Terrazzo flooring field guide: install, grind, and polish

What terrazzo is, and why the slab and the strips decide it

Terrazzo is a poured composite floor. A binder matrix, either epoxy resin or portland cement, is filled with decorative chips of marble, glass, or granite, placed in the field, then ground and polished smooth so the chips show as a continuous surface of stone set in color. There are no tiles and no grout lines in the field. The floor is one continuous surface, and it lasts for decades, which is why it shows up in airports, schools, hospitals, and the lobbies of buildings meant to stand a long time.

Two things decide whether a terrazzo job comes out right, and neither is the chip color the owner picked. The first is the substrate. Modern epoxy terrazzo is a thin topping bonded directly to the concrete slab, so if the slab is not sound, flat, cured, and dry, the floor debonds or cracks no matter how good the pour looked. The second is the divider strips. Terrazzo shrinks and the slab under it moves, so the floor is going to crack. The strips decide whether it cracks along a clean metal line you laid out on purpose or in a random map across the field.

Get the slab and the strips right and the rest is craft you can control: lay the strips, mix and place the matrix and the chips, then grind, grout, and polish to expose the aggregate. Get either one wrong and you own a callback that grinding cannot fix. The polished concrete and resinous flooring guides cover the two related floors that share this same slab. This one covers the chip-filled topping that gets ground to stone.

What terrazzo actually is: matrix and aggregate

Strip the job to its parts and terrazzo is a matrix plus an aggregate. The matrix is the binder that holds everything together and carries the background color. The aggregate is the decorative chip, the part you actually see, sized and blended to the design. In the wet state it looks like a colored slurry packed with stone. None of the stone shows until the floor is ground.

The grinding is what makes it terrazzo and not just a filled topping. You grind the cured surface down far enough to slice the tops off the chips, opening them as flat polished faces flush with the matrix around them. Then you fill the tiny holes the grinding opens and polish the whole field to a gloss. What you end up with is a hard, dense, continuous floor with stone set in color, refined to a shine, and no joints in the field to trap dirt or break under a cart.

That combination is why terrazzo earns its place in high-traffic public buildings. It takes foot traffic for fifty years and more, it cleans with a mop, and a worn floor can be ground and polished back to new instead of torn out. The cost up front is real. The floor still serving after five decades is the reason owners pay it.

The slab and the divider strips decide the job

Spend your worry where the failures come from. On epoxy terrazzo that is the slab and the strips, in that order, and almost everything else is recoverable craft.

The slab is what the whole system bonds to. Epoxy terrazzo is a thin resin topping, nominally 1/4 to 3/8 in thick, glued to the concrete with nothing under it but the bond. A slab that is unsound, dusting, contaminated, out of flat, still curing, or driving moisture takes that bond down with it. You cannot grind your way out of a debonded floor. You prep and test the slab before you pour, every time, and the same floor-prep and moisture work the resinous flooring guide covers in full applies here, because epoxy terrazzo lives by the same rules as any bonded resin floor.

The strips are the crack plan. Terrazzo is a rigid topping over a slab that shrinks, curls, and moves at its joints, so the topping will crack. Divider strips give the crack a place to go. Lay a strip over every slab control and construction joint and along the layout grid, and the floor cracks at the strip line where nobody notices, instead of wandering across the field. Miss a slab joint with your strips and the slab telegraphs that joint straight up through the terrazzo as a random crack. The NTMA, the manufacturer, and the project specification set how both of these are detailed, and the spec is the contract for what the floor has to be.

Epoxy or cementitious terrazzo?

Terrazzo runs as two families of systems, and the binder is what splits them. Epoxy terrazzo uses a resin matrix. Cementitious terrazzo uses a portland cement matrix. They look similar once polished, but they install differently, sit at different thicknesses, and fail in different ways, so the choice is made by the project, not by taste.

Epoxy terrazzo is the modern default and the most common system poured today for interior floors. It is thin, nominally 1/4 to 3/8 in, bonded straight to the slab, it takes almost any color, and it cures fast enough to hold a schedule. What it cannot do is breathe or move much, so it depends on a dry, sound slab and on the strips and the crack detailing to handle movement.

Cementitious terrazzo is the traditional system, and it is thicker because the cement matrix needs the mass. It comes in versions that isolate the topping from the slab so the floor can move independently, which is why it still wins for exterior work, for slabs that move, and for matching historic floors. It is slower and heavier, and it is the right call exactly where epoxy's thin bonded film would fight the substrate.

Pick the system for the slab and the service, then build it to the NTMA method and the manufacturer for that binder.

Epoxy terrazzo: the modern thin-set system

Epoxy terrazzo is a thin-set resin system, and it is what most people mean now when they say terrazzo. An epoxy binder, pigmented to the background color, is mixed with the chip blend and troweled over the prepared slab at a nominal 1/4 to 3/8 in. It bonds to the concrete through a primer, cures in hours to a day rather than the days a cement matrix needs, and it holds bright, consistent color that cement cannot match. That color range and the thin profile are why it dominates interior work in airports, schools, hospitals, and retail.

Because the film is thin and bonded, the slab under it controls the outcome. There is no sand cushion or mass to absorb slab movement, so epoxy terrazzo leans entirely on the substrate being sound and dry and on the divider strips and crack isolation to manage cracking. The same moisture and prep rules that govern any bonded epoxy floor govern this one. The resinous flooring guide covers that testing and prep, and it applies here without exception.

The strips for epoxy terrazzo are usually set in epoxy adhesive to the slab, most often aluminum because it is inexpensive, carries recycled content for LEED, and bends easily for curved patterns. Follow the NTMA epoxy terrazzo guide specification and the binder manufacturer for the primer, the matrix ratio, the strip setting, and the cure.

Cementitious terrazzo: monolithic, bonded, and sand cushion

Cementitious terrazzo uses a portland cement matrix, and it splits into a few methods by how the topping relates to the slab. Each handles slab movement differently, which is the whole reason to choose one over another.

Monolithic terrazzo is a 1/2 in cement topping placed directly on the concrete slab. It is the thinnest and cheapest cement system, and it rides entirely on the quality of that slab for flatness and crack control, so it suits a sound, flat interior slab and little else. Bonded terrazzo adds an underbed bonded to the slab, bringing the total to roughly 1-3/4 to 2-1/4 in including the 1/2 in topping, which gives more tolerance for a slab that is not dead flat.

Sand cushion terrazzo is the premium cement system and the one that handles movement. It sets the topping over reinforcing wire and a dividing membrane on a thin sand bed, isolating the floor from the slab so the slab can move, crack, and shift underneath without telegraphing up through the terrazzo. It runs about 2-1/2 to 3 in total. That isolation is why cement terrazzo, and sand cushion in particular, is still the answer for exterior work, for floors over slabs expected to move, and for matching historic installations. The thicknesses and reinforcing are set by the NTMA method for each system, so confirm the build against it and the spec.

The substrate: the slab the floor bonds to

The slab is where epoxy terrazzo is won or lost, so treat the substrate as the first deliverable, not a surface to pour on. The concrete has to be sound, clean, flat, fully cured, and dry before any primer goes down. Sound means no laitance, no dusting, no delaminated patches. Clean means no curing compound, sealer, paint, glue, or oil, because a bonded resin floor over a bond-breaker peels in sheets, and the bond-breaker is right there on the back when it lifts.

Flatness is a number, not a judgment call. The NTMA and common terrazzo specifications call for a steel-troweled slab held to about 1/4 in variation in a 10 ft span, finished without troweling to a hard burnished sheen and without curing agents that would have to be ground off later. Cured matters because a green slab is still shrinking and still wet, and both fight a thin bonded topping. On a slab on grade, an under-slab vapor barrier conforming to ASTM E1745 Class A directly beneath the concrete is what keeps the slab dry enough to take epoxy in the first place, and that is a design decision made before the pour.

Prep is mechanical. The slab gets shot-blasted or ground to strip contaminants and open a profile the primer can key into, then vacuumed clean, exactly the way the resinous flooring guide lays out for any bonded resin floor. Skip the prep or pour on a slab that is not sound and dry, and you own the flooring failure. The NTMA, the manufacturer, and the spec set the requirements. Meet the strictest of the three.

Do you need a moisture test before epoxy terrazzo?

Yes, and it is the single test that prevents the most expensive failure on the job. Moisture driving up out of the slab is the number one cause of epoxy terrazzo debonding. Concrete that is still drying pushes water vapor toward the surface, the epoxy topping seals that surface, the vapor pressure builds under the bonded film, and it lifts. You get blisters, then debonding, then a floor you grind off and pour again. No resin and no primer fixes a slab driving moisture. You test it before you prep, and you respect the number.

Two ASTM methods do the work, the same two the resinous flooring guide covers in detail. ASTM F2170 reads relative humidity inside the slab with in-situ probes and is the more trusted test, because it tells you what the slab interior will do once the top is sealed. ASTM F1869 is the calcium chloride test, reading the moisture vapor emission rate from the top of the slab in pounds per 1000 square feet over 24 hours. Common epoxy terrazzo limits run around a maximum 3 lb per 1000 sq ft per 24 hours by F1869 and below about 80 percent RH by F2170, but those are starting points. The binder manufacturer's data sheet sets the number the warranty is written against, so use that one.

If the slab reads wet, you install a moisture mitigation membrane, a 100 percent solids epoxy vapor barrier rated to hold back the drive, then build the terrazzo on top. Price that the day the test comes back high, not after the floor blisters. Skip the test and you own the failure on the schedule's terms, not yours.

Slab flatness, profile, and prep

On a real slab you almost never get the flat, clean, profiled substrate the detail assumes, so prep is the work of making the slab into one. Start by reading it. Walk it with a straightedge to find the high spots and the low spots, check it for old coatings and contamination, and map the cracks and the joints before any equipment runs.

Shot blasting is the primary tool for the open field. It fires steel media at the slab, stripping laitance, curing compound, and contamination while it cuts a profile, and it leaves no dust ground into the surface. Diamond grinding handles the edges, the corners, the columns, and the high spots that have to come down to bring the floor into flatness, and it is also how you knock down a slab that is out of the 1/4 in in 10 ft tolerance. Grinding drives fines into the pores, so you vacuum and clean before priming or you have built your own bond-breaker. Low spots and spalls get filled with a patch rated to take the system and ground flush.

Cracks in the slab get addressed before the terrazzo, not after they reappear through it. Static cracks are routed, cleaned, and filled rigid. Working cracks and the slab joints get the strip-over-the-joint detail or a flexible crack isolation membrane, both covered in the next sections. The profile and the cleanliness are two separate checks. A slab that is rough enough but still has dust in the valleys fails the bond just the same.

The divider strips: what they actually do

Divider strips are the metal or plastic strips set on edge into the terrazzo, and they do three jobs that no other part of the system does. They control where the floor cracks, they make the pattern and the color breaks, and they handle the movement at the slab joints. Of the three, crack control is the one that keeps the floor whole.

Terrazzo cracks. A rigid topping over a slab that shrinks and moves is going to crack somewhere, and the strip decides where. This is old, settled knowledge: a terrazzo contractor patented the metal divider strip back in 1921 specifically to localize cracking to the strip lines instead of letting it wander the field. Set the strips on the grid and over the joints and the topping cracks at the strip, along a clean line the eye reads as part of the pattern. Leave a stretch of field with no strip over a moving slab joint and the crack shows up there anyway, raw and random.

Strips come in zinc, brass, aluminum, and plastic. Aluminum is the common choice for epoxy terrazzo, cheap and easy to bend for curves and carrying recycled content. Zinc was the traditional cement-terrazzo strip. Brass is back in fashion for design work, with one caution: the copper in brass can bleed a blue stain into a light-colored matrix, so it is a risk in pale floors. Match the strip material, height, and gauge to the system and the design, per the NTMA and the spec.

Laying out and setting the strips

The strip layout is the floor's design and its crack plan in one drawing, so it gets set before a drop of matrix is placed. The grid, the pattern, the logos, the color breaks, and the borders are all defined by where the strips go. On a plain floor the layout is a simple grid. On a feature floor it is the artwork itself, with curved aluminum strips bending around shapes the design calls for.

Two lines have to be in the layout no matter what the pattern is: the building's column lines and every slab joint. The column lines are where structural movement concentrates, and the slab joints are where the slab is already designed to move. Run strips along both, then fill in the decorative grid between them. For traditional cement terrazzo the NTMA method historically calls for divider strips on roughly a 4 ft by 4 ft maximum grid to counter shrinkage and movement, which is a useful reference for how tight the field grid wants to be even when the design would space the visible pattern wider.

Setting is method-specific. For epoxy terrazzo the strips are bedded in epoxy adhesive to the slab with no voids under the strip, so the matrix cannot undercut them. For cement systems they are set in the underbed. Either way the strips have to be straight, true to height, and locked down hard, because the grinder is going to ride across their top edges and anything proud or loose shows in the finished floor.

Strips and isolation over the slab joints

This is the detail that separates a floor that stays whole from one that cracks in the first year. Every control joint and construction joint in the slab is a line the slab is built to move along, and that movement transfers straight up into a thin bonded topping. If nothing in the terrazzo is detailed to take it, the slab cracks the floor right over the joint.

The standard answer is a divider strip directly over every slab control and construction joint. The strip gives the topping a built-in break at the same line the slab moves on, so the floor opens at the strip instead of tearing through the field. This is not optional on a bonded epoxy floor, and it is the first thing an experienced installer checks against the slab joint plan before laying out the decorative grid.

Where a strip cannot sit over a joint, or as added protection, the NTMA crack detailing for epoxy terrazzo allows a flexible crack isolation membrane over the joint or crack instead. It is a flexible epoxy applied at a nominal 40 mils, engineered to stretch and keep a substrate crack from reflecting up into the terrazzo. Understand its limit. It accommodates horizontal movement, not vertical movement from settlement or slab deflection. A joint with vertical shear needs more than a membrane. Detail the joints to the NTMA method and the spec, and treat any slab joint your strips miss as a crack you scheduled.

The pour: matrix, chips, and the trowel

With the strips set and the slab primed, the pour is mixing the matrix with the aggregate and placing it between the strips. The binder, epoxy or cement, is pigmented to the background color and mixed with the chip blend at the proportions the design and the method call for, then spread into the bays the strips form and troweled out level with the top of the strips. The strips become the screed line, which is one more reason they have to be set true to height.

Seeding is the step that makes the ground floor read full of stone. After the matrix is placed, the crew broadcasts extra chips onto the wet surface and trowels them in, so when the grinder cuts the top off the floor there is aggregate everywhere it cuts, not just the chips that happened to float up in the mix. Seeding is how you hit the dense, stone-packed look the sample promised instead of a thin scatter of chips in a sea of matrix.

Then it cures, and the matrix decides how long. Epoxy reaches grinding hardness in roughly a day, depending on temperature and the product. Cement needs to hydrate and gain strength over days before it will take a grind without tearing chips loose. Place it, trowel it flush, seed it, and let it cure to the manufacturer's hardness before anyone drops a grinder on it. Grind it green and you pluck chips and chew the matrix.

The aggregate: marble, glass, and the chip blend

The chips are the part everyone looks at, and the blend is where the floor's appearance is designed. Terrazzo aggregate is most traditionally marble, prized because it grinds and polishes to a clean flat face, but it also runs to granite and quartz for hardness, recycled glass for color and sustainability credit, and mother-of-pearl, mirror, or porcelain chips for special effects. Epoxy terrazzo in particular takes glass and bright aggregate well, because the resin can be tinted to any background behind it.

The look is set by chip size, the blend of sizes, the color mix, and how much aggregate the matrix carries. Chips come graded by number, from fine through large, and a design might run a single size for a tight uniform field or a blend of sizes for depth. More aggregate and larger chips read as a busier, stonier floor. Less aggregate and finer chips read calmer. Cement and epoxy carry different aggregate proportions, so follow the method for the system.

Recycled glass deserves a note because it does double duty. Post-consumer and pre-consumer glass chips give colors marble cannot, and the recycled content earns LEED credit, which is part of why glass-heavy epoxy terrazzo shows up in institutional and green building work. Mock up the actual blend on a sample panel and get it signed, because a chip blend on a board in the showroom and the same blend ground into a floor under the building's own light are two different things.

Grinding to expose the chips

Grinding is the step that turns a cured slurry of stone into terrazzo, and it is the heart of the trade. You grind the cured surface down with diamonds until you cut the tops off the chips, opening them as flat faces flush with the matrix. The first cut is the one that matters most, because it sets how much aggregate you expose. Grind too shallow and the chips barely show, leaving a matrix-heavy floor that misses the design. That first pass commits the exposure.

A planetary grinder does the work, its heads counter-rotating so the scratch pattern crosses itself instead of digging tracks in one direction. You start with a coarse diamond to cut, level, and open the aggregate, then step the grit up through progressively finer diamonds, the same discipline of never skipping a grit that the polished concrete guide lays out. Each grit only removes the scratch pattern the one before it left. Skip a step and you polish the coarse scratches in instead of out. The cut throws a slurry you have to manage and clean as you go.

Here is the number that keeps you honest: the terrazzo topping is thin. Epoxy terrazzo is only 1/4 to 3/8 in, and a cement topping is about 1/2 in, so there is not much depth to give. Grind too aggressively or chase a level on a wavy floor and you cut clean through the topping into the underbed or the slab, which is a patch that never disappears. Read the floor, set the exposure on the first cut, and take the rest in controlled steps.

The grout coat: filling the pinholes

Grinding opens the chips, and it also opens thousands of tiny pinholes and voids, air pockets in the matrix and gaps right at the edges of the chips. Leave them and the finished floor is a polished surface full of little holes that catch dirt and read as a haze of dots up close. The grout coat is the step that closes them, and it is what separates a dense, refined terrazzo from a pitted one.

After the first grind, you spread a grout coat over the whole floor, a matching matrix, epoxy grout with marble flour for an epoxy floor or a cement grout for a cement floor, pressed into the pinholes with a trowel or a grout pan so it packs the voids, then squeegeed off the high surface. It cures, usually overnight, and you regrind. The regrind cuts the grout back off the field and leaves it only where it belongs, down in the holes. What you get is a tight, void-free surface ready to polish.

This is the step crews skip when the schedule is tight, and it is a false economy. A floor that was not grouted shows its pinholes forever, and the only fix is to go back, grout, and regrind anyway, now around furniture and traffic. Grout the pinholes, cure, regrind, then polish. The grout coat is what makes the finish look poured instead of patched together.

Polishing to the finish

Polishing is the back half of the diamond sequence, climbing the grit from the regrind up to the gloss the spec calls for. Where the coarse diamonds cut and exposed the stone, the fine resin-bond diamonds and polishing pads close the surface and bring up the shine, refining the matrix and the chip faces together until they reflect as one continuous surface. The discipline is the same as the grind: step the grit up without skipping, and finish each step across the whole floor before moving up, or the haze of the lower grit stays under the gloss.

The finish level is a spec, not a feeling. A honed finish is matte to satin, ground and refined but stopped short of high gloss, and it hides scuffs and gives a little more grip. A polished finish climbs higher up the grit, often to 1500 or 3000 and beyond, for a reflective, glassy surface. Higher gloss shows more and reflects more. It also shows wear and scuffing sooner. Both are legitimate, and the project picks one.

Agree on the finish by the chart and a sample, not by an adjective, the same way the polished concrete guide handles gloss. One person's polished is another's honed. Grind a sample panel to the specified finish, get it signed, and make that panel the standard the floor is judged against.

Sealing the floor and the slip-resistance call

A finished terrazzo floor gets a penetrating sealer to protect it and to set how it cleans, and the sealer is also where slip resistance is decided. Polished terrazzo is a smooth, dense surface. The building's occupants are walking on whatever sealer or finish is on top of it, so the sealer choice, not the stone, governs traction. A penetrating sealer soaks in to resist staining without building a film that would itself wear and have to be stripped.

Slip resistance is a safety call with numbers behind it. ANSI A137.1 sets a minimum wet dynamic coefficient of friction of 0.42 for floors expected to be walked on wet, and the terrazzo industry has long tested coefficient of friction by ASTM D2047. The ADA does not enforce a single number today, but designers commonly target the higher historic references, around 0.6 for level walking surfaces, to stay clear of slip-and-fall liability. The number that controls a given floor is the project spec and the authority having jurisdiction, so confirm it rather than assuming the polished default passes.

Where a floor will be wet, oily, or sloped, build the traction in. An abrasive additive broadcast or mixed into the sealer gives a smooth terrazzo the texture it needs for a ramp, an entry, a kitchen, or a wash area. The trade-off is the same as any textured floor: more grip catches more dirt and cleans harder, so match the texture to the wettest realistic condition, not to the showroom.

Cure and the project schedule

Terrazzo is a slow, multi-step trade, and the schedule has to respect the cures or every step downstream pays for it. The matrix has to cure to grinding hardness before the first cut, the grout coat has to cure before the regrind, and the floor has to be clean and dry before the sealer. Rush any of those and you tear chips, smear grout, or seal in haze.

The binder sets the pace. Epoxy terrazzo moves fast. The matrix is often grindable within a day, the grout cures overnight, and the whole floor can be poured, ground, grouted, polished, and sealed in a compressed window, which is a large part of why epoxy displaced cement for interior work on a clock. Cement terrazzo is slow by comparison, needing days for the topping to hydrate and gain strength before it will take a grind, which is one of the trade-offs you accept for the movement tolerance and exterior service a cement system buys.

Sequence terrazzo near the end of the build, after the wet trades overhead are done and before the final finishes, and protect the cured floor from traffic and dropped material until the sealer is down. A floor walked on green or dusted with drywall sanding before it is sealed carries that abuse into the finish. Build the cures into the schedule up front. They are not delays you can compress away.

Maintaining a terrazzo floor

Maintenance is one of terrazzo's real advantages, and it is genuinely light for a floor this hard. Day to day, a terrazzo floor is dust-mopped to keep grit off it and damp-mopped or auto-scrubbed with a neutral-pH cleaner and clean water. That is most of the program. The dense, sealed surface does not need the stripping and waxing cycle a resilient floor demands, and a floor that is never waxed never builds the yellowed wax layer that ruins the look of so many old institutional floors.

Two habits protect it and two habits wreck it. Grit is the enemy. Sand and dirt dragged underfoot or under a worn pad scratch the polish over time, so walk-off mats at the entries and regular dust-mopping earn their keep. The other killer is the wrong cleaner. An acidic cleaner etches the matrix and the marble, and a high-pH stripper dulls the surface, so a neutral cleaner is not a preference. It is the rule.

On a cycle the sealer gets refreshed and the floor can be re-polished to bring the gloss back. Hand the owner a written maintenance plan with the cleaner type, the pad grade, and the reseal interval. A terrazzo floor will look like the floor you installed for decades, but only if whoever cleans it knows not to reach for the acid and the wax.

Restoration: why old terrazzo comes back

Terrazzo is restorable, and that is the property that makes a fifty-year-old floor worth keeping instead of demolishing. Because the floor is solid stone in matrix and the finish is ground into it rather than coated on top, a dull, scratched, stained terrazzo floor can be ground and polished again, back to a surface that looks new. There is no coating to strip and recoat, no wear layer to run out. You are refinishing the same material the original crew poured.

The work is the back half of a new install run again. You grind off the worn, etched, or waxed-over surface, repair cracks and missing chips with a matching matrix and aggregate, regrout the pinholes that open up, polish back through the grits, and reseal. Decades of grime, old wax, and surface damage come off, and the stone underneath is the stone it always was. This is exactly why historic terrazzo in courthouses, schools, and transit buildings gets restored rather than replaced.

This restorability is the heart of terrazzo's lifecycle case. Other floors wear out and get torn out. Terrazzo wears at the surface and gets ground back, again and again, which is why the same floor can serve through generations of a building. Match the original chips and matrix on a restoration, and confirm the method against the NTMA, because the historic system may not be the one you would pour new.

Cost and lifecycle value

Terrazzo costs more up front than almost any floor it competes with, and that number scares owners comparing first cost. The install is labor-intensive and slow, it takes a specialty crew, and the materials are not cheap. There is no honest way to make terrazzo a low-bid floor, and trying to value-engineer it into one usually means cutting the substrate prep, the strips, or the grout coat, which is cutting the parts that make it work.

The case for terrazzo is the lifecycle, not the bid day. A terrazzo floor routinely serves fifty years and more, it cleans with a mop and a neutral cleaner instead of a strip-and-wax program, and when it finally dulls it gets ground back instead of replaced. Spread the first cost across that life and the floor that looked expensive becomes the cheap one, which is why owners who plan to hold a building for the long term keep choosing it for the lobbies and corridors that take the most traffic.

Run the comparison on lifecycle cost, not first cost, the same way the polished concrete guide frames its own case. For a building meant to last, the math favors the floor you grind back to new over the one you tear out and replace on a cycle.

Terrazzo, polished concrete, or a resinous coating?

These three floors share the same concrete slab and get confused for one another, so it helps to say plainly what each one is. Terrazzo is a decorative topping, stone chips in a matrix, placed on and ground to expose the aggregate, chosen for appearance and a decades-long life. Polished concrete is the slab itself, ground and densified with no topping, chosen when the slab is the finish and cost and durability beat appearance. A resinous or epoxy coating is a resin film bonded to the slab, chosen for chemical resistance, washdown, and a joint-free surface in labs, kitchens, and plants.

The slab decides what is even possible across all three. A slab driving moisture can usually be polished, because polished concrete breathes, but it will debond an epoxy coating and an epoxy terrazzo topping alike, which is why the moisture test is non-negotiable for the two bonded resin systems. The polished concrete guide and the resinous flooring guide cover those two siblings in full. If appearance and longevity are not the point, one of them may be the better floor.

Pick by what the room needs. For a showpiece public floor meant to last, terrazzo. For a hard, cheap, breathing floor where the slab can be the finish, polished concrete. For chemical resistance and washdown, a resinous coating, with the moisture test first.

What to document

A terrazzo floor is judged on appearance and it has to last decades, which makes the record both a defense against a dispute and a gift to whoever restores it later. The moisture test result is the first thing the manufacturer's rep asks for on a debond claim, and the chip and matrix blend is what a restoration crew needs to match the floor in thirty years.

Capture it by area, because a building can carry different mixes and systems room to room. Record the substrate prep and profile, the moisture test method and result against the limit, the divider strip material and layout, the matrix and chip blend with proportions, the grind and grout sequence and the finish achieved, and the sealer. Photograph the signed sample panel and keep it as the standard the floor was accepted against. A field tool like FieldOS keeps those records and the photos tied to the area, so the closeout package and the future restoration both have what they need.

Common mistakes

• Pouring epoxy terrazzo over a slab that is too wet, so the bonded topping blisters and debonds.
• Skipping the slab moisture test and finding out the slab was driving vapor after the floor lifts.
• Not running divider strips over the slab control and construction joints, so the slab cracks the floor at random.
• Poor substrate prep, leaving curing compound, laitance, or contamination under a bonded resin topping.
• Skipping the grout coat, so the finished floor stays full of pinholes that catch dirt and read hazy.
• Grinding too shallow on the first cut, leaving the chips barely exposed and the floor matrix-heavy.
• Grinding too deep or chasing a level on a thin topping, cutting through into the underbed or the slab.
• Setting brass strips in a pale matrix without accounting for the blue copper stain they can bleed.
• Handing over the floor with no maintenance plan, so it gets stripped with acid or built up with wax.

Field checklist

Standards and references

The National Terrazzo and Mosaic Association, the NTMA, is the reference body for terrazzo in North America, and its guide specifications and system reference guide define the methods for epoxy and cementitious terrazzo, the system thicknesses, the divider strip standards, and the crack detailing over joints. An architect cites the NTMA method and system in the project specification, and that specification is the contract for what the floor has to be. Confirm the current edition and the selected system, and do not quote a section number you do not have in front of you.

Moisture testing runs on the ASTM methods the bonded resin trades all use: ASTM F2170 for in-situ relative humidity and ASTM F1869 for the calcium chloride moisture vapor emission rate, with the acceptance limit set by the binder manufacturer, not the test method. The under-slab vapor barrier references ASTM E1745, commonly Class A directly beneath a slab on grade. Slip resistance references ANSI A137.1 for the wet dynamic coefficient of friction and ASTM D2047 for the terrazzo industry's coefficient-of-friction test, with the ADA and the authority having jurisdiction governing the requirement on a given floor.

The binder manufacturer's data sheet controls the primer, the matrix ratio, the moisture limit, and the cure for the specific product, and where the project specification is stricter, the spec wins. Cite the standard that controls the point, follow the NTMA method for the system and the manufacturer for the numbers, and verify current editions before relying on any of them on a submittal.

Units, terms, and definitions

Terrazzo borrows vocabulary from concrete, stone, and coatings, so the same floor reads differently across a spec, an NTMA method, and a manufacturer's data sheet.

System thickness is given in inches, from the nominal 1/4 to 3/8 in of a thin-set epoxy topping up through the 2-1/2 to 3 in of a sand cushion cement system. Chip size is given by aggregate number, from fine to large. Slab flatness is given as a tolerance over a span, commonly 1/4 in in 10 ft. Moisture is read two ways that do not convert: relative humidity as a percentage inside the slab by F2170, and moisture vapor emission rate in pounds per 1000 square feet over 24 hours by F1869. Slip resistance is given as a coefficient of friction, with the wet dynamic value by ANSI A137.1 the common number.

Terrazzo

A poured composite floor of decorative stone or glass chips set in a matrix, ground and polished to expose the chips

Epoxy vs cementitious terrazzo

Epoxy uses a thin bonded resin matrix; cementitious uses a thicker portland cement matrix, bonded or isolated from the slab

Divider strip

A zinc, brass, aluminum, or plastic strip set on edge that controls cracking, makes the pattern, and handles joint movement

Matrix and aggregate

The matrix is the binder and background color; the aggregate is the decorative chip, marble, glass, granite, or recycled glass

Moisture test (MVER / RH)

Slab moisture measured by ASTM F1869 emission rate or ASTM F2170 in-situ relative humidity before a bonded pour

Grout coat

A matching matrix spread after the first grind to fill the pinholes and voids, cured, then ground back off the field

Grind and polish

Cutting the cured topping with progressively finer diamonds to expose the chips and bring the surface to the specified finish

Restoration

Re-grinding and re-polishing an existing terrazzo floor back to a new finish, the reason terrazzo lasts for generations

Related tools

Calculators and readiness checks for this work

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FAQ

People also ask

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