When water is actively coming through a specific crack in concrete — a basement wall, a water tank, a foundation — the go-to repair in most cases isn't to re-waterproof the whole surface, it's to inject the crack itself. Polyurethane resin injection works by pumping a liquid resin directly into the crack under pressure, where it reacts, expands to several times its original volume, and cures into a flexible, watertight seal that can move with the structure afterward.

There are two broad approaches. Hydrophilic polyurethane actively seeks out moisture — on contact with water it foams and expands, which makes it ideal for cracks that are actively leaking right now, since the water itself triggers the seal. Hydrophobic polyurethane instead repels water, forming a barrier on the outside face of the crack; it's typically used for curtain grouting around structures with many small cracks, sealing them from the exterior without excavation.

Injection is done through small ports drilled along the crack, using either a hand-pumped gun for low-pressure work or an electric pump for higher-pressure applications on wider or deeper cracks. Because it's a targeted repair rather than a surface treatment, it's typically a fraction of the cost of excavating and re-waterproofing a wall from the outside — and because the cured resin stays flexible, it accommodates the ongoing minor movement that caused the crack in the first place, rather than cracking again at the same spot.

It's worth being clear about scope: polyurethane injection seals non-structural cracks against water. Wide cracks caused by genuine structural movement — settlement, overloading, foundation failure — need a structural assessment first; sealing the symptom without addressing the structural cause just delays the next, likely larger, repair.