What gets harder when terrazzo gets thin

Thin terrazzo is usually presented as an achievement. It is more usefully presented as a set of trade-offs, because each one is written down somewhere and most of them are arithmetic.

The aggregate ceiling

JC/T 507-2022 clause 5.3: the largest chip may not exceed two thirds of the topping thickness.

Topping	Largest chip permitted	What that means
8 mm	about 5.3 mm	fine aggregate, light duty or over an existing floor
10 mm	about 6.7 mm	fine to medium
12 mm	about 8 mm	medium aggregate; the practical limit for a normal floor
18 mm	12 mm	the top of the published range

Below about 8 mm you are no longer choosing an aggregate. You are choosing a surface treatment.

The wearing depth

A thin topping is nearly all wearing surface. GB 50209-2010 clause 5.4.1 puts a cement-bound topping at 12 to 18 mm, and the grinding process removes part of that before anybody walks on it.

So a thin floor is not a thick floor with material removed. It is a different amount of material to begin with, and the amount that survives to be walked on is what has to last.

The binder is the clock

A resin sets in hours and a cement gains strength over weeks. That difference decides the installation sequence, the wet trades, the flexibility, the repairability and the answer to the renovation question — all of it follows from one property of the binder.

What thin does not remove

Grinding, and the flatness that has to be achieved before the floor is finished. A 3 mm allowance over a two-metre straightedge is the same figure whether the floor is 8 mm or 30 mm, and a thin floor has less material in which to absorb a substrate that is out of tolerance.

Which is why a thin floor over a perfect substrate is routine, and a thin floor over a marginal one is not.

What we do not say here: we do not quote a flexural strength, a weight or a span for our own thin band, because we hold no test report for it. The arithmetic above is ours and the clauses are named; the figures are theirs.

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