Frequently Asked Questions

Can Sikaflex 291 be applied underwater or when the joint is wet, or does the concrete need to be dry before application?

Sikaflex 291 cannot be applied directly underwater or onto a fully submerged, water-filled joint during normal construction conditions. The product cures by reaction with moisture — but there is a critical difference between moisture-curing (surface dampness) and underwater application (fully submerged in flowing water). For Sikaflex 291 application in wet environments: the joint substrate should be as dry as possible on the sealant contact faces — ideally surface-dry or at worst damp-to-touch without dripping or pooled water on the joint face. Remove standing water from the joint by compressed air blowing or mopping. Apply Sika Primer 209 or Sika Aktivator-205 to the dry joint faces and allow to flash off (2 to 5 minutes) before sealant application — do not allow the primer to become wet after application. Gun Sikaflex 291 into the prepared, primed joint and tool immediately. Allow minimum 2 hours of open-air curing above the water level before the joint is submerged. For joints that cannot be dewatered (active tidal zone, live pipe systems that cannot be shut down), specialised underwater-cure sealants or epoxy injection systems are required — Sikaflex 291 is not the appropriate product for injection into actively flowing or constantly submerged joints without dewatering. Contact Space Arc Engineering for guidance on the correct sealant or injection system for specific underwater sealing conditions.

What is the chemical resistance of Sikaflex 291 to the wastewater at a Delhi NCR sewage treatment plant — sulphide gases, organic acids, and chlorinated effluent?

Sikaflex 291 has been tested for chemical resistance in standard sewage and wastewater treatment environments and is suitable for most STP (sewage treatment plant) applications in Delhi NCR. For the specific chemicals encountered in Indian STPs: Hydrogen sulphide (H2S gas): Sikaflex 291 has moderate resistance to H2S gas in the vapour phase — in the sewer headspace above the liquid level where H2S concentrations can be high (100 to 1000 ppm in raw sewer), the sealant may experience surface chalking and slight softening over many years of exposure but this does not cause joint failure in most cases. Short-chain organic acids (acetic, lactic): good resistance in dilute concentrations typical of biological treatment. Long-term resistance in concentrated anaerobic fermentation conditions may be reduced. Sodium hypochlorite (chlorination effluent, 5 to 10 ppm free chlorine): good resistance at typical treatment-level chlorination. Higher concentrations (above 50 ppm, as in disinfection tanks) may cause surface degradation over extended immersion. Sulphuric acid from biogenic sulphide corrosion: in concrete sumps where H2S is converted to sulphuric acid by Thiobacillus bacteria on concrete surfaces (the infamous biogenic acid corrosion in wet wells and inlet chambers), both the Sikaflex 291 sealant and the concrete can be attacked. Sikaflex 291 has better resistance than concrete to sulphuric acid at below 5 per cent concentration, but for severe biogenic acid environments (wet well inlet chambers), consult Space Arc Engineering for a complete chemical-resistant lining system rather than sealant alone.

What is the correct joint preparation and prime coat for Sikaflex 291 sealing the horizontal deck joint between precast concrete STP (sewage treatment plant) wall panels?

For Sikaflex 291 sealing of a horizontal deck joint between precast concrete STP wall panels, the preparation and prime coat sequence is: Step 1 — Joint cleaning: clean the joint faces thoroughly by wire brushing or angle grinder to remove all surface contamination, cement slurry, form release oil, and any previous sealant residue. Blow clean with compressed air. Step 2 — Backing rod: install a polyethylene (PE) closed-cell foam backing rod of the correct diameter (10 to 15 per cent larger than the joint width to ensure a snug fit) to the required depth — the backing rod controls sealant depth and prevents three-sided bond (Sikaflex 291 must only bond to the two joint faces, not the backing rod). For a 20 mm wide joint, specify 15 mm deep x 20 mm wide PE backing rod to give 1.5:1 width-to-depth ratio. Step 3 — Prime coat: apply Sika Primer 209 D or Sika Primer N by cotton-cloth wipe or brush to both concrete joint faces to a thin, even film. Allow the primer to flash off for a minimum of 20 minutes but maximum 2 hours (in Indian conditions at 35 degrees C, do not allow the primer to remain on the concrete for more than 2 hours without applying sealant as the primer film effectiveness decreases). On steel or fibreglass elements adjacent to the concrete panel joint, apply Sika Aktivator-205 by wipe. Step 4 — Masking tape: apply masking tape to both concrete face surfaces adjacent to the joint edge to ensure clean sealant lines. Step 5 — Sealant gun: apply Sikaflex 291 in a continuous bead filling the joint to 5 to 10 mm above the joint face, tool concave with a dampened stainless steel spatula, and remove masking tape immediately.