Crack injection: epoxy or polyurethane, which one to choose?

Inject epoxy into a dormant, dry crack (or a slightly damp one, depending on the product) when the concrete must be bonded back together and its continuity restored; inject polyurethane into a wet crack, one that leaks or still moves, when water must be stopped while keeping flexibility. Epoxy does not stop an active leak and polyurethane does not restore structural strength.

A crack in a slab, a foundation wall, a basin or a gallery does not always call for the same answer. The right product depends on two questions: does the crack still move, and is there water? This guide helps maintenance managers and project managers of industrial, mining, hydroelectric and water treatment facilities in Quebec and Ontario choose the process. It complements our guide to concrete repair before coating.

What is the difference between epoxy and polyurethane injection?

Epoxy is a rigid resin that bonds both faces of the crack and restores the concrete's continuity; polyurethane is a resin that reacts with water, expands into a flexible foam or gel and blocks the water path. One repairs, the other seals.

ASTM C881 classifies epoxy resins for concrete by type and viscosity; a low-viscosity resin is chosen to penetrate fine cracks.

How do you know whether a crack is dormant or active?

Observe the crack over time and look for its cause: a crack whose width does not change, whose cause has ended (shrinkage, past settlement) and which stays dry is dormant; a crack that opens and closes with temperature, loads or water levels is active. ACI 224.1R describes the causes and evaluation of cracks.

Elements of the diagnosis:

• Width and length, measured and plotted on a drawing.
• Change over time, tracked with tell-tales or gauges placed across the crack.
• Presence of water, calcite, rust stains or efflorescence.
• Probable cause: shrinkage, overload, settlement, freeze-thaw, rebar corrosion, concrete reaction.
• Structural role of the cracked member: a crack in a load-bearing member needs a structural assessment by a qualified professional before the repair is chosen.

If the crack is caused by rebar corrosion, injection alone solves nothing: the steel is treated and the concrete repaired.

How does epoxy injection work?

Epoxy injection has five stages: cleaning the crack, placing injection ports and the surface seal, injecting resin from port to port until full, curing, then removing the seal. ACI RAP-1 and the ACI 503.7 specification describe this process.

• Cleaning. Dust, laitance and contaminants are removed from the crack with dry compressed air or vacuum.
• Ports and surface seal. Ports are bonded along the crack, then the surface is sealed with an epoxy paste so the resin does not escape.
• Injection. Resin is injected at low pressure at the first port until it appears at the next one, then injection moves to that port. On a wall, work starts at the bottom.
• Curing. The resin cures according to temperature; the member is not loaded before the data sheet time.
• Finishing. Ports and seal are removed by grinding if the surface will be coated or must stay clean.

How does polyurethane injection work?

Polyurethane injection uses angled drill holes that cross the crack, on both sides, fitted with packers. The crack is flushed with water if needed, then resin is injected under pressure until it comes out of the crack and the leak stops.

Steps:

• Drilling. Holes staggered on both sides, angled to cross the crack within the concrete thickness, at a spacing set by the member thickness.
• Packers. Mechanical, tightened in the holes, with a check valve.
• Flushing. With water, to clean the crack and supply reaction water if needed.
• Injection. From the lowest point upward, until refusal or until resin shows at the surface.
• Finishing. Packers removed or cut, holes filled with mortar, surface resin trimmed.

Hydrophilic or hydrophobic polyurethane: which one?

Hydrophobic polyurethane suits cracks that can dry at times and heavy water inflows, because it expands little after cure and stays stable; hydrophilic polyurethane suits cracks that are always wet, because it absorbs water and forms a flexible gel or foam, but it can shrink if it dries out. ICRI 340.1 helps select the grout by leak type and environment.

• Hydrophobic. Fast reaction, stiffer foam, good behaviour through wet-dry cycles. Often chosen for foundation walls and galleries.
• Hydrophilic. Penetrates wet cracks well and follows movement, but needs an environment that stays wet.
• Compatibility. The water in contact (potable water, process water, loaded water) may call for a specific resin; the data sheet and specification say so.

What are the limits of each injection?

Epoxy does not tolerate movement and stops an active leak poorly; polyurethane provides no structural strength and does not replace a chemical-resistant coating. No injection corrects the cause of a crack.

• Epoxy in an active crack. The crack reopens next to the repair, because concrete gives way where it is weakest.
• Epoxy in a running crack. Water flushes the resin out or prevents bond; stop the water first.
• Polyurethane as a structural repair. Flexible foam does not transfer loads.
• Very fine cracks. They may need a very low-viscosity resin or another method, such as a surface sealer.
• Chemical exposure. In an aggressive basin or pit, injection is completed with a suitable coating; see our article on secondary containment coating.

How do you check the injection and prepare for coating?

Check an epoxy injection with the injection log and, if the specification requires it, with cores to verify penetration; check a polyurethane injection by confirming the leak has stopped under normal water conditions. The surface is then prepared like the rest of the structure before coating.

The log records ports, quantities injected, pressures and conditions. Before coating, residues are removed, the surface is prepared to the required profile as in our guide to concrete surface preparation, and active cracks are treated as flexible joints. In wastewater structures, also check for the concrete attack described in our article on H2S concrete corrosion.

How do you plan injection and get a quote?

Plan from a crack survey, with length, condition and presence of water, then choose the process crack by crack. One structure often combines both: polyurethane where water enters, epoxy where concrete must be bonded.

Our concrete crack injection service covers epoxy and polyurethane, with the repair, concrete protection and protective coating that follow. For hydroelectric structures, see also our hydroelectric coatings, and for basins, our article on water treatment plants. Our crews and qualified subcontractors work from LaSalle, including in Saguenay-Lac-Saint-Jean, Montreal and Ontario. Send photos, crack lengths, presence of water and access through the quote request: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Can epoxy be injected into a damp crack?

Some epoxy resins tolerate a damp crack, per their data sheet, but not a running one. If water is flowing, stop it first, often with polyurethane.

Is polyurethane injection permanent?

It is designed to last if the resin suits the environment and the crack stays within expected conditions. A hydrophilic polyurethane that dries out for long periods can shrink and let water through again.

Should a crack be injected before coating a slab?

Not always. A dormant crack with no structural role can be filled with resin before coating. Injection is used when the crack runs through the member, leaks or must be bonded.

How do you know an epoxy injection filled the crack?

From the injection log and from resin appearing at the next ports during injection. If the specification requires it, cores taken after cure show penetration.