PRC.60 · Construction Process

Terrazzo Flooring with Brass Inlays

Terrazzo is not a floor that gets laid: it is a floor cast in place and then uncovered. The surface that will be seen does not yet exist when the mix is poured — it sits inside the mix, a couple of millimetres lower — and grinding is not a finish but the moment the design becomes visible. That is why the only drawing that counts is the one of the brass strips: they are the one thing decided beforehand that cannot be corrected afterwards.

Function of the brass strip

crack control joint and drawing, at the same time

When the design appears

at grinding, not at pouring

Grouting between passes

part of the cycle, not a remedy for a defect

Final protection

open-pore treatment or wax, never a film

Overview

TAV. 00

The process covers cast-in-place terrazzo flooring with brass inlay strips, from the strip layout through to final protection and maintenance. The particularity governing every phase is the dual nature of the strip: it is at once the drawing that will be seen and the joint controlling cracking in a continuous slab cast over tens of square metres. Both functions must be satisfied at once, and when they conflict — a layout wanting large fields in a long narrow room — the joint wins, because a crack cuts across the design far more than an extra strip disturbs it. The rest of the process is a sequence of waits: the mix has to cure before it can be ground, and every grinding pass uncovers a surface that must be grouted before moving to the next.

The site score

TAV. H

The detail of the craftWhere the strip has to pass anyway

01Weeks 1-2

Draw the strips: joint and drawing at once

The strip layout is built by overlaying two criteria. The first is compositional: fields, borders, thresholds, any lettering. The second is technical: the strip must pass through every point where the slab will tend to crack — re-entrant corners, narrowings in plan, door thresholds, changes of use, joints already present in the substrate — and must enclose fields of contained size and proportion, avoiding very elongated shapes. Where the two criteria conflict the second one wins, and the design is reorganised around the obligatory strips.

Why it is done this wayTerrazzo is a continuous slab that shrinks as it cures. Shrinkage looks for the points where geometry concentrates stress, and it always finds them in the same places: a re-entrant corner is a trigger, a long narrow field is a trigger, a door threshold is a trigger. The brass strip placed there does not prevent the movement: it collects it in a line the design had already planned.

2 · criteria overlaid in the layouttutti · re-entrant corners to be covered by a stripil giunto · criterion that wins in case of conflict

In handOverlaid architectural and structural plansSurvey of existing joints in the substrateScaled working drawing of the stripsSamples of aggregates and bindersStrip schedule with lengths and radii

Layout frozen before ordering: shaped strips are made to measure

The mistake that costs dearlyLaying out the fields on compositional grounds alone and leaving it to the installer to add a few. The installer adds them where it is convenient to work, that is almost never at re-entrant corners, which are exactly where the floor will crack.

Site supervision checkOverlay the strip layout on the structural plan and check that every re-entrant corner and every substrate joint has a strip above it: these are checks made in ten minutes on the drawing and never again afterwards.

Preparation of the brass inlay strip layout with verification of its crack-control function, including obligatory passage at re-entrant corners, narrowings, thresholds and substrate joints.site notebook — editorial synthesis

The beats — with site daysbeat 1 of 5

3-4 weeksCuring of the mix before the first grinding: on a binder not yet cured the abrasive tears the aggregate out instead of cutting it.

24-48 hoursHardening of the grouting slurry between one grit and the next, for every grinding cycle planned.

click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work

PLATE 01

The strip is the drawing and the joint at once

TAV. G1

A continuous slab shrinks as it cures, and shrinkage always looks for the same points. The brass strip is the one occasion when a compositional choice and a technical necessity coincide — provided the layout is built on both.

The diagram illustrates the logic of the layout. Field sizes, permitted proportions and thicknesses are to be taken from the design and the guidance for the system used.

PLATE 02

The surface sits two millimetres lower

TAV. G2

At the pour the floor is a closed surface of binder. The aggregate — that is, the design — lies below and appears only when the abrasive goes down. From then on every pass uncovers something and every grouting closes it.

The diagram is qualitative. Thicknesses, number of passes and grits are to be taken from the design and the guidance for the system used.

How it ages (and what betrays it)

TAV. P

PT.01Cracking outside the layout. The slab shrinks and looks for re-entrant corners, narrowings in plan and thresholds: where it finds no strip, it opens a line of its own. The crack cuts across the fields ignoring the composition, and it is the defect that more than any other makes a newly finished terrazzo unrecoverable.

PT.02Permanently stained brass. Ammonia products used to brighten metals and some aggressive detergents darken brass irreversibly, often in irregular patches along a single run of strip. The damage cannot be repaired: it is reground, that is floor thickness is consumed over a cleaning question.

PT.03Unfilled surface voids. Bubbles left in the mix are opened by grinding and, if there is no grouting between grits, remain as a diffuse stippling that collects dirt. They are millimetre-scale defects, invisible one by one and very evident across a whole surface.

PT.04Aggregate torn out by early grinding. On a binder that has not yet cured the abrasive does not cut the grain but pulls it out, leaving a crater the size of the grain itself. It is a defect that cannot be grouted invisibly, because the crater has the shape of what is missing and is larger than air voids.

PT.05Substrate crack copied into the floor. Without an uncoupling membrane the terrazzo bonds to the slab and follows its movements: every crack in the substrate reproduces itself identically at the surface, running in a way that bears no relation to the strip layout.

Questions from the site

TAV. Q

Why can strips not be added afterwards?

Because they are not applied on top of the floor: they run through its full thickness and are embedded in the mix at the moment of the pour. Adding one later means cutting the terrazzo, gluing a strip into the cut and grouting: the result is a line distinguishable from all the others, because it has two side joints the others do not.

Does the brass oxidise? Is it suitable for a bathroom?

Yes, it oxidises, and in most cases that is exactly what is wanted: it darkens where little walked on and stays bright where people pass, returning a map of how the space is used. In damp environments the process is faster and less even. If stable brass is wanted, a surface-treated strip must be chosen, and decided before the pour rather than after.

How long before the floor can be walked on?

Walking on it carefully is possible after a few days, but the useful question is another: how long before grinding. There the times are weeks and cannot be compressed, because they depend on binder curing. In programming it is best to place the pour very early and the grinding close to handover, so the wait falls inside the other trades.

Can it be laid over an existing floor?

Yes, and it is a common route in refurbishment, but one important thing changes: the available thickness. A thin terrazzo has less reserve for future regrinding and less margin to absorb substrate irregularities. If the existing floor is cracked, the uncoupling membrane is not an option but the condition for the work to make sense at all.

Materials involved

TAV. M

MAT.69/92A

Copper-Zinc Alloy (Cu-Zn)

Intrusive Igneous Rock

Granite

MAT.34/57C

Natural Hydraulic Mortar

Cocciopesto (Opus Signinum)

MAT.27/50S

Repair mortar class R4 (EN 1504-3)

Structural Repair Mortar (R4)

Stratigraphy

TAV. S

↑ OuterInner ↓

Ground surface treated with wax

It is the level at which grinding was stopped: a few tenths higher the aggregate is still covered, a few tenths lower and the pattern of the terrazzo changes.

Brass inlay strip

It runs through the full thickness of the mix and reaches the surface: it is not applied on top, it is embedded from the pour onwards.

Mix of aggregate chips and cementitious binder

It must be compacted until the air is driven out: every bubble left becomes a void that will appear at the surface only during grinding.

Uncoupling membrane over the substrate

It separates the terrazzo from movements of the slab: without it, every crack in the substrate copies itself identically into the finished floor.

Load-bearing reinforced concrete slab

Its flatness decides the actual thickness of the mix: an out-of-plane slab is paid for in material and in cracking risk.

Advantages

TAV. V

The surface is continuous and jointless: no edges where dirt collects and no joint degrading before the rest of the floor.
The design has no modular limits: curves, irregular fields, lettering and drawn thresholds are obtained by bending a strip, not by cutting an element.
The thickness removed by grinding is a reserve: terrazzo can be reground decades later and comes back new, which no floor with a thin wear layer allows.
The aggregate can come from sawmill offcuts and quarry recovery: it is one of the few high-quality floors in which the noble material is literally a by-product.
The brass ages with the floor: it oxidises, darkens where little walked on and stays bright where people pass, over time returning a map of how the space is actually used.