut out and stitch across it with 6 mm galvanised twisted bars at 300 mm vertical spacing, embedded 250 mm each side in 1:4 mortar, then fill with non-shrink grout and finish flush. If active, stop and refer for structural assessment.
5. Efflorescence: dry-brush off the salts, never wash them back into the wall, and do not apply any coating until the wall has dried to below 12% moisture.
3. What workmanship checks and acceptance criteria would you record before signing off the repaired masonry?
Read the worked answer
Record measured values, not opinions, on a one-page checklist signed by the mason and the supervisor:
- Plumb: check with spirit level and plumb bob at every lift - tolerance ±5 mm in 3 m and not more than 10 mm over the full 4.5 m storey height.
- Line and level: string line on every course; bed joints level and of uniform 10 mm thickness.
- Joint fill: probe vertical joints at three random spots per square metre; no visible unfilled perpends on the exposed face.
- Brick quality: one sample per 1,000 bricks for absorption (limit 20%) and visual soundness; reject cracked or under-burnt bricks on the stack, not on the wall.
- Mortar: measure the 1:6 mix by a gauge box, not by shovel count; record the cement bag count against the mortar volume placed.
- Curing: log the 7-day curing regime with dates and the person responsible.
- Crack monitoring: retain tell-tales in place for at least one monsoon and record readings monthly before removing them and accepting the wall.
- Damp-proofing and drainage: confirm the new plinth protection has the specified fall, and that the downpipe now discharges clear of the wall.
Finally, photograph the rebuilt panel and the stitched crack before plastering, since the evidence is buried afterwards and any defect claim will rest on those records.