International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
|Start > Contributions >Lectures > Structures 2:|
NDT used in the Netherlands From a principal point of viewF.J. Postema, Ministry of Transport, Public Works and Water Management, The Netherlands
A. van Beek, Ministry of Transport, Public Works and Water Management, The Netherlands
Being responsible for the condition of a wide range of structures, Public Works in the Netherlands prefers to inspect these structures in a non destructive manner. The development of non-destructive inspection techniques seems to fulfil this whish. The applications of these techniques, however, has not always been successful.
Public Works surveys its structures on a regular basis. Most of these surveys are visually. If more in depth information is required special surveys are performed. The inspection techniques used for these special surveys often damage the structure. Therefore we try to use non-destructive techniques as often as possible. If companies develop new techniques we try to offer the possibility for them to gain practical experiences.
In this paper we will show some examples of non-destructive techniques used, for example radar, potential mapping, Consensor. It will be shown why some of these projects were successful and which lessons were learned. With these examples we try to show the need for NDT for some problems we have while surveying structures. This may offer the NDT companies and developers better understanding for the need for NDT techniques from a principals point of view.
Inspections on structures are performed to determine the maintenance needed. Information about maintenance is needed to predict the costs to keep the infrastructure functional in the future . The inspections determine the condition of our structures. Inspections are also required if damage, for example due to collisions, is observed and has to be repaired. These inspections are focussed on the cause of the damage, which has to result in a repair method.
The inspections should preferably be non-destructive. The use of non-destructive techniques however is minor. Most inspections are firstly visual. If damage is observed, most inspections require taking samples of the structure, for example drilling cores from a concrete bridge deck. Taking samples should be prevented because:
Public works started a research project in 1999 which had to result in improving the use of non-destructive testing within the organisation. The project started with a research to the use of non-destructive test techniques at that moment and the problems which were observed when the techniques were used. This research resulted in recommendations to improve the use on non-destructive inspection techniques. These recommendations were embedded in the project which is still in operation and will continue till 2004.
2 Research using NDT at public works in 2000
2.1 Results of the research
In 2000 most NDT projects were focussed on steel structures. Standard methods were used in this field, for example: magnetic and ultrasonic. For concrete structures NDT is hardly used. From interviews, it was observed that there is a need for using NDT techniques on concrete structures. One of the aspects for not using the techniques available was lack of knowledge.
The knowledge and experience of working with certain techniques is spread within the organisation. It was recommended to collect this knowledge and to spread this knowledge with the intranet of the organisation. The knowledge will be stored and spread with an electronic centre of knowledge. This centre of knowledge is focussed on the role of a principal in NDT projects. Therefore, the database contains mainly practical knowledge.
For the development of new knowledge and techniques it is important that the obtained knowledge will be directed to practice. The knowledge from practice has to be updated regularly. The centre of knowledge on the intranet will be used to send the information into the organisation. The information is the database is collected from projects. Some examples of these projects are presented in the following sections.
3 Using NDT
In the figure below a mind map is presented, which can be the basis for decisions on using NDT techniques. With this mind map the principal can decide if NDT might be a solution. It helps him in discussions while working with NDT contactors. Important topics for a NDT project are:
By discussing these topics with the NDT contractor the expectations of the techniques used will be clear to both. This will minimize the risk of having good measurements but poor results. In the following sections some NDT project in the Netherlands will be outlined. These sections will not be focussed on the techniques alone but also on practical experiences obtained.
With potential measurements the electrical potential is measured between a so called reference electrode on the concrete surface and the reinforcement by a high impedance voltmeter. There will be no disturbance of the electrical field because an electrical field will not be applied. A low potential is an indication for corrosion. In the Netherlands potential surveys are often used on reinforced concrete structures, steel structures in soil and in seawater. For cathodic protection of concrete structures potential surveys are essential to determine the effectiveness of the system.
At the Ketelbrug there was a need for information about the condition of the prestress tendons. The potential survey at the concrete surface was used. On forehand it was known that the interpretation of the results would be difficult. Although many systems are available to determine corrosion the only reliable method seamed to be the potential survey. The result of the bridge deck through the asphalt layer were even less reliable. The measurements through the asphalt layer were more practical because otherwise the layer had to be remove first. Experience for measurements through asphalt are lacking. Due to the increased distance between surface and reinforcement the potential gradients will be less pronounced. The interpretation of the results in terms of risk for corrosion is therefore difficult.
3.3 Monitoring of ASR affected bridges
The measurements are based on Time Domain Reflectrometry (TDR), with which an electromagnetic wave will be reflected by discontinuities in the electrical impedance of the material. The dielectric constant of the material can be determined with this technique. In porous materials the dielectric constant is related to the moisture content, so is the case in concrete. By measuring the speed of propagation of an electromagnetic wave in a material the dielectric constant can be determined. The TRIME sensor operates with an electromagnetic field at a frequency of 0.6 to1.2 GHz.
The sensor can be used for moisture measurements on concrete, rock materials and sand. The sensors used in concrete were originally designed to be used in soil. TDR was applied by Public Works together with Delft University of Technology, Universiteit van Amsterdam (UvA) and Wageningen University to determine moisture in soil underneath asphalt layers.
The determination of strength of young concrete is still under development. To obtain practical experiences Public Works used the method at the Suurhofbrug. At this bridge a sensor was used called the Consensor. Two electrodes attached to the formwork. The electrodes will be embedded in the concrete after casting. An electrical field between the two electrodes will measure the changes in the dielectric properties of the concrete. The electrical conductivity of the concrete changes during the hydration of the cement. During hydration the strength of the concrete will changes as well. In other words:
High conductivity --> low strength
The relation between conductivity and strength is determined in a laboratory and differs for different concrete mixtures.
The results of the Consensor were compared to the results of the Dutch maturity method. This comparison gave the developers of the system new input for further developments.
4 NDT at Public Works 2003
4.1 Centre of knowledge
These methods have been used in practice or in research project at Public Works. For these methods the current state of the art of the method is mentioned in the database. Details of the measurements methods given in the database are:
The use of the centre of knowledge is limited but appreciated. It is seen as an example of knowledge sharing within the organisation. In practice not only technical and practical information about techniques are requested. An important aspect of working with NDT is making the choice between the techniques given by suppliers and consultants.
4.2 Selection of techniques
This are only a few aspects which are important for a NDT project. Not only Public Works has the need for objective selection criteria. The HERMES project of Delft Cluster, the Netherlands, develops a method to design monitoring systems for construction and maintenance . Their aim to develop an objective methodology for design is close to the aim of Public Works to have an objective methodology of choosing a NDT system. Both methodologies should be a tool to facilitate decisions regarding further construction and maintenance.
Also international the need of objective selection criteria is presented. In the recently published 'Guide to testing and monitoring the durability of concrete structures' of the Concrete Bridge Development Group of the UK  a methodology for choosing inspection tools is given. This report shows when NDT can give valuable information in the inspection process. The relation between type of damage and possible inspection technique is given. These techniques are related to the practical application and differ from for example visual inspection to taking cores and radar surveys. NDT, however, will have to prove its use in practice to become a standard option for inspection techniques. The latest developments in techniques should therefore be given a change to be tested in practice. These practical research projects enhance the development of NDT techniques to a practical measurement devices.
4.3 Practical research projects
Not only the data from the device is an important aspect of the quality of the measurements. Also the interpretation of the expert and his presentation of the results to the principal is important. For the principal its hard to have knowledge about all systems. This knowledge, however, is often required to make an objective choice between the systems offered and to understand the results presented. The results of the inspection are often used for important decisions concerning, economics and safety. The verification of the end results should therefore be a standard procedure in NDT projects. This will mean that a limited number of (destructive) tests should be done to confirm the NDT results.
In these latest development a shift has been observed in the need for measuring and monitoring durability parameters. An example is the need of monitoring parameters which are related to the ingress of chlorides. This can be the material parameter chloride diffusion coefficient, which can be determined with a electrical resistance measurement. This method might replace the Rapid Chloride Migration test . It can also be used as a test for accepting concrete products, like tunnel elements for a tunnel lining. Another parameter that can be monitored is the real chloride ingress in practice to determine the moment to activate the cathodic protection system.
The use of NDT in practice is limited. Only in special cases NDT has been used. At some of these projects the results of the measurements gave valuable information about the condition of the structure. Some projects, however, were less successful. To increase the success rate of NDT projects Public Works has activities on:
The aging of our structures will increase the need of maintenance. An effective maintenance plan will depend on reliable inspection data. This need for reliable inspection data will increase the need for Non Destructive Test methods and monitoring techniques. One of the areas of interest will, therefore, be the durability of structures. Its therefore logical to focus on measurement techniques for durability parameters.