It might seem counterintuitive that a little extra “stuffing” is necessary to allow a structure to move appropriately. However, the best expansion joint installations use filler material that accommodates the “extra” – like how a great Thanksgiving dinner tightens our waistlines, yet leaves room for dessert.
This material is necessary to occupy the space left between the other elements, letting the other components do their job.
There are a variety of options when it comes to expansion joint filler material. We’ll walk you through the criteria to consider when pulling together project materials, as well as the pros and cons of each filler type.
Filler ≠ Sealant (≠ Sealer)⚠️ Before you go deeper, note that we’re not discussing sealants, which go on the outside of the joint. Filler is what goes in the middle, after which you might apply a sealant to keep out water and debris. Joint filler also has to act as a bond breaker for the sealant. To add to the confusion, many industry folks use “sealer” and “sealant” interchangeably, even though they’re separate elements of an installation. Concrete joint sealer is an optional step (either before or after sealant application) for improving watertightness, UV resistance, or aesthetics. |
You might also know expansion joints by another name: movement joints. The overall product fills the gap between slabs or masonry, while the filler material is sandwiched inside the joint edges.
Joint filler’s #1 responsibility is physically supporting the sealant. Because your sealant should only go a certain amount of depth into the joint, it needs another product behind or below it to hold it in place. By separating the sealant from the joint cavity’s bottom, filler material also prevents three-sided adhesion, which could restrict the sealant’s ability to flex.
It’s a simple life, but fillers need other key traits to keep them from becoming a hindrance rather than a help. Use these product parameters as you determine how to fill expansion joints in your concrete application:
There are many types of concrete expansion joint filler material, but not all are suitable for commercial projects. Naturally, some materials are also better suited for your job than others.
Let’s review these materials in the context of heavy-duty use, i.e. situations where the structure handles high traffic at least occasionally.
This product is famous for its ability to compress and expand while maintaining its shape. You can find it all over America in facade and pavement joints.
The foam is made up of tiny cells (usually polyethylene or polypropylene), each sealed off from its neighbors so that nothing can pass through easily. The structure of this material also makes it more rigid than your average filler.
Pros: Closed-cell polyethylene foam provides durability and flexibility while standing up to moisture, chemicals, and temperature changes. The material is lightweight and easy to cut.
Cons: You’ll need to take extra care during installation. If you rupture the skin of the material, it can fill with air bubbles that’ll compromise performance.
Ethylene propylene diene monomer (EPDM) is a type of rubber foam. It’s most common in challenging applications like bridges, roads, and big buildings.
Pros: EPDM foam stands strong against moisture, temperature changes, UV rays, and many chemicals. Its elasticity also helps it compress and recover effectively in times of movement. It’s even available in an impressive variety of widths.
Cons: Installation is tricky; in particular you must position the foam precisely to maximize its effectiveness without stretching or compressing it. It’s also like that rich Aunt Marguerite you only see at Thanksgiving: expensive taste, and doesn’t mesh well with others (sealants).
There’s an exploding market for epoxy products, especially in industrial flooring (i.e. an Amazon warehouse). In the case of expansion joint filler, we’re talking about low-viscosity epoxy resin.
Application of this product happens via squeegee or trowel. The epoxy bonds the concrete as it hardens.
Pros: Epoxy is strong and durable, keeping out dirt and debris. It’s also easy to apply.
Cons: Special epoxy is necessary to address commercial-level exposure. Ordinary epoxies lack the flexibility and strength to endure continual weight and traffic. For those who use regular epoxy, watch out for cracking, peeling and other joint failure concerns.
This broad-spanning category consists of a blend of securely bonded wood fibers and additives. The two subtypes you should know are:
Pros: Fiber offers impressive resilience in compressibility and movement absorption vs. other products. Cork, in particular, is a natural and renewable material. Surprisingly, it’s cost-effective in a pinch, too.
Cons: Fiber snaps easily and typically requires maintenance faster than other expansion joint filler materials. It also turns other materials black when they come in contact.
This extruded, tube-like material is essentially an extruded plastic. You might associate it with wing gland expansion joints.
As for specific applications, it’s an option for sidewalks and driveways, swimming pool decks, treatment systems, and facades.
Pros: This material is durable, even in exposure to chemicals, sunlight, and water. Some varieties are pretty flexible and easy to handle.
Cons: The product comes in preset heights and widths, so it might not completely seal your joint. In regions with rapid freeze-thaw cycles, it tends to break down more quickly than other fillers.
Someone has to be the black sheep at the family gathering. We usually don’t recommend these as concrete or masonry expansion joint fillers:
The most common joint filler material is still wood. Decades ago, the world thought using wood to fill concrete expansion joints was a good idea, but contractors are phasing it out in modern commercial structures.
Pros: Similar to concrete, wood expands during exposure to temperature changes or moisture.
Cons: Wood doesn’t create a good seal with the concrete, allowing moisture to penetrate the joint. There’s also that whole rotting thing.
If you ever encounter this as a concrete expansion joint filler material, it was probably an emergency fix.
Pros: … None?
Cons: Grout and mortar don’t expand or contract during temperature changes. Instead, they crack and crumble, requiring immediate maintenance.
Is your “sandwich” missing a little something? Certain installation materials can support the filler’s ability to, in turn, support the sealant. Sometimes, subcontractors use multiple installation supports in the same joint.
The top tools to know are:
Backer rods are flexible, round objects that create a backstop in the expansion joint. They take sealant control a step further, helping you use the right amount without it seeping too deeply into the gap. This also prevents three-sided adhesion to the joint structure.
Choose this product when you need to fill a large joint (0.5” or more). Irregular joint shapes can also benefit from backer rods.
Backer rod products usually come in a spool of foam. They come in a wide variety of lengths and diameters, so you should easily be able to find a size for your needs.
Don’t be alarmed if a supplier’s recommendation seems too big for the joint – that’s intentional. For example, a 0.5” joint should use a 0.675” backer rod. This ensures it doesn’t fall out of the joint, but also keeps the sealant from trespassing behind the rod. You should be able to compress the backer slightly, then shoot the sealant over it.
These products come in three forms: open-cell backer rods, closed-cell backer rods, and hybrid (soft-cell) products.
| Type |
Function |
Drawback |
Where to Use |
|
Open-Cell |
Interconnected cells allow air & moisture to pass |
Wicks ponding water to the sealant’s underside |
Closed metal-to-metal joints (when you need curing on both sides) |
|
Closed-Cell |
Rigid cells don’t allow air or moisture to pass |
Little margin for error – if you puncture it, it the sealant will bubble |
Wet areas that could hamper sealant adhesion & performance |
|
Soft-Cell |
Open, compressible cells; closed exterior won’t wick water through itself |
Limited resistance to chemicals & oils |
Horizontal & other flat work |
Another product that can support the assembly is bond-breaker tape. This product goes in the back of a joint, keeping the caulking from sticking to the joint. It’s another option for stopping three-sided adhesion (and ripping your seal in half).
Use bond-breaker tape when the joint is too shallow for a backer rod. In high-movement environments, it’s also sometimes best to pair highly elastic sealants with bond-breaker tape to improve their stretchability without sticking.
Polyethylene is far and away the most common material for the tape. You can add a primer to the sides of the joints to promote bonding with the tape.
It isn’t hard to compromise the integrity of your construction – all it takes is a little cracking, water ingress, or some other type of damage.
Installing the best concrete expansion joint filler material for your project makes it easier for the sealant – and the overall installation – to do its job. Consider your job site’s climate, joint and sealant type, and expected movement. The best material for a bridge will be different than for a masonry wall or a parking garage.
A technical product expert (or the manufacturer itself) can bring a lot to the table regarding choosing a site-specific solution. You’ll give thanks – trust us.
To learn more about product solutions, see our manufacturer page: