What is stress relaxation?
Stress relaxation is the gradual drop in stress in a material that is held at a constant deformation over a long period. The member gets neither longer nor shorter, but the force it exerts falls over time.
Relaxation or creep?
Relaxation and creep are two sides of the same time-dependent behaviour and are often confused:
| What stays constant? | What changes over time? | |
|---|---|---|
| Creep | The stress (load) | The deformation increases |
| Relaxation | The deformation (strain) | The stress decreases |
A beam that keeps sagging under its own weight for years is creeping. A steel wire tensioned to a fixed length that slowly loses force is relaxing.
Where does relaxation matter?
- Prestressing steel. This is where relaxation matters most. Strands or wires are tensioned and anchored in the concrete, so they are held at a fixed length. Relaxation causes part of the prestressing force to be lost, on top of losses from creep and shrinkage of the concrete.
- Concrete. When concrete is held at an imposed deformation, for example by differential settlement or restrained movement, the stress in it partly relaxes. That usually works in your favour: some of the restraint stress disappears.
- Rubber and seals. Rubber gaskets and seals that stay compressed for a long time lose clamping force. A seal can start leaking while still looking fine.
Relaxation classes in Eurocode 2
BS EN 1992-1-1 (clause 3.3.2) defines three relaxation classes for prestressing steel:
- Class 1: wire or strand, ordinary relaxation
- Class 2: wire or strand, low relaxation
- Class 3: hot-rolled and processed bars
The key value is ρ1000: the relaxation loss in percent 1000 hours after tensioning at a mean temperature of 20 °C. The code allows you to assume 8% for Class 1, 2.5% for Class 2 and 4% for Class 3, or to take the value from the supplier’s certificate.
Practical tips
- Ask which relaxation class is being supplied for prestressing work and check the certificate, which states the relaxation value of the steel delivered.
- Don’t assess relaxation in isolation from creep and shrinkage. The losses interact. Have the structural engineer determine them together.
- Allow for loss of clamping force in rubber gaskets and seals, especially in joints that also move.
Related terms
- Creep
- Concrete shrinkage
- Modulus of elasticity
- Elasticity
- Reinforced concrete
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