- 2071 Bhadra · 8 marks
Painting on masonry; causes and remedies of cracks
Short answer
Masonry painting: let plaster cure 3-4 weeks, clean, repair, apply alkali primer and wall putty, then two coats of paint (cement paint wetted and cured). Causes: moisture and thermal movement, settlement, elastic deformation, corrosion, chemical attack, vegetation, earthquakes, poor work. Remedial: hairline – cement slurry; small – rake and mortar; medium – epoxy injection; wide – stitch/jacket; settlement – underpinning.
Explain the process of painting works on masonry surfaces. What may be the causes of cracks occurred in a building and what are remedial measure to cracks?
Painting on masonry surfaces
- Preparation: plaster must be at least 3-4 weeks old and fully dry. Remove dust, oil, loose particles and efflorescence with a wire brush and wash.
- Repair: fill cracks and holes with cement mortar or putty and let them dry.
- Primer: apply an alkali-resistant primer.
- Putty: apply wall putty in two coats; rub down with sandpaper after each.
- Painting: apply two coats of emulsion, distemper or cement paint with a brush or roller. The second coat is applied after 4-6 hours. For cement paint, wet the surface first, and cure the paint for 2 days.
Causes of cracks in buildings
| Cause | How it cracks the building |
|---|---|
| Moisture movement | Bricks, blocks and concrete expand on wetting and shrink on drying; mortar and plaster shrinkage |
| Thermal movement | Expansion and contraction with temperature in roofs, long walls, parapets |
| Settlement of foundation | Unequal settlement due to poor or non-uniform soil, wrong foundation level, change in moisture, or inadequate depth |
| Elastic deformation | Overload, deflection of beams and slabs, bending and shear |
| Corrosion of reinforcement | Rust expands and splits the concrete cover |
| Chemical action | Sulphate attack, alkali-aggregate reaction, carbonation |
| Vegetation | Roots of trees near walls and foundations |
| Earthquake and wind | Lateral forces on weak walls and joints |
| Poor workmanship and materials | Poor curing, mix, bonding, early removal of formwork, missing joints |
| Overloading and design error | Excess load, wrong detailing |
| Fire, vibration | Heat damage, machine vibration |
Remedial measures
| Crack width | Remedy |
|---|---|
| Hairline (< 0.3 mm) | Fill with cement slurry or paint-grade filler; fresh plaster |
| Small (0.3-1.0 mm) | Rake out, fill with cement-sand mortar (1:3) or polymer-modified mortar |
| Medium (1-3 mm) | Epoxy injection or cement grout through packers |
| Wide (> 3 mm) | Remove and rebuild, stitching with steel clamps, epoxy mortar, jacketing |
| Foundation settlement | Underpinning, grouting, soil improvement |
| RCC cracks | Epoxy injection, grouting, jacketing, bonding steel plates |
| Roof cracks | Waterproof treatment, expansion joints, sealants |
✓ Answer checked by Claude Opus 5.5 (high effort)
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