Guides
How to read a terrazzo grading sheet in five minutes
A data sheet is an argument dressed as a table. Five questions separate a sheet that tells you something from one that only fills a space.
Most terrazzo data sheets carry the same six rows. The difference between a useful one and a decorative one is whether the row says where the number came from.
One: is this a requirement or a result?
JC/T 507-2022 table 7 gives four grades with their own pairs of requirements: water absorption of 8.0, 6.0, 4.0 and 1.0 per cent, and minimum average flexural strength of 5.0, 8.0, 10.0 and 15.0 MPa.
These are requirements, not results. A sheet that quotes one of those four numbers as “our absorption” is quoting a grade, not a measurement. Ask which grade, then ask for the test that established it.
Two: what test produced the number?
Water absorption has methods and they disagree. Flexural strength has methods and they disagree. A number without the method beside it is not a measurement; it is a number with a favourable method chosen for it.
Three: what tolerance, over what distance?
Flatness is always two numbers: a deviation and a span. GB 50209-2010 table 5.1.7 allows 3 mm on a standard floor and 2 mm on a high-performance one, measured with a two-metre straightedge and a wedge gauge.
North America states the same pair differently — 3 mm over a six-foot straightedge — and the two are the same figure. What matters is that both numbers are given, because “flat to 3 mm” on its own is not a specification.
Four: is the thickness a topping or an assembly?
This is where sheets mislead most often. A cement-bound topping is 12 to 18 mm. An NTMA cement topping is half an inch, which is 12.7 mm — almost the same figure. But the sand-cushion assembly is three inches in total.
So “our floor is 13 mm thick” and “our floor is 76 mm thick” can describe the same topping. One of those sentences is hiding a quantity the buyer needs, and it is usually hidden by accident.
Five: is there anything marked not tested?
A sheet where every row has a number is not a better sheet. It is a sheet where nothing was measured.
Six UNVERIFIED rows and forty sourced ones are the same kind of sheet. What makes a data sheet usable is that you can tell which is which.
What we do not say here: no test result for our own material, and no claim that any figure on this page describes a batch we supplied. Every figure above is a published requirement with the clause named.
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