What is compressive strength?
Compressive strength is the stress a material can carry before it fails under compression, expressed in newtons per square millimetre (N/mm²) or megapascals (MPa). It is one of the most important properties of building materials such as concrete, brick, natural stone, mortar and rigid foam.
How is compressive strength measured?
In the laboratory a test specimen is compressed between two plates until it fails. The maximum load divided by the cross-section gives the compressive strength. For concrete a cube or cylinder is usually tested after 28 days.
Each material has its characteristic values:
- Concrete — strength class such as C20/25 (20 N/mm² cylinder, 25 N/mm² cube)
- Brick — usually between 10 and 60 N/mm² depending on type
- Mortar — typically 2.5 to 15 N/mm²
- Aerated concrete — 2 to 7 N/mm²
- PIR and PUR insulation — 0.1 to 0.5 N/mm²
Application
- Choosing concrete — strength class matched to the structural demand
- Selecting mortar — paired with the strength of the bricks
- Foundation design — concrete strong enough for the loads
- Road and pavement work — paviors and slabs with appropriate strength
- Compression-resistant insulation — beneath floor slabs and on flat roofs
Points of attention
- Compressive strength tells you nothing about tensile or flexural strength
- Measured values depend on the shape, size and age of the specimen
- Concrete strength is reported as both cube and cylinder values
- Use an appropriate safety factor in design
- Higher strength often means higher cost and more brittleness
Related terms
- Concrete
- Flexural strength
- Structural concrete
- Concrete quality
- Modulus of elasticity
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