Choosing the Right Chemical Anchor: Materials, Technologies and Safe Design

A chemical anchor is only as good as the material, the specification and the installation behind it. Get any one of the three wrong and the consequences can be severe — as several well-known failures show.

When anchoring fails: three real cases

Case 1: the Boston tunnel ceiling collapse (2006)

On 10 July 2006, in the I-90 connector tunnel in Boston (part of the “Big Dig”), some 26 tonnes of concrete ceiling panels came away from their epoxy-anchored supports and fell onto a passing car, killing a passenger. Responsibility was disputed: the client argued the material had failed, while the supplier (Powers Fasteners) argued that the installation had not followed the specification, and the contractor argued that the definitions in the design were not clear. The NTSB investigation concluded that the underlying cause was the use of a fast-curing epoxy with poor creep resistance — an adhesive not capable of carrying a sustained load over time. The supplier’s own creep tests, dating from the 1990s, had already shown that the product was unsuitable for sustained loads. An aggravating factor: no pull-out tests were carried out before the tunnel was opened to the public.

Case 2: stress-corrosion cracking of stainless anchors

Stainless-steel wedge anchors (SS304) made by Fischer and Hilti were installed in an exposed environment. After a few years the anchors cracked as a result of stress-corrosion cracking, and several people narrowly escaped injury. The lesson: SS304 is not suitable for every environment — in some, SS316 is the minimum required.

Case 3: hot-cutting of threaded rods at the Be’er Sheva theatre

At the Be’er Sheva theatre, M24 rods were installed with an epoxy adhesive (Hilti HIT-RE 500). The contractor inserted over-long rods and cut them to length after installation — heating each rod while the adhesive was already bonded to it. The heat damaged the bond, and in the pull-out tests afterwards the rods pulled out at only about 30% of the expected load. Tellingly, the sample installations carried out correctly before the work began were entirely sound.

Whose responsibility is it?

The designer

• Be explicit: give every installation detail — diameter, embedment depth, type of material, and conditions of execution.
• Choose the anchor to suit the specific conditions of the application (moisture, temperature, loads, chemical environment).
• Require pull-out tests in accordance with the standard.

The contractor

• Order the product specified in the drawings, or an alternative approved in writing by the designer.
• Install exactly according to the designer’s and the manufacturer’s instructions — even where it looks as though another way would work just as well.

The three chemical-anchoring technologies

Chemical anchoring is built on three material families — polyester, vinylester and epoxy — delivered by three installation technologies. Anchoring as a whole divides into two broad fields, chemical anchoring and mechanical anchoring; this article deals with chemical anchoring.

1. Hand-mixed materials (A + B)

Supplied in two separate tubs and mixed by hand before use. Their only advantage is a low price; the drawbacks are many. Working time is limited to 5-20 minutes once mixed, mixing quality is inconsistent so load capacities vary unpredictably, and the cheap epoxies generally carry temperature limits and prohibit diamond drilling and wet holes.

The drawbacks of hand-mixed materials

• Limited working time. Once mixed you have 5 to 20 minutes. Past that the adhesive is weakened, and there is no way to detect the problem other than a pull-out test.
• Mixing-ratio errors. If the installer does not mix in the correct ratio, mixes A with A instead of A with B, or does not mix long enough, the bond strength is reduced or zero.
• High waste. Material that is mixed and not used goes in the bin.
• Downward installations only. The material is poured into the hole, so a horizontal installation in a wall is not possible.
• The hole is never properly filled. Installers who dip the rod in the material and hammer it into the wall do not produce a uniform anchorage, and the pull-out capacity varies unpredictably.
• Severe restrictions. Standard epoxies from Chinese sources usually carry temperature limits and prohibit diamond drilling and wet holes.

2. Capsule materials

Supplied in a glass capsule with a second, smaller capsule inside it; breaking both mixes the components and starts the cure. Hammer-in capsules are easy to install but the components are only partially mixed, so strength is reduced. Screw-in capsules are driven in with a rotary hammer using a screwing motion — but a common site error is to hammer them in instead, which cuts the anchor’s strength by 60-70%. All capsules contain liquid epoxy, so they must not be used overhead, and the fixed quantity of material means the hole must be drilled exactly to specification. They are expensive, with no real advantage over injection.

3. Injection materials (the recommended method)

Material supplied in cartridges and injected with a dispensing gun through static mixing nozzles. This is the best technology available today, and it comes in the three material families:

Adit CT50Pro (polyester)

a basic material with standard strength at an attractive price; the best-selling product in Israel over the last decade. Initial cure about 3 minutes, full cure about an hour. ETA for concrete and for hollow blocks. Not for use in a wet hole, above 40 C, or with diamond drilling.

Adit Chemfix200 (hybrid vinylester)

an advanced hybrid, around 150% of the strength of polyester, and the best-seller in Israel in its category. Initial cure about 3 minutes, full cure about an hour. ETA for cracked concrete and for seismic use (C1/C2). May be used in a wet hole, up to 100 C, and with diamond drilling, subject to the instructions.

Adit Chemfix100 (epoxy)

strength comparable to vinylester, with excellent long-term durability and performance in extreme environments. Initial cure 20-30 minutes, full cure 8-12 hours. ETA for cracked concrete, for seismic use (C1/C2), and for dynamic and shock loads. Not for use above 40 C; check the instructions for wet holes and diamond drilling.

Comparing the three injection materials

Installing it correctly

Even an excellent material will fail if it is installed badly. The correct sequence is:

• Drill to the correct diameter and depth, per the manufacturer’s table for the rod diameter and material in use.
• Clean the borehole thoroughly — a dedicated wire brush at least twice, plus compressed air. This is the single most critical stage: drilling dust left in the hole stops the adhesive gripping the concrete and can cut strength by tens of percent.
• Purge the nozzle — inject a little material to waste first, to confirm the mixer is blending both components to a uniform colour.
• Inject from the bottom of the hole upwards, filling about two thirds of its volume.
• Insert the rod with a slow twisting motion, to coat it evenly and let trapped air escape.
• Respect the curing time — do not load the anchor before the minimum time required for the temperature.

Hollow blocks — special instructions

Principles for safe design

Guiding principles