Key points
- All concrete tends to crack and completely crack-free concrete can't be produced, but cracking can be reduced and controlled.
- Most random early cracks are unsightly but rarely affect structural integrity or service life.
- Plastic shrinkage cracks form when surface water evaporates faster than bleed water rises; wind above about 8 km/h, heat and low humidity increase the risk.
- Cracks along reinforcing bars, cracks that keep widening, or cracks with tilting or settlement need a structural engineer.
Concrete, like any building material, expands and contracts with changes in moisture and temperature, and deflects under load and support conditions. When provisions for these movements are not made in design and construction, cracks appear1. The US National Ready Mixed Concrete Association states that all concrete tends to crack and it is not possible to produce completely crack-free concrete, but cracking can be reduced and controlled with good practice1.
1) Common crack forms
| Type | Appearance and cause |
|---|---|
| Plastic shrinkage | Shallow, often parallel surface cracks appearing in fresh concrete soon after placing, on horizontal surfaces2. |
| Improper jointing | Missing or incorrectly executed contraction or isolation joints1. |
| External restraint | E.g. a cast-in-place wall restrained along the bottom edge by the footing1. |
| Crazing | A network of very fine surface cracks1. |
| Settlement | From settlement of the soil or the concrete1. |
The association explains that most random cracks appearing early, although unsightly, rarely affect structural integrity or service life, with exceptions including cracking from alkali-aggregate reaction, which leads to long-term structural damage1.
2) Why does concrete crack?
Most cracks come from improper design and construction practices1:
- Omitting isolation and contraction joints, or improper jointing.
- Poor subgrade preparation under slabs.
- High-slump concrete or excessive water added on site.
- Improper finishing.
- Rapid loss of moisture from fresh concrete in dry conditions.
- Inadequate or no curing.
3) Hot weather and plastic shrinkage
Plastic shrinkage cracks occur when water evaporates from the surface faster than rising bleed water can replace it, before the concrete has set2. Conditions that raise evaporation include wind above 5 mph (about 8 km/h), low relative humidity, and high air or concrete temperatures2 — normal summer conditions in Saudi Arabia.
Precautions recommended by the association2:
- Windbreaks and sunshades; in very hot conditions, early-morning placing to control concrete temperature.
- Fog sprays above the concrete to reduce evaporation until curing begins.
- Dampening the subgrade, forms and reinforcement before placing, without standing water.
- Covering the concrete with wet burlap or polyethylene between finishing operations.
- Starting curing immediately: curing compound or wet burlap kept moist for at least 3 days.
- Considering synthetic fibres.
4) Joints in ground slabs
The association recommends contraction joints cut or tooled to ¼ to ⅓ of the slab thickness, spaced 24 to 36 times the thickness, with roughly square panels no longer than 1.5 times their width, and full-depth isolation joints where floors meet walls and columns1. Example: a 10 cm slab → joints every 2.4 to 3.6 m.
It also notes that wire mesh in slabs cannot prevent cracking, but reduces crack width when placed correctly1.
5) When is a crack worrying?
Practical warning signs that call for a structural engineer's inspection; not a substitute for one.
- A crack running along a reinforcing bar in a column, beam or slab, or with rust stains (see rebar corrosion).
- A crack that is widening over time. Mark its ends and the date, and measure its width periodically.
- Diagonal (45°) cracks near door and window corners, with doors or windows sticking.
- A sinking floor or a leaning wall.
- Cracks in a building on expansive soil or sabkha (see soil and foundations).
6) Repair in brief
- Dormant, non-structural hairline cracks: suitable filler before painting.
- Moving cracks: treated as a joint and filled with a flexible sealant. See joint sealants.
- Structural cracks: as the engineer's report specifies (epoxy injection, strengthening, or treating the cause in the soil).
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1CIP 4 — Cracking Concrete Surfaces — National Ready Mixed Concrete Association (NRMCA)
- 2CIP 5 — Plastic Shrinkage Cracking — National Ready Mixed Concrete Association (NRMCA)