At a glance
- Bond is the whole game: externally bonded FRP fails chiefly by debonding — a 375-beam European database found 100% debonded — so a sound, prepared substrate is the precondition for everything.
- Hit the targets: pull-off tensile strength ≥ 1.5 MPa, moisture < 4%, an abraded CSP 3 profile with aggregate exposed, corners rounded to R ≥ 20 mm, and cracks > 0.3 mm injected first.
- Prove it: a pull-off test passes only if the load exceeds 200 psi (1.4 MPa) and the fracture is inside the concrete — not at the bond line.
Surface preparation is not a preliminary to a CFRP job — it is the job. A carbon laminate is only as good as the concrete it is bonded to, and the most common way strengthening fails is the FRP peeling off a substrate that was never sound or clean enough. This guide covers the targets and the tests, traced to ACI 440.2R, ACI SPEC-440.12-22, and the FidStrong data sheets. It pairs with the full installation guide.
Why does surface preparation decide whether CFRP works?
Because debonding, not fiber rupture, is the dominant failure mode of externally bonded FRP. A European test database of 375 unanchored FRP-strengthened flexural beams found that 100% failed by debonding — 70% by intermediate-crack debonding and 30% at the ends. In every case the laminate separated from the substrate before the carbon did any real work. That is why a strengthening design assumes a sound bond, and why the whole outcome rests on how the surface is prepared. For the bigger picture, see what CFRP strengthening is.
How sound and rough does the concrete need to be?
Two things: strong enough, and rough enough. The substrate must reach a pull-off tensile strength of at least 1.5 MPa per the FidStrong data sheet — a deliberate margin above ACI 440.2R’s code floor of 200 psi (1.4 MPa, tested to ASTM C1583). And it must be mechanically abraded — ground or abrasive-blasted per ASTM D4259 — to strip the weak surface laitance and expose sound aggregate to a minimum profile of CSP 3 (ICRI 310.2). A smooth or laitance-covered face gives the primer nothing to key into.
| Substrate target | Requirement | Basis |
|---|---|---|
| Pull-off strength | ≥ 1.5 MPa (ACI floor 1.4 MPa) | FidStrong TDS / ACI 440.2R §1.2.1.4 |
| Surface profile | ≥ CSP 3, aggregate exposed | ICRI 310.2 / ASTM D4259 |
| Moisture content | < 4% | ASTM D4263 |
| Corner radius | R ≥ 20 mm (ACI floor 13 mm) | FidStrong TDS / ACI SPEC-440.12-22 §3.1.7 |
| Cracks | Inject those > 0.3 mm first | ACI 440.2R §6.4.1 |
How dry does it need to be, and why?
Substrate moisture content must be below 4% (ASTM D4263) for the full bonded system — the primer alone can go onto a damp (not wet) surface, but the complete fabric system cannot. The reason is outgassing: moisture vapor driving out of the concrete through the uncured resin forms surface blisters and destroys the FRP-to-substrate bond (ACI 440.2R §6.2). Primer is formulated to penetrate and seal the substrate against exactly this, and porous or heavily carbonated concrete gets a second thin primer coat once the first is tack-free.
What about corners, cracks, and unevenness?
Three details finish the prep. Outside corners the fabric wraps must be rounded to a radius of at least 20 mm (FidStrong’s house standard, above the 13 mm ACI floor) so the fiber is not kinked or cut at the bend. Cracks wider than 0.3 mm must be pressure-injected with epoxy before the FRP goes on, or they will propagate and delaminate the laminate. And surface deviations greater than 1 mm are filled with FSE502 leveling putty (full cure ≥ 24 h before priming) so the fabric beds down in full contact rather than bridging a void. Work throughout within a substrate and ambient temperature of +5 to +35 °C, above the dew point.
How do you prove the bond is good?
You test it. On bond-critical work, pull-off testing to ASTM D7522 is run at 3 tests per 1,000 ft², minimum 3 per day. The pass criterion has two parts that must both hold: the load must exceed 200 psi (1.4 MPa), and the failure must occur inside the concrete substrate (a “concrete-mode” failure). A high reading that breaks cleanly at the bond line is a fail — it means the adhesion, not the concrete, was the weak link.
FAQ
What pull-off strength does the concrete need before installing CFRP?
FidStrong’s data sheet requires at least 1.5 MPa substrate pull-off tensile strength, slightly above ACI 440.2R’s code minimum of 200 psi (1.4 MPa), tested per ASTM C1583. On site, ACI SPEC-440.12-22 requires the pull-off to exceed 200 psi and fail inside the concrete itself, not at the bond line.
How rough does the concrete surface need to be?
It must be mechanically abraded — ground or abrasive-blasted per ASTM D4259 — to remove laitance and expose sound aggregate, reaching a minimum profile of CSP 3 per ICRI 310.2. That roughened, contaminant-free surface is what lets the primer and saturating resin develop full mechanical and chemical bond.
What moisture content is acceptable before priming?
The full bonded fabric system requires substrate moisture below 4%, measured per ASTM D4263. The primer alone tolerates a damp (not wet) surface, but the complete system — primer, saturating epoxy, and fabric — needs the substrate under 4% to avoid vapor-driven blistering that destroys the bond.
How flat does the surface need to be before laminating?
Irregularities greater than 1 mm must be filled and leveled with a repair adhesive before priming (the threshold is a tighter 0.5 mm for rigid plate bonding). FidStrong’s FSE502 leveling putty needs a full 24-hour cure before the next step, and deep repairs over 5 mm are built up in layers.
Why does the outside corner need to be rounded before wrapping?
Sharp corners create stress concentrations that can crease or cut the fiber during wrap-around, causing localized failure. FidStrong specifies a minimum 20 mm radius, above the ACI SPEC-440.12-22 code floor of 13 mm — a smoother radius protects fiber integrity at the bend.
What happens to existing cracks before CFRP goes on?
Per ACI 440.2R §6.4.1, any crack wider than 0.3 mm must be pressure-injected with epoxy before FRP installation — an unaddressed crack under the laminate can propagate and delaminate the bonded system. FidStrong’s FSE523 crack-injection epoxy is the product used for this pre-treatment. See common installation mistakes.
The resins that go onto this prepared surface are covered in our structural epoxy adhesives guide.
FidStrong manufactures FSC carbon fiber fabric and the FSE primer, leveling, and crack-injection epoxies used in substrate preparation, under ISO 9001, 14001, and 45001 quality systems. Figures here are drawn from the product data sheets, ACI 440.2R, and ACI SPEC-440.12-22; follow the project specification and a qualified engineer’s requirements.