As of Mai 2010 there are exactly 38.525 bridges along the German Autobahns and motor highways. 53.8% of the bridges are made of armoured concrete, 37.8% from prestressed concrete, 6.1% from steel and 2.3% from composite steel materials.
Working steel into concrete makes it an ideal construction material for bridges. The basic idea with armoured concrete is to produce a material that combines the high resistance of steel against tensile stress with the concrete’s high pressure resistance.
The German Federal Agency for Roads (Bundesamt für Straßenwesen, BASt) estimates the lifetime of bridges between 80 and 100 years, depending on the type of construction. About one half of the German Autobahn and motorway bridges already are more than 30 years old. Over time, water perculates into constructions made of armoured concrete, concrete additives react chemically with the metal. The steel becomes brittle; rust also breaks the surrounding concrete. Steel ropes must be replaced in many bridges made of armoured and prestressed concrete because of age. Alternatives, including the support of bridges with modern construction materials such as textile concrete or carbon fibre are still expensive and therefore are presently only used in a small number of highly individual cases.
35% of the German bridges were rated as “very good” in 1999 according to the German motor club ADAC, but in 2008 it was only 16%. Bridge conservation used 350 million Euros in the same year. 17 million vehicles were moved on German roads in 1970; today the number is 50 million. The German Federal Ministry of Traffic predicts that truck traffic alone will increase by 85% until the year 2025. Many bridges need urgent refurbishment. Even today 15% of the bridges are in a critical to poor state. Altogether, 46% of the bridges are in urgent need of repair as a result of age and increasing traffic.
Experts of the ADAC motor club state that around 5 to 7 billion Euros are required in order to secure long-term usability of bridges in Germany.
Should so-called giga-liners – trucks with up to 60 tons of weight resulting in respectively high pressure on the roads – be allowed in the future, the majority of bridges will not be capable to support them, according to a study published by the Federal Agency for Road Affairs. It will not be possible to use most bridges with the next generation of trucks before an additional eleven billion Euros are invested into their reconstruction.
Against this background of issues traffic planners with any bridge will sooner or later be faced with the following alternative: refurbishment and additional support of the existing structure, or demolition and replacement with a completely new structure. In view of limited public resources one thing is for sure: it will not be possible to carry out any of the two alternatives on all bridges at the same time. Besides aspect of traffic safety this is the reason for an increasing demand in information on bridges: Knowledge on the status of individual structures helps directing the investment into repairs and replacements effectively.
All bridges are controlled quarterly regarding aspects of traffic safety. Functional security of parts such as pavement transitions, drainage devices, protection devices (banisters, railings, noise protection walls) is checked. Once a year, or more frequently after events such as inundations or damage resulting from an accident, every bridge is visited more intensively. Every third year more comprehensive control is carried out that includes checks of bearing capacity, structural stability and status of construction elements. All upper parts and foundations are controlled; all functional parts, equipment, security measures and infrastructure such as cables or pipes conveyed over the bridge are checked. Major control is carried out every sixth year. All parts of the structure are checked.
Control works go from knocking on concrete surfaces, to checking the secure fastening of combining elements, to measuring crack widths and deformations. In this context Especially ALLSAT Global Monitoring becomes more and more important.
ALLSAT Global Monitoring helps direct investments effectively
Despite periodical control the behaviour of many bridges after decades of use with increasing traffic load is not known. Is structural movement increasing? Is there a hazard that tendons break, e.g. in box-girder bridges made of armoured steel from the 1960ies and 1970ies?
To answer these and other questions the State of Lower Saxony’s Agency for Road Construction and Traffic in 2008 has equipped bridges of different types with permanent monitoring systems. ALLSAT Global Monitoring is used on a typical three-field box-girder bridge from the 1960ies. A Leica Geosystems total station is used to permanently record the movement of selected observation points on the structure in all three spatial directions. The data are transferred into the next regional office of the agency via a wireless mobile phone connection. Because the bridge is in a remote location, a photovoltaic device is used for energy supply independent from the power grid.
ALLSAT messaging software GART-2000® geomon is installed in the traffic control centre. Thus the agency keeps control, seven days a week, 24 hours a day.
Points equipped with prisms, mounted under a box-girder Autobahn bridge for permanent observation with ALLSAT Global Monitoring.