Polyurethane crack injection is the standard way to stop water coming through a crack in a slab, wall or tank. A coating on the surface cannot hold back water that already travels inside the concrete. Injection, by contrast, fills the path itself. This guide explains how to diagnose the crack, which resin to choose and how Target tests the result.
Contents
- Diagnose the crack before choosing a resin
- How polyurethane crack injection works
- Acrylic curtain injection: Sika Injection-306
- Packers, ports and the injection sequence
- Polyurethane crack injection: execution steps
- Common mistakes
- When a structural check comes first
- FAQ
- Conclusion
Diagnose the crack before choosing a resin
Not every crack needs the same treatment. So the first visit is about reading the crack, not drilling it.
- Is it wet? Damp, dripping or running water each call for a different start.
- Is it moving? A crack that opens and closes needs an elastic filler.
- Is it structural? Wide, growing or stepped cracks may signal a load or settlement problem.
- What is behind it? Soil, a tank or another room changes the access and the method.
Water-bearing cracks are a common form of concrete degradation in basements, tanks and roofs. Equally, honeycombed concrete at poor compaction spots can carry water in the same way.
How polyurethane crack injection works
Polyurethane resins react with water. When the liquid meets moisture inside the crack, it foams and expands to fill the gap. You can read about the chemistry of polyurethane in general.
MasterInject 1335 (now SikaInject-1335)
MasterInject 1335, formerly known as Concresive 1335, is a solvent-free polyurethane injection foam. Sika’s MBS product list now names it SikaInject-1335. Its submittal title covers slab cracks between 0.05 mm and 5 mm wide.
The data sheet lists pre-injection of cracks and joints, sealing flowing or leaking water, and filling water-bearing voids. It also states that the product reacts well in moist surroundings and bonds to wet surfaces. In addition, the sheet cites certification for contact with drinking water.
Key values from the sheet:
- Accelerator dosage: 2–10%, chosen by the reaction time you need.
- Viscosity: 700–900 cps at 25 °C; density 1.16 g/cm³.
- Application temperature: +5 °C to +40 °C.
- Free foam expansion: about 1:20–30 at 10% accelerator.
- Reaction at 20 °C with 10% accelerator: starts after about 15 s and ends after about 60 s.
Reaction speed depends on both the structure and the product temperature. For this reason, the sheet asks for site trials before work starts.
Acrylic curtain injection: Sika Injection-306
Sometimes the crack is only part of the problem. Water may run behind the wall through soil, or along a failed membrane. In that case, an acrylic resin can seal the zone, not just the crack.
Sika Injection-306 is a very low-viscosity, elastic polyacrylic resin. Its sheet gives a viscosity of about 3–11 mPa·s, close to water. The crew can adjust its curing time between 10 and 60 minutes. Moreover, it can reversibly absorb and release moisture, so it tolerates minor movement.
Its uses include water-bearing cracks, leaking elements in saturated ground and the repair of damaged waterproofing membranes. However, the sheet says only experienced professionals may use it. Before curtain injection near a building, the team must survey the site and the ground. The crew must also confirm there are no drains or open pipes near the injection area.
| Property (from data sheets) | MasterInject 1335 | Sika Injection-306 |
|---|---|---|
| Resin type | Polyurethane foam | Elastic polyacrylic gel |
| Viscosity | 700–900 cps at 25 °C | ~3–11 mPa·s at 20 °C |
| Reaction control | 2–10% accelerator | 10–60 min via accelerator chart |
| Typical use | Cracks 0.05–5 mm, running water, voids | Cracks, joints, curtain and membrane repair |
| Temperature range | +5 to +40 °C | +5 to +40 °C |
Packers, ports and the injection sequence
The MasterInject 1335 method statement sets out the steps. First, the crew cleans the crack with high-pressure water or a suitable solvent. Next, they drill holes into the crack for entry ports. The sheet puts port spacing at 150–500 mm, depending on crack width and depth.
For a vertical crack, injection starts at the lowest port. When resin comes out of the next port up, the crew caps the lower one and moves up. For horizontal cracks, the work runs from one end to the other.
The method statement gives a simple test. The crack is full only when the pump can hold pressure. If pressure drops, the resin is still flowing into the crack or escaping somewhere else.
Plugging and surface sealing
Where water is running hard, a fast plug comes first. Prokem’s Waterplug sets in 3–5 minutes and stops running water through concrete and masonry. After that, injection can proceed without washing the resin out.
On the surface, MasterProtect 2020 is a non-slump epoxy mortar. Its sheet lists the fixing of surface ports for crack injection and the repair of fine cracks and spalls. It reaches 75 N/mm² at 7 days (ASTM D695) and needs no primer.
Polyurethane crack injection: execution steps
At Target, a typical job follows these steps:
- Map every crack and record width, length and water flow.
- Plug strong leaks with a fast-setting hydraulic cement.
- Clean the crack and, if needed, seal its face.
- Drill and fit ports at the spacing set for the crack.
- Mix only the resin you can inject, with the accelerator chosen from site trials.
- Inject from the lowest port upward until each crack holds pressure.
- Remove the ports, fill the holes and grind the surface where needed.
- Flush the pump: the 1335 sheet asks for xylene, then flushing or hydraulic oil.
How we test the result
The first test happens during injection, since each crack must hold pump pressure. After that, we inspect the wall or slab face for damp patches after the next wet period. For tanks, we refill the tank and watch the outer face before handover. If a damp spot appears, we inject again from an extra port close to it.
Common mistakes
- Injecting without a trial. Reaction times change with temperature.
- Too much accelerator in summer. The foam may set before it fills the crack.
- Ports too far apart. Resin will not travel the full length.
- Stopping when resin first appears. Hold pressure to confirm the crack is full.
- Leaving resin in the pump. It reacts with air humidity and blocks the lines.
- Treating a structural crack as a leak only. Foam stops water, not structural movement.
When a structural check comes first
Polyurethane crack injection seals water paths. It does not restore load-bearing capacity. So when cracks are wide, growing, or appear near supports, Target asks for a structural assessment first.
In those cases, the fix may involve structural repair or strengthening, as described on our repair and rehabilitation page. Sealing the water comes after the structure is safe. You can also view the type of high-rise work we deliver, such as the Iconic Tower project.
FAQ
How long does polyurethane crack injection take to set?
With 10% accelerator at 20 °C, the MasterInject 1335 sheet shows the reaction ends in about 60 seconds. Lower temperatures slow it down.
Can you inject a crack that is still leaking?
Yes. MasterInject 1335 works in wet conditions and bonds to wet surfaces. Strong flows may need a fast plug first.
Is injected foam safe in drinking-water tanks?
The MasterInject 1335 sheet states it is certified for contact with drinking water. Always check the current certificate for your project.
What is grout curtain injection?
It is injection into the soil or layers behind a wall to form a sealed zone. It is a form of grouting, and Sika Injection-306 lists it among its uses.
Conclusion
A surface coat rarely solves a leaking crack. Polyurethane crack injection, or acrylic curtain injection when water travels behind the wall, seals the path itself. Success depends on diagnosis, port spacing, trial mixes and patient injection.
For a site survey and an injection method statement, contact our injection works team.
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