These first appeared in 1963 when Upat GmbH & Co of Germany invented 'Bonded Anchors' that would withstand exceptionally high loads for the construction industry.
Resin bolt / glue-in / chemically bonded anchor are all interchangeable terms that describe the same type of fixing and the other major type of rock anchor used by climbers.
Glue-in anchors require:
- a clean, secure bond between the adhesive and rock inside the drilled hole and
- features of the anchor that prevent movement by mechanical advantage.
While adhesive may appear to bond with the anchor itself, in reality this 'bond' is very weak and for satisfactory glue-in bolt performance, the adhesive must be able to prevent movement of the bolt by mechanical advantage.
Examples of features that create mechanical advantage include grooves stamped into the shaft, or twists in the shaft, that provide mechanical resistance against the cured adhesive.
There are 4 principle types of glue-in anchor in use for rock climbing:
- Bent rod
- Welded eye
- U-bolt or staple
- Forged eye
Bent rod glue-in anchors
The first bent rod glue-in anchors were developed by the late Frankenjura climber Georg Bühler to be easily manufactured in a home setting and the design is a very common throughout German climbing areas.
There are 5 bent glue-in anchors of which Bühler was the first:
- Bolt Products twist leg
- ClimbTech Wave
- Titan Climbing ‘Eterna’
The Bühler bolt offers an inherent resistance to rotational forces by way of its design and therefore does not need to be countersunk. This gives this design a fairly straightforward installation process however larger versions fabricated from 8mm bar, have a large profile and counter sinking in this instance reduces the appearance.
An improvement on the original design was an 'inference' fit to retain the anchor in position as the adhesive gels for glue-ins installed inverted or at an angle that would otherwise result in the anchor sliding out of the hole.
Examples of commercially available Bühler bolts are the quality innovative twisted-leg design from Bolt Products and the American designed Climb Tech Wave Bolt.
Homemade Bühler glue-in anchor
Bolt Products twist leg glue-in anchor
ClimbTech 'wave' glue-in anchor
AustriAlpin glue-in anchor
Titan Climbing titanium 'Eterna' glue-in anchor
Metal bar is bent to form an eye and the loop closed by a welded joint.
There are several drawbacks inherent to the design:
- Weld quality
- Achieving sufficient mechanical advantage at the adhesive / shaft interface
- Potential corrosion initiation point around / in the weld
- Welding increases the fabrication cost
Correct material selection, welding performed by competent persons and controlled cooling is essential for achieving strong welds. This applies particularly for welding titanium in which an oxygen free atmosphere is mandatory otherwise weak joints will occur (the discontinued Ushba titanium bolt was known to have this issue).
Unlike twist leg designs that create a mechanical keying effect from the twist, welded eyebolts require shaft preparation in order for the adhesive to gain a mechanical advantage.
Stamping, cutting and knurling all achieve the end result however stamping increases tooling costs, cutting can introduce weakness not least a corrosion initiation point and knurling is less effective with certain types of adhesives. Some adhesives are characteristically gritty from the inclusion of fine aggregate and have reduced flow arising from the increased viscosity of the adhesive. The result of which can be less engagement of the adhesive with the features of the anchor designed to produced the mechanical advantage. For knurled shafts this is results in markedly lower pull out results if the particle size is wider than that of the knurled indentation.
U Bolts or Staples
U-bolts are by far the cheapest design of any glue-in bolt and as the name implies, consist of a single length of metal bar bent into a U shape and fixed into two holes drilled parallel to one another. The cost benefit alone would suggest that staples would be the default glue-in anchor choice so why are they less prevalent than single hole glue-in anchors? The principle issue, which concerns the requirement for two holes and the spacing that exists between them, raises issues about strength and ease of placement in all types of placement. The close spacing between each leg requires very accurate drilling to ensure that upon installation, the adhesive is equally displaced around each leg and per placement, potentially more adhesive and drilling is needed. Staples also result in the quickdraw lying flat against the rock so require the rock surface around the staple to be free of any rock protrusions.
With holes drilled correctly i.e. parallel, staples have no means of hole retention rendering their use problematic on steep rock. Certainly climbers have found ways to prevent staples from slumping or dropping out, however other glue-in designs are free of this issue. There is a higher risk of the quick draw unclipping itself from the bolt should it rotate around the placement when the rope puts tension on a quick draw (and not necessarily in a fall). The Fixe staple has a pronounced dropped nose for this reason.
As for any welded eyebolt, shaft preparation is crucial and staples cannot be rotated to improve the flow of adhesive over the legs. Like stamped grooves or knurling employed for single shaft designs, adhesive viscosity is particularly relevant in ensuring the adhesive flows in and around the legs. Epoxies have a clear advantage in this regard. Due to the low cost and simplicity of design, many staples are homemade thereby lacking the design features and assurance with a certified anchor.
Ultimately the key issue often cited regarding staples is strength however this is very much subject to the anchor and rock in which it is being placed. In Australia, Steve Hawkshaw’s thesis demonstrated that staples are capable of sustaining acceptable loads, however, radial pull tests conducted by us on British Portland staples, a once popular home made staple resulted in full extraction in soft limestone at loads no higher than 12kN which is well below either of the current standards (EN959 / UIAA 123).
Whilst failure cones for climbing bolts originate closer to the rock surface than a comparison test in concrete (the matrix failure is different) there is the potential for stress cones to overlap for staple designs that feature closely spaced legs. In very soft rock, staples have failed because of this close distance. However, in most cases the failure is the result of insufficient mechanical resistance due to the design (poor keying of adhesive to the legs) rather than all out rock failure and staples typically fail at one leg rather than both. With sufficient leg spacing and adhesive of low viscosity (e.g. pure epoxy), this becomes less of an issue.
Our in-house tensile testing performed on the Titan Climbing staples we sell resulted in loads well above 30kN and this is due in part to the use of thicker rod, deeper leg embedment and wide leg spacing.
Staples combined with chain are particularly useful when requiring a theft proof anchor configuration and a cheaper type of glue-in anchor when equipping routes that are not overhanging.
Titan Climbing titanium glue-in staple
Petzl produce the iconic forged ringbolt in two sizes: the Collinox, rated to 25kN and the Batinox, which is rated to 40kN. They are inherently robust mechanically, fabricated from high quality marine grade stainless steel (316) and have the advantage of using a relatively small drilled hole.
Petzl Collinox forged glue-in ring
In France, ringbolts are manufactured specifically for the FFME and distributed to their equippers. They are hot dipped zinc galvanised steel, not stainless but have the unfortunate effect of killing lichen on the rock surface as zinc leaches out of the bolt, leading to the distinct vertical streaks common to French sport cliffs.
French FFME Galvanised forged glue-in ring
Fixe have fairly recently produced the EPXL HELY glue-in bolt that is fabricated from duplex 2304 stainless steel. This product essentially counters the 904L hanger and 926 stainless steel expansion bolt produced by Petzl, with both manufacturers intending their respective products to be used where marine grade stainless steel products (316) suffer corrosion.
Fixe 'Hely' forged glue-in ring
Similar in design function to a Bolt Products twist leg glue-in, the rod design prevents bubbles forming and ensures that the glue is distributed evenly, without the need to turn the tensioner or drill an oversize hole. This glue-in anchor is designed for holes of 10mm (3/8 inch) diameter, reducing drill bit wear but lacks an inference fit that aids retention in overhanging placements.
Forged glue-in anchors tend to be prohibitively expensive compared to other types, lack in-hole retention features and require careful manufacturing if material defects are to be prevented immediately after forging.
Glue-in Anchor Advantages:
- Significantly stronger than a comparable depth expansion bolt.
- Cannot be stolen.
- No parts that can rotate loose.
- No karabiner damage.
- Can be abseiled from by threading the rope through the eye.
- Typically have greater corrosion resistance - this a general statement however crevice corrosion is less of an issue with a glue-in, as is the interstitial stress.
- Single piece unit - dissimilar metal corrosion is not an issue.
- Can improve holding power by installing a longer bolt - off the shelf, longer expansion bolts are not available in comparable lengths.
- Best overall system for long life.
- The only sustainable fixed protection system for regions affected by SCC (stress corrosion cracking).
- The only type of fixing suitable for ALL rock types.
- Once cured they form a tough, chemical resistant, stress free fixing making them ideal for ‘close-to-edge’ fixings – a major advantage over traditional expansion bolts that cannot be located as close to edges due to the constant expansion forces generated by the fixing.
- Visually less obtrusive than an expansion bolt and hanger.
- Can respond to irregular hole quality - any internal pits are filled with adhesive.
- Generally the cheapest fixing when used in bulk.
Glue-in Anchor Disadvantages:
- More equipment required (hole cleaning brush, blower, adhesive dispenser).
- Hole preparation (removal of dust) is crucial to installing a safe bolt.
- Can be messy using bulk adhesive dispensing systems depending on how well the user has been trained (and is organised).
- Adhesive has a limited shelf life.
- Cannot immediately load the bolt (adhesive cure times vary from a couple of hours to a day).
- Not as easy to replace versus an expansion bolt.
We are often asked about whether to counter sink glue-in anchors.
Generally glue-in twist leg anchors do not require the eye to be recessed because the opposing metal legs provide high resistance to torque loading. The EN 959 certification standard for glue-in anchors specifically requires resistance to torque loading where the anchor eye is not recessed. The loads to achieve failure through torsion have to be high and in many cases the attachment link e.g. karabiner, would fail beforehand.
A single shaft glue-in anchor is more likely to twist so certain anchors such as the Petzl Collinox and Batinox rings have oval shaft profiles, prominent ribs and the shaft end cut at a 450angle to resist torsion.
The principle reason for counter sinking is to reduce cracking of the adhesive bond around the anchor eye and provide support to glue-in anchors that use a welded eye. The welds are so stiff that they create a shear point in the Heat Affected Zone (HAZ) and this creates a stress raiser.
For large glue-in anchors countersinking reduces the visual impact and reduces the potential for a rope to become caught underneath the eye.
Aside from additional installation time, the only major disadvantage concerns anchor replacement and if the eye is recessed then the task becomes more complicated than simply cutting the eye away with a cordless angle grinder and coring out the shaft.
Techniques for creating notches:
- Drill short stab holes in a vertical row then break them through.
- Run the drill bit up and down vertically with sufficient chuck speed to engage the hammer action and create a notch that way.