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Retrofitting need; repairing cracks of different width

Short answer

Retrofitting is necessary to strengthen existing structures against earthquakes, damage, increased loads, or updated codes. Crack repair methods: hairline cracks (<0.3 mm) are surface-sealed; 0.3-1 mm cracks are raked and filled with mortar; 1-3 mm cracks are injected with epoxy/cement grout; 3-10 mm cracks are V-grooved and stitched; cracks >10 mm require rebuilding and jacketing.

Why retrofitting is necessary? Explain methods of repairing crack of different width.

Need for retrofitting

Retrofitting is the strengthening of an existing structure to make it safer and serviceable. It is necessary because:

  • Many old buildings were built without earthquake-resistant design and may collapse in future earthquakes (as in Nepal 2015).
  • Damage from earthquake, fire, corrosion and ageing reduces strength.
  • Loads or building use change (extra storeys).
  • Design or construction errors are found.
  • Codes (NBC 105) have been revised, and retrofitting is cheaper than demolishing.

Repair of cracks by width

WidthMethod
Hairline, < 0.3 mmSurface painting with cement slurry or sealant
0.3 to 1 mmRake and fill with cement-sand mortar 1:3, or polymer mortar
1 to 3 mmEpoxy or cement grout injection through packers at 150-300 mm spacing
3 to 10 mmRake into V groove, epoxy mortar, with stitching by clamps
> 10 mmRemove and rebuild the portion; add jackets, bands or mesh; stitching

General steps: remove the cause (settlement, leakage), clean the crack with compressed air, seal the surface, inject or fill, then cure and finish.

✓ Answer checked by Claude Opus 5.5 (high effort)

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