How to choose the right chemical anchor

Choosing the right Chemical anchoring – Types of materials, technologies and principles for safe planning

Choosing the right chemical anchor — materials, technologies and principles for safe design

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 between three parties: the client (MHD) argued the material had failed; the supplier (Powers Fasteners) argued the installation was faulty and had not followed the specification; the contractor argued the design specification was not clear. The NTSB investigation concluded that the underlying cause was the use of a fast-set epoxy with poor creep resistance — an adhesive that could not sustain a long-term load. The supplier's own creep tests from the mid-1990s had already shown the product was unsuitable for sustained loading. An aggravating factor: no pull-out tests were carried out before the tunnel was opened to the public.

Case 2 — corrosion of climbing anchors in South Africa

At climbing crags in South Africa (Montagu, Silvermine Crag, The Mine and Peers Cave), SS304 stainless steel wedge anchors made by Fischer and Hilti were installed. After a few years the anchors cracked as a result of stress corrosion cracking, and several climbers narrowly escaped with their lives. The problem: SS304 stainless steel is not suitable for every environment — in some environments 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 — that is, he heated the rod while the adhesive was already bonded to it. The heat damaged the bond, and in the pull-out tests carried out afterwards the rods pulled out at only about 30% of the expected load. Worth noting: the sample installations carried out correctly before the work began were entirely sound.

These are only a few of the cases that illustrate the failures that recur in this field. The conclusion is plain: choosing a suitable material, installing it correctly, and carrying out pull-out tests are not optional — they are an engineering obligation.

The principles these failures teach

The designer's duties
Be explicit in specifying the anchoring, and give every installation detail — diameter, embedment depth, type of material, conditions of execution.
Choose the anchor that suits the specific conditions of the application (moisture, temperature, loads, chemical environment).
Require pull-out tests in accordance with the standard.
A specification such as “16 mm epoxy”, with no statement of the anchor type, the grade of steel or the embedment depth, is an invitation to failure.

The contractor's duties
Order the product specified in the drawings, or an alternative approved in writing by the designer.
Carry out the installation according to the designer's and the manufacturer's instructions — even where it looks as though another way would work just as well.

Types of chemical anchoring

Post-installed anchoring divides into two broad fields: chemical anchoring and mechanical anchoring. This article deals with chemical anchoring, which is built on three main material families — polyester, vinylester and epoxy — and three installation technologies.

1. Hand-mixed materials (A+B)

These are supplied in two separate tubs (component A and component B), mixed by hand before use. Their only advantage is a low price; the drawbacks are many and serious:

Drawbacks of hand-mixed materials

Limited working time: once mixed, you have only 5–20 minutes to use the material. Beyond that the adhesive weakens — and there is no way to detect the problem other than a pull-out test.
Mixing-ratio problems: if the installer does not mix in the correct ratio, mixes A+A instead of A+B, or does not mix thoroughly enough, the bond strength will be reduced or nil.
High wastage: any material mixed and not used goes in the bin.
Limited to floor installations: the material is poured into the hole, so a horizontal installation into a wall is not possible.
The borehole cannot be filled properly: installers who dip the rod in the material and hammer it into the wall do not produce a uniform bond. Pull-out capacities then vary unpredictably.
Serious restrictions: the standard epoxies from cheap sources generally carry temperature limits, and prohibit diamond drilling and wet holes.

2. Capsule materials

Supplied in a glass capsule containing a second, smaller capsule inside it. Breaking both mixes the components and starts the cure. There are two kinds:

Hammer-in capsules: easy to install, but the components are only partially mixed, so the strength is reduced.
Screw-in capsules: the rod is fitted to a rotary hammer and driven in with a screwing motion. A common failure: many installers hammer them in instead, contrary to the instructions — which reduces the anchor's strength by 60–70%. In addition, all capsules contain liquid epoxy, so they must not be used overhead; and the quantity of material is fixed, which means the hole must be drilled exactly to the manufacturer's specification.

Capsules are expensive, and there is no real advantage in using them over injection materials.

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. There are three main material types:

polyester

Adit CT50Pro

A polyester chemical anchor — a basic material with standard strength at an attractive price. The best-selling product in Israel over the last decade.

Initial cure in about 3 minutes; full cure in about an hour
European Technical Assessment (ETA) for concrete and for hollow blocks
Limitations: not for use in a wet hole, above 40°C, or with diamond drilling

vinylester hybrid

Adit Chemfix200

An advanced hybrid vinylester chemical anchor — 150% of the strength of polyester. The best-selling product in Israel in its category.

Initial cure in about 3 minutes; full cure in 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)

epoxy

Adit Chemfix100

An advanced epoxy adhesive — strength comparable to vinylester, with excellent long-term durability and performance in extreme environments.

Initial cure in 20–30 minutes; full cure in 8–12 hours
ETA for cracked concrete and seismic use C1/C2, and for dynamic and shock loads
Limitations: not for use above 40°C. Check the manufacturer's instructions for use in the wet and with diamond drilling

Comparing the three injection chemical anchors

Property	CT50Pro — polyester	Chemfix200 — hybrid vinylester	Chemfix100 — epoxy
Relative strength	100% (baseline)	~150%	~150%
Initial cure	~3 minutes	~3 minutes	20–30 minutes
Full cure	~1 hour	~1 hour	8–12 hours
Wet hole	No	Yes	Check with the manufacturer
Diamond drilling	No	Yes (with reduction factors)	Check with the manufacturer
Maximum temperature	40°C	100°C	40°C
Cracked concrete	No	Yes (ETA)	Yes (ETA)
Seismic	No	C1/C2	C1/C2
Dynamic / shock loads	No	Yes	Yes
Approval for hollow blocks	Yes (ETA)	Yes (ETA)	No

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, according to the manufacturer's table for the rod diameter and the 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 walls of the concrete, and can reduce the strength by tens of percent.
Purge the nozzle — inject a little material to waste before injecting into the hole, to confirm that the mixer is blending both components to a uniform colour.
Inject from the bottom upwards — filling about two thirds of the volume of the borehole.
Insert the rod with a slow twisting motion — to ensure it is evenly coated and to 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

When anchoring into hollow blocks (Ytong, pumice, or hollow concrete block) you must use a dedicated mesh sleeve, inserted into the hole before injecting. The mesh stops the material running away into the void of the block, and allows a wide “chemical plug” to form that grips the walls of the block. Without a mesh sleeve there is no effective anchoring in a hollow block. In addition, do not use hammer action when drilling into blocks — drill without percussion, so as not to shatter the walls of the block.

In summary — guiding principles for choosing a chemical anchor

Guiding principles
A suitable material for each application: polyester for routine work; hybrid vinylester for harsh environments and seismic use; epoxy for high loads and extreme environments.
Injection materials (cartridge + mixing nozzle) are preferable in every situation to hand mixing or capsules.
Cleaning the borehole is a critical stage, and must never be skipped.
Require pull-out tests on site after installation, particularly on structural projects.
The designer must specify the type of material, the diameter, the embedment depth and the conditions of execution — a generic specification is an invitation to failure.

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