When water is seeping through a concrete crack under pressure, or when loose soil beneath a foundation needs to be stabilised quickly, there is really no better tool in the contractor's arsenal than polyurethane grouting. Polyurethane — or PU — grouting is a versatile injection technique used for stopping water ingress, consolidating soil, filling voids, and even lifting settled concrete slabs. I have used PU grouting on projects ranging from basement waterproofing in Delhi NCR to dam seepage control in Himachal, and it rarely disappoints when applied correctly.
What Is Polyurethane Grouting?
Polyurethane grout is a two-component resin system that reacts to form a rigid or flexible foam. When injected into cracks, joints, or soil, it expands many times its original volume — typically 5 to 30 times — filling every void and creating a watertight seal. The reaction can be triggered by moisture (for hydrophilic systems) or by a chemical catalyst (for hydrophobic systems).
The key advantage of PU grouting over cementitious grouting is its ability to penetrate fine cracks and voids that cement particles simply cannot reach. Cement grout particles are typically 30–50 microns in size; PU resin can penetrate gaps as small as 0.1 mm. For stopping active water leaks under pressure, PU works where nothing else will.
Hydrophilic vs Hydrophobic PU Grouts
This is where many specifiers get confused. Hydrophilic PU grouts are designed to absorb water and swell, forming a flexible gel. They are ideal for sealing construction joints, cracks in water-retaining structures, and areas subject to movement. The swelling action continues as long as moisture is present, which makes these grouts self-healing to some degree — if a small gap opens up, the gel swells to fill it.
Hydrophobic PU grouts, on the other hand, repel water and form a rigid, closed-cell foam. They react almost instantly upon contact with water, achieving high strength in minutes. These are best for stopping high-flow leaks, stabilising soil, and void filling. I have used hydrophobic PU to stop a 20-litre-per-minute leak through a concrete retaining wall — within 90 seconds of injection, the flow had stopped completely. Hydrophilic grouts would have washed out under that flow rate.
Applications in Foundations and Tunnels
PU grouting is widely used in foundation repair. When a building's foundation settles due to loose or washed-out soil, PU grout can be injected through small-diameter ports to densify the soil and lift the foundation back to its original level. This technique, often called "soil stabilization grouting" or "compaction grouting," is far less invasive than underpinning and can be completed in days rather than weeks.
In tunnelling, PU grouting is used to pre-treat the ground ahead of excavation, sealing off groundwater and stabilising loose rock or soil before the tunnel boring machine arrives. I consulted on a tunnel project in the Himalayas where the water inflow was so severe that the TBM was about to be abandoned. We pre-injected hydrophobic PU from the surface at 20-metre intervals ahead of the face, and the inflow dropped from 600 litres per minute to negligible levels within three days.
Injection Methodology
Correct injection technique matters as much as material selection. The grout must be injected at the right pressure — typically between 5 and 20 bar depending on the substrate and crack width — and the injection ports must be spaced correctly. For concrete crack injection, ports are usually spaced at 150–200 mm intervals along the crack. For soil stabilisation, injection lances are driven to depth and withdrawn gradually as grout is pumped.
Always start injecting from the lowest point or the widest crack, working upward and across. This ensures the grout displaces water rather than trapping it. And never inject PU grout without wearing proper PPE — the isocyanates in the resin can cause severe respiratory sensitization. A full-face respirator with organic vapour cartridges, nitrile gloves, and chemical-resistant coveralls are non-negotiable.
Common Mistakes to Avoid
The most common mistake I see is using hydrophilic grout in a high-flow situation. It simply washes out before it can react. Match the grout type to the water flow — hydrophobic for moving water, hydrophilic for damp or intermittent seepage.
Another frequent error is under-pumping. PU grout needs to be injected until it exits through adjacent ports or the crack is fully saturated. Stopping too early leaves voids that will leak again. A good rule of thumb: pump until you see clean grout at the next port, then move to that port and continue.
Lastly, do not ignore the substrate temperature. PU grouts have an optimal application temperature range of 10°C to 40°C. Below 5°C, the reaction slows dramatically and the grout may not cure properly. I have seen a winter job fail because the contractor injected PU at 3°C and the grout remained tacky for three days.
Can PU grouting lift a settled concrete slab?
Yes. This is called slab jacking or polyurethane lifting. High-expansion hydrophobic PU is injected beneath the slab through small holes. The expanding foam creates a lifting force that can raise slabs by 25–75 mm in a single injection session.
Is PU grouting permanent?
Hydrophobic PU grouts are chemically stable and can last 20+ years underground. Hydrophilic grouts may degrade over time in dry conditions because they need moisture to stay swollen. For permanent water shut-off, hydrophobic is generally preferred.
How long does PU grout take to cure?
Hydrophobic PU grouts can cure in 30 seconds to 5 minutes depending on the formulation. Hydrophilic grouts take longer — typically 15–45 minutes. Full strength development occurs within 24 hours for both types.