European infrastructure is ageing fast. In the Netherlands too, many bridges and viaducts were built in the 1960s and 1970s and they are approaching the end of their (design) service life. Replacing them would be costly, as a result of which the construction of new civil structures is making way for keeping existing structures safe. Clear information about the current state of the structures and how it is likely to change is needed in order to maintain it in an efficient manner. TNO is developing integrated innovative monitoring and inspection techniques for this purpose. Work can be prioritized on the basis of this information. Network managers can target their repair and replacement tasks more effectively. This reduces maintenance costs and limits inconvenience to the general public.
How do the authorities ensure that the structural safety of civil structures is sufficient? Safety features are incorporated into the design and assessment of bridges and viaducts. The motto in the Netherlands is ‘better safe than sorry’ - in other words, it is better to err on the side of caution. Nonetheless, there is always a very small probability that a bridge may fail. Reducing such likelihood to zero would be impossible and unaffordable. Looking at the condition of our bridges and viaducts, we note two developments of significance. The first is that they are ageing, with many of them approaching the end of their (design) service life. The second is that the volume of traffic has increased in recent decades, and exceeds that for which the structures have been designed. Because of the high cost of replacing existing bridges and viaducts, and of those that bear heavy loads, various methods have been developed to give them a second life without lacking the required structural reliability. The cost of replacing just the 3,700 bridges and viaducts managed by the Directorate-General for Public Works and Water Management is estimated at 16 billion euros. The total value of all the approximately 40,000 bridges in the Netherlands is many times greater.
End of service life
Many of the bridges and viaducts in the Netherlands were built in the 1960s and 1970s. Around 70% of the bridges and viaducts that is managed by the Directorate-General for Public Works and Water Management was built before 1980. At that time, the service life for structures was estimated at 50 to 80 years. However, this does not imply that the structural safety of these structures is insufficient when they reach this age. After all, structures built by the Romans shows that some objects last much longer. On the other hand, there are cases of bridges that have actually failed (collapsed or severely damaged). Since 2000, some ten bridges in Western industrialized countries have collapsed as a result of a combination of inadequate strength and heavy loading conditions. A tragic example is that of a concrete bridge near Milan that collapsed late last year, killing one person and injuring five. In the Netherlands, there are around 30 cases a year involving damaged bridges and viaducts. This was revealed in an inventory carried out in 2009 by the Inspectorate of the then Ministry of Housing, Spatial Planning, and the Environment.
“The value of the entire civil structures in the Netherlands is almost as great as the national debt and is the largest public property”
Increasing traffic loads
As well as the fact that the various structures have aged, the burden of traffic has also increased over time. The greater number of heavy trucks in particular has affected the condition of bridges and viaducts. Compared to other countries, the policies operated by the Netherlands regarding heavy trucks weighing between 50 and 100 tons are generous. New types of axle and fewer tyres per axle have also been introduced without any account being taken of the greater burdens this places on bridges. When it comes to assessing the structural safety, cars are not taken into account because of their negligible weight.
Each with their own set of problems
The combination of ageing and heavy traffic loads may lead to a variety of problems. Some parts of steel bridges on busy stretches of the national road network are showing symptoms of fatigue, as a result of which their capacity to bear loads is diminishing in the face of the large numbers of trucks passing them. Small cracks increase in size with the number of trucks passing. So far, this has only been observed in the Netherlands in certain types of road bridges. Concrete viaducts may be affected by corrosion of their reinforcement, which in turn weakens the structure. Particularly in cases where joints leak, corrosion and damage to concrete may occur at the points where slabs and beams are supported. TNO expects that around 5% of concrete structures will have to be repaired after 40 years, and as many as 50% after 70 years. Given that the construction of bridges and viaducts reached its peak in about 1970, we envisage a rise in the number of cases where concrete will have to be repaired.
Some urban bridges are considerably older than those in the national road network. Although the burden of traffic is usually lower in cities, the authorities need to be alert to any problems, because there is sometimes insufficient information about the condition of the bridges. In addition to the 30 (on average) cases of damage every year, there are another 20 instances annually of restrictions being imposed, which apply mostly to trucks. This lowers the probability for damage. In some cases, the reinforcements corrodes, possibly leading to spalling of the concrete cover. Chemical reactions in the concrete may also lead to restraint swelling, causing internal cracking. The archives relating to old bridges are incomplete in many cases. As a result, calculations and drawings of the design may be missing, so there is no knowledge of the quantity, origin, and the quality of the materials used. Also, information about traffic loads in decades gone by and ‘damage histories’ may be difficult to trace. This lack of knowledge increases the uncertainties about the reliability of the structures, which possibly leads to conservative assessments. Unnecessary inspections and premature replacement costs can be avoided by finding out more about the actual conditions of bridges.
“The service life of steel bridges can be extended by post-treating vulnerable welding joints”
By no means does every case of damage lead to dangerous situations. Very strict requirements apply to the safety of structures. The requirements with respect to the failure probability for a new structure is about 1/1.000.000. The requirements for existing structures are less strict because strengthening an existing structure is much more expensive than making structures stronger that are yet to be built. These safety requirements acquire greater context if we compare them to other norms. For example, the new Water Act sets the maximum likelihood of dying as a result of flooding at 1/100.000 per year. This requirement is similar to that for existing buildings, and is therefore less strict than those for new structures. Another comparison is that with the chances of a person having a fatal accident - this amounts to around 1/10.000 a year. From this, it can be concluded that structural failure is more or less completely unacceptable.
Safe extension of service life
Because the design codes are developed for applying to a large number of types of structures, several conservatisms are incorporated. In practice, when considering an individual structure, this results in quite some ‘hidden strength’ that was not taken into account in the original design (structures may appear stronger compared to what was calculated for). This hidden strength can be revealed through further examination or more detailed calculations. Measurements have shown, for example, that after 20 years, concrete is much stronger than should be assumed according to the rules of calculation. Identifying this hidden strength has meant that around 600 concrete bridges that were built and designed before 1974 were not replaced prematurely. This prevented the unnecessary expenditure of billions of euros.
Measuring and monitoring the actual condition of bridges and viaducts results in a need for fewer inspections and a more accurate assessment of the residual service life. Fatigue cracks in steel, for example, are easier to detect and it is then possible to measure any further deterioration on a periodic or even permanent basis. This has been successfully tested on two Dutch bridges. Monitoring lays the solid basis for reliable predictions of the remaining service life and for decisions on whether repairs or renovations are needed. Thanks to new measuring methods, that among others involve acoustic emission and satellites, there is smaller probability that the structure in question may fail unexpectedly.
Finally, we refer to new and improved repair techniques that can extend the service life. An example of this is repairs to concrete in combination with cathodic protection. Putting a small electric current through concrete doubles the lifetime of concrete repairs, without large-scale additional costs being incurred. The service life of steel bridges can be extended by post-treating vulnerable welding joints.
“Thanks to new measuring methods, that among others involve acoustic emission and satellites, there is smaller probability that the structure in question may fail unexpectedly”
Maintaining civil structures
In summary, TNO has identified several areas of concern and developments for maintaining Dutch civil structures safely and affordably. As established by the Inspectorate of the then Ministry of Housing, Spatial Planning and the Environment in 2011, local authorities in particular need to pay greater attention to the safety of structures. And action has indeed been taken on this front, such as through the creation of joint initiatives for establishing an effective approach. New measuring techniques are now available and offer ways of better determining and monitoring the condition of civil structures. Together with advanced models for predicting service life, they have laid the basis for further improvements to the asset management carried out by the authorities. It is also important to continue the process of developing effective repair techniques for existing structures. Consideration could be given as well to tougher policies on allowing the use of heavy trucks of between 50 and 100 tons because of the cost savings this may produce in terms of maintaining civil structures. Finally, it is important that levels of expertise among those responsible for managing civil structures be maintained or brought up to the required standard, in order that they are able to properly fulfil their role as commissioning parties and make sound decisions regarding this complex matter. One way of achieving this is for small and medium-sized local authorities to pool their expertise into one joint operation.
The closure of the Merwedebrug bridge to heavy trucks is a sign that we have to continue being careful with our ageing and more heavily burdened bridges and viaducts. The value of the all civil structures in the Netherlands is almost as great as the national debt and is the largest public property. Such an asset deserves responsible management. Safety should be carefully monitored and overdue maintenance only means shifting the bill onto future generations.
Ir. Arie Bleijenberg, Prof. Dr. Ir. Johan Maljaars, Prof. Dr. Rob Polder, Prof. Dr. Ir Raphaël Steenbergen, Dr. Ir. Adri Vervuurt
- Buildings, Infrastructure & Maritime