Injection Grouting Explained: PU, Epoxy, Acrylic and Cement Compared

Injection grouting is the technique that separates contractors who can stop an active leak from those who can only coat a dry surface. It is also the area where the wrong material choice produces the most expensive failures, because a badly injected crack is harder to treat the second time.

Why injection exists

Every surface coating shares one limitation: it must be applied to a dry surface, and it works by keeping water away from a defect. When water is already coming through a crack under pressure — a basement wall below the water table, a lift pit, a live construction joint — there is no dry surface to coat and no way to hold a film against the flow. Injection solves the problem from the other side. Resin is pumped into the crack under pressure and seals it inside the structure, where the water is, not where the stain is.

The four chemistries

PU: hydrophobic or hydrophilic

Both react with water, and the difference matters. Hydrophobic resin cures to a closed-cell foam that is dimensionally stable and does not shrink back when the crack later dries out. That makes it the safer default for a permanent seal in a structure that will go through wet and dry seasons. Hydrophilic resin cures to a flexible gel that swells further on rewetting, which suits a joint that genuinely moves and is wet seasonally — but it shrinks in the dry season, so choosing it where hydrophobic was needed produces a seal that reopens and leaks again the following monsoon.

Epoxy: a strength repair, not a leak repair

Structural epoxy injection restores continuity across a crack. The cured resin exceeds the compressive strength of the surrounding concrete, so the cracked member behaves as a monolith again and the load path is re-established. That makes it correct for a structural crack in a beam, column or slab where capacity matters, consistent with the repair principles underlying IS 456.

Two conditions must both hold. The crack must be dry, because epoxy will not bond through water. And it must be dormant, because epoxy is rigid — inject a crack that is still moving and the failure simply relocates to fresh concrete a few millimetres away. Establishing dormancy means monitoring with a crack gauge across a temperature cycle, not judging by eye on the day of the survey.

Acrylic gel: for seepage without a crack

A common and frustrating situation is a basement wall that is not leaking through any definable crack but weeping across a broad area. Crack injection has nothing to inject into. Acrylic gels have a viscosity close to water and a controllable gel time, so they can be injected through the wall into the soil behind it, forming a continuous impermeable curtain outside the structure. This is the correct treatment for diffuse seepage and it is not achievable with PU, whose foaming action cannot penetrate fine soil porosity.

Cement grout: for volume

Where the void is large — honeycombed concrete behind a shutter, a gap under a machine base, a fissured rock face — resin becomes prohibitively expensive and cementitious grout is the sensible answer. Microfine cements, ground far below ordinary Portland cement particle size, penetrate fissures that standard grout cannot enter while keeping the cost and compressive strength advantages. It is common to use cement grout as the bulk fill and follow with a resin to seal the residual fine pathways.

How the work is actually done

1. The crack is mapped and its width and activity assessed.
2. Ports are drilled at roughly 45 degrees, staggered either side of the crack, so each hole intersects the crack partway through the section. Drilling straight into the crack face is a common error that leaves the interior untreated.
3. Mechanical packers are set and tightened.
4. Resin is pumped from the lowest port upward, at pressures typically between 80 and 200 bar for PU work, until it appears at the next port.
5. Packers are removed after cure and the holes are made good.
6. Where required, a surface treatment or coating follows once the leak has stopped.

Under-powered equipment is a real problem in this trade. A pump that cannot sustain pressure will not drive resin along the full depth of a crack, producing a repair that seals the visible face and leaves the interior open. The leak returns, and the next contractor has a harder job.

Frequently asked questions

When is injection grouting needed?

Whenever water is actively coming through a crack under pressure, because coatings require a dry surface and cannot hold against flow.

PU or epoxy for a leaking basement?

PU. Epoxy will not bond through water and is a structural repair for dry, dormant cracks.

How is it priced?

Per injection point — roughly ₹650–1,400 for PU and ₹750–1,600 for structural epoxy — with the number of points established at inspection.