Yield stress
Yield stress is the shear stress that must be exceeded before a fresh cementitious material begins to flow, below which the material behaves as a solid.
Updated
Yield stress is the shear stress that must be exceeded before a fresh cementitious material begins to flow; below that threshold it behaves as a solid. It is the defining parameter in the Bingham and Herschel-Bulkley models commonly used to describe fresh concrete, alongside plastic viscosity.
In practice
Printable mortars need two different yield stress values at once. The dynamic value, governing flow in the hose and through the nozzle, must stay low enough that the pump can move the material without excessive pressure. The static value, governing the deposited filament, must be high enough immediately after extrusion to resist the weight of the layers above, and must keep rising as printing continues. Yield stress is measured with a rheometer or a vane shear device, and estimated on site with simpler tools such as a slump or slump-flow test. Superplasticisers lower it, while clays, viscosity-modifying admixtures and accelerators raise it or speed its recovery.
Related terms
- Thixotropy — the time-dependent recovery of static yield stress at rest.
- Buildability — the practical outcome of adequate yield stress development.
- Superplasticiser — an admixture that reduces yield stress without adding water.
Frequently asked questions
- What is the difference between static and dynamic yield stress?
- Static yield stress is the stress needed to start a material at rest moving, and it grows with resting time as the internal structure rebuilds. Dynamic yield stress is the stress needed to keep an already flowing material moving, and it is normally the lower of the two. Printable mortars are designed for a low dynamic value, so pumping stays feasible, and a rapidly rising static value, so deposited layers hold their shape.