About Fosroc Supercast SW20

Fosroc Supercast SW20 is a hydrophilic (water-swellable) waterstop strip used to waterproof construction joints in cast in-situ concrete. Unlike a gun- or pouring-grade joint sealant, it is not applied into a visible movement gap in a finished surface — it is cast into the body of the concrete at the interface between an old pour and a fresh pour, where it acts as an internal water barrier. The strip is manufactured from a high-performance synthetic elastomer containing hydrophilic groups built into the polymer’s molecular structure. When these groups meet water, the strip swells and increases in volume by up to 200%, generating positive pressure against both faces of the concrete joint and closing off any capillary path through which water could track. Because the hydrophilic groups are part of the basic polymer rather than a soluble additive, they are not leached out or exhausted by repeated wetting and drying, which gives the product its long service life. The SW20 designation refers to the 10mm x 20mm cross-section, the larger of the two strips in the Supercast SW family (its companion, SW10, is a smaller section for lighter-duty construction joints). The bigger section presents a greater bonded surface area to the surrounding concrete, which is why SW20 is specified for higher-risk and higher-grade waterproofing situations — for example BS 8102 Grade 3 (habitable, dry) basements. SW20 is supplied in convenient 5-metre rolls and is BBA-approved; the Supercast SW system is also accepted for contact with potable water under the Water Regulations Advisory Scheme (WRAS). Critically for detailing, SW20 is designed to be positively linked into conventional Supercast PVC waterstop networks using Supercast SWX swellable adhesive, so designers can mix rigid PVC waterbars at expansion/movement joints with swellable strip at construction joints inside one coordinated, continuous waterproofing line.

The technical insight

The mechanism that makes SW20 reliable is also its main design constraint: the swell is driven by water uptake, so it must be fully encapsulated in concrete to perform. The strip seals because expansion is restrained on all sides by surrounding concrete, converting volume increase into sealing pressure at the joint faces. If the strip is left exposed in a gap, or installed too close to a free edge, the same swell simply pushes outward, can spall thin concrete cover, and does not develop the pressure needed to seal. Fosroc therefore requires adequate concrete cover (typically a minimum of around 80mm) to the strip on all sides and continuous, gap-free bonding to a sound, set substrate. The second engineering subtlety is the delayed-swell coating: Supercast SW10 and SW20 carry a coating that postpones the onset of swelling. This is deliberate — on a wet Indian site, rain, ground water or curing water sitting on a kicker could otherwise pre-activate the strip before the next pour, wasting its expansion capacity and leaving little swell in reserve once cast. The delay gives a practical installation window. The 1 bar (approximately 10m head) pressure resistance quoted by Fosroc is achieved after just one day cast in concrete, but engineers should treat that as a single-line construction-joint figure, not a substitute for sound concrete, correct cover, and good joint preparation. For deep basements or high water tables common in NCR and coastal projects, SW20 is frequently used in combination with an external membrane or a PVC waterbar rather than as a sole defence.

Where it is used

• Horizontal and vertical construction (day-work) joints in in-situ reinforced concrete
• Basement retaining walls, rafts and wall-to-base kicker joints (BS 8102 Grade 2 and Grade 3 structures)
• Pile caps and capping beams where reinforcement continuity prevents a PVC waterbar being threaded through
• Water-retaining and water-excluding structures — tanks, reservoirs, sumps, lift pits and underground service chambers
• Casting fresh concrete against existing/old hardened concrete where no provision for a conventional waterstop was made
• Sealing around pipe and service penetrations passing through concrete walls and slabs
• Tie-in detailing where a Supercast PVC expansion-joint waterbar must be linked to construction-joint waterproofing via Supercast SWX
• Sewage and effluent treatment works and other below-ground civil structures requiring durable joint sealing

Key advantages

• Self-activating — swells up to 200% on water contact to apply continuous sealing pressure across the joint
• Simple, fast installation: bonded to the surface or into a groove with no on-site welding, heat or specialist labour
• Delayed-swell coating tolerates wet site conditions and prevents premature activation before the next pour
• Achieves resistance to 1 bar water pressure after only one day cast in concrete
• Hydrophilic groups are part of the base polymer, so performance survives repeated wet/dry cycling without being leached out
• Links positively into Supercast PVC waterstop networks (via Supercast SWX) for a single continuous waterproofing system
• Larger 10x20mm section gives greater bonded contact area, suiting higher-risk and Grade 3 basement waterproofing
• BBA approved and WRAS accepted for contact with potable water
• Flexible strip follows complex joint geometry and detailing around penetrations and corners

Limitations & considerations

• Seals construction joints only — it is NOT a movement/expansion-joint product and cannot accommodate ongoing joint cycling; use a PVC expansion waterbar or an elastomeric sealant (e.g. Thioflex 600/Nitoseal) for moving joints
• Must be fully encapsulated in concrete with adequate cover (around 80mm minimum); exposed or low-cover installation can cause spalling and will not seal
• Requires a sound, clean, set substrate and continuous gap-free bonding — honeycombing, laitance or dust at the joint compromises performance
• Premature wetting before the second pour wastes swell capacity; the delay coating helps but site protection is still needed
• Pressure rating (1 bar after 1 day) is a single-strip construction-joint figure; deep or high-water-table structures need a belt-and-braces approach with a membrane or PVC waterbar
• Not a substitute for good concrete quality, correct cover, or proper joint design and waterproofing grade selection
• Fixed 10x20mm section and 5m roll format; very large joint volumes or pre-formed gaps may suit other systems better

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Supercast SW20

Within the Fosroc range, Supercast SW20 sits in the waterstop family alongside the smaller-section Supercast SW10 and the rigid Supercast PVC waterbars — and the three are designed to work together (linked via Supercast SWX) rather than compete, with SW20 handling construction joints and the PVC bars handling expansion/movement joints. It should not be confused with Fosroc’s elastomeric joint sealants, which solve a different problem: Thioflex 600 (two-part polysulphide, gun and pouring grades) for general building and water-retaining movement joints, Nitoseal PU sealants (polyurethane) for facade, floor and perimeter joints, and Colpor 200PF (cold-applied pavement-grade) for trafficked and fuel-resistant joints such as runways and hardstandings — all of which seal visible moving joints rather than internal construction joints. For projects that prefer an alternative supply chain, comparable swellable/hydrophilic waterstops include Sika’s SikaSwell range, BASF/MBCC MasterSeal swellable profiles, and Indian-manufactured hydrophilic strips and PVC waterbars from STP (Supreme), CICO and similar; for the moving-joint sealants, credible equivalents are Sika Sikaflex (PU) and Sikadur, and MasterSeal sealants. As an authorised Fosroc distributor and applicator serving Delhi NCR and Pan-India, Space Arc Engineering supplies genuine Supercast SW20 with the matching SWX adhesive and PVC waterbar accessories, and can advise on the correct combination of swellable strip, PVC waterbar and surface sealant for a given basement grade, water table and joint type.

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