Fosroc Proofex Swellmastic
A single-component, gun-applied hydrophilic mastic that expands on contact with water to seal construction joints, pipe penetrations and rough concrete interfaces from the inside.
What is it?
A grey, elastomeric water-swellable paste from Fosroc, applied like a sealant from a standard cartridge gun. It is a hydrophilic waterstop that expands roughly 200% by volume when it meets water.
Where is it used?
Non-movement construction joints, cold joints, pipe and conduit penetrations, precast segment interfaces and rough surfaces in basements, water tanks, tunnels, manholes and below-ground concrete.
Why use it?
It seals irregular gaps that strip waterstops cannot follow, needs no mixing, cures fast enough to concrete over within hours, and holds its swelling performance through repeated wet and dry cycles.
How is it applied?
Extruded as a continuous bead of at least 10 mm diameter onto clean, damp-tolerant concrete, then cast into the joint with a minimum 50 mm concrete cover on all sides.
About Fosroc Proofex Swellmastic
Proofex Swellmastic is Fosroc’s problem-solving answer to the joints a rigid strip waterstop simply cannot handle. On any real site, construction joints are rarely clean and straight. Pipes push through retaining walls at awkward angles, precast segments meet with uneven faces, and slurry walls butt against slabs with gaps that no preformed strip will sit tight against. This gun-grade hydrophilic mastic is designed for exactly those situations. It is a grey elastomeric paste that comes ready to use in a cartridge, so a single applicator can gun it into a recess, around a penetration or along a rough cold joint and move on, without the mixing, priming or timing headaches of a two-part system.
The performance comes from its hydrophilic chemistry. Once the surrounding concrete is cast and groundwater eventually reaches the seal, the mastic absorbs water and expands by around 200% of its volume. Because that expansion is confined within the concrete, it develops a positive compressive pressure against the joint faces and physically blocks the water’s path. Just as importantly, this swelling is reversible and durable. The material takes up water in the wet season and gives it back in the dry, and independent-standard wet and dry cycling shows it retains its elastomeric character and swelling capacity over these cycles rather than degrading. For Indian basements, STPs, lift pits and metro structures that see monsoon-driven water table swings every year, that repeatable behaviour is what separates a lasting seal from a one-season fix.
The technical insight
The mechanism is compression sealing driven by controlled swelling, not a chemical bond to water. The polymer carries hydrophilic groups that draw water into the material, and as it takes on water it grows to roughly twice its original volume. In free air this would just be an expanding blob, but cast inside concrete the expansion has nowhere to go, so it converts into pressure against the joint faces and squeezes any potential leak path shut. Two design numbers govern whether this works on site. First, the mastic must be given a minimum of 50 mm of concrete cover on all sides so the young concrete can restrain the swelling pressure instead of spalling under it. Second, the timing of the concrete pour matters. The mastic is tack-free in about an hour at 20 degrees C, and Fosroc allows hand-placed concrete cover from 2 hours, but recommends waiting up to 8 hours before large pours where fresh concrete may fall directly onto the seal. Pour too early, or expose the fresh bead to heavy rain before it has skinned, and it can begin to swell prematurely and lose its position. Handled correctly, the same forgiving paste that plugs inequalities in rough concrete becomes a permanent, self-activating gasket inside the joint.
Where it is used
- Sealing pipe, cable and conduit penetrations through basement and retaining walls
- Sealing joints in precast manhole rings, cable ducts and inspection chambers
- Bonding and sealing precast segment interfaces in tunnels and box structures
- Sealing rough interfaces such as slurry (diaphragm) walls meeting concrete slabs
- Sealing around H-beams, kingposts and other steel penetrations through concrete
- Belt-and-braces sealing alongside conventional rubber or PVC waterstops before a pour
- Filling and sealing irregular cold and construction joints that strip waterstops cannot follow
- Adhering Supercast SW swellable strip to concrete while filling undulations beneath it
Key advantages
- Gun-applied and single-component – no mixing, no primer, minimal wastage
- Swells about 200% by volume to develop a positive, self-activating compression seal
- Plugs inequalities in rough or uneven concrete where rigid strips leave gaps
- Adheres to damp, uneven surfaces and stays put during the concrete pour
- Fast curing – allows hand-placed concrete cover from 2 hours on emergency repairs
- Retains swelling and elastomeric performance through repeated wet and dry cycling
- Works as both a stand-alone hydrophilic waterstop and a swellable adhesive for SW strip
- Backed by Fosroc’s BS EN ISO 9001 registered quality system and India technical support
Limitations & considerations
- Not for expansion joints or joints subject to significant repetitive movement
- Requires a minimum of 50 mm concrete cover on all sides to restrain swelling pressure
- Not intended as a general building sealant – it is a below-grade waterstop, not a facade sealant
- Must be protected from heavy rainfall while curing to prevent premature expansion
- Do not pour concrete directly onto the fresh bead – allow the recommended 2 to 8 hours first
- May cause skin sensitisation – gloves and eye protection are required during application
Selection & best practice
Start with a sound substrate. Remove loose debris, soil, moss and organic growth, and brush away any standing or seeping water – the mastic tolerates damp concrete but not running or ponded water, so divert active channels away from the area first. Cut the cartridge tip, fit the nozzle and gun a continuous bead of not less than 10 mm diameter, with no breaks along its length; step up to a 15 mm bead when using it to bed Supercast SW10 or SW20 strip. Keep the bead positioned so it will finish with at least 50 mm of concrete cover all round. Allow about 30 minutes before re-establishing any diverted water channels, then leave a minimum of 2 hours before hand-placed concrete and up to 8 hours before large pours that could drop concrete straight onto the seal. Shield the fresh bead from heavy rain while it skins. Store cartridges unopened in cool, dry conditions and use within the 9-month shelf life.
Specifications
| Form | Elastomeric water-swellable paste (gun-grade) |
| Colour | Grey |
| Chemistry | Single-component hydrophilic swellable polymer |
| Swell ratio (volume) | Approximately 200% |
| Tack-free time | Approx. 1 hour at 20 C / 50% RH |
| Cure rate | 3 mm in 24 hours |
| Hardness, Shore A | 30 |
| Minimum bead diameter | 10 mm (15 mm as SW strip adhesive) |
| Minimum concrete cover | 50 mm on all sides |
| Coverage | ~3.9 m per cartridge at 10 mm bead; ~1.75 m at 15 mm bead |
| Pack size | 310 ml cartridge (single or box of 20) |
| Shelf life | 9 months in original unopened cartridges, cool and dry |
Industry use cases
Basements & Podiums
Seals pipe penetrations, tie-rod holes and rough cold joints in raft and retaining-wall construction where monsoon groundwater loads the structure.
Water & Sewage (STP/WTP)
Provides a durable internal seal at construction joints and penetrations in tanks and sumps that face constant wet and dry cycling.
Tunnels & Metro
Seals precast segment interfaces and kingpost or H-beam penetrations, and beds swellable strip against uneven cast-in-situ concrete.
Infrastructure Precast
Seals joints in precast manhole rings, cable ducts and box culverts where segment faces are rough and access is tight.
Repair & Remediation
Fast cure allows hand-placed concrete cover from 2 hours, making it suited to emergency joint sealing and leak remediation on live structures.
Frequently asked questions
Is Proofex Swellmastic the same as Fosroc Supercast SWX?
How much does one cartridge cover?
Can I apply it to damp concrete?
Why does it need 50 mm of concrete cover?
Can it be used in an expansion joint?
How soon can we pour concrete over it?
How we position Fosroc Proofex Swellmastic
Proofex Swellmastic is the paste-form member of Fosroc’s hydrophilic waterstop range, best where joints are irregular, congested or penetration-heavy. On long, straight, formed construction joints, a strip waterstop is more economical – pair it with, or step up to, Fosroc Supercast SW swellable strip, which this mastic is also designed to bed and seal against undulating concrete. For conventional dumbbell-profile joints, Fosroc’s Supercast PVC waterstops remain the workhorse. Where the issue is an already-cracked or leaking joint rather than a fresh pour, look instead at injection systems from our polyurethane and acrylic grout range. Across brands, teams standardising on a single supplier can also match this against MC-Bauchemie and Sika hydrophilic swellable profiles – talk to our Delhi NCR technical desk and we will map the right waterstop strategy for your structure.
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