Causes and prevention of cracks in buildings

Structural cracks

Structural cracks occur from faults in design, construction or loading that affect the strength and stability of the structure. They are wide, progressive and often pass through the full thickness.

• Causes: overloading, differential foundation settlement, poor design or detailing, shear and bending failure, corrosion of reinforcement, earthquake, defective materials.
• Appearance: diagonal or step cracks near supports, vertical cracks in the middle of beams, cracks near column-beam joints, cracks at the soffit of slabs.
• Danger: may cause collapse. They need immediate investigation and repair.

Non-structural cracks

Non-structural cracks come from internal stresses in building materials. They do not affect the strength directly but allow water entry and look bad.

• Causes: shrinkage, thermal expansion, moisture movement, chemical reaction, poor curing, vegetation.
• Appearance: fine, hairline, random (map) cracks, in plaster, floors and walls.
• Remedy: cement slurry or sealant, fresh plaster, expansion joints.

Cracks are repaired by the method that suits their type, width and cause. The cause (such as settlement) should be removed first.

Repair by crack width

Methods in brief

1. Surface sealing: hairline cracks are filled with cement slurry or polymer-modified paste.
2. Raking and filling: crack is widened into a V groove (10-20 mm), cleaned, and filled with cement-sand mortar 1:3 or epoxy mortar.
3. Epoxy injection: low-viscosity epoxy is injected through packers fixed along the crack; used for structural cracks in RCC.
4. Grouting: cement or chemical grout pumped into wide cracks and voids.
5. Stitching: U-shaped metal clamps fixed across the crack and embedded in epoxy or mortar.
6. Jacketing / section enlargement: extra concrete cover with new reinforcement on columns and beams.
7. Bonding steel plates or FRP on the tension face.
8. Underpinning or soil improvement for settlement cracks.
9. Re-plastering with chicken mesh over joints between different materials.
10. Expansion joints / sealants for thermal cracks.

A crack is a visible separation in a wall, slab, plaster or any building element. Cracks can be structural or non-structural. The main causes are as follows.

Prevention

• Proper investigation and design of foundation for uniform settlement.
• Provide expansion, contraction and construction joints.
• Use good materials and proper mortar mix; cure at least 7 days.
• Allow lintels and bands (plinth, lintel, roof band).
• Protect reinforcement with enough cover.
• Remove vegetation near the foundation.
• Waterproofing the roof and walls.

Cracks in buildings arise from the following causes.

Remedial measures

Additional measures: provide movement joints, good drainage, proper curing, tie beams, and good-quality materials.

Why joints are necessary

Buildings change in size because of temperature and moisture, and settle unevenly. Without joints, these movements create stresses that crack the structure. Joints are provided:

• To allow expansion and contraction due to temperature changes.
• To permit shrinkage movement of concrete and masonry.
• To allow differential settlement of different parts.
• To control the location of cracks.
• To limit the size of pours of concrete (construction joints).
• To separate parts with different heights or loads.

Expansion joint

Expansion joint: a gap of 20-25 mm, provided through the full depth of the structure above the foundation to allow expansion and contraction due to temperature, so that cracks do not form. Used at intervals of 30-45 m in buildings, in long walls, slabs, roofs and parapets.

   Expansion joint in a wall/slab
    slab |  25 mm gap  | slab
    =====|             |=====
    wall |  filler     | wall
         |  (bitumen)  |
         | sealant on top
   foundation continuous below

The gap is filled with a compressible filler (bitumen-impregnated fibre board, cork, rubber) and sealed on top with sealant. The two parts are fully separated, often by a double column or wall.

Why joints are necessary

Buildings are exposed to temperature changes, moisture, shrinkage, settlement and earthquake movement. Joints give space for these movements, so that the structure does not crack. Joints also make casting in stages possible.

Types of joints and their uses

1. Expansion joint Expansion joint: a gap of 20-25 mm, provided through the full depth of the structure above the foundation to allow expansion and contraction due to temperature, so that cracks do not form. Used at intervals of 30-45 m in buildings, in long walls, slabs, roofs and parapets.

   Expansion joint in a wall/slab
    slab |  25 mm gap  | slab
    =====|             |=====
    wall |  filler     | wall
         |  (bitumen)  |
         | sealant on top
   foundation continuous below

The gap is filled with a compressible filler (bitumen-impregnated fibre board, cork, rubber) and sealed on top with sealant. The two parts are fully separated, often by a double column or wall.

2. Construction joint Construction joint: a joint made where concrete placing is stopped and resumed after it has set, because the structure cannot be cast in a single operation. It must be located where shear and bending are small and must provide good bonding.

Positions (as per IS 456):

• Slabs: at the middle third of span, parallel to the main reinforcement.
• Beams: at the middle third of span (also at secondary beams and girders: at a distance of twice beam width from the main beam).
• Columns: at the top of the footing, and just below the beam/slab soffit (or below the haunch/capital).
• Walls: at openings, at the junctions, and in the middle third.

  Slab/beam                 Column
  ___________|___________    |slab|
  |<-L/3->|joint|<-L/3->|    | beam
                             | col  ---- joint just below beam
                            ===footing=== joint at top of footing

3. Contraction (control) joint, sliding joint and settlement joint Contraction (control) joint: a groove cut in the concrete at fixed spacing (e.g. 3-6 m in pavements and floors) so that shrinkage cracks form along the joint line. Sliding joint: allows horizontal movement between a slab and a wall, with a bitumen layer or plain sheets. Settlement (isolation) joint: between parts with different foundations or heights so that settlement does not crack the building.

4. Isolation/separation joint: between a slab and a column or wall, so that each moves independently.

5. Seismic (separation) joint: a wider gap (as per IS 4326 / NBC 105) between adjacent buildings or blocks to prevent pounding in an earthquake.

Construction joint

Construction joint: a joint made where concrete placing is stopped and resumed after it has set, because the structure cannot be cast in a single operation. It must be located where shear and bending are small and must provide good bonding.

Positions of construction joints

Positions (as per IS 456):

• Slabs: at the middle third of span, parallel to the main reinforcement.
• Beams: at the middle third of span (also at secondary beams and girders: at a distance of twice beam width from the main beam).
• Columns: at the top of the footing, and just below the beam/slab soffit (or below the haunch/capital).
• Walls: at openings, at the junctions, and in the middle third.

  Slab/beam                 Column
  ___________|___________    |slab|
  |<-L/3->|joint|<-L/3->|    | beam
                             | col  ---- joint just below beam
                            ===footing=== joint at top of footing

Methods of joining new concrete to old concrete

1. Cleaning: remove all loose and laitance material from old concrete, and roughen the surface by hacking, wire brushing or sand blasting to expose the aggregate.
2. Wetting: saturate the old surface with water for several hours, and remove the free water before concreting.
3. Bonding coat: apply a layer of neat cement slurry or 10-15 mm of rich cement mortar (1:2) just before placing new concrete.
4. Bonding agent: apply an epoxy or latex (SBR) bonding agent for better adhesion.
5. Shear keys or dowels: form a groove (key) in the old concrete or fix dowel bars across the joint for load transfer.
6. Placing new concrete: place in layers and compact thoroughly with a vibrator; cure well.
7. Waterstops (PVC or rubber strips) in water-retaining structures.

 Structural (settlement)     Non-structural (shrinkage)
  _______________              _______________
 |  \  step crack |           | ~~ map cracks  |
 |   \_           |           |  ~~            |
 |     \_  wide   |           |                |
 |_______\________|           |________________|
   foundation settles

Causes

Structural: overloading, unequal settlement of foundations, poor design and detailing, shear and bending failure, corrosion of steel, earthquake and wind, early removal of formwork. Non-structural: drying shrinkage of mortar, plaster and concrete, thermal expansion and contraction, moisture movement of bricks, chemical action (sulphate attack), vegetation, poor materials and workmanship.

Crazing

Crazing is the formation of a network of fine, shallow, hair-like cracks on the surface of plaster, concrete, paint or glaze, looking like a spider web or map. It is caused by shrinkage of the surface layer due to rapid drying, rich mix, too much trowelling, or sudden temperature change. It affects appearance only and is repaired by filling with cement slurry or repainting.

Common problems of cracks on roofs

• Thermal expansion and contraction of a slab or parapet in sun, causing cracks and leaks.
• Shrinkage cracks in the terrace screed or slab due to poor curing.
• Deflection and overload causing cracks in slabs.
• Corrosion of reinforcement with spalling at the soffit.
• Differential movement of the parapet and slab causing horizontal cracks.
• Poor waterproofing and ponding, giving leakage and dampness.
• Lack of expansion joints in large terraces.

Prevention: good curing, expansion joints, insulation and waterproofing, correct slope, proper reinforcement cover.

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.

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

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

A joint is a planned break or gap in a structure to allow movement or to stop concreting. It reduces random cracking.

Types of joints

1. Expansion joint: allows thermal expansion.
2. Contraction (control) joint: controls shrinkage cracks.
3. Construction joint: at stopping place of concreting.
4. Settlement joint: between parts that settle differently.
5. Sliding joint: between slab and wall for horizontal movement.
6. Seismic joint: between blocks, to avoid pounding.

Expansion joint

Expansion joint: a gap of 20-25 mm, provided through the full depth of the structure above the foundation to allow expansion and contraction due to temperature, so that cracks do not form. Used at intervals of 30-45 m in buildings, in long walls, slabs, roofs and parapets.

   Expansion joint in a wall/slab
    slab |  25 mm gap  | slab
    =====|             |=====
    wall |  filler     | wall
         |  (bitumen)  |
         | sealant on top
   foundation continuous below

The gap is filled with a compressible filler (bitumen-impregnated fibre board, cork, rubber) and sealed on top with sealant. The two parts are fully separated, often by a double column or wall.

Positions: at 30-45 m intervals along long buildings; where the plan changes shape (L, T, U); where height or foundation level changes; between wings and old and new portions; in long parapets, roofs, retaining walls and pavements.

Construction joint

Construction joint: a joint made where concrete placing is stopped and resumed after it has set, because the structure cannot be cast in a single operation. It must be located where shear and bending are small and must provide good bonding.

Positions (as per IS 456):

• Slabs: at the middle third of span, parallel to the main reinforcement.
• Beams: at the middle third of span (also at secondary beams and girders: at a distance of twice beam width from the main beam).
• Columns: at the top of the footing, and just below the beam/slab soffit (or below the haunch/capital).
• Walls: at openings, at the junctions, and in the middle third.

  Slab/beam                 Column
  ___________|___________    |slab|
  |<-L/3->|joint|<-L/3->|    | beam
                             | col  ---- joint just below beam
                            ===footing=== joint at top of footing

Why joints are necessary

Concrete, brick and other materials expand with a rise of temperature and moisture and contract when cooled or dried (concrete also shrinks while curing). If a long structure is not free to move, tensile and compressive stresses develop and cause cracks. Joints are provided to:

• Allow free thermal expansion and contraction, and shrinkage.
• Prevent cracking, buckling and spalling of walls, slabs and floors.
• Accommodate differential settlement and small movements (a separate joint type for structures with different loads or foundation levels).
• Divide a long or irregular building into independent blocks.

Expansion joint: a full-depth gap (25-40 mm) through the building from roof to plinth, usually at 30-45 m intervals (IS 3414 / IS 456 suggests about 45 m) or where the plan changes shape (L, T, U), or the height changes. The foundation is generally not divided unless settlement difference is expected, but here the joint is carried to the foundation top. Filler (bitumen-impregnated fibre board, cork or expanded polystyrene) is placed in the gap and the surface sealed with a sealant, plus a cover strip (flashing/GI or copper strip).

Sketch: section through a 350 mm brick wall at the expansion joint

  Terrace  |<-25->|  Terrace
 ==========|filler|==========  roof slab (cover strip)
  [ beam ] |      | [ beam ]
  ---------|      |---------  lintel level
   350 wall|      |350 wall   (double wall, gap)
  ---------|      |---------  floor (sealant+strip)
   plinth  |      |  plinth   DPC both sides
  ---------|      |---------
  footing  |      |  footing  (joint to foundation)

Details of the joint at each level

Good detailing keeps the gap free of mortar and debris and watertight.