Phofo: Colourbox

Corrugated tube helps prevent cracks in concrete

Tuesday 06 Feb 18
|
by Morten Andersen

Contact

Ole Mejlhede Jensen
professor
DTU Civil Engineering
+45 45 25 18 63
It took 30 years to develop. But a DTU method for measuring contraction in concrete is now used world-wide.

Singapore is currently spending EUR 6,7 billion on engineering works for wastewater discharge, and it is a requirement that the concrete quality be documented using a measurement method developed at DTU. This is just one example of the impact of the Auto-Shrink method.

“We have really sold a lot of Auto-Shrink systems, especially to India, China, and several South American countries. We have sold them to virtually every country in the world,” says Claus Germann Petersen, CEO and owner of Germann Instruments in Copenhagen, and a DTU graduate.

Concrete contracts as it hardens. The extent of this contraction depends on the precise composition of the concrete, and the local temperature, humidity, and other conditions. Conditions are rarely the same at the construction site as in the laboratory, and there are many examples of past construction projects that held unpleasant surprises for contractors and developers. Excessive contraction causes cracks in the concrete, and the cost of rectifying these can run into millions.

Fierce debate among researchers

In 1984, Per Freiesleben Hansen (1936-2002), a professor at DIA (later DTU), received enquiries from Danish companies looking for a reliable measurement method. The task was left to Ole Mejlhede Jensen, an engineering student at DIA. In consultation with Per Freiesleben Hansen, he decided to build a dilatometer—an instrument that measures changes in the volume of a substance.

"After we had built 30 dilatometers for colleagues in various countries, we felt it was taking up too much of our time. After all, selling instruments is not what we are here to do."
Professor Ole Mejlhede Jensen, DTU Civil Engineering

The best-known example of a dilatometer is the mercury thermometer, which measures the expansion of the enclosed mercury in response to the heat the tip of the thermometer is exposed to.

“Auto-Shrink may seem very simple, but make no mistake. There are many years of hard work behind the status the method has achieved. My original project has been followed by many others. A large number of scientific articles have been published, which have led to fierce debate within the international research community,” says Ole Mejlhede Jensen, who is now a professor at DTU Civil Engineering.

The first version of the dilatometer was made in 1988. A number of adjustments have since been made. For example, it was found that it is useful to pour the liquid concrete sample into a corrugated tube (a tube with wave-shaped grooves, like flexipipe). This allows measurements to be made immediately. You do not need to wait for the concrete mix to harden enough to be handled. It is important to be able to read the early measurements. The corrugated tube also functions as good encapsulation, so that no water evaporates from the concrete sample during measurement. This would otherwise distort the measurements.

Gave idea away to company

A series of tests showed that the new instrument was highly useful. Although there were still critical voices in the research community, DTU Civil Engineering received more and more requests from other universities and laboratories. The department therefore began to sell the instrument in 2001.

“After we had built 30 dilatometers for colleagues in various countries, we felt it was taking up too much of our time. After all, selling instruments is not what we are here to do,” notes Ole Mejlhede Jensen.

He therefore decided to give the invention to Germann Instruments in 2006, a company which was already making instruments for measuring concrete. The company immediately began to sell the instruments all over the world under the name Auto-Shrink.

Illustration: Marianne Rom Andersen
This mysterious instrument is a dilatometer, which can be used to measure changes in the volume of a substance. This dilatometer is used to measure and predict how much concrete contracts as it hardens. It thereby helps prevent cracks in the final concrete structure. Illustration: Marianne Rom Andersen, DTU.