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Choose a concrete joint sealant by the movement the joint must accommodate and its finished geometry, then check orientation, exposure, traffic, temperature, substrate compatibility and the project specification. The sealant’s ASTM C920 class must meet or exceed the joint’s calculated movement; chemistry alone is not a selection method.
For many painted, above-grade concrete and masonry joints, a polyurethane such as Sikaflex® + Construction Sealant is a practical starting point. Neutral-cure silicone is better where very high movement, ultraviolet exposure or temperature cycling dominates and paintability is not required. Polysulfide systems are candidates for submerged, intermittently submerged, fuel or chemical service only when the exact product is approved for that environment. Correct backer-rod sizing and sealant depth are part of the joint design, not cosmetic finishing steps.
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1. Calculate movement before choosing a sealant
An expansion joint opens and closes as temperature, drying shrinkage and structural movement change. Select the sealant only after the design team has estimated the largest expected opening and closing. A product rated for less movement than the joint requires is not an acceptable selection, even if its label says it is intended for concrete.
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- Thermal movement: Long slabs, walls and pavements move as their temperature changes. The calculation normally uses the member length, the material’s thermal expansion coefficient and the design temperature range.
- Drying shrinkage and moisture movement: New concrete can continue to shrink after placement; adjacent masonry and concrete may move at different rates.
- Structural movement: Seismic drift, frame deflection, settlement, vibration and repeated loading can add movement that is not visible in a simple temperature calculation.
- Construction tolerances: The installed joint may be narrower, wider or offset from the nominal drawing dimension. The movement calculation should use the as-built geometry where it is known.
Have the engineer of record or the project specification establish the design movement where structural or seismic movement is possible. For a simple thermal check, the change in length can be expressed as ΔL = α × L × ΔT, but that result is only one part of the design. Add the other credible movements and compare both opening and closing directions with the sealant rating.
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Read the ASTM C920 designation as a set
ASTM C920 information should be read as type, grade, class and use designations rather than as a single marketing number. The class identifies the movement capability stated by the manufacturer or specification; the grade tells you how the material behaves during placement.
| Designation or requirement | What it controls | Selection implication |
|---|---|---|
| Class 25 | Movement capability of 25 percent, as stated for the tested sealant | Use only when the calculated joint movement and the project specification are within that capability. Idaho Transportation Department’s exterior, non-traffic joint provision accepts silicone, polyurethane or polysulfide meeting ASTM C920 Grade NS, Class 25. |
| Class 35 | Movement capability of 35 percent | A documented example is Sikaflex® + Construction Sealant, ASTM C920 Type S, Grade NS, Class 35, with ±35 percent movement capability (Sika Product Data Sheet, September 2024). |
| Class 100/50 | A high-movement designation stated by the applicable product or specification | Do not infer suitability from the number alone; verify the product’s tested direction of movement, substrate, joint geometry and exposure approvals. |
| Type S | Single-component sealant classification | Confirm curing method, packaging, shelf life and installation conditions in the current technical data sheet. |
| Grade NS | Non-sag behavior | Use for vertical or overhead joints where a pourable material would run. |
| Grade P | Pourable behavior for suitable horizontal joints | Use a pourable or self-leveling product only where the joint is horizontal and the specification permits it. |
Class is not a substitute for joint design. A Class 100/50 sealant can still fail if it is installed too deep, bonded to three sides, applied to a contaminated substrate or exposed to a chemical that the manufacturer has not approved.
2. Size the joint and control its geometry
Width must accommodate movement
Joint width is established from expected movement, installation temperature and the sealant’s permitted movement percentage. A narrow joint with large movement demands a larger strain than a wider joint carrying the same absolute opening. Follow the project drawings and the product data sheet for minimum and maximum width; do not widen or narrow a joint arbitrarily in the field.
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Use a backer rod or approved bond breaker
Install a closed-cell polyethylene backer rod, or another backing system specifically approved for the sealant, to control depth and prevent three-sided adhesion. The rod should be continuous, clean and sized from the joint design rather than forced into place with gaps or twists. If the joint is too shallow for a rod, use the bond-breaker method required by the technical data sheet.
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- The backer rod creates the designed sealant profile so the material can stretch and compress instead of being restrained by excessive depth.
- It keeps sealant from adhering to the rear face of the joint, which creates three-sided stress and can cause cohesive or adhesive failure.
- It reduces material consumption and makes the finished bead easier to tool consistently.
- It must be compatible with the sealant, primers, coatings and adjacent materials; some solvents or uncured coatings can damage or soften backing materials.
Follow the product data sheet for the permitted sealant depth, width-to-depth relationship and minimum edge distance. Do not use a generic filler, newspaper, rope or absorbent insulation as a substitute for an approved backer rod.
3. Choose non-sag or self-leveling grade for the orientation
Vertical and overhead joints: Grade NS
Grade NS material stays in the joint after gunning and tooling. It is the normal choice for façade, wall, vertical slab and overhead work. A non-sag sealant also gives the installer time to tool a tooled concave profile without the bead slumping onto adjacent surfaces.
Horizontal joints: pourable or self-leveling grade
Horizontal pavement, floor and plaza joints may accept a pourable or self-leveling product when the specification allows it. These materials can fill a long joint quickly, but they require sound joint edges, controlled depth and temporary dams at corners or changes in elevation. Confirm that the product is rated for pedestrian or vehicle traffic if the joint will be loaded; a wall-grade non-sag sealant is not automatically traffic-rated.
Do not confuse flow with traffic performance
Self-leveling describes installation behavior, not resistance to tires, abrasion, fuel or impact. A horizontal joint in a parking deck, warehouse or loading area may require a traffic-rated formulation and a different preparation method. Check the manufacturer’s traffic, abrasion, cure and hardness information before selecting a pourable material.
4. Match the chemistry to exposure and performance
| Chemistry | Where it fits | Strengths | Important limits and checks |
|---|---|---|---|
| Polyurethane | General concrete and masonry movement joints, especially above grade | Commonly available, paintable when the product permits it, and resistant to ordinary abrasion. Sikaflex® + Construction Sealant is documented for concrete expansion joints as ASTM C920 Type S, Grade NS, Class 35, with ±35 percent movement capability (Sika Product Data Sheet, September 2024). | Verify movement class, cure conditions, primer requirements and compatibility with the intended coating. Some polyurethane products are unsuitable for continuous immersion or aggressive chemicals. |
| Neutral-cure silicone | Very high movement, ultraviolet exposure, weathering and severe temperature cycling | High elasticity and durable outdoor performance without the corrosive by-products associated with some other silicone cure systems. Sikasil®-728 NS is documented as a neutral-cure, non-sag silicone for dynamic concrete joints with ASTM C920 Class 100/50. | Most silicone sealants are not paintable with ordinary architectural coatings. Confirm adhesion to porous concrete, the need for primer, and resistance to the specific chemicals present. |
| Polysulfide | Submerged or intermittently submerged joints and selected fuel or chemical-service joints | Established industrial category for demanding liquid exposure when the exact formulation is approved for that service. | Approval must be product-specific. HRSD specifies polysulfide systems for submerged or chemically exposed conditions, and an EPA project manual requires submerged sealants to be approved by the manufacturer for wastewater. Do not infer wastewater or fuel resistance from the word “polysulfide” alone. |
| Hybrid or MS polymer | High-movement construction joints where the project specification permits a silyl-terminated-polyether system | Can combine non-sag installation with high movement capability. HRSD lists a hybrid polyurethane system as ASTM C920 Type S, Grade NS, Class 100/50 for high-movement vertical applications. | Verify the exact product’s ASTM classification, joint-width limit, substrate approvals, immersion resistance and coating compatibility before substituting it for a specified sealant. |
When polyurethane is the practical default
Use a documented polyurethane when the joint is an ordinary exterior concrete or masonry movement joint, the calculated movement is within its class, the bead must be painted, and abrasion resistance matters. This is a performance-based choice, not a rule that every concrete joint should receive polyurethane.
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When silicone is the better fit
Choose neutral-cure silicone when the joint experiences high movement, strong sunlight or repeated temperature cycling and the finished bead does not need to accept paint. Confirm that the specific silicone is intended for porous concrete and dynamic joints; silicone chemistry by itself does not guarantee adhesion to a dusty or damp slab.
When polysulfide is justified
Consider polysulfide for submerged, intermittently submerged, fuel or chemical exposure only after the owner, engineer and manufacturer confirm the exact service. Obtain written approval for the wastewater, fuel or chemical involved, expected concentration, temperature, immersion duration and cleaning regime. A generic exterior-joint approval is not enough.
5. Check the substrate and adjacent materials
Concrete is not a single uniform substrate. New and mature concrete, precast panels, painted surfaces, masonry, metal frames and glass can require different cleaning and priming. Before ordering material, confirm the following with the current technical data sheet:
- Adhesion to the actual concrete finish, including power-troweled, broom-finished, damp or previously sealed surfaces.
- Compatibility with adjacent aluminum, stainless steel, galvanized metal, masonry, glass, insulation and waterproofing membranes.
- Whether a primer is mandatory, optional or prohibited on each substrate.
- Whether existing coatings, curing compounds, release agents or efflorescence must be removed mechanically.
- Compatibility with the selected closed-cell backer rod and any bond-breaker tape.
- Whether the cured sealant can be painted and which coating chemistry and cure time are acceptable.
Perform an adhesion test on representative, prepared substrates when the work is large, high consequence or exposed to immersion. Test the actual sealant, primer, backer and coating combination; a successful test on bare concrete does not prove that the same system will adhere to a membrane or painted edge.
6. Account for traffic, water and mechanical loading
Concrete joints in walls and façades mainly experience weather and movement. Horizontal joints in floors, ramps, parking structures, bridge approaches and loading docks also experience wheel loads, abrasion, impact, dirt packing and repeated cleaning. The sealant must be selected for those forces, and the joint edges must be strong enough to support them.
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- For pedestrian areas, verify resistance to foot traffic, abrasion and dirt accumulation.
- For vehicle areas, verify traffic rating, tire shear resistance, fuel or oil exposure, cure time and whether a recess is required to protect the sealant from plows and tires.
- For submerged joints, verify continuous or intermittent immersion, hydrostatic pressure, wastewater constituents and the manufacturer’s approved application method.
- For chemical service, identify the actual chemical, concentration, temperature and contact time rather than relying on a broad label such as “chemical resistant.”
Idaho Transportation Department’s specification separates general exterior, non-traffic joints from other expansion-joint systems and also identifies pre-compressed polyurethane foam and hollow neoprene compression gaskets. That distinction matters: a liquid sealant is not automatically interchangeable with a pre-compressed gasket or a mechanically retained joint system.
7. Verify the project specification and product documentation
Before purchase, assemble the joint schedule, drawings, engineer’s notes and the latest technical data sheet. The submittal should identify the product, ASTM C920 type and grade, movement class, intended use, joint-width limits, color, primer, backer rod and cure requirements.
Questions the submittal must answer
- Does the ASTM C920 class meet the calculated opening and closing movement?
- Is the grade NS or P appropriate for the joint orientation?
- Is the product approved for concrete, masonry and every adjacent material in the detail?
- Is it approved for the actual water, wastewater, fuel or chemical exposure?
- Is the joint horizontal and traffic-bearing, and if so, is the product specifically traffic-rated?
- What primer, cleaner, masking method and backer rod are required?
- What are the minimum application temperature, substrate moisture limits, cure time and protection period?
- What coating can be applied, if any, and after what cure interval?
Use the current manufacturer documentation, not an old carton label or a distributor summary. Product formulations, colors, approvals and installation limits can change even when the product name remains similar.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.8. Install the sealant as a joint system
- Confirm the design: Measure representative joint widths, verify the calculated movement, and confirm the selected ASTM class, grade and exposure approvals with the project specification.
- Plan access and weather: Schedule work within the product’s temperature and moisture limits. Protect the joint from rain, wash water, dust and traffic during installation and cure.
- Prepare the joint: Remove laitance, loose concrete, old sealant, dust, oil, curing compounds and other contaminants. Repair weak or spalled edges with a compatible repair material and allow it to cure as specified.
- Install the backing: Place the approved closed-cell polyethylene backer rod continuously at the designed depth. Do not puncture it, stretch it or leave voids at splices and corners.
- Prime if required: Apply only the specified primer, at the stated coverage and flash-off time. Keep primer off the visible face of the concrete and do not substitute a general-purpose solvent.
- Mask the edges: Use tape where needed to keep the sealant off finished surfaces. Remove tape while the sealant is still workable and before a skin forms at the edge.
- Gun or pour the sealant: Use Grade NS for vertical or overhead work and an approved pourable or self-leveling Grade P product for suitable horizontal work. Fill from the deepest point forward to avoid entrapped air.
- Tool the bead: Tool the sealant against both joint faces to eliminate voids and establish the profile required by the data sheet. Do not use a wetting solution or solvent unless the manufacturer expressly permits it.
- Protect the installation: Keep water, dust, movement, pedestrians and vehicles away for the full stated cure period. Cure time varies with chemistry, temperature, humidity, bead size and ventilation.
- Inspect and document: Check for gaps, bubbles, fish-mouths, adhesion loss, sagging, contamination and inconsistent depth. Record product lot, primer, weather, installer and repair locations for the closeout file.
9. Understand total installed cost, not just cartridge price
The lowest material price can be the most expensive choice if it requires extensive preparation, fails to cure in the available weather window or must be replaced after immersion or traffic exposure. Compare the complete installed system:
| Cost driver | Why it changes the budget |
|---|---|
| Joint preparation | Removing failed sealant, grinding contaminated concrete, repairing edges and drying damp joints can exceed the cost of the new sealant. |
| Backer rod and primer | Correct depth control and adhesion promotion are separate materials and labor items, not included automatically in a sealant cartridge. |
| Application rate | Joint width, depth, corners and waste determine the amount of sealant consumed. A deeper-than-designed bead increases cost without improving movement performance. |
| Access and protection | Lift equipment, traffic control, shutdowns, temporary dams and cure protection can dominate a horizontal or elevated project. |
| Downtime and replacement risk | A slower-curing or incompatible product may delay reopening; premature failure can require removal, disposal and reinstallation. |
| Service life | A product with documented resistance to the actual ultraviolet, immersion, chemical and traffic exposure may cost more initially but reduce maintenance interventions. |
Obtain a quantity takeoff from the joint dimensions and the manufacturer’s coverage guidance, then add backer rod, primer, preparation, labor, access and protection. Do not compare a premium immersion system with a general-purpose exterior sealant solely by price per tube.
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10. Troubleshoot common joint-sealant failures
Sealant pulls away from one or both edges
Likely causes include dust, laitance, oil, excess moisture, incompatible coating, missing primer or movement beyond the product’s class. Remove the failed material, identify the adhesion problem, prepare a representative test area and reinstall only after the corrected preparation is approved.
The bead splits down the middle
Excessive joint movement, an incorrect width-to-depth profile, three-sided adhesion or an under-rated product can produce cohesive failure. Recalculate movement, inspect the backer rod and select a sealant whose class matches the opening and closing demand.
The sealant sags or runs
A pourable product may have been used in a vertical joint, or a non-sag product may have exceeded its application-temperature limit. Use Grade NS for vertical and overhead work and verify that the substrate and sealant are within the manufacturer’s temperature range.
Water leaks through an apparently intact bead
Look for pinholes, untooled sections, gaps at corners, discontinuity behind the sealant, cracks in the concrete edge and water bypass at an adjacent membrane. Sealant cannot repair a failed joint edge or a discontinuous waterproofing layer; repair the complete joint assembly.
The sealant will not accept paint
Many neutral-cure silicones are not paintable. If a painted finish is required, choose a product documented as paintable and verify coating compatibility and cure time before installation. Do not assume that a coating will bond because it is labeled for exterior use.
The joint fails after immersion or chemical cleaning
Confirm the exact liquid, concentration, temperature and exposure duration against the manufacturer’s approval. Replace a general exterior sealant with a product specifically approved for the service; a chemistry name alone is not evidence of wastewater, fuel or chemical resistance.
11. Field decision checklist
- Calculated opening and closing movement are documented.
- ASTM C920 class meets or exceeds that movement.
- Grade NS or Grade P matches the joint orientation.
- Joint width and sealant depth comply with the current product data sheet.
- Closed-cell polyethylene backer rod or another approved bond breaker is installed continuously.
- Concrete, masonry, metal, glass, coatings and membranes have confirmed compatibility.
- Primer, if required, is identified and included in the submittal.
- Traffic, abrasion, ultraviolet exposure, temperature cycling, immersion and chemicals are addressed individually.
- Any wastewater, fuel or chemical exposure has product-specific written approval.
- Weather, cure, protection and reopening requirements are scheduled.
- Preparation, installation and inspection records are retained.
The right sealant is therefore the one whose documented movement class, grade, geometry, adhesion and exposure approvals fit this joint—not simply the product with the strongest-sounding chemistry or the highest movement number.
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