A hairline crack shows up in a concrete beam, wall, or slab, and the reflex on many sites is to reach for an epoxy injection kit. Sometimes that reflex is correct: a dormant structural crack with a diagnosed cause is exactly what low-viscosity crack injection resin was formulated to fill. Sometimes the same reflex is wrong: injecting a crack still opening and closing with temperature or load does not fix anything — the concrete simply cracks again at the edge of a now-rigid repair. The resin is not the variable that decides success; whether the crack has stopped moving, and why the crack opened, decides success.
At a glance
- Epoxy injection is for dormant cracks only. A crack still moving from thermal cycling, load reversal, or settlement needs a flexible sealing system, not a rigid epoxy — rigid injection of a live crack causes re-cracking next to the repair (FidStrong TDS-FSE523-V202605).
- Crack width sets the method. Low-viscosity injection resin, mixed viscosity ≤300 mPa·s, fills cracks down to 0.05 mm by pressure injection or 0.1 mm by gravity feed (FidStrong TDS-FSE523-V202605).
- Bond, not just fill, is what gets tested. ASTM C882 slant-shear testing requires the test report to state whether a specimen failed in the epoxy, in the concrete, or at the interface — a well-formulated injection resin should force failure into the concrete (ASTM C882 §13.1).
Which cracks can be repaired by epoxy injection?
Epoxy injection is the right repair for dormant, non-moving structural cracks in beams, columns, slabs, walls, and bridge members, and for filling honeycombing and voids left by poor consolidation (FidStrong TDS-FSE523-V202605). The scope is set by the formulation of the low-viscosity crack injection epoxy itself: a two-component, solvent-free system with mixed viscosity of 300 mPa·s or less, thin enough to travel into a fine, dormant crack under gravity or low pressure and cure into a rigid, load-transferring solid (FidStrong TDS-FSE523-V202605).
After a full 7-day cure at 23 °C, the cured resin measured at 23 °C / 50% relative humidity reaches 25 MPa tensile strength with a tensile modulus of 1.5 GPa and 1.8% elongation at break (ASTM D638), 30 MPa flexural strength (ASTM D790), 80 MPa compressive strength (ASTM D695), and 25 MPa steel-to-steel shear strength (ASTM D1002) — properties sized to restore load transfer across a crack, not just to keep water out of it (FidStrong TDS-FSE523-V202605). Typical applications include structural cracks in bridge girders, deck slabs, and shear walls, and restoring load transfer across a dormant crack before a member is strengthened externally, an approach covered in strengthening bridge girders with CFRP.
How can you tell whether a crack is dormant or still moving?
A crack is dormant when its width stops changing across a full seasonal and loading cycle, and a crack is still moving when its width measurably opens and closes with temperature, live load, or ongoing settlement — the thermal-cycling, load-reversal and settlement cases that the resin data sheet excludes from rigid injection (FidStrong TDS-FSE523-V202605). A crack monitor or a painted witness mark checked over several weeks, spanning a hot and a cold spell, tells the two apart; a single site visit cannot, because a dormant crack and a slowly moving one look identical in one snapshot. The same question is also the starting point of post-earthquake damage assessment, where a crack that keeps working after the shaking stops is treated as an unresolved structural problem, not as a crack ready for injection.
Does a structural crack need more than injection?
Yes — a structural crack needs its root cause diagnosed first, because overload, insufficient reinforcement, and foundation movement each call for a different follow-up repair, and epoxy injection by itself restores continuity across the crack without adding capacity that was missing when the crack formed (FidStrong TDS-FSE523-V202605). Where the diagnosis shows a member lacks capacity rather than simply having aged, injection restores load transfer across the crack but adds no capacity; the added capacity comes from external strengthening applied after the injected resin has cured, which is why crack injection is commonly paired with external CFRP strengthening whenever the root cause is a capacity shortfall (FidStrong TDS-FSE523-V202605).
How wide does a crack have to be before injection can fill it?
Epoxy injection resin reliably fills cracks down to 0.05 mm wide by pressure injection and down to 0.1 mm wide by gravity feed, because a mixed viscosity of 300 mPa·s or less is thin enough to penetrate a fine, dormant crack once enough pressure head or applied pressure is present to drive it in (FidStrong TDS-FSE523-V202605). Width is not the only variable that decides whether resin reaches every part of a crack: ports set at 200–300 mm spacing along the crack deliver resin along its full length, because a single injection point can only push resin so far before the applied pressure drops off (FidStrong TDS-FSE523-V202605).
Pressure injection or gravity feed: which one fits the crack?
Crack orientation decides the method: vertical, inclined, and overhead cracks are pressure injected through ports, while horizontal, open-top cracks such as slab surfaces are filled by gravity feed (FidStrong TDS-FSE523-V202605).
| Property | Pressure injection | Gravity feed |
|---|---|---|
| Crack orientation | Vertical, inclined, and overhead cracks | Horizontal, open-top cracks (e.g. slab surfaces) |
| Minimum crack width | 0.05 mm | 0.1 mm |
| Sealing before filling | Surface seal band and bedded ports, FSE 502 paste | Underside sealed first, only if the crack is visible on the slab soffit |
| Equipment | Two-component gun, static mixer, flow restrictor, low-pressure injection ports | V-groove cut along the crack, oil-free compressed air |
| Source | FidStrong TDS-FSE523-V202605 | FidStrong TDS-FSE523-V202605 |
What is the step-by-step low-pressure injection procedure?
Low-pressure injection follows a fixed sequence: clean the crack, fix ports at 200–300 mm spacing, seal the surface, let the seal cure, then inject port by port at low pressure (FidStrong TDS-FSE523-V202605); on a vertical crack, work from the bottom up and stop each port when resin shows at the next one. Surface seal and port bedding is done with FidStrong's epoxy repair and leveling paste, not the injection resin itself; FSE 502 is a repair and leveling paste, not a structural adhesive, and is not intended for load-bearing cavities deeper than 20 mm without engineering approval, with deep repairs over 5 mm built up in layers (FidStrong TDS-FSE502-V202605). At 23 °C the paste is tack-free in about 1.5 hours, and the data sheet specifies waiting at least 24 hours at 23 °C before continuing work over it (FidStrong TDS-FSE502-V202605).
- Clean the crack with a wire brush and oil-free compressed air until the crack is free of water, oil, and loose material (FidStrong TDS-FSE523-V202605).
- Fix injection ports along the crack at 200–300 mm spacing, using transparent, reusable low-pressure injection ports so resin travel can be watched during injection (FidStrong TDS-FSE523-V202605; FidStrong crack injector product data).
- Seal the crack surface and bed each port with FSE 502 repair and leveling paste (FidStrong TDS-FSE502-V202605).
- Wait for the FSE 502 seal to cure before applying any injection pressure against it: the paste is tack-free in about 1.5 hours at 23 °C, and the data sheet calls for at least 24 hours at 23 °C before work continues over it (FidStrong TDS-FSE502-V202605).
- Load the cartridge: remove the cap, fit a flow restrictor and static mixer, and load the assembly into a two-component gun (FidStrong TDS-FSE523-V202605).
- Purge the gun by holding the nozzle upward and discarding the first portion of mixed material before it reaches the port (FidStrong TDS-FSE523-V202605).
- Inject FSE 523 resin at low pressure so the ultra-low-viscosity resin travels along the crack (FidStrong TDS-FSE523-V202605). On a vertical crack, start at the lowest port so displaced air escapes upward, and stop when resin shows at the next port up.
- Move to the next port and repeat the injection sequence, working up the crack until the last port is reached and the full crack length is filled.
Gravity feed follows a shorter sequence for horizontal, open cracks such as a slab top surface: cut a V-groove along the crack, blow it clean with oil-free compressed air, and pour resin slowly until the groove is full; if the crack is visible on the slab soffit, the underside is sealed first so poured resin does not run through (FidStrong TDS-FSE523-V202605).
How is the epoxy's bond to concrete actually verified?
The epoxy's bond to concrete is verified with ASTM C882 slant-shear testing, which bonds two halves of a 75 × 150 mm mortar cylinder across a plane inclined 30° to the vertical and loads the bonded cylinder in compression until it fails (ASTM C882 §4.1). That 30° angle puts the bond line under combined shear and compression and makes the bonded area exactly twice the cylinder's cross-section (ASTM C882 Note 2); bond strength is then calculated as failure load divided by that bond area (ASTM C882 §12.1). Three specimens are tested per resin type (ASTM C882 §10.1), and the mortar itself must reach at least 31 MPa compressive strength at 28 days (ASTM C882 §7.1.2); for the result to count as quantitative rather than pass/fail, the mortar strength should also exceed 2.5 times the expected bond strength (ASTM C882 Note 1).
Tested this way, FidStrong's crack injection epoxy bonds to dry concrete at 2.5 MPa and to wet concrete at 1.8 MPa, with the specimen failing in the concrete itself rather than in the adhesive in both cases (FidStrong TDS-FSE523-V202605). ASTM C882 requires the report to state exactly where a specimen failed — in the adhesive, in the mortar, or at the interface (ASTM C882 §13.1); failure in the mortar means the cured epoxy is stronger than the surrounding concrete, so the concrete would fail before the bond does in an actual repaired crack. ASTM C882 produces its wet-concrete condition by soaking the mortar halves in water for 24 hours before bonding them (ASTM C882 §10.2.1.2) — the closest a lab test gets to injecting a crack that has not fully dried out. For a flowable resin, ASTM C882 also allows the two mortar halves to be set about 0.5 mm apart and the resin poured into that gap vertically (ASTM C882 §10.3.2), which resembles how an injection resin actually fills a crack. The same discipline carries over from surface preparation for CFRP: a bond is only as strong as the weaker material at the interface, which is why the crack is cleaned before injection just as concrete is prepared before bonding a CFRP laminate.
How much working time does the injection resin actually give on site?
Working time is set almost entirely by ambient temperature: FidStrong's crack injection epoxy has a pot life of about 150 minutes at 10 °C, 40 minutes at 23 °C, and 30 minutes at 30 °C, and above 30 °C the pot life shortens sharply enough that batches need to be mixed smaller and used faster (FidStrong TDS-FSE523-V202605). The resin is applied at substrate and ambient temperatures from −5 °C to +35 °C and, once cured, serves in a temperature range of −5 °C to +40 °C (FidStrong TDS-FSE523-V202605). It is mixed at a 2:1 ratio of A to B by weight, supplied as 20 kg + 10 kg sets for larger pours or in cartridges for gun injection, with an 18-month shelf life when stored unopened between +4 °C and +32 °C (FidStrong TDS-FSE523-V202605).
All strength, bond, and timing values in this guide come from FidStrong's own technical data sheets, tested to the ASTM methods named throughout; FidStrong is ISO 9001, ISO 14001, and ISO 45001 certified.
FAQ
Can a crack that is still moving be repaired with epoxy injection?
No — a crack still moving from thermal cycling, load reversal, or differential settlement needs a flexible sealing system, not a rigid epoxy. Injecting a rigid resin into a live crack does not stop the movement; the concrete simply re-cracks next to the repair once movement resumes (FidStrong TDS-FSE523-V202605).
What is the narrowest crack that epoxy injection can fill?
FidStrong's crack injection epoxy, at a mixed viscosity of 300 mPa·s or less, reliably fills cracks down to 0.05 mm wide by pressure injection and down to 0.1 mm wide by gravity feed; narrower cracks fall outside the resin's stated scope (FidStrong TDS-FSE523-V202605).
How long does the injection resin stay workable once it is mixed?
Pot life is about 150 minutes at 10 °C, 40 minutes at 23 °C, and 30 minutes at 30 °C, and shortens sharply above 30 °C, which is why hot-weather work calls for smaller batches mixed more often rather than one large batch (FidStrong TDS-FSE523-V202605).
Does epoxy injection make a repaired crack as strong as the surrounding concrete?
Tested by ASTM C882 slant-shear, FidStrong's crack injection epoxy bonds to dry concrete at 2.5 MPa and to wet concrete at 1.8 MPa, with the specimen failing in the concrete rather than in the adhesive in both cases — meaning the bond itself is not the weak point (FidStrong TDS-FSE523-V202605; ASTM C882 §13.1).
Should a structural crack be injected before or after CFRP strengthening is applied?
Injection comes first: injecting the dormant crack restores load transfer across it and gives external CFRP strengthening a continuous substrate to bond to, rather than a crack that could open further once the new reinforcement is under load (FidStrong TDS-FSE523-V202605).