Green strength

Green strength is the strength of a cementitious material in its fresh, unhardened state, which determines how much load a deposited layer can carry before the binder has set. It is not the same property as hardened compressive strength and is measured in different units and by different methods, usually as a shear or yield stress in the range of kilopascals rather than megapascals.

In practice

Green strength is not a fixed value but a curve: it rises from the moment of deposition, first through thixotropic structural build-up and then through early hydration. Printing succeeds when that curve stays ahead of the load accumulating from the layers above, and fails as buckling or plastic collapse when it does not. Green strength is typically assessed with vane shear tests, penetrometers or uniaxial compression on fresh cylinders, repeated at intervals after mixing. It is sensitive to temperature, to admixture dosage and to the time the material has spent in the hose, which is why mixes are usually re-checked on site rather than trusted from laboratory data alone.

Related terms

• Buildability — the number of layers a mix can support without deforming.
• Yield stress — the stress at which fresh material starts to flow.
• Thixotropy — the recovery of stiffness once the material stops moving.