Sika SikaProof-312 HDPE SG W IN
A pre-applied, fully bonded HDPE sheet membrane that fuses to freshly poured concrete, sealing basements and below-ground structures against ground water without any lateral seepage path.
What is it?
A virgin-HDPE self-adhesive sheet membrane, coated with a pressure-sensitive adhesive and a protective sand layer, with a heat-weldable selvedge along one edge. Effective composite thickness is greater than 1.5 mm.
Where is it used?
Below-ground reinforced concrete: base slabs, footings, grade slabs, confined single-faced-formwork walls, basements, subways and tunnels where external access is lost after casting.
Why use it?
The adhesive activates when concrete is cast against it and forms a permanent, monolithic bond, so water cannot track between the membrane and the structure even if a puncture occurs.
How is it applied?
Loose-laid on horizontal or inclined substrates and fixed to vertical formwork, adhesive side up; laps are hot-air welded through the selvedge, then reinforcement and concrete are placed directly on top within 45 days.
About Sika SikaProof-312 HDPE SG W IN
SikaProof-312 HDPE SG W IN is Sika’s high-performance pre-applied waterproofing membrane for reinforced concrete structures that sit below the water table. It is built around a virgin (non-recycled) high-density polyethylene film, coated with a high-bond pressure-sensitive adhesive and finished with an inorganic sand-sprinkled layer that protects the adhesive from site traffic and weather during construction. What sets the “W” variant apart from the plain SG grade is the factory heat-weldable selvedge running along the sheet edge, which lets crews thermally weld overlaps into a continuous, mechanically fused joint rather than relying on tape alone. The membrane is loose-laid over blinding, formwork, insulation or plywood before the reinforcement cage is fixed, so there is no waiting for a substrate to cure and no protective screed to pour.
The defining behaviour happens at the pour. When fresh concrete is placed directly against the adhesive face, the cement paste keys into it and cures into a permanent, intimate bond with the finished structure. Because the membrane is married to the concrete rather than loosely draped behind it, there is no void for water to migrate through, which is the failure mode that lets water find a slab crack metres away from where it entered. That “no lateral migration” property is why pre-applied systems are specified for blindside and deep basement work where you can never get back to the outside face to repair a post-applied coating. For engineers, contractors and waterproofing consultants across Delhi NCR and pan-India, Space Arc Engineering supplies the membrane with the matching Sika sealing tapes and detailing accessories needed to close out a complete, warrantable system.
The technical insight
Post-applied tanking sheets sit against the concrete but are never truly bonded to it; if water breaches one point, it spreads through the gap behind the sheet until it finds the nearest construction joint or crack, so a pinhole at the corner can surface as a leak in the middle of the slab. SikaProof-312 HDPE SG W IN inverts that logic. The pressure-sensitive adhesive, protected on site by the sand layer, is chemically and mechanically gripped by the cement paste as the concrete hydrates, producing a peel bond of at least 1500 N/m measured at seven days. That continuous bond compartmentalises the interface: any local damage stays local because there is no continuous water plane behind the sheet. The HDPE film itself carries the mechanical duty, with tensile strength of at least 25 MPa, elongation over 500% to bridge shrinkage and settlement cracks, and static puncture resistance around 1000 N to survive rebar chairs and foot traffic before the pour. Water tightness is rated above 71 m of hydrostatic head, and the film stays foldable and workable down to -23°C, so cold-store or high-water-table sites are within scope.
Where it is used
- Reinforced concrete base slabs and raft foundations below the water table
- Confined basement and retaining walls cast against single-faced formwork or piling (blindside)
- Reinforced concrete footings and grade slabs
- Subway, metro and road tunnel below-ground concrete
- Extension and reconstruction works tying into existing below-ground structures
- Prefabricated and precast below-ground elements
- Deep pits, tank bases and lift/plant-room shafts
- Dampproofing and concrete protection against aggressive ground water and soils
Key advantages
- Pre-applied and fixed before reinforcement, so no substrate cure wait and no protective screed is needed
- Bonds permanently to fresh concrete, eliminating the lateral water-migration path behind the sheet
- Heat-weldable selvedge produces continuous, mechanically fused laps stronger than tape-only joints
- High elongation (>500%) bridges shrinkage and settlement cracks in the structure
- Sand-protected adhesive tolerates site traffic and temporary weather/UV exposure during the build
- Foldable and workable down to -23°C, suiting cold and high-water-table sites
- Chlorine-, plasticiser- and phthalate-free virgin HDPE for long-term durability and eco compliance
- Declared to IS 16471:2017 and tested to a full suite of ASTM methods for specification confidence
Limitations & considerations
- Not a permanent finish: continuous UV exposure degrades it, so concrete must be cast within 45 days and backfill protection applied promptly
- Best suited to single-faced formwork; with double-faced formwork the membrane can debond from concrete (watertightness is retained, but Sika prefers post-applied systems here)
- Requires trained or Sika-approved applicators and thermal welding kit (hot-air gun, Leister-type jointing machine) for reliable seams
- Substrate must be smooth, stable and free of standing water; gaps of 12-15 mm or more must be filled before laying
- Application window is +5°C to +40°C ambient and substrate; below +5°C needs Sika Technical Services clearance and special precautions
- Penetrations, construction joints and detailing still depend on the correct Sika ancillary sealing products to be watertight
Selection & best practice
Lay the membrane adhesive-side up on a clean, stable, standing-water-free substrate, filling any voids of 12-15 mm or larger first; the substrate may be damp but not wet. Overlap sheets 80 mm in the provided selvedge and weld longitudinal laps with a hot-air machine set between +275°C and +375°C at 0.8-2 m/min, cleaning the selvedge of dust and oil before welding and adjusting temperature/speed to site conditions. For transverse laps, first scrape off the sand layer with a hot-air tool, then overlap 80 mm and weld the same way. Detail all penetrations and construction joints with the matching SikaProof ancillary products, carry out a final inspection to confirm the adhesive face is clean and undamaged, then place reinforcement and concrete directly on the membrane, with no mortar or screed, within 45 days. After formwork removal, seal shutter-anchor points and any damage, and protect the membrane before backfilling.
Specifications
| Chemical base | HDPE sheet membrane with pressure-sensitive adhesive layer and sand topping |
| Effective thickness | HDPE composite > 1.5 mm (ASTM D3767) |
| Roll size | 20 m length; 1.2 m, 1.5 m or 2.0 m width |
| Tensile strength | ≥ 25 MPa, film (ASTM D412) |
| Elongation | > 500% (ASTM D412) |
| Static puncture resistance | 1000 ± 50 N (ASTM E154) |
| Water tightness | > 71 m of hydrostatic head (ASTM D5385) |
| Adhesion in peel | ≥ 1500 N/m at 7 days (ASTM D903) |
| Standard / declaration | Bonded sheet membrane to IS 16471:2017; foldable, unaffected at -23°C (ASTM D1970) |
Industry use cases
Commercial & residential basements
Deep multi-level basements below the water table use the pre-applied bond so a puncture during the build cannot become a wandering leak later, and the slab is castable without a protective screed.
Metro & tunnel infrastructure
Below-ground metro slabs and cut-and-cover tunnel sections rely on the >71 m hydrostatic rating and welded selvedge joints for continuous, high-head watertightness.
Blindside / single-faced formwork walls
Walls cast against piling or shoring where the outer face is inaccessible after pour are the textbook case for a fully bonded pre-applied membrane.
Industrial pits, tanks & plant rooms
Lift pits, tank bases and below-grade plant rooms in industrial builds benefit from the puncture resistance and chemical-resistant virgin HDPE against aggressive ground water.
Cold-climate & high-water-table projects
Foldability down to -23°C and tolerance of damp substrates suit hill-station, cold-store and high-water-table sites where other membranes stiffen or lift.
Frequently asked questions
What is the difference between SikaProof-312 HDPE SG W IN and the plain SG grade?
How soon must concrete be cast after installing the membrane?
Do I need a protective screed or mortar over the membrane before concreting?
Can it be used with double-faced formwork?
What temperature range and site conditions are needed for application?
How are the sheet joints sealed?
How we position Sika SikaProof-312 HDPE SG W IN
SikaProof-312 HDPE SG W IN is the welded-selvedge, high-performance tier of Sika’s pre-applied range; the plain SikaProof-312 HDPE SG IN is the tape-jointed option for less demanding heads, while the SikaProof-312 IN HDPE is the base variant. It works as a system with SikaProof Sandwich Tape-100 IN or SikaProof-936 double-sided tape plus the SikaProof detailing accessories, which Space Arc supplies alongside the rolls. For post-applied situations, or double-faced formwork where a pre-applied membrane can debond, pair it with Sika liquid-applied or preformed membranes from our Sika catalogue. Where a specifier prefers a mechanically-keyed pre-applied system, Fosroc Proofex Engage is the closest cross-brand equivalent in our range.
Fosroc Proofex Engage — Choose Proofex Engage when the specification calls for a mechanically interlocking pre-applied membrane whose 3D cell mesh keys into the concrete, rather than a pressure-sensitive adhesive bond, on blindside and deep-raft work.
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