Fosroc Hydrocell XL
A cross-linked, closed-cell polyethylene joint filler board that stays put under hydrostatic pressure and backs up your sealant without soaking up water.
What is it?
A semi-rigid, cross-linked, closed-cell polyethylene sheet used to form or fill expansion joints and to back up gun-applied sealants.
Where is it used?
Expansion and isolation joints in water-retaining structures, basements, subways, reservoirs, canals, culverts, roads and runways.
Why use it?
Its closed-cell structure will not absorb water and resists lateral and hydrostatic pressure, so the joint compresses and recovers cleanly without extruding.
How is it applied?
Cut to the required depth, pin to the joint face with nails at roughly 100 mm centres, then strip the top layer to leave a clean bond-breaking base for the sealant.
About Fosroc Hydrocell XL
Fosroc Hydrocell XL is a high-density, cross-linked, closed-cell polyethylene board engineered to form and fill expansion joints in concrete, brickwork and blockwork. On Indian sites it is most often specified as the compressible filler that occupies a movement joint and as the firm backing that supports a gun-grade sealant above it. Because every cell in the material is individually cross-linked and sealed, the board does not draw water in the way an open-cell foam or a fibreboard would. That single property is what makes it dependable in water-retaining and water-excluding work, where a filler that swells or rots would compromise the joint over time.
For specifiers, Hydrocell XL is positioned as a modern replacement for the older sponge-rubber and cork fillers described in ASTM D1752 (Type 1 and Type 2). It behaves predictably under load: it compresses when the structure closes, recovers when the joint opens, and does not extrude out of the joint face. It is bitumen-free, rot-proof, non-tainting and chemically resistant, which is why it turns up repeatedly in potable-water reservoirs, sewage tanks, basements, subways and airfield pavements. Space Arc Engineering supplies it as an authorised Fosroc distributor across Delhi NCR and Pan-India, and our applicators can advise on the right density and thickness for the movement you are designing for.
The technical insight
The performance of Hydrocell XL comes down to its cell geometry. In a conventional open-cell foam the cells interconnect, so water wicks through and the material loses stiffness and can rot. Hydrocell XL is cross-linked, meaning the polyethylene chains are chemically bonded into a three-dimensional network, and each cell is closed and independent. Water absorption stays below 0.05% by volume, and because the cell walls do not collapse under pressure the board resists lateral and hydrostatic loads rather than being squeezed flat. That is why it holds up in deep basements and reservoirs where the water table is pushing on the joint. Mechanically it recovers 98% of its original thickness after 50% compression and carries a compressive strength of about 0.15 N/mm2 (ASTM D3575), so it accommodates joint movement while still giving a sealant enough back-pressure to tool against. The result is a filler that stays in the joint, keeps its shape through repeated open-close cycles, and never becomes a path for water.
Where it is used
- Expansion and isolation joints in reinforced concrete water-retaining structures
- Potable-water reservoirs, service tanks and treatment plant walls
- Sewage and effluent tanks where chemical and biological resistance matters
- Basement rafts, retaining walls and subway boxes below the water table
- Canals, culverts, spillways and other hydraulic structures
- Roads, hardstandings, runways, taxiways and aprons
- Movement joints in brickwork and blockwork facades
- Firm backing and bond-breaker beneath gun-applied polysulfide or PU sealants
Key advantages
- Closed-cell, non-absorbent structure keeps water absorption below 0.05% by volume
- Cross-linked cells resist lateral and hydrostatic pressure without collapsing
- Recovers 98% of thickness after 50% compression, so it tracks joint movement
- Non-extruding under load, unlike softer sponge or foam fillers
- Rot-proof, non-tainting and safe for potable-water contact
- Chemically resistant to acids, alkalis and oxidising agents
- Bitumen-free formulation with a natural bond-breaking surface for sealants
- Direct high-performance replacement for cork and sponge-rubber fillers (ASTM D1752)
Limitations & considerations
- It is a joint filler and sealant backing, not a standalone waterproofing membrane or an active swelling waterstop
- The exposed joint face still needs a suitable gun-grade sealant on top to be watertight
- It should not be used where the service temperature is continuously outside roughly -70°C to 100°C
- Semi-rigid boards must be cut and pinned accurately; gaps or overlaps at the filler leave weak points for the sealant
- For heavy traffic or point loads, joint density and thickness must be matched to the design load rather than assumed
Selection & best practice
Choose the density to suit the duty: the lighter 40 kg/m3 grade suits general movement joints, while the 100 kg/m3 grade gives firmer sealant support and better load transfer for trafficked or heavily loaded joints. Cut the board to the required joint depth and pin it to the prepared concrete or masonry face with nails at approximately 100 mm centres, keeping the filler flush and continuous with no gaps at butt joints. Where the board is also acting as sealant backing, install it to leave the correct sealant depth-to-width ratio, then remove the strippable top layer to expose a clean bond-breaking surface before priming and applying the sealant. Keep the joint face dry and free of laitance, dust and curing compound so the sealant bonds to the arris rather than to debris. On water-retaining work, pair Hydrocell XL with a compatible Fosroc sealant such as Thioflex 600 to complete the joint.
Specifications
| Material | Cross-linked, semi-rigid, closed-cell polyethylene |
| Nominal densities | 40 kg/m3 and 100 kg/m3 |
| Available thicknesses | 10, 15, 20 and 25 mm |
| Sheet size | 1000 x 2000 mm (1 m x 2 m) |
| Compressive strength | 0.15 N/mm2 (ASTM D3575) |
| Compression recovery | 98% after 50% compression |
| Water absorption | Less than 0.05% by volume |
| Service temperature range | -70°C to 100°C |
| Replaces | Sponge rubber (ASTM D1752 Type 1) and cork (ASTM D1752 Type 2) fillers |
Industry use cases
Water & Wastewater
Expansion joints in reservoirs, service tanks and sewage structures where a non-absorbent, rot-proof filler is essential for long-term watertightness.
Basements & Underground
Isolation and movement joints in basement rafts, retaining walls and subway boxes sitting below the water table under sustained hydrostatic pressure.
Infrastructure & Hydraulic
Joints in canals, culverts, spillways and bridge substructures that need a filler resistant to lateral pressure and chemical attack.
Pavements & Airfields
Firm, non-extruding filler and sealant backing for expansion joints in roads, hardstandings, runways, taxiways and aprons.
Building Envelope
Movement joints in brickwork and blockwork facades where the board backs up the sealant and prevents three-sided adhesion.
Frequently asked questions
Is Hydrocell XL a waterstop that swells in water?
Does Hydrocell XL make the joint watertight on its own?
Which density should I specify, 40 or 100 kg/m3?
How is it fixed in the joint?
Can it be used in potable-water tanks?
What is the difference between Hydrocell XL and ordinary cork or sponge fillers?
How we position Fosroc Hydrocell XL
In a movement joint, think of Hydrocell XL as the base and the sealant as the finish: the board fills and backs the joint, and a Fosroc sealant such as Thioflex 600 (two-part polysulfide) or a Nitoseal PU grade sits on top to make it watertight. If your real need is an active waterstop for construction joints rather than a filler for movement joints, look at Fosroc’s Supercast range (hydrophilic and PVC waterstops) instead. Where the wider waterproofing system is being specified, our Fosroc crystalline and membrane products handle the slab and wall faces while Hydrocell XL and the sealant handle the joints. We also stock comparable joint fillers and sealants from Sika, MC-Bauchemie and STP, so if a project already runs on one of those systems we can keep the whole joint detail within a single manufacturer for warranty and compatibility.
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Authorised distribution, technical advice & professional application across Delhi NCR and Pan-India.