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Answers

What size should the chips in terrazzo be?

Both systems number their grades rather than quote millimetres, and both tie the grade to the thickness of the topping rather than to appearance alone. NTMA runs from number 0, which passes a one-eighth inch screen, up to number 5, which passes three quarters of an inch. The Chinese standard specifies 6 to 16 millimetres for a floor topping.
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

NTMA number Passes screen, inches Retained on screen, inches Customary use
0 1/8 1/16 epoxy, fine build-ups
1 1/4 1/8 cement and epoxy
2 3/8 1/4 cement and epoxy
3 1/2 3/8 larger castings
4 5/8 1/2 deep castings
5 3/4 5/8 Venetian and deep castings

Why the processes are not interchangeable

Chip size is chosen backwards from the product. The topping has to be at least one and a half times the chip, so the thickness sets the ceiling and the chip follows from what the system can carry. Within that ceiling the chip size is an appearance and performance decision: small aggregate reads as a fine speckle and grinds to a flatter surface, large aggregate reads as a statement piece and leaves deeper pockets that hold dirt. NTMA lists customary practice per system, and the interesting line in it is cement at one-eighth inch and Venetian at five-eighths for the same nominal topping, which is the clearest published statement that aggregate size is a choice within a fixed envelope rather than a fixed value.

What these figures are, and what they are not

The NTMA grading and the customary-use table are from the NTMA Terrazzo Systems Reference Guide, given in inches. The 6 to 16 millimetre figure is GB 50209-2010 clause 5.4.8; note that the older 1991 edition of that code said 6 to 15 millimetres, and we quote the current edition. ⛔ Millimetre equivalents of the NTMA numbers are conversions, not quotations, and are not published in the guide.

Measured data sheets and test reports

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