What does the floor have to be before terrazzo goes down?

Flat, clean, and strong enough: the binding layer under a cement-bound terrazzo is not less than M10 with a consistency of 30 to 35 millimetres. Where there is no built-up system and the terrazzo is bonded straight down, the slab has to be flat enough that the finished floor hits its 3 millimetre tolerance after the topping is added — which means the slab is held to a tighter number than a tile installation would need.

The six processes drawn to one scale. Layer thicknesses are published ranges.

The figures, and where each one comes from

In the Chinese codes	Figure	Source
Binding layer strength	not less than M10	GB 50209 cl.5.4.3
Binding layer consistency	30 to 35 mm	GB 50209 cl.5.4.3
Finished floor flatness	3 mm standard, 2 mm high-performance	GB 50209 table 5.1.7
Topping thickness	12 to 18 mm	GB 50209 cl.5.4.1
Slab moisture before an antistatic test	under 8%	GB 50944 cl.4.5.1
In the North American codes	Figure	Source
Mechanical preparation	blast, grind or scarify	ASTM F710
Acceptance profile	ICRI CSP 3 to 4	ASTM F710
Concrete strength	3,626 psi, which is 25 MPa	ACI 302.1R
Moisture before a resin floor	over 80% RH needs a barrier	ASTM F2170

Why the processes are not interchangeable

The tolerance arithmetic is what surprises people. A cement-bound terrazzo floor is ground together with its substrate, so the finished flatness is the substrate tolerance plus whatever the topping adds, plus the effect of grinding — and the code allows three millimetres on the finished floor. That means the substrate cannot be allowed the same slack it would get under a tile, because a dip that is invisible under a 9 millimetre tile telegraphs straight through a ground surface. Where a slab cannot be brought to that condition, the choice is not to try harder on the terrazzo but to change system: an epoxy floor is thin and bonded, and it tolerates a substrate that is sound but not perfect.

What these figures are, and what they are not

Every figure on this page is a published requirement or a published test method, named beside it. ⛔ THE TWO MOISTURE FIGURES ARE NOT THE SAME MEASUREMENT and must not be compared or substituted: the Chinese 8 per cent is a water content on the slab and the American 80 per cent RH is a relative humidity reading through it. ⛔ We have not prepared a substrate, inspected a slab or tested a floor we supplied; substrate condition is established by testing on site, by the party installing.

Measured data sheets and test reports

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