ASTM C920: Classification String, Testing, and Labeling

ASTM C920 is the Standard Specification for Elastomeric Joint Sealants, the document that sets the baseline physical-property requirements a chemically curing sealant must meet before it can be sold for use in buildings, plazas, parking decks, and similar construction. The current designation is C920-18, reapproved in November 2024. Its practical value is the classification system it creates: once you can read the Type, Grade, Class, and Use letters printed on a sealant label, you can tell at a glance whether the product in front of you matches the joint you’re trying to fill.

What the Standard Covers and What It Leaves Out

C920 applies to cold-applied, single-component and multi-component elastomeric sealants used for sealing, caulking, or glazing on buildings, plazas, and decks intended for vehicular or pedestrian use. It also covers other construction besides highway and airfield pavements and bridges, which are governed by separate specifications.¹ The chemistries that fall under it include silicone, polyurethane, polysulfide, and hybrid formulations.

One limit matters more than any other. C920 evaluates the physical properties of a sealant at the time of purchase. It does not predict how that sealant will hold up in a poorly designed joint, against an incompatible substrate, or under decades of UV and thermal cycling. Passing C920 means the material left the factory with the right makeup to do its job. It does not guarantee a successful installation, and the standard is explicit about that boundary.

The International Building Code references C920 in Chapter 35, in connection with IBC sections 703.7 and Table 2506.2.² An inspector checking sealant compliance will usually ask for documentation that the product meets the classification called for in the project specification.

How to Read a C920 Classification String

Every compliant sealant carries a classification string along the lines of “Type S, Grade NS, Class 25, Use NT, M, G, A.” Four categories, in order: Type, Grade, Class, Use. Reading the string correctly is the single most important step in sealant selection, because every letter narrows what the product is designed to do.

Type: Single or Multi-Component

Type S is single-component and comes ready to apply from the cartridge or sausage pack with no mixing. Type M is multi-component, furnished in two or more parts that must be mixed thoroughly before application to trigger curing.¹ Single-component sealants dominate smaller projects because they remove the chance of mixing errors. Multi-component products show up on larger commercial work where faster cure and specific performance characteristics justify the extra handling.

Grade: Pourable or Non-Sag

Grade P (pourable) sealants are self-leveling and belong in horizontal joints: sidewalk control joints, plaza deck joints, flatwork of any kind. Grade NS (non-sag) sealants hold their shape on vertical and overhead surfaces and are the default for wall joints, window perimeters, and curtain-wall assemblies.¹ Pourable sealant in a vertical joint will run out before it cures. It is one of the more common field mistakes.

Class: Movement Capability

Class indicates the percentage of joint movement a sealant can handle without adhesive or cohesive failure. This is where specification errors concentrate. The standard defines five classes:

Class 12.5 handles ±12.5% movement, for joints with minimal thermal or structural movement.
Class 25 handles ±25%, the most commonly specified class for general building joints.
Class 35 handles ±35%, for moderate movement.
Class 50 handles ±50%, typical for high-movement curtain walls and glazing systems.
Class 100/50 handles +100% extension and −50% compression, an asymmetric rating for joints that open significantly more than they close, common on high-rise facade and curtain-wall stack joints.¹

Working out the actual movement a joint will see requires the temperature range the building will experience, the coefficient of thermal expansion for the substrate materials, and the joint width. Specifying a Class 25 sealant on a joint that actually moves ±40% guarantees failure no matter how clean the installation.

Use: Substrate and Environment

The Use designation tells you where the sealant can go and what it bonds to. The common categories:

Use T (Traffic): for joints in pedestrian and vehicular traffic areas such as walkways, plazas, decks, and parking garages. Requires a harder cured sealant, Shore A hardness between 25 and 50.
Use NT (Non-Traffic): for joints not subject to foot or vehicle traffic. Allows a softer cured sealant, Shore A hardness between 15 and 50.³
Use M: tested for bonding to mortar and masonry.
Use G: tested for bonding to glass.
Use A: tested for bonding to aluminum.
Use I (Immersion): for joints submerged in water, such as swimming pools, fountains, and storage tanks.
Use O: for specialty substrates beyond the standard mortar, glass, and aluminum test panels. When a project involves an unusual substrate, the specification can require additional testing on that specific material.¹

Every letter in the Use list carries weight. A sealant rated for Use G but not Use M has been tested on glass but not on masonry. Putting it on a brick-to-glass transition joint and hoping the masonry side holds creates a real liability.

The Testing Behind the Label

The classification string is backed by a battery of lab tests. Flow testing confirms that Grade P sealants self-level and that Grade NS sealants resist sagging. Extrusion rate testing verifies that single-component sealants dispense smoothly and that multi-component sealants remain workable for at least three hours after mixing.¹ Tack-free time measures how quickly the surface skins over enough to resist picking up dust.

Hardness testing uses a Shore A durometer. Traffic-rated sealants must reach at least 25 Shore A so they can take foot and vehicle loads; non-traffic sealants can be as soft as 15 Shore A; neither can exceed 50 Shore A, because above that the material loses the flexibility it needs to accommodate joint movement.³ Heat aging exposes cured sealant to elevated temperatures and measures weight loss, which cannot exceed 7%, and the sample cannot crack or chalk.¹ Excessive weight loss means volatiles are escaping, which leads to shrinkage and eventual failure in the field.

The most demanding test runs under a separate standard, ASTM C719, the Standard Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement. Specimens are subjected to water immersion, cyclic extension and compression, and temperature changes.⁴ The sealant either keeps its bond and internal integrity through the full cycle or it fails, either by separating from the substrate (adhesive failure) or tearing within itself (cohesive failure). C719 evaluates the sealant when properly installed with recommended primers, and does not account for failures caused by bad joint design, excessive movement, or poor application.

Separately, ASTM C794 measures peel adhesion by pulling a thin strip of cured sealant away from its substrate at a controlled angle. The passing threshold is 5 pounds of force per linear inch of width. Peel testing is particularly useful before a project begins: contractors can test the sealant against the actual substrates they’ll meet on site and decide whether a primer is needed.

Shelf Life and Storage

C920 requires that both single-component and multi-component sealants remain capable of meeting all specification requirements for at least six months after delivery, provided the material stays in its original unopened container at temperatures no higher than 27°C (80°F).³ That ceiling is lower than many job-site trailers reach during summer. Sealant baking in a hot trailer on a Phoenix site in July is not stored per the specification, and both the manufacturer warranty and the C920 compliance claim become questionable. Checking lot dates on delivery and keeping storage temperature-controlled is a basic quality-control step that gets skipped more often than it should.

Labeling and Submittals

Products claiming C920 compliance must display the manufacturer name, product name, and a unique lot number on the packaging, with the full classification string visible so that an inspector or contractor can verify the material against the specification without opening the container.¹ The lot number is the traceability link back to a production batch. If a failure surfaces months or years later, the lot number lets investigators determine whether a defective batch was involved and whether other projects pulled from the same run.

On commercial jobs, submittals typically pair the product label with the manufacturer’s technical data sheet. The data sheet should list the test results that correspond to each C920 requirement, along with the classification string. The architect’s job on review is to compare the classification on the data sheet against what the specification actually calls for. A common rejection: submitting a Class 25 product for a joint that was specified as Class 50.

Independent Validation

C920 compliance is a manufacturer’s self-declaration. The standard does not require independent third-party certification. The Sealant, Waterproofing and Restoration Institute runs a voluntary Product Validation Program that adds an outside layer: approved independent labs run the tests, and the results are compared against the performance data the manufacturer has published on its data sheet.⁵ Products that pass receive a Validation Seal with a specific expiration date, and manufacturers must resubmit if the seal expires or if they change the formulation. The C719 cyclic movement test, which backs the highest movement class a manufacturer claims, must also be performed to support those claims.⁶ For specifiers who want more than self-reported numbers, the Validation Seal is the closest thing to an independent stamp in the sealant industry.

Companion Standards and the Limits of C920

C920 does not stand alone. Several related standards cover the things it deliberately leaves out:

ASTM C719: the cyclic movement test method used to validate adhesion and cohesion under repeated joint movement.⁴
ASTM C794: the peel adhesion test with its 5-pound-per-inch threshold.
ASTM C1193: the guide for use of joint sealants, covering joint design, surface preparation, primer selection, and application procedures.

When a sealant fails in service, the root cause is far more often a joint-design or installation issue that falls under C1193 than a material deficiency that falls under C920. Specifying the right C920 classification is necessary. It is not sufficient. Correct joint dimensions, proper backer rod, the right primer, and a trained applicator are what turn a compliant sealant into a working seal.

1
ASTM International. ASTM C920-18 – Standard Specification for Elastomeric Joint Sealants
2
International Code Council. 2024 International Building Code – Chapter 35 Referenced Standards
3
iTeh Standards. Standard Specification for Elastomeric Joint Sealants
4
ASTM International. ASTM C719-22 – Standard Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement (Hockman Cycle)
5
SWR Institute. Product Validation
6
SWR Institute. Sealants