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Horizontal crack below RCC slab

Short answer

Horizontal cracks below RCC slabs are caused by thermal expansion of the slab dragging the wall, slab deflection, shrinkage, and lack of a sliding layer. Preventive measures include providing a sliding joint (bitumen/polythene) between slab and wall, thermal insulation, a ring beam, expansion joints, and adequate bearing.

Explain the causes and preventive measures for the development of horizontal crack in load bearing wall just below the RCC slab.

A horizontal crack often forms in a load-bearing wall just below the RCC roof slab.

Causes

  • Thermal movement: the RCC slab, heated by the sun, expands more than the wall below, and drags the top of the wall outwards. In cooling, it contracts and the wall cracks horizontally.
  • Different expansion of concrete and masonry.
  • Slab deflection causing rotation at the support and lifting of the wall edges.
  • Shrinkage of the slab in the first few months.
  • Poor bearing or lack of a sliding layer between slab and wall.
  • Moisture movement of the wall below.
   ------ RCC slab ------>  expands
   ====================== horizontal crack
   |  brick wall        |

Preventive measures

  • Provide a sliding joint (bitumen coat with polythene, or two layers of tarred paper) between the slab and the wall top.
  • Provide thermal insulation or a reflective finish (lime concrete terracing, white tiles) on the roof.
  • Provide a ring beam (roof band) to tie the wall.
  • Provide expansion joints in long slabs.
  • Use proper curing, good mortar, adequate bearing (at least 150-200 mm).
  • Use a soft flexible pad or a bearing plate where the slab ends on the wall.

✓ Answer checked by Claude Opus 5.5 (high effort)

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