Compressive strength
Materials used for structural purposes are usually classified according to their resistance to basic stresses such as compression, tension and shear.
Compression is a force that pushes the particles of a material closer together. For example, when a column supports a load, it is under compression and its height shortens, albeit often imperceivably. The opposite is tensile force which tends to elongate a material.
All materials can, to a certain degree, withstand compressive forces before they fail and it is at this point that compressive strength is measured. Therefore, the compressive strength of a material is usually stated as the maximum compression that the material can stand before failure.
Materials which can resist high, applied compressive forces before failure are said to have high compressive strengths.
Some materials are better than others at withstanding compression before failure occurs. Steel can withstand relatively high compressive forces. Other materials, such as concrete and ceramics, typically show much higher compressive strengths than tensile strengths. Depending on the material, failure can comprise fracture at the compressive strength limit or irreversible deformation.
[edit] Measuring compressive strength
It is possible to measure precisely the compressive strength of materials by conducting a compressive test under carefully controlled conditions using a universal testing machine. This can typically have testing capacities of up to 53 mega Newtons (MN) which is equal to a 5,404 ton force.
In building construction, testing the compressive strength of concrete is usually undertaken at different stages after it has been poured in order to allow sufficient time for strength development (eg after 28 days). Typically, a cube (or cylinder) of concrete is used as a test specimen, ensuring that the top and bottom surfaces are flat and parallel, and that both faces are a perfect cross-section, ie, at right angles to the vertical axis of the cube.
A compressive force is applied to the specimen gradually by the testing mechanism. Measuring the compressive strength using this method requires:
- The cross-sectional area of one of the cube’s faces, top or bottom (they should be identical), and
- The compressive force applied at the time of failure (defined as permanent deformation - ie an inability to assume its former shape once the compressive force is removed).
Once these measurements are available, the compressive strength (C or σc) can be calculated as:
[edit] C = F/A
where F is the maximum force (load) applied at the point of failure and A is the cross-sectional area of the specimen before the force was applied. It can be expressed in terms of N/m² or Pascals (where 1 Pascal (Pa) = 1 N/m²).
It is sometimes difficult to measure the compressive strength of ductile metals, such as mild steel, which have high compressive strengths. This is due to the failure mode of such materials. Typically, under a compressive load, mild steel deforms elastically up to a point; this is followed by plastic deformation and ultimately the specimen may be flattened without significant evidence of fracture. It can therefore be difficult to measure the precise point of compressive failure. For this reason, it is more common to quote the tensile strength of mild steel which is easier to obtain; as its tensile strength is always lower than its compressive strength, it can be used as the basis for calculations.
[edit] Related articles on Designing Buildings
- Arch.
- Barrel vault.
- Compressive strength of timber lattice columns for low-rise construction.
- Concrete.
- Dome.
- Elements of structure in buildings
- Engineer.
- Flying buttress.
- Mass concrete.
- Steel.
- Structural engineer.
- Structural principles.
- Substructure.
- Superstructure.
- Tower.
- Tensile strength.
- Tension.
- Types of structure.
- Voussoir.
Featured articles and news
CIOB launches global mental health survey
To address the silent mental health crisis in construction.
New categories in sustainability, health and safety, and emerging talent.
Key takeaways from the BSRIA Briefing 2024
Not just waiting for Net Zero, but driving it.
The ISO answer to what is a digital twin
Talking about digital twins in a more consistent manner.
Top tips and risks to look out for.
New Code of Practice for fire and escape door hardware
Published by GAI and DHF.
Retrofit of Buildings, a CIOB Technical Publication
Pertinent technical issues, retrofit measures and the roles involved.
New alliance will tackle skills shortage in greater Manchester
The pioneering Electrotechnical Training and Careers Alliance.
Drone data at the edge: three steps to better AI insights
Offering greater accuracy and quicker access to insights.
From fit-out to higher-risk buildings.
Heritage conservation in Calgary
The triple bottom line.
College of West Anglia apprentice wins SkillELECTRIC gold.
Scottish government launch delivery plan
To strengthen planning and tackle the housing emergency.
How people react in ways which tend to restore their comfort.
Comfort is a crucial missing piece of the puzzle.
ECA launches Recharging Electrical Skills Charter in Wales
Best solutions for the industry and electrical skills in Wales.
New homebuilding skills hub launch and industry response
Working with CITB and NHBC to launch fast track training.