Wiring chaos in the warning system called for creative thinking when corrosion monitoring on the Øresund Bridge short circuited.

The answer is blowing in the wind - or to be more precise, in the weather. Nobody at FORCE Technology knew that when the Concrete Department was asked to supervise the corrosion monitoring system. During the inspection of the corrosion warning system on the Øresund Bridge the engineers at FORCE Technology came across data that were dubious and incomprehensible.

Sensors have been cast into a number of bridge piers to monitor the ingress of chloride ions from the seawater into the concrete. This is of interest as increased chloride content results in corrosion of the reinforcement. Corrosion of the reinforcement leads to expansion, causing cracking and spalling of the concrete cover. The sensors monitor when the chlorides penetrate to the reinforcement and thus provide ample warning to take preventive measures before corrosion will cause serious damage.

This data was debatable in the case of bridge pier W1. Initial measurements from 2001 and measurements performed in 2005 with portable equipment by FORCE Technology confirmed that there were serious faults. The faults were caused by short circuiting in the cables of the corrosion sensors.

The sensors are mounted in four groups on pier W1, one group located to more or less face one corner of the world. The sensors are all located 0.5 metre above water level where the concrete is subjected to great impact due to the alternation of wet and dry surfaces - referred to as the “splash zone”.

Each group consists of three sets of sensors each connected to 7 wires - a total of 84 wires, which is quite challenging to keep track of as Jesper Stærke Clausen from FORCE Technology explained.

Deep down

Supervisor Tonny Egebjerg Jensen, from MT Højgaard a/s, has been working on the bridge since the start of construction, and he knows the Øresund Bridge. He tells us, “Deep down in the bridge piers all these wires are joined in an epoxy sealed connection box. After many trials and tribulations we managed to open the connection box and here we found a chaotic jumble of more – or especially less – identifiable wires. Of the 12 sets of sensors, we managed to definitely identify nine of them and the other three to a reasonable degree of certainty, but it was impossible to determine which set belong to which corner of the world".

“We attempted to identify the location of each group of sensors by sending an electrical impulse through the wires and thus measure the distance from the connection box to the sensors. But as the distance to the sensors were almost equal, we were on very uncertain ground. There had to be another method. But which?” pondered Jesper Stærke Clausen.

DMI data

As the temperature sensors, which are part of each corrosion sensor, react quickly to temperature changes in the concrete, the answer was to monitor the warming and cooling of the concrete surface during the journey of the sun around the bridge. This creative solution consisted of connecting each temperature sensor to a data logger and then measure the temperature each hour for a month. The collected data was analysed and data from two days was chosen.

“We compared our data with temperature measurements from the Danish Meteorological Institute (DMI) and this soon provided a clear picture of how the sensors and wires matched up. When we supplemented this data with DMI’s cloud cover measurements from a weather station located near the bridge at the main land, there was no doubt. The sun and clouds had given us our answer”, Jesper Stærke Clausen says in conclusion.

Creativity during chaos

It is just as important that creativity does not get killed by thinking in grooves as it is to prevent the corrosion of the reinforcement in the Øresund Bridge.

“We enjoyed good teamwork and collaboration with FORCE Technology in the solution of this problem. It is always exciting to walk down new roads and build bridges between problems and solutions", says Tonny Egebjerg Jensen.