World-class Danish measuring equipment

Germann Instruments sells equipment for testing concrete quality all over the world.

It all started in the 1970s at DIA, which became part of DTU in 1995. A research group at DIA, headed by Professor Herbert Krenchel (the man who designed the Krenit bowl) and Professor Ervin Poulsen, was working with concrete. Together with Peter Kierkegaard-Hansen (MSc Eng), they developed a method for measuring the compressive strength of concrete.

This information is crucial for knowing when it is safe to remove moulds around a freshly cast concrete structure. Removing the moulds too early can be disastrous, but you do not want to wait longer than necessary because it delays the construction process. This means that being able to precisely determine the best time can save a lot of money.

The method conceived by the Danish engineers was to cast a number of steel discs in the concrete during construction. After hardening, a pulling force is applied to the disc using a lifting jack. The force is gradually increased until the concrete fails. The force required to reach this point can be directly correlated to the compressive strength of the concrete.

Patent put to work

This method for measuring the compressive strength of concrete was called the LOK-test. The name makes reference to the old Danish word for punching out a material (‘lokning’).

Peter Kierkegaard-Hansen patented the method. However, he needed a device that could perform the measurements at the construction sites. This was developed by Claus Germann Petersen, a mechanical engineering graduate from DIA.

Together they founded Germann Instruments, and started making the devices. The year was 1974.

In addition to the original LOK-test, Claus Germann Petersen developed a derived method for situations where no steel discs had been cast in place during construction. This involves drilling a hole in the concrete and inserting a steel ring, which is expanded. You can then make a measurement in the same way as for the LOK-test. This method is called CAPO (Cut And Pull Out). Both methods were used to check the surface layer on the Great Belt bridge—the 18 km bridge that links east and west Denmark.

Trims months off construction projects

Three serious accidents in the USA and Canada, where moulds had been removed prematurely leading to loss of human life, meant that the North American market was particularly interested in the new method.

Germann Instruments has sold around 8,000 devices to Canada over the years, and the LOK-test has been performed on over 800 major construction projects.

“The Canadians have told us that they can reduce a typical building project by a few months. Using the LOK-test, which they call ‘the holy grail’, they can be sure the concrete is strong enough, and do not need to wait for it to harden further. The time saved corresponds to 3-4 million dollars on a typical project,” says Claus Germann Petersen.

In other words, the LOK-test method has led to total savings of around 3 billion dollars in Canada alone. It also saves other economic and human costs by preventing accidents.

The company also became popular in the USA. Claus Germann Petersen has been a member of the relevant working group under the American Concrete Institute for many years. He received the definitive accolade a few years ago when he became a partner of the US National Institute of Standards and Technology (NIST), the world’s leading institution for measurement technology.

		One method—the LOK-test—was developed at DIA, which later became part of DTU, in the years up to 1975. A steel disc cast into the concrete is pulled towards a backstop placed on the surface. During this process, the concrete between the disc and the backstop is compressed to the point of failure. The maximum pull force is a direct measure of the concentrated compressive stresses (marked with yellow arrows). This is the reason for the very close correlation between the pull-out force and the compressive strength of the concrete.
The LOK-test requires that steel discs be cast into the concrete during the construction of a structure. For situations where people want to determine the compressive strength of the concrete in a structure, without having cast steel discs in it beforehand, Germann Instruments refined the method. This involves cutting a hole in the concrete—a cut recess—and expanding a steel ring within it. You can then measure the compressive strength in the same way as under the LOK-test. This method is called a CAPO (Cut And Pull Out) test. Illustrations: Marianne Rom Andersen, DTU.