Installing a chemical anchor on sound concrete and installing the same anchor on cracked concrete are two operations with very different requirements. Cracked concrete alters the stress distribution around the anchor, reduces the load transfer area, and exposes the resin to movements that the intact material does not experience. The resins, anchor depths, and safety coefficients change, and ignoring these parameters leads to undersized fixings.
ETA Approval on Cracked Concrete: What the “cracked concrete” Label Really Changes
The distinction between cracked and uncracked concrete is not just a technical detail. European Technical Assessments (ETA) classify anchoring resins according to options that condition their field of use. Only ETA-approved resins option 1 or 2 according to the EAD can be used on cracked concrete for structural safety fixings. A resin approved only for uncracked concrete loses all performance guarantees as soon as a crack crosses the anchoring zone.
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Several manufacturers (Hilti, Fischer, Würth, INDEX) publish ETAs where this mention is clearly stated. Before purchasing a cartridge, checking the approval class on the product sheet remains the first reflex. An article detailing chemical anchoring for cracked concrete on Easy Home specifies the checks to be made on this specific point.
In the absence of this approval, the allowable load values indicated by the manufacturer simply do not apply. An anchor may seem to hold in the short term, but the resin will not have been tested to withstand the cyclic movements of an active crack.
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Characterization of Concrete Cracks Before Chemical Anchoring
Recent versions of Eurocode 2, Part 4 (EN 1992-4), impose a specific check of the dispersion and opening of cracks before any chemical anchor sizing. The crack width, its position relative to the anchor point, and its nature (permanent or evolving) determine the calculation.
Crack Width and Nature
A stabilized shrinkage crack for years does not behave like a structural crack that continues to open under load. Identifying whether the crack is active or passive conditions the choice of resin and the sizing of the anchor. On an active crack, conventional rigid resins (polyester) risk debonding due to mechanical fatigue.
Field feedback varies on the acceptable limit width. ETA sheets mention reference crack openings used during tests, but each site presents different conditions. A visual survey is not always sufficient: a crack meter or a dial gauge allows for quantifying the evolution over time.
Position of the Crack Relative to the Drilling
A crack that crosses the drilling hole in its middle has a much more penalizing effect than a tangential crack. The resin must then fill a variable space and maintain adhesion on two lips that may move independently. Cleaning the hole also becomes more delicate, as drilling debris accumulates in the crack and contaminates the resin-concrete interface.
Anchor Depth and Safety Coefficients on Cracked Concrete
Technical guides for chemical anchors (INDEX Fixing Systems, among others) indicate that on cracked concrete, the anchor depth must be increased or the allowable loads reduced compared to the same product in sound concrete. The partial safety coefficients derived from European evaluations are more unfavorable, which translates concretely into longer threaded rods or larger diameters.
Applying the load values from the “uncracked concrete” table of a technical sheet to a cracked substrate amounts to undersizing the fixing. This error is common among installers who only consult the first page of the product sheet without checking the validity conditions.
- Check in the ETA the minimum anchor depth specific to cracked concrete, which often exceeds that indicated for sound concrete
- Apply the increased partial safety coefficients provided by Eurocode 2, Part 4, for anchors in cracked zones
- Adapt the drilling diameter and the length of the threaded rod (or stud) to the manufacturer’s corrected values
Vinylester Hybrid Resins Without Styrene: Real Contribution on Cracked Concrete
Next-generation resins, particularly hybrid vinylester technologies without styrene, exhibit mechanical and chemical properties that make them more suitable for cracked concrete than conventional polyesters. Their residual flexibility after polymerization allows them to better absorb the micro-movements of a crack without abrupt loss of adhesion.
These resins also have the advantage of a wider temperature range for application and often a shorter polymerization time at low temperatures. On an outdoor site in winter, the behavioral difference between a standard polyester resin and a hybrid vinylester can be significant.
However, the additional cost per cartridge is notable. For a non-structural fixing (small equipment, signage), a standard resin approved for cracked concrete may suffice. The choice of resin depends on the load-cracking couple, not just the price.
Cleaning the Drilling Hole: Increased Requirement on Cracked Substrate
On sound concrete, blowing and brushing in two passes are generally sufficient. On cracked concrete, drilling dust migrates into the crevices of the crack and remains trapped against the walls of the hole. If this layer of debris is not removed, the resin adheres to the dust rather than the concrete.
- Use a manual blowing pump or a dry air compressor (no oiled air) to expel fine debris
- Brush with a metal brush suitable for the diameter of the hole, making at least two passes back and forth
- Repeat the blowing-brushing cycle at least twice: the first pass detaches, the second evacuates
- Visually check that the bottom of the hole is free of compacted dust before injecting the resin
An injection sieve (filter sleeve) is necessary in hollow or severely degraded materials to hold the resin in place during polymerization. On cracked concrete with open cavities, the sieve prevents the resin from leaking through the crack before it hardens.
Chemical anchoring on cracked concrete does not tolerate approximation. Every parameter, from resin approval to hole cleaning, interacts with the others. Neglecting one of them nullifies the precautions taken on the rest, and the fixing becomes a structural unknown rather than a calculated anchor.