Research in medicine and bridges has a fairly gruesome detail in common: certain things can only be known after a body has been dissected. In order to find out more about the bowels of a bridge, during a period of seven years civil engineers in the service of ETH Zurich examined diverse objects during demolition. This was a good opportunity for specialists to complete and improve their expertise on the skeleton of these bridges, because it is a difficult and time-consuming job to assess their condition as long as they're "alive".

"Parallels to medicine certainly exist," says head of the study, Professor Thomas Vogel from the ETH Institute of Structural Engineering (IBK) with a grin. The study was carried out for the Swiss Federal Roads Authority (Astra) within the framework of the ZEBRA project (Assessment of Bridge Condition During Demolition). Non-invasive investigation methods, such as radar, X-rays or ultrasonic measurements are technically possible. "But X-rays couldn't be used on the sites because of the radiation dose it requires," he explains.

Demolition delivers insights

Between 1997 and 2004 89 bridges were demolished and surveyed during demolition on behalf of ETH Zurich. The scientists involved had no time to lose; some of their study objects were "spirited away" over night or at weekends in order to keep traffic flow disruptions to a minimum. There was often only time for a rapid inspection. However some of the bridgs were sawn off and temporarily deposited in gravel-pits. This gave the researchers more time to examine the condition of the bridges. Within the framework of the ZEBRA project they accurately analysed 36 objects in all.

What especially interested the engineers was the condition of the citical and endangered supporting elements of the bridge wich are difficult to access, such as tendons. Because more than half of the objects they examined were so-called pre-stressed concrete bridges where concrete-cast tendons provide adequate carrying capacity of the construction. These comprise bracings and ducts made of sheet metal, which contain multiple wires or strands of high-strength steel. Thanks to these tendons the concrete is provided with the necessary drag to support the weight of the bridge. The builders pump liquid mortar into the ducts that surround the steel tendon to protect it against corrosion.

Condition: good

Overall, the results of the research bear witness to the constructors' good work. Most of the tendons were usually in good condition. There were hardly any nasty surprises according to Thomas Vogel. "On the whole the tendon systems had stood up well to the test of time," he says. A big problem, however, was their susceptibility to corrosion. Due to leaky splices, sealing or carriage ways, road salt can permeate the concrete with the melt water, possibly all the way to the bridge's reinforcements. The consequence: braces, ducts and, in the end, the steel tendon itself started to rust.

"Old bridges in particular are often not sufficiently sealed. This is why their tendons were more heavily corroded, " says the professor for structural engineering. The reverse, however, did not hold true. Even if the construction was not watertight it did not necessarily mean that a bridge would show corrosion damage. It also depended on the condition of the concrete. The salt water could only penetrate easily if the concrete was porous.

Pre-stressed concrete joints for bridges, before being put in (above) and after demolition (below). On the top picture the duct–cut open to make it visible–is intact; on the bottom picture, after years in its concrete casing, it is totally rusted in parts (Pictures: top, P. Rüegg; bottom, IBK). large

One tensioning system had not lived up to expectations as far as this was concerned, but constructors stopped using this system in the 1960s. These tensioning elements tended to corrode very strongly because too much hollow space ensued when the mortar was pressed into the ducts. Salt water was subsequently able to collect in the ducts and heavily corrode the tendons.

Expiry date mostly not reached

Why did all these bridges have to come down? Only 15 of the 89 bridges examined were really delapidated. The remainder of the bridges had not reached their expiry date yet, however. After all, road bridges are built to last for around 50 to 70 years, while railway bridges are expected to be serviceable for 100 years. "The main reason for demolishing bridges is due to new routeing of roads and rail" says Vogel. Bridges often had to give way, for instance, if they were above a motorway that was being widened or was being lead into a tunnel.

The data collected from the demolition also afforded the researchers insight into the advantages and disadvantaged of various demolition and procedure methods that will prove useful for building new bridges and maintaining others. More and more, for example, synthetics that do not corrode are being used instead of metal for the ducts. This means, however, that builders have to be more careful when fitting the tendons in order not to damage the plastic tubes. In addition, the results of the research carried out with chisel and crusher provide valuable conclusions on non-destructive methods of investigation. Vogel and his team will continue their work in this area. They are currently concentrating on the further development of a non-invasive ultrasonic approach.