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Determining a Tension Test Load

Tension loads for adhesive anchors have to be tested, as it is almost impossible to determine the strength of an anchor visually. Small mistakes during installation like damp concrete or an improperly cleaned borehole or guide hole could result in an anchor being much weaker than it should be according to its design value.

Additionally, the International Building Code and many state building codes require a minimum of spot checks for tension loads in certain public buildings like schools and hospitals. Horizontal overhead mechanical anchors are also more likely to require testing according to local building codes, while a certain percentage of anchors will need to be tested (between 10% and 100%) depending on the type of job and whether or not the anchors are structural.

Why Is Determining a Tension Test Load for Adhesive Anchors Important?

Test loads, otherwise known as pull tests, are a key part of good construction practice whether or not they are required in local building codes.

Pull tests or test loads can tell us a lot about how well anchors have been installed and the conditions affecting installation you may have missed. If there is any possibility that the epoxy or other adhesive you used was improperly mixed, there’s a high chance the anchor will have a low load strength.

Determining a tension test load is particularly important when anchors are installed in nonstandard substrates. If installed into rock such as limestone or sandstone, for example, the design value for an anchor probably won’t be specified so you will have nothing to go on except your tension testing. While you could simply hope for the best, the failure of an adhesive anchor could be disastrous given some of their infrastructure applications.

Torque vs. Tension Testing

It should be noted that torque testing is not an alternative to tension testing. Since it doesn’t require any equipment and is relatively fast, testing with a calibrated torque wrench is standard for post-installation expansion anchors that have a specific torque-to-tension relationship. Adhesive anchors don’t share this relationship with torque, and their tensile strength can vary hugely based on whether proper curing and bonding have occurred. For this reason, a torque test isn’t helpful for standard adhesive anchors.

Two Options for Determining a Tension Test Load

There are two generally accepted options for testing tension loads of adhesive anchors. They are "twice the maximum allowable tension load" and the "hydraulic ram method." Let’s take a look at them in greater detail.
Adhesive Anchors - Determining Tension Test Load

Using Strength Design Values

Design strength is the highest load that can be applied under given conditions without a part — in this case an adhesive anchor — without that part failing. This is usually calculated based on past experience with similar materials. In the '80s and '90s, allowable stress design was used. Modern measures of strength design values take into account both fixed and variable factors for safety, and load combinations are considered.

Realistically, "twice the maximum allowable tension" means finding out the allowable load of an anchor and then multiplying by two. This will be more complicated for anchors placed close to edges, closely spaced anchors, and anchors in cracked concrete.

Using the Hydraulic Ram Method

The second option for determining tension load in adhesive anchors is by using a hydraulic jack or spring-loaded device. When testing an anchor in this way, the anchor has to maintain the test load for 15 seconds at a minimum. If there is any movement of the anchor during these 15 seconds, the test is failed, and you will have to remove and reset your anchor and retest.

To use the hydraulic ram method, you will need a hydraulic cylinder supported on a loading plate. The loading plate should have a hole that’s between 1.5 and 2 times the width of the borehole.

Test loads should be at least twice the max tension load or 1.5 times the maximum design strength of the anchor.

Accounting for Early Age or "Green" Concrete

When you’re on a job with a lot of teams who need to start work once an element of construction is completed or a job with a tight turnaround time, there is every likelihood you may need to place your adhesive anchors into early-age concrete, otherwise known as "green" concrete. This is any concrete that has been cured for less than 28 days, at which point it will have reached 99% of its final psi.

In progressive laboratory experiments carried out by DEWALT, concrete with 7–21 days of curing time was tested for bond strength. For each of the adhesives tested (all DEWALT brand), bond strength was reduced by a factor between 1.0 and 0.7. The tested adhesives included epoxies and acrylics, both fast and standard cures. DEWALT’s findings can be applied broadly to other adhesives, though factors like temperature, moisture, and the make-up of specific concrete may change how quickly and how well adhesive bonds.

Why White Cap?

White Cap has everything you need to correctly install and test adhesive anchors in stock. Whether you shop online or visit one of our 400+ brick-and-mortar stores, you’ll find a huge selection of tools for the preparation of your boreholes, anchors, and adhesives to maximize the strength of your anchor installations.

If you need help working out how to determine a tension test load in your newly placed adhesive anchors, choosing a concrete mix, or any other part of placing successful structural or nonstructural adhesive anchors, you can get in touch with a local White Cap Expert today. After filling out a short online form, you’ll get a phone call to chat about your needs, followed by ongoing support. This means an expert can help you through designing the placement, placing, and testing your anchors. And, as our experts are all local to the area in which you’re working, they can help you understand how weather conditions might affect the curing of both adhesives and concrete.
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