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Answers

Thin terrazzo on a stair: what keeps it from moving?

Two requirements and they are not the same one. The tread wants grip, so it takes a honed or textured finish rather than a polish — a polished terrazzo tread is a slippery stair. The riser and the fixing want a mechanical anchor through the panel, because a tread that is only bonded will work loose at the nosing under load.
Chip size against topping thicknessA horizontal scale in millimetres. For each topping thickness from 8 to 18 millimetres, a bar shows the topping and a second bar shows the largest stone chip permitted, which is two thirds of the thickness. Derived from JC/T 507-2022 clause 5.3.0481216208 mm5.3 mmthe band we supply10 mm6.7 mm12 mm8 mm15 mm10 mm18 mm12 mmTopping thickness (solid) against the largest chip permitted (accent), on one millimetre scale.Chip ceiling is two thirds of the thickness (JC/T 507-2022 cl.5.3).Topping range from GB 50209-2010 cl.5.4.1; the arithmetic is ours.
Chip size against topping thickness, on one millimetre scale. The chip ceiling is two thirds of the thickness, from JC/T 507-2022 clause 5.3; the arithmetic on the thicknesses is ours.

The figures, and where each one comes from

Where on the stair What it needs
Tread honed or textured, and a nosing profile, not a polish
Nosing specified as a profile, because it is the edge that takes the load
Fixing mechanical through the panel, not bond alone at the nosing
Under it a substrate flat enough that the panel is not bridging a hollow
The band 8 to 12 mm gives less section for the anchor than a 20 mm panel does

Why the processes are not interchangeable

A stair is the one place on this site where the finish and the fixing are decided together, because the failure is the same failure. A polished tread is slippery when wet, and a bonded nosing works loose, and both complaints arrive as "the stair is dangerous" from the person using it rather than from the inspector. What the thin band changes is the section available to the anchor: at 8 millimetres there is less material to fix into than a conventional 20 millimetre panel, which is not an argument against it so much as a reason to specify the fixing rather than leave it to the installer by default.

What these figures are, and what they are not

The chip ceiling at each thickness is our arithmetic from JC/T 507-2022 clause 5.3. ⛔ No grip value, no slip number and no fixing specification appears here: a slip rating would have to come from a tested surface and a fixing capacity from a tested assembly, and we hold neither for this product. ⛔ That a stair needs a mechanical fixing rather than bond alone is stated as general practice, not as a figure from any standard.

Measured data sheets and test reports

If you are specifying this

These are the forms we actually supply for this question, with production photographs where we have them:

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