1 Scope
NOTE This standard governs the materials and installation of poured-in-place terrazzo flooring and base — the epoxy (resin matrix) thin-set system and the cementitious systems — over concrete substrates in commercial and institutional construction. (1.1)
NOTE Terrazzo is a composite finish: a matrix of resin or cement is placed, seeded with decorative aggregate, cured, and then ground and polished to expose the aggregate and produce a dense, monolithic, jointless wearing surface. (1.2)
NOTE It is specified where the owner wants a finish that lasts the life of the building, carries heavy foot traffic and rolling loads without wearing through, and can be cleaned and maintained indefinitely without recoating or replacement — lobbies, concourses, corridors, atria, museums, and transportation terminals are its natural home. (1.3)
NOTE A terrazzo floor is a system consisting of the structural slab, any crack-isolation or moisture-mitigation membrane, the matrix and its aggregate, the divider and control strips that define the field and accommodate movement, the integral or applied base, and the sealer. (1.4)
NOTE The single most important decision is the matrix system, because it determines the system thickness, the substrate it can be placed over, the moisture tolerance, the crack behavior, and the cost. (1.5)
NOTE Epoxy thin-set terrazzo, placed at a nominal 3/8 inch directly on a level interior slab, is the dominant modern system and is the default of this standard; the cementitious systems remain the correct choice for exterior work, for wet areas, for very large monolithic fields, and where structural movement demands a sand-cushion isolation layer. (1.6)
NOTE The defining characteristic of terrazzo is that its service life is measured in decades rather than years, which is why the quality of the substrate preparation and the crack-isolation detailing matters far more than for any replaceable floor covering: a crack telegraphed up from the slab into a terrazzo floor is permanent and cannot be patched invisibly. (1.7)
1.8The Contractor shall treat the floor as a system.
1.9The Contractor shall confirm that the matrix, aggregate, strips, membrane, and base selected are mutually compatible and approved by the matrix manufacturer for the measured substrate condition.
1.10The Contractor shall not begin placement until the substrate has passed the flatness and moisture acceptance criteria of this standard.
1.11The Contractor shall coordinate the structural slab, its control-joint layout, its surface finish, and its under-slab vapor retarder with Cast In Place Concrete so that the slab control joints and construction joints are located and detailed before terrazzo work begins and crack-isolation membrane or divider strips can be placed directly over them. 1.12The Contractor shall coordinate transitions to adjacent finishes with Resilient Flooring and Ceramic Tile so that thresholds and finish-floor elevations reconcile. NOTE Terrazzo crack control is governed by what is underneath, not by the terrazzo itself; epoxy terrazzo is a thin rigid veneer fused to the slab, and any crack that opens in the slab will reflect through the terrazzo unless it is isolated. (1.13)
2 Referenced Standards
| Standard |
Title |
| ASTM C307 |
Standard Test Method for Tensile Strength of Chemical-Resistant Mortar, Grouts, and Monolithic Surfacings |
| ASTM C579 |
Standard Test Methods for Compressive Strength of Chemical-Resistant Mortars, Grouts, Monolithic Surfacings, and Polymer Concretes |
| ASTM C580 |
Standard Test Method for Flexural Strength and Modulus of Elasticity of Chemical-Resistant Mortars, Grouts, Monolithic Surfacings, and Polymer Concretes |
| ASTM C413 |
Standard Test Method for Absorption of Chemical-Resistant Mortars, Grouts, Monolithic Surfacings, and Polymer Concretes |
| ASTM C1353 |
Standard Test Method for Abrasion Resistance of Dimension Stone Subjected to Foot Traffic Using a Rotary Platform, Double-Head Abraser |
| ASTM D4541 |
Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers |
| ASTM E84 |
Standard Test Method for Surface Burning Characteristics of Building Materials |
| ASTM C97 |
Standard Test Methods for Absorption and Bulk Specific Gravity of Dimension Stone (marble aggregate) |
| ASTM C503 |
Standard Specification for Marble Dimension Stone |
| ANSI A326.3 |
Test Method for Measuring Dynamic Coefficient of Friction of Hard Surface Flooring Materials |
| ASTM F1869 |
Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride |
| ASTM F2170 |
Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes |
| ASTM E1745 |
Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs |
| ASTM C150 |
Standard Specification for Portland Cement (cementitious matrix systems) |
| NTMA |
National Terrazzo and Mosaic Association technical data, system definitions, and details |
| IBC |
International Building Code (current edition adopted by jurisdiction) |
2.1All materials, testing, and installation shall comply with the latest edition adopted by the Authority Having Jurisdiction for each of the foregoing standards.
2.2Where the contract documents, a referenced standard, or the matrix manufacturer's written instructions impose a more stringent requirement than the minimum of any other standard, the more stringent requirement governs unless the Architect of Record directs otherwise in writing.
2.3The Contractor shall follow both the NTMA technical data and details and the matrix manufacturer's written instructions in addition to this standard.
NOTE For terrazzo, the NTMA technical data, system definitions, and details are the recognized industry reference, and the matrix manufacturer's written instructions define the conditions under which the matrix warranty is valid. (2.4)
2.5ASTM C1028, formerly cited for terrazzo slip resistance, has been withdrawn and shall not be specified for acceptance; dynamic coefficient of friction is now measured per ANSI A326.3.
NOTE The chemical-resistant mortar test methods (ASTM C307, C579, C580, C413) are the methods by which epoxy terrazzo matrix physical properties are reported, because a cured epoxy terrazzo matrix is a polymer concrete in the meaning of those standards. (2.6)
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Architect's review prior to procurement and installation:
- Product data for the matrix system, identifying the system type, nominal thickness, mix design, and the matrix manufacturer's written installation instructions and physical-property data
- Product data for the aggregate, identifying type (marble, granite, glass, recycled, or mother-of-pearl), source, and gradation by NTMA size number
- Product data for divider strips, control-joint strips, and base strips, identifying material, gauge, profile (L-type or T-type), and depth
- Product data for the crack-isolation / membrane system, primer, grout, and sealer
- Samples of the cured terrazzo, not less than 6 by 6 inches, ground and polished and sealed, for each color and aggregate blend, showing the actual aggregate, matrix color, and finish
- Samples of each divider, control, and base strip in the specified material and finish
- Moisture and relative-humidity test reports for the actual slab, conducted in accordance with ASTM F2170 (relative humidity) and, where used, ASTM F1869 (moisture vapor emission rate), identifying test locations and ambient conditions
- Shop drawings showing the divider-strip and control-joint layout, the location of all slab control and construction joints to be isolated, aggregate-blend zones, patterns, logos, and base details, coordinated with the finish plan and architectural details
- Maintenance instructions describing initial cleaning, sealing, and the recommended periodic maintenance program
☐ Matrix system product data and physical-property data
☐ Aggregate product data (type, source, gradation)
☐ Divider, control, and base strip product data
☐ Membrane, primer, grout, and sealer product data
☐ Cured terrazzo samples (each color/blend, ground and sealed)
☐ Strip samples (each material and finish)
☐ Slab moisture and RH test reports (F2170 / F1869)
☐ Strip-layout and pattern shop drawings
☐ Maintenance instructions
3.1.2Installation shall not begin until the moisture-test reports have been submitted and reviewed.
NOTE The moisture condition of the slab determines both the matrix compatibility and the mitigation requirement, and a moisture-related epoxy terrazzo failure is a full-depth replacement, not a repair. (3.1.3)
3.2 Closeout Submittals
NOTE Provide the following at project closeout: (3.2.1)
- Matrix manufacturer and installer warranty documentation, executed in the Owner's name
- Record of the final slab moisture and RH test results, the crack-isolation method installed, and the matrix and sealer used, retained for warranty purposes
- Attic-stock transmittal documenting the quantity, matrix color, and aggregate blend of spare material delivered to the Owner
☐ Matrix manufacturer and installer warranty documentation (executed in Owner's name)
☐ Record of final slab moisture/RH results, crack-isolation method, and matrix and sealer used
☐ Attic-stock transmittal (quantity, matrix color, aggregate blend)
4 Quality Assurance
4.1 Installer Qualifications
○ NTMA member contractor
○ Five years documented commercial terrazzo experience in the specified system
○ Either NTMA member or five years documented experience
4.1.1Terrazzo shall be installed by a contractor in good standing with the National Terrazzo and Mosaic Association, or by an installer with documented experience completing not fewer than five commercial terrazzo installations of the system type and scale required within the preceding five years.
4.1.2The installer shall employ mechanics experienced in the placement, grinding, and finishing of the specified matrix system.
4.1.3The Contractor shall not assign terrazzo placement or finishing to labor inexperienced in the specified system.
NOTE Terrazzo is a craft trade in which the visible result depends almost entirely on workmanship — aggregate seeding density, grinding depth, grout filling of pinholes, and polishing all determine the appearance of a finish that the owner will live with for decades. (4.1.4)
4.2 Mock-Up
○ Yes — install a representative field area including a divider strip, a control-joint detail, and base
○ No
4.2.1Where a mock-up is required, the Contractor shall install a representative field area of each terrazzo type at a location and size directed by the Architect, fully ground, grouted, polished, and sealed, and including at least one divider strip, one control-joint detail, one aggregate-blend transition where blends change, and the integral or applied base.
4.2.2The mock-up shall establish the acceptable standard for aggregate distribution and density, matrix color, grinding and polishing quality, strip alignment, and base detailing, and shall remain available for comparison throughout the work.
NOTE Because aggregate seeding density and grinding depth are judgment calls that vary between crews, the mock-up is the only reliable way to fix the acceptable appearance before the full floor is committed. (4.2.3)
4.3 Pre-Installation Conference
4.3.1Before installation begins, the Contractor shall hold a pre-installation conference with the Architect, the terrazzo installer, and the concrete contractor to review the slab moisture-test results, the slab flatness and control-joint locations, the crack-isolation requirement, the divider-strip and pattern layout, the aggregate blends, the matrix and sealer selection, and the environmental conditions.
NOTE The most common cause of an epoxy terrazzo crack is a slab control or construction joint that was not honored with a divider strip or membrane directly above it; the conference exists to reconcile the slab joint layout with the terrazzo layout before any matrix is placed. (4.3.2)
5 Environmental and Service Conditions
5.1 Temperature During Installation
5.1.1The installation area shall be enclosed and maintained at a minimum of 60 °F (16 °C) and a maximum of 90 °F (32 °C) for 48 hours before, during, and for not less than 72 hours after placement of an epoxy matrix, and within the range the matrix manufacturer specifies.
5.1.2The permanent HVAC system or equivalent temporary conditioning shall maintain the space.
5.1.3The slab surface temperature, not only the air temperature, shall be within the manufacturer's range, because a cold slab cools the matrix at the bond line regardless of air temperature.
NOTE Epoxy matrix cure is temperature-dependent: below the minimum, the resin will not cure to its specified physical properties and remains soft and grindable only with difficulty; above the maximum, pot life shortens and the placed matrix can flash-cure before it is properly leveled and seeded. (5.1.4)
5.2 Slab Moisture and Relative Humidity Limits
5.2.1The acceptable slab moisture condition shall be established by test before placement and confirmed against both the matrix manufacturer's limit and the limit of this standard.
5.2.2Internal relative humidity measured by in-situ probe per ASTM F2170 shall not exceed the limit stated, and the lower of the manufacturer's limit and the project limit governs.
5.2.3An epoxy primer or vapor-mitigation coating rated for the measured relative humidity shall be installed where the slab exceeds the matrix limit.
NOTE Epoxy terrazzo is more moisture-tolerant than most resilient flooring, and many epoxy terrazzo matrix systems are warranted to a maximum internal relative humidity of 85 to 90 percent; internal relative humidity per F2170 is a more reliable predictor of long-term moisture behavior than a surface emission measurement because it reflects the moisture distributed through the slab thickness. (5.2.4)
5.3 Slab Moisture Vapor Emission Limit
35
Default: 5 lb/1000 sq ft/24 hr
5.3.1Where moisture vapor emission rate is used as a screening or supplementary measure per ASTM F1869, the rate shall not exceed the matrix manufacturer's limit, commonly 3 to 5 pounds per 1,000 square feet per 24 hours for epoxy terrazzo.
5.3.2The calcium chloride method shall not be used as the sole acceptance criterion for slabs on grade or below grade — F2170 internal relative humidity is required for those conditions.
NOTE The calcium chloride method measures only the surface condition, which can read deceptively low while the slab interior remains wet, and moisture trapped beneath a cured epoxy veneer drives osmotic blistering and bond loss. (5.3.3)
5.4 Under-Slab Vapor Retarder
○ ASTM E1745 Class A vapor retarder beneath slab (required for slab on grade)
○ Not applicable — slab above grade / suspended
NOTE The under-slab vapor retarder is the single most effective moisture-control measure for slab-on-grade terrazzo and is placed long before terrazzo work begins; its presence or absence is verified during slab review, not at the time of terrazzo placement, but it governs whether a vapor-mitigation coating will later be required. (5.4.2)
5.5 Lighting for Inspection
5.5.1Permanent or equivalent temporary lighting shall be operating during finishing and inspection so that aggregate distribution, grinding uniformity, residual pinholes, scratch patterns, and strip alignment can be evaluated under realistic conditions.
NOTE The floor shall be inspected under the lighting in which it will be viewed in service, because terrazzo is frequently accepted under raking construction lighting that either conceals or exaggerates surface defects. (5.5.2)
6 Terrazzo System
6.1 Matrix System
Epoxy (resin matrix) thin-set — nominal 3/8 in over level interior slab
Polyacrylate (polymer-modified cement) — nominal 3/8 in over level interior slab
Monolithic cement — 1/2 in cement matrix bonded to interior slab
Bonded cement — 1/2 in topping over a bonded underbed (1-3/4 to 2-1/4 in total)
Sand-cushion cement — 1/2 in topping over reinforced underbed, isolation membrane, and sand layer (2-1/2 to 3 in total)
6.1.1The matrix system is the primary selection and determines the system thickness, the substrate it may be placed over, the moisture and crack behavior, the interior-versus-exterior suitability, and the cost.
6.1.2The matrix system shall be selected for the substrate condition, the field size, the exposure, and the design intent, and shall be indicated on the finish schedule. NOTE Epoxy thin-set terrazzo, a nominal 3/8-inch resin matrix veneer placed directly on a level interior slab, is the default of this standard because it is the thinnest, lightest, and most chemically resistant system, offers the widest color range, cures quickly, and accepts dense aggregate seeding; it is an interior, above-grade-or-on-grade system only and is not used for exterior or continuously wet areas. (6.1.3)
NOTE Polyacrylate terrazzo is a polymer-modified cementitious veneer of similar nominal thickness used where a cement matrix is preferred but slab depth is limited; the polymer permits the thin section and improves moisture-vapor tolerance over a plain cement topping. (6.1.4)
NOTE Monolithic terrazzo is a 1/2-inch cement matrix bonded directly to the slab and depends entirely on the quality and crack-control of that slab for its own crack behavior. (6.1.5)
NOTE Bonded terrazzo adds a bonded sand-cement underbed to build up a recessed slab and to provide a controlled bonding bed, and is suited to interior and protected exterior work. (6.1.6)
NOTE Sand-cushion terrazzo — a 1/2-inch topping over a wire-reinforced underbed placed on an isolation membrane over a sand dusting layer — is the most movement-tolerant system because the sand-and-membrane layer mechanically isolates the terrazzo from the structure, which makes it the choice for buildings with anticipated structural movement and for the largest jointless fields, at the cost of the greatest thickness and dead load. (6.1.7)
6.2 System Thickness
1/4 in — epoxy thin-set, fine aggregate only
3/8 in — epoxy thin-set, standard
1/2 in — monolithic / polyacrylate topping
1-3/4 to 2-1/4 in — bonded system total
2-1/2 to 3 in — sand-cushion system total
6.2.1Epoxy terrazzo shall be placed at a nominal 3/8 inch, which accommodates aggregate up to NTMA size No. 2; a 1/4-inch placement shall be used only where the recess depth is limited and the aggregate is restricted to the finer sizes.
6.2.2The matrix manufacturer's minimum thickness for the selected aggregate gradation governs.
6.2.3Cementitious system thicknesses are dictated by the system definition and the available slab recess.
NOTE There must be sufficient matrix depth above the slab to embed and lock the largest aggregate; placing too thin for the aggregate size leaves chips that loosen and pop out during grinding and service. (6.2.4)
6.3 Aggregate Type
Marble chips
Granite chips
Recycled glass
Marble and recycled-glass blend
Mother-of-pearl / specialty (accent only)
6.3.1All aggregate shall be the calcareous or siliceous stone or glass capable of being ground and taking a polish appropriate to the matrix.
6.3.2Certain glass colors are sensitive to the high alkalinity of cement matrices and shall be confirmed compatible with a cementitious matrix before use.
NOTE Marble chip is the traditional and most common terrazzo aggregate; it grinds and polishes readily to a high luster and is the basis of the widest range of standard colors, which makes it the default. (6.3.3)
NOTE Granite and other hard silica aggregates are harder to grind but are specified for added abrasion resistance and a distinct appearance, and are common in epoxy systems where the resin matrix tolerates the harder grind. (6.3.4)
NOTE Recycled glass — including post-consumer and post-industrial glass — is widely used both for appearance and for recycled-content credit in green-building programs, and is fully compatible with the epoxy matrix. (6.3.5)
NOTE Mother-of-pearl and other specialty aggregates are typically used as accents within a field of marble or glass rather than as the full blend. (6.3.6)
6.4 Aggregate Gradation
No. 0 (1/8 in and finer)
No. 1 (1/8 to 1/4 in)
No. 2 (1/4 to 3/8 in)
Blend of No. 1 and No. 2 (standard)
Blend of No. 0 through No. 2
6.4.1The maximum aggregate size shall not exceed the limit the matrix thickness can embed — a No. 2 chip requires the full 3/8-inch epoxy section.
6.4.2Larger decorative aggregate, where used in palladiana or feature work, shall be set and detailed individually and is outside the standard gradation.
NOTE Terrazzo aggregate is graded by NTMA size number from No. 0 (the finest, 1/8 inch and smaller) through No. 2 (3/8 inch); the conventional standard terrazzo appearance uses a blend of No. 1 and No. 2 chips, while epoxy terrazzo commonly uses blends spanning No. 0 through No. 2 to achieve a dense, fully-seeded surface with minimal visible matrix between chips. (6.4.3)
6.5 Matrix Color
NOTE Epoxy matrices accept a far wider and more saturated color range than cement matrices, including deep and bright colors that cement cannot achieve; this color flexibility is one of the principal reasons epoxy terrazzo dominates contemporary design work. (6.5.2)
7 Resin Matrix Physical Properties (Epoxy System)
7.1 Compressive Strength
800012000
10000
Default: 10000 psi
7.1.1The cured epoxy terrazzo matrix shall develop a compressive strength of not less than the value stated when tested per ASTM C579.
NOTE A compressive strength on the order of 10,000 psi is characteristic of a properly formulated epoxy terrazzo matrix and is what allows a 3/8-inch veneer to carry the rolling loads and point loads of a high-traffic public floor without crushing or indenting. (7.1.2)
7.2 Tensile Strength
15003000
2500
Default: 2500 psi
7.2.1The cured matrix shall develop a tensile strength of not less than the value stated when tested per ASTM C307.
NOTE Tensile strength governs the matrix's resistance to cracking when the slab beneath it moves or shrinks; a higher tensile strength lets the thin veneer bridge minor substrate movement that would crack a brittle cement topping, but it does not substitute for crack isolation over an active slab joint. (7.2.2)
7.3 Flexural Strength
25004000
3500
Default: 3500 psi
7.3.1The cured matrix shall develop a flexural strength of not less than the value stated when tested per ASTM C580.
NOTE Flexural strength reflects the matrix's ability to span and resist bending stresses, which matters where the floor bridges minor substrate irregularities and at the edges of divider strips. (7.3.2)
7.4 Water Absorption
7.4.1The cured matrix shall exhibit water absorption not greater than the value stated when tested per ASTM C413.
NOTE Low absorption — typically below 0.5 percent for epoxy terrazzo — is what makes the floor effectively non-porous, stain-resistant, and cleanable, and is a principal advantage of the resin matrix over a cementitious matrix, which is comparatively porous and requires more diligent sealing. (7.4.2)
7.5 Bond Strength
200400
200300
Default: 200 psi
7.5.1The cured matrix system, including its primer, shall develop a pull-off bond strength to the prepared slab of not less than the value stated when tested per ASTM D4541, or shall fail in the concrete substrate rather than at the bond line.
NOTE Bond is the foundation of a thin-set system: an epoxy terrazzo veneer that debonds becomes a drum-hollow, cracking shell, and adequate bond depends on correct surface preparation, a clean dry slab, and the manufacturer's primer applied at the specified rate. (7.5.2)
7.6 Abrasion Resistance
○ Required — report Taber abrasion resistance for the matrix and aggregate blend
○ Not separately required
7.6.1Where abrasion resistance is a governing requirement — in transportation, retail, and other very-high-traffic floors — the terrazzo abrasion resistance shall be reported per ASTM C1353.
NOTE Terrazzo is among the most abrasion-resistant of all floor finishes, and for most projects the system selection alone assures adequate wear life; explicit abrasion testing is reserved for the highest-traffic applications or where a hard aggregate such as granite is being substituted for marble. (7.6.2)
7.7 Surface Burning Characteristics
○ Class A — flame spread index 0-25, smoke developed index 0-450
○ Not separately regulated at this location
7.7.1Where the building code regulates the surface burning characteristics of the finish, the terrazzo system shall be tested per ASTM E84 and shall achieve a Class A rating (flame spread index not greater than 25 and smoke developed index not greater than 450).
7.7.2The product data shall document the tested indices.
NOTE Properly formulated epoxy terrazzo is inherently Class A because the cured matrix is highly filled with mineral aggregate and contributes little fuel. (7.7.3)
7.8 Slip Resistance
0.420.55
0.420.55
Default: 0.42 DCOF
7.8.1The dynamic coefficient of friction shall be measured per ANSI A326.3 and shall meet the minimum for the applicable use classification — not less than 0.42 for level interior spaces expected to be walked on when wet.
7.8.2Where slip resistance is a concern the finish may be honed rather than polished to a high gloss, or an abrasive aggregate or anti-slip additive may be incorporated at entries, ramps, and wet areas.
7.8.3The specified value applies to the floor as installed and as maintained per the manufacturer's program.
NOTE A polished terrazzo surface can be slick when wet, and slip resistance is a function of both the finish and its maintained sealer. (7.8.4)
8 Strips and Joints
8.1 Divider Strips
White zinc alloy
Brass
Aluminum
Plastic
○ L-type (single angle, standard for thin-set epoxy)
○ T-type (for thicker sections and double-pour cement systems)
NOTE Divider strips define the field of the terrazzo, separate aggregate blends and colors, form patterns and logos, and provide a screed line that controls matrix thickness and flatness during placement. (8.1.1)
8.1.2The strip layout, including patterns and aggregate-blend boundaries, shall be as shown on the terrazzo layout plan. 8.1.3Zinc and aluminum strips shall not be placed in continuous wet exposure where they would corrode.
8.1.4Divider strip gauge is specified in Brown & Sharpe (B&S) gauge, and the strip depth shall match the nominal system thickness so that the top of the strip finishes flush with the ground terrazzo surface.
8.1.5Epoxy terrazzo may also be poured as a seamless free-form field with no divider strips where the design and the slab crack-control permit.
NOTE For epoxy thin-set terrazzo the most common divider strip is an L-type strip in white zinc alloy, which is corrosion-resistant, takes the grind cleanly, and leaves a neutral line; brass is specified where a warmer metallic line is desired, aluminum where a bright line and lower cost are acceptable, and plastic where a colored line matched to the matrix is desired or where dissimilar-metal corrosion must be avoided. (8.1.6)
NOTE 16 gauge is the standard for thin-set epoxy terrazzo, with 14 gauge used for heavier or wider strips and 18 gauge for the finest lines. (8.1.7)
8.2 Control-Joint Strips
○ Divider strip directly over every slab control and construction joint
○ Flexible epoxy crack-isolation membrane over slab joints in lieu of strips
○ Both — membrane plus aligned divider strip
8.2.1Every slab control joint and construction joint shall be honored in the terrazzo above it, because the joint will move and any movement that is not relieved at the surface will crack the terrazzo.
8.2.2The standard treatment is a divider strip placed directly over and aligned with each slab joint, so that the joint in the finish coincides with the joint in the slab.
8.2.3As an alternative or supplement, a flexible epoxy crack-isolation membrane may be applied over the slab joints and known cracks beneath the terrazzo to absorb movement; this is required where the design calls for a jointless field across a slab that is jointed.
8.2.4The Contractor shall not place terrazzo across a slab control joint without either an aligned divider strip or a crack-isolation membrane.
8.3 Expansion Joints
○ Carry building expansion joints full-depth through the terrazzo with a compressible joint and cover
○ No building expansion joints within the terrazzo area
8.3.1Building expansion joints shall be carried full-depth through the terrazzo system and finished with a compressible joint filler and an appropriate joint cover.
8.3.2Terrazzo, divider strips, and matrix shall never bridge a building expansion joint, because the differential movement at an expansion joint is far greater than any surface strip or membrane can absorb.
9 Crack Isolation and Substrate Preparation
9.1 Slab Preparation
9.1.1The slab shall be prepared in accordance with the matrix manufacturer's instructions and NTMA recommendations: structurally sound, fully cured, clean, and free of dust, paint, oil, grease, curing and sealing compounds, laitance, and any other bond-inhibiting substance.
9.1.2The standard preparation for epoxy terrazzo shall be mechanical abrasion — shot-blasting or diamond grinding — to remove surface contaminants and laitance and to open the surface to a profile that the primer can key into.
9.1.3Acid etching shall not be substituted for mechanical preparation for epoxy terrazzo, because etching leaves residue and does not reliably remove curing compounds.
9.1.4The condition of the slab is the Contractor's responsibility to verify before placement.
NOTE Placing terrazzo over a noncompliant slab transfers a known defect into a finish that cannot later be lifted and corrected without full removal. (9.1.5)
9.2 Flatness
○ 1/4 in in 10 ft (standard for thin-set epoxy)
○ 1/8 in in 10 ft (where a tighter finish is required)
9.2.1The slab to receive epoxy or other thin-set terrazzo shall be flat within a maximum variation of 1/4 inch in 10 feet and finished with a steel trowel.
9.2.2High spots shall be ground down and low spots filled with a patching compound compatible with the matrix and primer.
9.2.3Where the slab cannot be brought into tolerance, a cementitious self-leveling underlayment approved by the matrix manufacturer shall be installed to establish the required plane before priming.
NOTE A thin-set system follows the slab and cannot correct slab unflatness — the terrazzo thickness is nearly constant, so a wavy slab produces a wavy floor. (9.2.4)
9.3 Crack-Isolation Membrane
○ Over slab joints and observed cracks only
○ Full-coverage crack-isolation / anti-fracture membrane over entire field
○ None — divider strips aligned to all slab joints
9.3.1A flexible epoxy crack-isolation (anti-fracture) membrane shall be applied over slab control joints, construction joints, and observed cracks before the terrazzo is placed, to absorb the small in-plane movement at those locations and prevent it from reflecting into the rigid terrazzo veneer.
9.3.2Full-coverage membrane over the entire field shall be specified where the slab is of uncertain crack history, where a continuous jointless appearance is required across a jointed slab, or where the matrix manufacturer requires it for the measured slab condition.
9.3.3Membrane shall be rated and installed per its manufacturer's instructions and shall be compatible with the matrix primer.
NOTE Full coverage adds cost but is the most reliable defense against reflective cracking. (9.3.4)
9.4 Moisture-Vapor Mitigation
○ None required — slab passes F2170/F1869 within matrix limits
○ Epoxy vapor-mitigation coating rated for measured slab RH
9.4.1Where the measured slab relative humidity or moisture vapor emission rate exceeds the matrix system's limit, an epoxy vapor-mitigation coating shall be applied to the prepared slab before priming and placement, rated by its manufacturer for the relative humidity actually measured at the slab.
9.4.2The mitigation coating shall be selected to the measured condition, not a generic value, and is part of the bonded system below the primer.
NOTE A coating rated to a lower relative humidity than the slab exhibits will blister and debond, taking the terrazzo with it. (9.4.3)
10 Installation
10.1 Layout
10.1.1The Contractor shall establish the divider-strip layout, pattern, logo, and aggregate-blend boundaries from the control lines and details shown on the terrazzo layout plan, coordinated with the slab joint locations so that control-joint strips align with slab joints and field strips produce a balanced pattern. 10.1.2Strips shall be set and secured before matrix placement and shall be dry-fit and approved where the pattern is intricate.
NOTE A terrazzo floor laid without a coordinated layout produces strips that miss the slab joints, unbalanced borders, and patterns that fail to align through openings. (10.1.3)
10.2 Matrix Placement and Seeding
10.2.1The matrix shall be mixed in the proportions and with the aggregate loading the manufacturer specifies, placed between the set strips, screeded to the top of the strips, and seeded with additional broadcast aggregate as required to achieve the specified aggregate density and distribution before the matrix gels.
10.2.2The Contractor shall mix only the quantity that can be placed, seeded, and rolled within the working time, and shall not retemper gelled material.
10.2.3The seeding rate shall match the approved sample and mock-up.
NOTE Epoxy matrix has a limited pot life and working time that shorten as temperature rises, and aggregate density at the surface is what gives terrazzo its characteristic appearance — under-seeding leaves excessive visible matrix between chips. (10.2.4)
10.3 Curing
10.3.1The placed matrix shall cure undisturbed for the period and at the conditions the manufacturer specifies before grinding begins.
10.3.2The Contractor shall confirm full cure by the manufacturer's method before the first grind.
NOTE Grinding into an under-cured epoxy matrix tears and smears rather than cutting, dislodges aggregate, and produces a defective surface. (10.3.3)
10.4 Grinding
○ Wet grinding
○ Dry grinding with dust collection
10.4.1After cure, the terrazzo shall be ground to expose the aggregate and produce a flat, uniform surface, beginning with a coarse abrasive to cut down to the aggregate and progressing through successively finer abrasives.
10.4.2The first grind shall remove enough matrix to expose the aggregate uniformly across the full field, including tight to strips and into corners, where a hand or edge grinder shall be used so that the edges match the field.
NOTE Wet grinding is the traditional method and controls dust and heat; dry grinding with continuous high-efficiency dust collection is used where water cannot be introduced into the space. (10.4.3)
10.5 Grouting
10.5.1After the initial grind, the surface shall be grouted with a matrix-compatible grout worked into the pinholes and voids opened by grinding, then cured, to produce a void-free surface.
10.5.2Cement-matrix systems shall be grouted with a cementitious grout; epoxy systems shall be grouted with a compatible epoxy grout.
NOTE Grinding inevitably opens small voids and pinholes at the aggregate-matrix interface, and grouting fills them so that the polished surface is dense and continuous rather than pitted. (10.5.3)
10.6 Polishing
○ Polished (high gloss)
○ Honed (satin, lower gloss for slip resistance)
10.6.1After grouting and cure, the floor shall be polished through progressively finer grits to the specified finish.
10.6.2At ramps, entries, and wet areas the finish should be honed or treated for slip resistance even where the field is polished.
10.6.3The final finish shall match the approved mock-up.
NOTE A high-gloss polished finish is the conventional terrazzo appearance and is the default; a honed satin finish is specified where a lower-gloss appearance or improved wet slip resistance is required. (10.6.4)
10.7 Sealing
10.7.1The finished terrazzo shall be cleaned and sealed with a sealer recommended by the matrix manufacturer for the system and finish, applied after the floor is fully cured and clean.
10.7.2The sealer shall be compatible with the matrix and shall not reduce the dynamic coefficient of friction below the specified slip-resistance value.
NOTE The sealer is what resists staining and eases maintenance; cementitious terrazzo, being more porous, depends more heavily on sealing than epoxy terrazzo, but both shall be sealed. (10.7.3)
11 Base and Stair Treads
11.1 Integral and Topset Base
Integral cove base (terrazzo coved up from floor)
Topset terrazzo base (precast or cast against wall)
Precast terrazzo base units
No terrazzo base
11.1.1Base height, profile, and cove radius shall be as shown on the base details. NOTE Where a terrazzo base is specified, an integral cove base — the floor terrazzo coved up the wall over a cove former and ground continuous with the floor — provides the most durable and cleanable wall-to-floor junction with no joint for dirt to collect, and is the default where the wall construction permits. (11.1.2)
NOTE A topset base is a separate terrazzo base set against the finished wall where an integral cove is not practical. (11.1.3)
NOTE Precast terrazzo base units are shop-cast, ground, and polished and set in place, and are used where field placement is impractical or where a crisp factory edge is required. (11.1.4)
11.2 Precast Terrazzo Treads and Risers
☐ Treads
☐ Risers
☐ Stringers / skirts
☐ Landings
☐ Abrasive nosing strips at treads
11.2.1Where terrazzo stairs are specified, treads, risers, stringers, and landings shall be precast, ground, and polished in the shop using the specified matrix and aggregate to match the field, and set in place.
11.2.2Treads shall incorporate an abrasive nosing strip or an abrasive aggregate band at the leading edge for slip resistance and code-compliant visual contrast.
11.2.3Precast component dimensions, profiles, and the nosing detail shall follow the stair details and comply with the applicable accessibility and stair-geometry requirements of the building code. NOTE A polished terrazzo stair nosing is otherwise slick and hazardous. (11.2.4)
12 Field Testing
12.1 Moisture Test Verification
12.1.1The Contractor shall not place terrazzo until the documented ASTM F2170 relative-humidity results, and ASTM F1869 emission results where used, confirm the slab is within the matrix manufacturer's governing limit, or until the specified vapor-mitigation coating has been installed and confirmed for the measured relative humidity.
12.1.2The moisture-test record is the documentary basis for the matrix warranty and shall be retained in the closeout submittals.
12.2 Bond and Surface Inspection
○ Yes — full-floor inspection under permanent lighting for bond, cracks, and finish
○ No
12.2.1After finishing and sealing, the floor shall be inspected under permanent or equivalent lighting for full bond (no drummy or hollow areas, verified by sounding), uniform aggregate exposure and density, absence of cracks and pinholes, flush and aligned strips, uniform color and finish, and a continuous base and tread detail.
12.2.2Drummy areas indicate a bond failure and shall be removed and replaced.
12.2.3Pinholes indicate incomplete grouting and shall be re-grouted and re-polished.
12.2.4Where bond strength is in question, a pull-off test per ASTM D4541 may be performed.
13 Cleaning and Protection
13.1After sealing, the floor shall be cleaned of all construction residue by the method the matrix manufacturer specifies and protected from traffic and other trades until the project is turned over, using a breathable protective covering that does not trap moisture against the floor or transfer color.
13.2Non-breathable sheeting taped directly to a freshly sealed floor shall not be used, because it can trap moisture and discolor the sealer.
13.3Staining from spilled construction materials and scratching from dragged equipment are the common construction-phase damages and shall be prevented by protection rather than corrected by re-grinding.
NOTE Terrazzo is durable but is finished early enough in many projects that it is exposed to heavy trade traffic. (13.4)
14 Delivery, Storage, and Handling
14.1Matrix components, aggregate, strips, membrane, and sealer shall be delivered in the manufacturer's original unopened packaging with labels intact, identifying product, color, and batch.
14.2Material that has frozen, exceeded its shelf life, or been stored outside its temperature range shall be discarded.
14.3Aggregate shall be kept dry and clean, because moisture and contamination on the aggregate interfere with matrix wetting and bond.
14.4All aggregate for a continuous area shall be from the same source and batch wherever possible, because natural aggregate color and pattern vary between batches and a batch change within a single visual field will be apparent.
NOTE Epoxy matrix components have a limited shelf life and a storage temperature range below and above which they are damaged. (14.5)
15 Warranty
1 year from substantial completion
2 years from substantial completion
15.1The Contractor shall warrant the terrazzo installation — including substrate preparation, crack isolation, bond, matrix placement and finishing, strips, base, and sealing — against defective workmanship for the period stated.
15.2The matrix manufacturer shall warrant the matrix system materials against defects.
15.3The Contractor shall retain and deliver the moisture-test record as part of the warranty basis, because most matrix warranties are void unless the slab moisture and relative-humidity conditions were within the system's stated limits and documented at the time of placement.
NOTE Reflective cracks arising from slab control or construction joints that were not isolated, and failures arising from subsequent structural movement, water intrusion, or maintenance contrary to the manufacturer's instructions, are evaluated against the as-built crack-isolation detailing. (15.4)
16 Spare and Extra Materials
16.1The Contractor shall deliver to the Owner spare matrix and aggregate of each color and blend installed, in the percentage of installed area stated, in unopened original packaging labeled with the matrix color and aggregate blend and batch.
16.2Spare material shall be stored by the Owner in the conditioned environment the manufacturer recommends.
NOTE Because terrazzo is repaired by grinding out and re-pouring a patch with matching matrix and aggregate, attic stock of the original materials allows a future repair to be blended into the floor; a later-purchased replacement aggregate from a different batch will not match the surrounding field. (16.3)