The Decisive Question Before You Pick Up a Cartridge
Every crack injection job starts with the same fork in the road. Epoxy injection restores load-bearing capacity to a dry, dormant crack by bonding the faces back to something approaching monolithic concrete. Polyurethane resin chases and seals active water ingress with a flexible foam or gel that accommodates ongoing movement. The two products are not interchangeable, and using the wrong one does not just waste material. Epoxy injected into a crack that is still moving will fracture again, usually alongside the repair, leaving you with two cracks where you had one.
This guide walks through crack assessment, port placement, surface sealing, injection sequence and verification. The Fosroc range covers both chemistries, and the choice between them follows directly from the assessment process below.
Crack Assessment: Four Things to Establish Before Specifying a Product
1. Is the crack dormant or active?
A dormant crack has stopped moving. The cause of movement, whether thermal cycling, shrinkage, overload or settlement, has either stabilised or been corrected. An active crack is still opening, closing or widening under ongoing load, moisture cycling or unresolved foundation movement.
The simplest field check is to glue a tell-tale (a small plaster or glass strip bridging the crack) and return after two to four weeks. Cracking in the tell-tale confirms activity. For structures with thermal exposure, check at both the hottest and coolest parts of the day. A crack that measures 0.2 mm in the morning and 0.4 mm in the afternoon is active by any practical definition.
If the cause of cracking has not been identified and addressed, injecting anything is premature.
2. What is the crack width?
Epoxy injection is effective in cracks from approximately 0.1 mm to 5 mm wide. Below 0.1 mm, penetration is unreliable regardless of viscosity. Above 5 mm, the economics favour repair mortar over injection resin. Polyurethane systems are less width-sensitive because the resin expands on contact with moisture, but very wide cracks may require multiple passes or a backing material.
Measure crack width at several points along its length. Width is rarely uniform, and the narrowest section governs product viscosity selection.
3. Is there active water ingress?
Water in a crack is not automatically a reason to choose polyurethane. The question is whether water is flowing or merely present. A damp crack in an otherwise stable structure can still be epoxy-injected if the faces are dried with compressed air and the product is a moisture-tolerant epoxy formulation. A crack with flowing water, hydrostatic pressure behind it, or tidal fluctuation is a polyurethane job. Epoxy will not cure properly against flowing water, and the bond will fail.
4. Does structural capacity need to be restored?
This is where the two chemistries diverge completely. Cured epoxy injection resin, per Fosroc product data, achieves tensile bond strength that can exceed the tensile strength of the parent concrete. The repaired section can carry load across the crack plane. Polyurethane foam or gel is flexible and compressible. It seals water. It does not restore load-bearing capacity across the crack, and specifying it in a load-bearing member without engineering sign-off is a liability issue, not just a technical one.
If the crack is in a load-bearing beam, column, slab or wall and structural continuity matters, the specification requires epoxy and an engineer's confirmation that injection alone is sufficient, or that supplementary reinforcement is needed.
Surface Port Placement and Spacing
Ports are the injection entry points. Their placement governs whether resin reaches the full depth of the crack or simply fills the surface zone.
For cracks accessible from one face only, surface-mounted ports (also called packers) are bonded directly over the crack. Spacing depends on crack depth and resin viscosity. A common starting point is port spacing equal to the estimated crack depth, so a crack 150 mm deep gets ports at roughly 150 mm centres. Low-viscosity epoxy travels further; high-viscosity product needs closer spacing.
For through-cracks accessible from both faces, or for cracks in thick sections, drilled ports angled at 45 degrees to intersect the crack plane at mid-depth give better penetration than surface ports alone.
Mark port locations before applying surface seal. Trying to find the crack line under a coat of epoxy paste is slower than it sounds.
Surface Sealing
The surface between ports is sealed with epoxy paste or a fast-setting repair mortar to contain injection pressure and force resin into the crack rather than back out the face. Apply the seal at least 25 mm wide on each side of the crack. Leave the ports open.
Allow the surface seal to reach handling strength before injecting. Injecting against a green seal wastes resin and pressure. For epoxy paste surface seals, this is typically two to four hours at 20°C, longer in cold conditions.
On vertical and overhead cracks, the seal also prevents resin from running out under gravity before it gels. On horizontal cracks injected from below, gravity assists penetration, which is one reason the injection sequence matters.
Injection Sequence: Low Point Upward
Always start at the lowest port and work upward. On vertical cracks, this means the bottom port first. On inclined cracks, start at the lowest end. On horizontal cracks in a slab soffit, start at one end and work systematically.
Inject at the first port until resin appears at the next port up the crack. Cap the first port, move to the second, and repeat. This sequence displaces air upward and out, rather than trapping it mid-crack. Trapped air voids are the most common cause of incomplete fill.
Injection pressure for epoxy in narrow cracks is typically 0.3 to 0.5 MPa for surface-mounted ports. Higher pressure risks blowing the surface seal or propagating the crack further. Polyurethane systems, particularly those that foam on contact with water, often require lower initial pressure because the expansion does the work of filling voids.
Injection rate matters too. Forcing resin in fast generates heat in epoxy systems and can cause premature gelation in the port before the crack is full. A slow, steady pressure hold is more effective than a rapid pump.
The Epoxy-in-a-Moving-Crack Mistake
This deserves its own section because it is the most common and most expensive error in crack injection work.
Epoxy cures rigid. Its bond strength is high, but its elongation at break is low, typically 1 to 2 percent depending on formulation. If the crack is still subject to thermal movement, live load cycling or ongoing settlement, the cured epoxy will fracture. The fracture does not follow the original crack plane exactly. It runs alongside the repair, through parent concrete that has not cracked before. The result is a wider, more complex defect than the original.
The tell-tale test described above is not optional. Neither is identifying and resolving the cause of movement before injection. If a crack is active but water exclusion is urgently needed, polyurethane is the holding measure. Epoxy is the structural repair, and it waits until the structure is stable.
Some cracks are permanently active, for example, movement joints that were detailed incorrectly as construction joints. These should not be epoxy-injected at all. The correct remediation is to convert them to proper movement joints with a flexible sealant and backer rod, a topic covered in our post on joint sealant selection.
Verification After Injection
Verification confirms that the crack is full and, for epoxy repairs, that bond has been achieved.
The simplest check is resin consumption against the theoretical crack volume. Calculate the approximate crack volume from length, average width and depth. If the volume of resin injected is substantially less than the theoretical volume, the crack is not full. Either ports were spaced too far apart, resin bypassed sections of the crack, or the crack geometry was more complex than the surface trace suggested.
For load-bearing structural repairs, visual and volumetric checks are not sufficient on their own. Coring through the repaired section and examining the core for resin fill, followed by testing of the core if required, gives direct evidence of fill quality. This is an engineer's call, not a contractor's, but the contractor should be prepared for it on structural work.
For polyurethane water-exclusion repairs, the functional test is straightforward: apply water to the crack face under pressure and check for ingress. Re-inject any sections that still pass water.
Product Selection Within Each Chemistry
Within epoxy injection, viscosity is the main variable. Ultra-low viscosity products (below 200 mPa·s) penetrate hairline cracks. Standard viscosity products suit cracks in the 0.3 to 2 mm range. The Fosroc Nitofill range covers these applications with published viscosity and bond strength data to support specification.
Within polyurethane, the choice is between hydrophilic foaming resins, which expand aggressively on contact with water and suit high-flow situations, and hydrophobic gels, which are less reactive and give a denser, more controlled seal in slower-moving water. Fosroc Nitofill PU products address both scenarios.
Neither chemistry should be selected from a catalogue without reference to the manufacturer's current technical datasheet. Formulations change, and the datasheet governs application temperatures, pot life, mixing ratios and surface preparation requirements.
Ordering Through CSA
Fosroc crack injection products are available through Construction Supplies Australia for trade account holders. Because job quantities vary and some projects require specific viscosity grades or packaging formats, the trade desk can assist with product selection based on crack width data and substrate conditions. Visit constructionsupplies.group/au or contact the trade account desk directly to discuss quantities and lead times before mobilising to site.