What is plasticity?
Plasticity is a material’s ability to deform permanently without breaking. As long as a deformation disappears once the load is removed, the behaviour is elastic. If the deformation remains, the material has deformed plastically. In construction the term turns up in three quite different contexts.
1. Plasticity of structural materials
In steel this is an advantage. Once the yield strength is exceeded, the section yields: it deforms permanently but does not fail suddenly. That reserve is why an overloaded steel beam visibly sags first. It also underpins plastic design, in which a structure is analysed through the formation of plastic hinges rather than through the first point to reach its limit.
The opposite is brittle behaviour: concrete, glass, cast iron and masonry fail in tension with almost no prior deformation. That is why concrete is reinforced — the reinforcing steel supplies the ductility the concrete itself lacks.
2. Plasticity of soil
In soil mechanics, plasticity describes how fine-grained soil behaves at different moisture contents. The Atterberg limits define when soil passes from liquid to mouldable and from mouldable to brittle:
- Liquid limit — the moisture content above which the soil behaves as a liquid.
- Plastic limit — the moisture content below which the soil can no longer be rolled out and crumbles.
- Plasticity index — the difference between the two. The higher the index, the more clayey the soil and the more strongly it reacts to moisture: swelling, shrinking and sticking to the spade.
That matters directly in building. Soil with a high plasticity index swells in wet weather and shrinks in drought, which can crack a shallow foundation. Workability in earthworks depends on it too: highly plastic soil is almost impossible to compact in the rain.
3. Plasticity of fresh mortar and concrete
In everyday site language, “plastic” refers to the workability of a fresh mix: how smoothly it handles and how well it holds its shape once the trowel leaves it. That consistency is not improved with more water — which weakens the mix — but with a plasticizer or a better graded aggregate.
Related terms
- Plastic deformation
- Plasticizer
- Consistence class
- Bearing capacity
- Settlement
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