Failure mechanism
Chemical anchor bond failure
A bonded anchor holds by adhesion rather than by expansion: the adhesive has to bond to the steel, bond to the concrete, and the concrete around the hole has to stay intact. It is the most installation-sensitive anchor type in common use, because the bond is made against dust, moisture and temperature rather than across a mechanical interface. A hole that was drilled and set in a few minutes can look perfect and carry very little.
How this failure works
The load path runs from the rod, through the adhesive, into the concrete at the hole wall, and then into the concrete mass by way of a cone or a splitting failure. Three interfaces can give way: the adhesive can separate from the steel, it can separate from the concrete, or the concrete itself can fail. Bond stress is developed over the bonded length and around the hole circumference, so capacity follows the embedment and the hole diameter rather than the rod diameter alone. The bond modes tend to be gradual, with the rod slipping a millimetre or two under load, while the concrete modes are sudden — and a rod that slips is telling you which mode is in play.
Hole cleaning is the dominant variable and it is the one most often done badly. Drilling leaves dust on the hole wall, and the dust is not harmless filler: it is a layer between the adhesive and the concrete, it consumes resin at the interface, and it becomes the plane along which the bond fails. The manufacturer prescribes a sequence, usually blowing, then brushing with a specified brush diameter for a specified number of strokes, then blowing again, and the sequence matters as much as the individual steps. The drilling method matters too: a hammer-drilled hole has a rougher and dustier wall than a diamond-cored one, and a cored hole is not interchangeable with a drilled hole for every product.
Cure is a chemical reaction and it runs at the temperature of the concrete, not the temperature in the site office. At low temperature the adhesive cures slowly, and loading or torquing a rod before the cure is complete breaks the bond before it has formed; at high temperature the working life shortens and the cartridge can gel in the nozzle. Water is the other contaminant — a hole open to rain, drilled upward into wet concrete, or set in a slab that has just been washed down will not bond with most products. Sustained load also has to be designed for separately, because structural adhesives creep under permanent tension and the long-term capacity is lower than the short-term value.
Two conditions change the design rather than the installation. In cracked concrete the failure surface is interrupted by the crack, so only products qualified for cracked concrete belong where a crack can open through the anchorage, and the design values come from that qualification even though the installation looks identical. Hole diameter carries its own tolerance: a bit a millimetre or two oversized leaves a thick annulus of poorly compacted adhesive, an undersized hole leaves none at all, and a worn bit does either without any warning on the drill. A hole that hits reinforcement takes the anchor off its intended position, and a repositioned anchor no longer has the embedment and edge distance that were analysed.
Illustration
Conceptual illustration: a bonded anchor in section showing the adhesive annulus, the two bond interfaces, and a failure surface following a hole wall that was not cleaned.
Typical causes
- A hole not cleaned in the manufacturer sequence — dust left on the wall is the single commonest reason a bonded anchor underperforms.
- A hole drilled with a worn or wrong diameter bit, leaving an annulus too thick to compact or too thin to exist.
- Loading or torquing the rod before the adhesive has cured for the actual concrete temperature.
- A damp or wet hole: drilling into wet concrete, a hole left open to rain, or a slab washed down before setting.
- An adhesive used outside its temperature range, past its shelf life, or through a mixer nozzle that was not renewed.
- A product qualified only for uncracked concrete used where a crack can open through the anchorage, or sustained tension designed from a short-term value.
- A hole that hit reinforcement, with the anchor repositioned at a rake or at reduced depth so the embedment and edge distance are no longer those analysed.
Warning signs
- A rod that turns or pulls out under a spanner, with adhesive still in the hole and little of it on the rod.
- Adhesive that is soft, tacky or crumbly when checked after the specified cure time for that temperature.
- A rod that went in with noticeably less resistance than the others in the group, or that will not reach the marked depth.
- Dust visible in the hole, or a heavy dust plume while drilling with no record that the hole was cleaned afterwards.
- A ring of dished or cracked concrete at the surface around the anchor, or spalling at the hole edge.
- Rust on the embedded length of a rod that has been withdrawn, showing that water was in the hole when it was set.
Inspection methods
- 01Review the installation record for each anchor: measured hole diameter and depth, the cleaning sequence actually carried out, and who set it.
- 02Check the bit diameter and condition against the product instruction at the start of each shift, not once at the start of the job.
- 03Check the cartridge expiry date, the mixing nozzle and the state of the adhesive before it is used.
- 04Verify embedment depth and rod position before cure, and inspect the cured anchor after the stated cure time at the measured temperature.
- 05Require proof load testing of a sample wherever the design specifies it, carried out by a qualified testing agency with load and movement recorded.
- 06Inspect the surrounding concrete for cracks, honeycombing and voids, and establish whether the anchorage sits in a zone the design treated as cracked.
Preventive measures
- Follow the product installation instruction exactly, and keep a copy at the work face rather than in the site office.
- Clean the hole in the specified sequence with the specified brush and air, and look into the hole before any adhesive goes in.
- Use a bit of the specified diameter, check it for wear at the start of every shift, and replace it before it drifts out of tolerance.
- Store the adhesive at the temperature the product requires, discard expired cartridges and fit a new mixing nozzle for each cartridge.
- Respect the cure time for the measured concrete temperature, and do not load, torque or disturb the anchor before it has elapsed.
- Select the product for the concrete condition — cracked or uncracked, dry or damp, sustained load or seismic — and take design values from that qualification.
- Treat a hole that has hit reinforcement as a design problem: set the anchor properly elsewhere rather than raking it or shortening the embedment.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- ETA — the European Technical Assessment for the adhesive product, which states the qualified conditions of use.
- EN 1992-4 — design of fastenings in concrete, including bond capacity and cracked concrete provisions.
- ACI 355.4 — qualification of adhesive anchors, which is what makes published bond values conditional on installation.
- ACI 318 — the concrete code within which the anchorage is designed.
- IS 456 — concrete quality, curing and the condition of the base material the anchor relies on.
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.