An imposed deformation is a change of shape that is not caused by external load but from within: by temperature difference, shrinkage, creep, moisture or differential settlement. As long as the structure can move freely, little happens. Restrain that movement and restraint stresses build up — and those cause cracks.
This is why engineers often prefer the term restrained deformation: the problem is not the movement itself, but the fact that the structure is not allowed to make it.
Where it comes from
- Temperature. A south-facing facade can be tens of degrees warmer in summer than in winter. Materials expand and contract accordingly.
- Shrinkage. Concrete and cement-based mortars shrink during and after curing. Calcium silicate and concrete blocks shrink; clay brick actually expands slightly after firing as it takes up moisture.
- Creep. Concrete and timber go on deforming slowly under sustained load.
- Differential settlement. If an extension is founded differently from the main house, the two parts settle by different amounts.
Coefficients of thermal expansion (indicative)
| Material | Linear expansion per °C |
|---|---|
| Clay brick | about 5-7 x 10⁻⁶ |
| Concrete | about 10-12 x 10⁻⁶ |
| Steel | about 12 x 10⁻⁶ |
| Aluminium | about 23 x 10⁻⁶ |
| PVC | about 70-80 x 10⁻⁶ |
Work it through: a 10 metre steel beam warming by 40 °C grows roughly 5 mm. Fix both ends and that 5 mm has to go somewhere — which takes enormous force.
How it is solved
The answer is almost always: let it move.
- Movement joints in masonry. As a rule of thumb for an outer leaf: clay brickwork roughly every 6 to 12 metres, calcium silicate and concrete blockwork rather closer (broadly 5 to 8 metres). The exact spacing depends on orientation, colour and unit type — follow the manufacturer’s guidance.
- Expansion joints in bridges and floors, so the deck can extend freely.
- Sliding bearings under beams, so one end can move.
- Shrinkage reinforcement in concrete: it does not stop the movement, but spreads the cracking into many fine cracks instead of a few wide ones.
- Separated junctions: an extension built with a joint against the existing wall rather than tied hard into it.
Classic renovation error: pouring a new underfloor-heating screed tight against every wall. Without an edge insulation strip and without bay joints, cracks in the screed are near certain.
Related terms
- Movement joint
- Shrinkage
- Structural engineer
- Calcium silicate block
- Screed
Learn more about building terms in our knowledge base at fredsdiyplans.com.