Expansion joint filler selection rarely gets the attention it deserves. The filler goes in first, gets covered by sealant, and disappears from sight, which is probably why it gets specified by habit. Bitumen board because that's what was on the last job. Closed-cell foam because it was on the shelf. The problem is that a poor filler choice can compromise the sealant above it, restrict the movement the joint was designed to accommodate, or introduce water into a substrate you've worked hard to protect.
This guide separates the two distinct functions in a joint assembly, compares the main filler materials on the properties that matter, and explains how to size the assembly for the design movement.
Filler vs Backer Rod: Two Different Jobs
These terms get used interchangeably on site, and they shouldn't be. They perform different functions at different depths in the joint.
Joint filler occupies the full depth of the joint from the bottom up to a set distance below the surface. Its job is to fill the void, support the sealant during installation, and accommodate the compressive and tensile movement the joint is designed to take. It is a structural component of the joint assembly.
Backer rod sits directly beneath the sealant and does two things: it controls the sealant depth-to-width ratio (the profile), and it prevents three-sided adhesion. Three-sided adhesion is the condition where sealant bonds to both joint faces and the bottom of the joint simultaneously. When the joint opens, a sealant bonded on three sides cannot deform freely; it tears at the centre rather than stretching uniformly. Backer rod breaks that bond at the base and gives the sealant a surface it won't adhere to.
In a shallow joint, backer rod may be the only backing material. In a deeper joint, filler occupies the lower portion and backer rod sits on top of it, setting the sealant depth. In either case, the backer rod must be the right type.
Why Open-Cell Backer Rod Causes Bubbling
Open-cell polyurethane foam rod holds air and moisture in its interconnected cell structure. When sealant is applied over it and the sun heats the joint, that trapped air expands and pushes through the uncured sealant, leaving pinholes and bubbles in the finished bead. Those defects are not cosmetic; they are pathways for water ingress and points of stress concentration when the joint moves.
Closed-cell backer rod, typically polyethylene, has a sealed cell structure that does not off-gas into the sealant. For any joint that will receive a poured or gunned sealant, closed-cell rod is the correct specification. Open-cell rod has limited legitimate use, mainly as a backing in joints where no sealant is applied.
The Main Filler Materials
Closed-Cell Polyethylene Foam Board
Manufactured from low-density polyethylene expanded to a closed-cell structure, this material is available in sheet and strip form in thicknesses from 10 mm to 50 mm or more. Key properties:
- Compressibility: typically 25 to 50 per cent compression at moderate loads, recovering to near original thickness when the load is released
- Recovery: good elastic recovery after compression, making it suitable for joints that cycle repeatedly
- Water absorption: very low, generally below 0.5 per cent by volume per AS 1580 equivalent test methods, because the closed-cell structure resists water ingress
- Sealant compatibility: compatible with most polyurethane and polysulphide sealants; confirm with the sealant manufacturer's data sheet for silicone systems
- Temperature range: performs from approximately -40°C to +70°C without significant change in properties
Closed-cell PE foam is well suited to slab-on-ground isolation joints, pavement joints and wall-to-slab joints where repeated thermal movement is expected. It does not rot, does not absorb water and does not leach compounds that might interfere with sealant cure.
Cross-Linked Polyethylene Foam
Cross-linking the polyethylene polymer during manufacture produces a denser, firmer foam with tighter cell structure. The practical differences compared to standard closed-cell PE:
- Higher compressive strength at the same thickness, useful where the filler must resist concrete pressure during pours
- Better dimensional stability under sustained load, meaning less creep over time
- Slightly lower compressibility percentage, so it is less forgiving in joints with large movement demands
- Water absorption remains very low
Cross-linked foam is often the better choice for formed joints in structural concrete where the filler is cast in place and must hold its position and thickness during the pour. It is also used as backer rod in larger diameter formats.
Bitumen-Impregnated Fibreboard
This material is made from compressed cellulose fibre (typically wood or cane fibre) saturated with bitumen. It has been used in Australian concrete construction for decades, which is partly why it still gets specified without much scrutiny. The actual performance profile:
- Compressibility: moderate, typically 25 to 50 per cent at higher loads, but recovery is poor. Once compressed, bitumen board does not spring back. This matters in joints that open and close with temperature cycles; a filler that doesn't recover leaves a void beneath the sealant when the joint opens.
- Recovery: low. This is the material's most significant limitation for dynamic joints.
- Water absorption: higher than foam alternatives. The cellulose fibre core absorbs moisture even with bitumen impregnation, particularly at cut edges. Sustained water contact can cause swelling or softening.
- Sealant compatibility: bitumen can migrate into some sealant types, particularly silicone, and interfere with adhesion or cure. A bond breaker or primer check is warranted before specifying bitumen board beneath a silicone sealant.
- Where it still makes sense: static or near-static joints where movement is minimal, or where the joint is sealed by a compression seal rather than a gunned sealant. Isolation joints around columns in slabs-on-ground, where the board is cast in and the joint sees little ongoing movement, are a reasonable application.
Cork
Cork expansion joint filler is less common than it once was, but it appears in specifications for certain applications, particularly around structures where chemical compatibility with foam is uncertain.
- Compressibility: good, with reasonable recovery for a natural material
- Recovery: moderate; better than bitumen board but below closed-cell foam
- Water absorption: cork absorbs water, which causes it to swell. In submerged or frequently wet joints this swelling can generate pressure against the joint faces, which may or may not be desirable depending on the application
- Sealant compatibility: generally compatible with most sealant chemistries; no bitumen migration risk
- Durability: cork can degrade over long periods in aggressive chemical environments
Cork finds occasional use in pool surrounds and water-retaining structures where its slight swell on wetting is seen as beneficial for maintaining a tight joint. For most standard concrete pavement and building applications, foam alternatives offer more predictable long-term performance.
Comparing the Materials Side by Side
| Property | Closed-Cell PE Foam | Cross-Linked PE Foam | Bitumen Fibreboard | Cork |
|---|---|---|---|---|
| Recovery after compression | Good | Good | Poor | Moderate |
| Water absorption | Very low | Very low | Moderate to high | Moderate |
| Compressive strength | Low to moderate | Moderate to high | Moderate | Moderate |
| Sealant compatibility | Broad | Broad | Check silicone | Broad |
| Rot / biological degradation | None | None | Possible | Possible |
| Suitable for dynamic joints | Yes | Yes | No | Marginal |
Sizing the Filler for Design Movement
The filler must accommodate the movement the joint is designed to take without bottoming out in compression or pulling apart in tension. The starting point is the design movement, which comes from the engineer's joint spacing and movement calculations, not from guesswork.
For a joint designed to accommodate, say, 20 per cent compressive strain, the filler must compress to 80 per cent of its original width without generating restraint forces that crack the adjacent concrete. If the joint is 20 mm wide at installation and is expected to close by 4 mm, a filler with a verified compressibility of 25 per cent or more at low load is adequate. If the joint is expected to open by 4 mm as well, the filler must not pull away from the joint faces and leave a void.
Practical sizing rules:
- Filler width: match the design joint width. The filler should be cut or supplied slightly wider than the joint so it sits in compression from the start, maintaining contact with both faces.
- Filler depth: typically fills the joint from the bottom to 20 to 25 mm below the surface, leaving room for backer rod and sealant.
- Backer rod diameter: select a rod diameter 25 to 30 per cent larger than the joint width so it sits in compression and stays in position. A 20 mm joint takes a 25 mm rod.
- Sealant depth: for most gunned sealants, the depth-to-width ratio should be 1:2 for joints up to 25 mm wide, meaning the sealant depth is half the joint width. The backer rod sets this dimension.
These are general guidelines. The joint design, including movement allowance, sealant selection and substrate conditions, should be confirmed by the specifying engineer or a waterproofing consultant before the joint assembly is finalised.
Sealant Compatibility Is Not Optional
The filler and backer rod are part of the same system as the sealant. Specifying them independently creates risk. Bitumen board under a silicone sealant, or an open-cell rod under any gunned sealant, are combinations that cause failures that are expensive to remediate once the joint is sealed and the slab is in service.
Fosroc produces technical data sheets for its sealant range that specify compatible backing materials and primers. Reading those sheets before ordering the filler is a straightforward step that eliminates a common source of joint failure.
Ordering Filler and Backer Rod
CSA stocks construction chemicals from the Fosroc range and can assist with specifying the right joint filler and backer rod for your application. Whether you're pricing a civil pavement contract, a warehouse slab or an isolation joint around a structural element, getting the filler specification right before the concrete goes down is considerably easier than cutting it out afterwards.
Visit constructionsupplies.group/au to browse the range or contact the trade desk for a quote on project quantities.