When water is leaking through a concrete crack, whether in a basement wall, a retaining structure, or a water tank, PU injection is often the fastest and most reliable repair method. Unlike epoxy injection, which restores structural strength but cannot stop flowing water, PU injection is designed specifically to seal cracks against water ingress. The polyurethane resin reacts with water to form a flexible, expanding foam that fills the crack from the inside out. I have used PU injection to stop leaks in situations where the water pressure was so high that the crack was spraying water 3 metres across the room — and within minutes, the flow stopped completely.
When to Use PU Injection
PU injection is the method of choice when: the crack is actively leaking water; the crack is subject to ongoing movement (thermal, structural); the structure is below grade and subject to hydrostatic pressure; or the crack is in a water-retaining structure like a tank, reservoir, or swimming pool. It can also be used as a pre-treatment before applying a waterproofing membrane — sealing the crack from the inside prevents water from migrating behind the membrane later.
For non-moving, dry cracks where structural restoration is needed, epoxy injection is a better choice. But for any crack with moisture or water flow, PU is the right material. I have tried epoxy on damp cracks, and the bond always fails because the moisture prevents the epoxy from adhering. PU, on the other hand, uses the water as a reactant.
Hydrophilic vs Hydrophobic PU for Cracks
Both hydrophilic and hydrophobic PU grouts are used for crack injection, but they serve different purposes. Hydrophilic PU absorbs water and swells into a flexible gel. It is ideal for cracks that may experience future movement — the gel remains flexible and can accommodate opening and closing cycles. It is also self-healing to some extent: if a small gap reopens, the gel re-swells when it contacts moisture.
Hydrophobic PU forms a rigid, closed-cell foam that does not re-emulsify or swell further once cured. It has much higher strength than hydrophilic grout and is preferred for cracks where water flow is high and immediate shut-off is needed. Hydrophobic PU achieves full cure in seconds to minutes when in contact with water, while hydrophilic grout may take 15–45 minutes.
For most structural crack repairs where some future movement is expected, I prefer hydrophilic PU. For high-flow leaks and void filling behind the crack, hydrophobic PU is more effective. Some contractors use a two-stage approach: hydrophobic PU first to stop the flow, then hydrophilic PU to ensure long-term flexibility.
Crack Preparation and Port Installation
Proper preparation is essential. The crack surface must be clean and free of loose material. For active leaks, I do not try to dry the crack — the PU needs water to react. But I do clean away any dirt, oil, or efflorescence that might block the resin's path.
Injection ports are installed along the crack at intervals of 150–200 mm. For each port, a hole is drilled about 25–40 mm deep into the crack, and a plastic or brass port is inserted and fixed with a rapid-setting cement plug or epoxy putty. The surface of the crack between the ports is sealed with a fast-setting cement or flexible epoxy to prevent the PU from exiting the surface before it has penetrated the full depth of the crack.
For cracks wider than 3 mm, a cementitious surface seal works well. For narrower cracks, an epoxy paste is better because it bonds more tightly to the concrete and will not crack under injection pressure. Let the surface seal cure for at least 1 hour before injection.
Injection Technique
Start injecting at the lowest port or the one with the most active water flow. The PU is injected using a manual or pneumatic injection gun connected to the port. Inject at a slow, steady pressure — typically 5–20 bar for concrete cracks. Watch for the PU to exit at adjacent ports or to appear at the crack surface. When it does, seal that point and move to the next port, continuing in sequence.
The injection should continue until the port refuses to accept more grout at the target pressure (refusal). This indicates that the crack is completely filled at that location. After all ports have been injected to refusal, allow the PU to cure (15–60 minutes depending on the formulation and temperature), then cut off the ports flush with the surface and patch with a repair mortar if needed for appearance.
One tip from experience: always have a container of water and a cleaning solvent (like acetone) ready. PU resin is incredibly sticky and will bond to everything it touches, including your gloves, tools, and clothing. Clean any spills or drips immediately before they cure.
Common Challenges and Solutions
The most common challenge is the PU resin flowing out of the crack before it has reacted enough to stay in place. This happens when the crack is too wide (over 5 mm) or the water flow is too strong. The solution is to use a faster-reacting hydrophobic PU or to pre-fill the crack with a coarse sand or sponge material to restrict the flow and give the PU time to react.
Another frequent problem is incomplete filling — the PU only fills part of the crack depth, leaving a void deeper inside. This is usually because the injection pressure was too low or the port spacing was too wide. I recommend reducing port spacing to 100 mm for deep cracks or cracks in thick walls, and increasing injection pressure gradually until refusal.
A third issue is that the PU foam re-emerges days or weeks later, pushed out by residual water pressure. This indicates that the crack was not fully filled to its deepest extent, and the water pressure behind the crack is forcing the foam out. The solution is to drill deeper injection ports that reach behind the crack plane and re-inject with a hydrophobic PU that will fill the void behind the crack.
Can PU injection be used for hairline cracks?
Yes, but the crack must be wide enough for the resin to enter. For cracks under 0.2 mm, the viscosity of even low-viscosity PU may be too high. In these cases, surface sealing with a flexible waterproof coating may be more practical than injection.
How soon after PU injection can the area be put back into service?
For hydrophobic PU, backfill or water contact can resume within 1 hour. For hydrophilic PU, allow 24 hours for full gel formation. If the repaired area will be coated or painted, wait at least 48 hours for any residual solvent or unreacted monomer to dissipate.
Does PU injection stop all types of leakage?
PU injection is highly effective for single-crack leaks and joint leaks. For diffuse seepage through porous concrete (where water is weeping across a large area rather than flowing from a single crack), PU injection is not appropriate — use a cementitious or crystalline waterproofing system instead.