Resin anchors are often blamed when bonded fixings fail on site, yet the root cause is frequently much earlier in the installation sequence. Before resin is injected or threaded rods are inserted, the performance of a bonded anchor system depends on a step that is often treated as optional: proper hole cleaning.
Under BS 8539, anchor installation must follow the manufacturer’s instructions and approval documentation, meaning the cleaning procedure defined within the system’s European Technical Assessment (ETA) forms part of the structural system itself.
The installation sequence typically begins not with resin, but with a simple but critical process:
Blow – Brush – Blow.
While resin anchors are often blamed when fixings fail, evidence shows that inadequate borehole cleaning and deviation from ETA-approved installation procedures are among the most common causes of bonded anchor performance failure.
Why Hole Cleaning Is Part of the Anchor System
For bonded anchors, the adhesive bond between resin and concrete is only as strong as the surface condition of the drilled hole.
Drilling generates fine concrete dust which coats the sides of the borehole. If this dust layer is not removed, the resin bonds to the dust rather than the structural concrete substrate.
This reduces:
- bond strength
- load capacity
- long-term reliability of the anchor
The cleaning sequence prescribed by most manufacturers therefore follows a basic physical logic:
Blow – remove loose drilling dust
Brush – break up dust adhered to the borehole wall
Blow again – remove the loosened material
If brushing is repeated, the hole must be blown again. Brushing simply dislodges the dust; without a second blow-out step the material remains inside the hole.
This is not a matter of installer preference. It is a direct cause-and-effect requirement of the anchoring system.
Cleaning Method Is Defined by the Anchor Approval
One of the most commonly misunderstood aspects of anchor installation is that not all bonded anchor systems accept the same cleaning method. Depending on the anchor type and the conditions defined in the ETA, acceptable cleaning equipment may include:
- manual blow-out pumps
- compressed air systems
- specific metal borehole brushes sized to the drill diameter
For larger borehole diameters or certain anchor approvals, manual pumps may not provide sufficient cleaning energy, meaning compressed air becomes mandatory.
Equally important is the correct brush type and diameter.
Improvised tools, such as general-purpose brushes carried in a vehicle, may not achieve proper wall contact and therefore cannot effectively remove adhered dust.
In practice, installers must use:
- manufacturer-specified brushes, or
- metal borehole brushes sized for the drilled diameter
Anything else risks leaving residual dust within the borehole.
Systems Designed to Reduce Installation Risk
Because borehole contamination is a known failure point in anchor installation, several technical solutions have been developed to reduce the risk.
Some anchor systems allow the use of hollow drill bits, which extract drilling dust during the drilling process itself. However, this method is only permitted when explicitly covered by the ETA and manufacturer’s instructions for that anchor system.
Capsule systems represent another approach. In these systems, the rotation of the threaded rod breaks the resin capsule and mixes the components directly inside the hole. Some capsule systems are more tolerant of minor dust contamination.
However, the same rule applies:
If the ETA does not state that cleaning can be omitted, the cleaning sequence remains mandatory.
Cleaning Is Not Just a “Resin Anchor Problem”
Another misconception on site is that borehole cleaning only applies to bonded anchors. In reality, many mechanical anchor systems also require hole cleaning as part of their installation procedure.
Dust inside the borehole can interfere with:
- expansion mechanisms
- sleeve friction
- anchor seating depth
As a result, proper cleaning is a common installation requirement across multiple anchor types, not just adhesive systems.
Why This Step Is Often Missed on Site
Hole cleaning failures usually occur for practical reasons rather than technical ones. The process is repetitive, produces no visible structural component, and is often performed quickly during installation. Under programme pressure, installers may shorten or omit parts of the sequence.
Yet from a structural reliability perspective, this step has a disproportionate influence on anchor performance. It is one of the few installation steps that applies across almost every anchor type used in construction.
Evidence-Based Summary
Anchor installation reliability is not driven by the resin itself but by the interaction between drilling, borehole condition and installation procedure. While failures are often attributed to adhesive performance, evidence shows that incomplete borehole cleaning and deviation from ETA-approved installation methods significantly reduce anchor bond capacity.
In practical terms, the simple blow–brush–blow sequence defined in manufacturer instructions remains one of the most critical controls for ensuring anchor performance in accordance with BS 8539.
|
Expert Verification & Authorship: Mihai Chelmus
Founder, London Construction Magazine | Construction Testing & Investigation Specialist |
