At a glance
- One epoxy per job, not one glue: primer seals the substrate, saturant wets out fabric, a stiffer paste adhesive (4.0 GPa) bonds plate across a 2–4 mm glue line, injection epoxy fills static cracks, and anchoring epoxy grips bars in holes.
- Each grade optimizes a different property — penetration, thixotropy, modulus, or pot life — so picking by function matters more than picking by headline strength.
- Two things govern them all: a sound substrate (pull-off ≥ 1.5 MPa) and temperature — pot life can halve above 30 °C, and service temperature must stay 15 °C below the resin’s glass-transition point.
A structural epoxy adhesive is the two-part resin that turns carbon fiber into a working part of a structure — but there is no single “CFRP epoxy.” FidStrong’s FSE range is a family of purpose-built grades, each tuned for one task. This guide explains what each does, the property it optimizes, and how substrate and temperature govern every one of them. Numbers come from the product data sheets and ACI 440.2R.
What is a structural epoxy, and why are there so many?
A structural epoxy is a two-component thermoset: a resin (part A) and a hardener (part B) mixed at a fixed ratio — 2:1 by weight for most FSE grades, 3:1 for the FSE323 saturants. Once mixed, it cures irreversibly into a rigid solid that carries load. The reason there are several is that the jobs are genuinely different: penetrating a hairline crack, wetting out a dry fabric, and bonding a rigid plate each demand opposite properties. A resin thin enough to seep into a 0.05 mm crack would sag off an overhead soffit; a paste stiff enough to hold a plate would never soak into fabric.
Primer, saturant, or paste adhesive — what is the difference?
These three do the bonding work in a CFRP system, in sequence. Primer (FSE302) is a low-viscosity resin (≤ 600 mPa·s) that penetrates and seals porous or carbonated concrete and stops outgassing; it does not bond fiber, and the next coat must go on within its 2–24 h recoat window. Saturant (FSE322/FSE323) is a thixotropic resin that wets out the fabric into a void-free laminate — FSE323H (4.0 GPa, high-thixotropy) for soffits and heavy fabric up to 600 g/m², FSE323L for fast, void-free wet-out of heavy or multi-ply lay-ups. Plate adhesive (FSE362) is a high-modulus paste that transfers load across a controlled 2–4 mm glue line, which is exactly why it is stiffer than the fabric saturant.
| Epoxy | Optimizes | Key number | Used for |
|---|---|---|---|
| Primer (FSE302) | Penetration / sealing | ≤ 600 mPa·s viscosity | Preparing the substrate |
| Saturant (FSE322/323) | Fiber wet-out | 3.1–4.0 GPa modulus | Bonding fabric |
| Plate adhesive (FSE362) | Glue-line stiffness | 4.0 GPa, glue line 2–4 mm | Bonding plate |
| Crack injection (FSE523) | Fine-gap flow | ≤ 300 mPa·s, fills to 0.05 mm | Static cracks |
| Anchoring (FISFIX) | Pull-out grip | Bond 11 MPa (C30) – 18 MPa (C60) | Bars / bolts in holes |
Values: FidStrong FSE / FISFIX product data sheets. Bond figures per ASTM C882 (slant-shear); anchoring bond by pull-out on a Ø25 mm bar.
What about crack injection and anchoring epoxies?
These two solve different problems again. Crack-injection epoxy (FSE523) is ultra-low viscosity (≤ 300 mPa·s), so it reliably fills cracks down to 0.1 mm by gravity or 0.05 mm under pressure, with near-zero shrinkage (0.3%). That same thinness means lower strength (25 MPa) and modulus (1.5 GPa) than a bonding grade, so it restores continuity across a static crack — it is not a load-transfer adhesive, and it must not be used on active, moving cracks. Chemical anchoring epoxy (the FISFIX family) is a high-thixotropy paste that grips a bar or bolt in a drilled hole, developing 11 MPa bond in C30 concrete and 18 MPa in C60, and reaching return-to-service cure in about 12 hours rather than seven days.
Why do the substrate and temperature matter so much?
Because every epoxy is only as strong as what it is bonded to, and how fast it sets. The substrate must reach a pull-off tensile strength of at least 1.5 MPa (ASTM C1583) and a moisture content below 4% (ASTM D4263); below that, the concrete becomes the weak link and the system fails by pulling off a layer of substrate, not by the epoxy letting go. Temperature then controls the clock: FSE362’s pot life falls from 150 minutes at 10 °C to just 40 minutes at 30 °C, and the data sheets flag that it drops sharply above 30 °C.
How do you read epoxy properties correctly?
Two cautions. First, test methods are not interchangeable: modulus and tensile strength come from ASTM D638, concrete bond from ASTM C882 (a slant-shear test where a good adhesive fails in the concrete, not the glue), and heat resistance from ASTM D648 (heat-deflection temperature, HDT). Second, mind the temperature rating: the FSE liquid epoxies report an HDT around 65 °C, which is not the same as a glass-transition temperature. As a design rule, ACI 440.2R requires service temperature to stay at least 15 °C below the cured system’s Tg, and FRP-system epoxies typically sit in a 60–82 °C Tg band — our deep-dive on structural epoxy Tg requirements works through why that margin matters.
FAQ
What is the difference between primer and saturating epoxy?
Primer (FSE302) is a low-viscosity, high-penetration resin that seals porous or carbonated concrete and stops outgassing — it does not bond fiber. Saturating epoxy (FSE322/323) is a thixotropic, higher-modulus resin (3.1–4.0 GPa) applied within primer’s 2–24 h recoat window to wet out and bond the CFRP fabric.
Why does the plate adhesive need a higher modulus than the fabric saturant?
CFRP plate transfers load through a controlled 2–4 mm glue line, so the adhesive itself must be stiff to limit shear deformation — FSE362 is rated 4.0 GPa. Fabric distributes load through a thin, fully impregnated laminate, so a 3.1 GPa saturating epoxy is sufficient there.
Can crack-injection epoxy be used as a structural bonding adhesive?
No. FSE523’s ultra-low viscosity (≤ 300 mPa·s), which lets it penetrate cracks to 0.05 mm, also means lower strength (25 MPa) and modulus (1.5 GPa) than a bonding grade. It restores load continuity across static cracks, not primary CFRP-to-substrate load transfer, and it is unsuitable for moving cracks.
Why must the substrate reach 1.5 MPa pull-off strength?
Every structural epoxy — primer, saturant, plate adhesive — is only as strong as what it bonds to. Below roughly 1.5 MPa pull-off tensile strength (ASTM C1583), the concrete becomes the weak link, and the system fails by pulling a layer of substrate off rather than by the epoxy debonding.
Does hot weather really matter that much for working time?
Yes. FSE362’s pot life drops from 150 minutes at 10 °C to 40 minutes at 30 °C, and the data sheet flags a sharp fall above 30 °C. On a hot site, mix smaller batches and place faster — the stated 23 °C pot life is not a reliable guide in the heat.
How long does a structural epoxy take to cure?
It depends on function. The structural bonding, saturating, and injection grades reach full cure in about 7 days at 23 °C, while the FISFIX anchoring epoxies are formulated for fast return-to-service — roughly 12 hours — because an anchor needs to carry load quickly.
For context on where these resins fit, see what CFRP strengthening is and why plate and fabric use different adhesives; for interpreting the property tables, see how to read a CFRP data sheet.
FidStrong manufactures the FSE structural epoxy range — primer, saturating resins, plate adhesive, repair and crack-injection grades — and the FISFIX chemical anchoring family, under ISO 9001, 14001, and 45001 quality systems. Values here are drawn from the product data sheets and ACI 440.2R; confirm project-specific values against the current TDS.