International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Non Destructive assessment of damaged reinforced concrete structures: from needs to answers. A national state of the art.BREYSSE Denys,
C.D.G.A., Université Bordeaux 1, France,firstname.lastname@example.org
L.C.P.C., Bouguenais, France, email@example.com
French national experts and practitioners of Non Destructive Evaluation have decided to contribute, under the auspices of AFGC (French Association of Civil Engineering) and COFREND (French Confederation for Non Destructive Testing), to write a book explaining what can be the interest of NDE for assessment of reinforced concrete structures, and how the NDE methods can be implemented and their results can be used.
This decision followed a statement:
The aim of the Working Group is to write a State of the Art, and guidelines on the use of NDE (when, where, how, for what...) and on the use of NDE results for answering the real building owners questions.
1. Working process - objectives
The Working Group has been created in September 2000. This paper will describe the methodology we have followed, the questions we have tried to answer and it will present the content of the book to be issued.
The guidelines are designed such as to serve as a practical help for the various professionals involved in the structural assessment process:
It is the reason why the Working Group was built with an equilibrated participation of professionals from these different fields: building owners, companies using NDE methods, experts of structural assessment, researchers (from universities or others institutes)... All meetings (about 15 during these 3 years) were plenary meetings, where all topics have been addressed in common. Regular exchanges have also been facilitated by creating a website (open to Working Group members, but also to external people interested in our work, after a simple agreement). This website contained all practical information (meeting minutes f.i.), but also the working versions of all sections of the book. This made possible the effective work in common, each section of the written book being under the responsibility of a co-ordinator, but with the practical help of several other participants.
From a scientific point a view, the first key question was to decide what kind of problem we intended to address. It was decided to focus on questions related to structural assessment of REAL structures, i.e. structures which:
The building owner wants his structure to fulfil given requirements in this context and asks if NDE can help him in assessing the structural condition. Thus, NDE must work with the same constraints and must give data which are of practical interest for the given problem.
Since we wanted to focus on methodology in practical cases better than on theoretical or laboratory results, it was decided, as far as possible, to illustrate the speech by many examples of on-site real problems.
It was also decided, at this preliminary stage, to focus on reinforced concrete, avoiding any scattering on other building materials (prestressed concrete, masonry). Exceptions were considered only when some methods have had few field applications on RC concrete and when we thought that the same method might apply in concrete.
We have decided to tackle important questions like:
It was chosen to divide the book into three main sections, each of them quoting many examples, taken as far as possible from real case studies.
Section A replaces the NDE within the general frame of maintenance/assessment, lists the practical questions NDE has currently to answer and shows how these questions can be, in practice, answered more or less satisfactorily.
Section B, after a general review of basic physical principles of the methods, is devoted to present 9 families of techniques, with several variants. For each family, real case studies are detailed, such as to discuss efficiency, field of use, precautions, limits...
Section C tries to draw a general synthesis. A defect/method matrix is proposed and the helpfulness of approaches which combine several methods is illustrated on several cases. Finally, it is shown how NDE results can be used for numerical assessment or coupled in a broader strategy, including long term structural monitoring.
This work is based on the expertise of the members of the Working Group as well as that existing in the international literature. This large amount of references is not quoted here but can be found in the book.
2. Non Destructive Testing within the frame of structural maintenance/assessment
Section A starts with two introductory chapters that define the words which are often used in the field of structural IMR (inspection - maintenance - repair) approaches: monitoring, measurement, diagnosis, evaluation, assessment... The different stages leading to a final forecast are described and the contribution of NDE is pointed at.
NDE methods appear as complementary of other approaches. Once it has been decided to use them, comes the question of the relevancy of the technique: what is the relation between the measured parameters and the diagnosis/forecast? These questions are often asked in a general way, like:
and if we try to find some answers, these questions must be analysed.
This section of the report tries to list in different chapters the various objects/properties for which NDE can be useful (and what are used today in practice) :
3. Physics and methods
Section B is centred on the NDE methods that are currently in use in the field of civil engineering.
It starts with a summary of the basic physical phenomena that intervene in methods using wave propagation and based on very low frequency phenomena. The objective is to give the reader an insight in the physics of the method to help him make appropriate technical choices when confronted to a problem. For instance, if a void is to be detected, his attention is drawn towards void size and void depth but also to wavelength in case of wave propagation. As far as possible the physical phenomena that are common to several methods are described only once in a synthetic manner, as in figure 2, that shows in a unique graph wavelengths for concrete and steel irrespective of the wave type. It is also aimed to help the reader to put aside methods that obviously will not perform well in the context of civil engineering applications. This chapter is the only one in Section B that does not rely on practical cases.
Before going into chapters dedicated to each method, all the requirements for the proper transfer of laboratory measurement techniques to in situ validated and efficient ones are described. The many changes from lab to in situ measurements survey are emphasised: limited interventions time, access difficulty, post interpretation... This chapter is particularly important in the case of NDE for civil engineering because almost no method is normalised, and research activity in this field is dynamic in designing new methods. At last, attention is drawn towards training and qualification level of the manpower.
The NDE methods are then gathered in six families and reviewed, respectively seismic, electromagnetic, infrared, nuclear, electric and optic methods. The chapter's structure for each family is similar. The application fields are described together with the measured observable with regards to the parameters that are deduced. Then the most common encountered application difficulties are listed including calibration, bias... For each method practical examples are given to illustrate those points. Those NDE results are furthermore always presented in the more general context of the diagnosis of the structure itself.
A detailed case studies for each method was not feasible (f.i. surface waves techniques are only briefly presented). Anyhow for all the methods that have known in situ applications, a synthetic form is filled. Each form has the same structure and ends with a set of three marks, awarded after the group consensus, about the adequacy, availability and efficiency of the technique.
4. From measurements to results
The last section synthesises the knowledge about the possibility (and difficulties) to deduce, from (good) data obtained after a NDE survey, a given structural assessment. This section contains the following chapters:
4.1. From measurement to assessment
4.2. Abilities of the various methods for diagnosis
The level of relevance of the method is discussed in relation with the expressed needs. This is illustrated on some typical objects / properties. Recommendations are given about the more efficient methods for given families of object / properties.
4.3. Complementarity of NDE methods
4.4. Contribution of laboratory tests and prototypes
Because of their history, their materials properties, the boundary conditions or loading are well-controlled, laboratory specimens and prototypes can be of a great help when calibrating methods, or when their sensitivity (or field of application) has to be carefully defined. Many examples are discussed, such as the careful study of the practical ability (and sensitivity) for radar reflectometry to estimate the depth of prestressed ducts, or for impact echo to detect voids.
4.5. Contribution of semi-destructive methods
4.6. Coupling NDE with monitoring and surveying
The specific contribution of monitoring with regard to structural evaluation(speaking about how the structure "works") is discussed on examples.
4.7. Structural and functional assessment
Depending on the kind of the required assessment, the weaknesses of some methods (f.i. difficulties for obtaining absolute values) can be cancelled. However, the spurious effects of some influence parameters (like temperature or humidity) remain and are a main obstacle. These points are illustrated on examples and the contribution of NDE methods to management strategy is shown.
4.8. Structural re-assessment
After three years work the AFGC-COFREND Working Group has produced a book that gives the present state of the art of non-destructive evaluation of concrete structures in France. An originality of this compendium is to clearly privilege a presentation anchored in the application field. It is expected that buildings owners and managers as well as NDE practitioners and diagnosis experts will find useful information as NDE has been positioned according to their different and respective points of view.
This book is thus a step towards a better integration of NDE in the life of structure. It fits the general trend that, due to economic pressure, asks for life extension of present and future structures under high security level for the users.
The authors have to thank all contributors of the Working Group who, by their regular participation have contributed to the success of this work, namely C. Abdunur, E. Antczak, J.P. Balayssac, S. Bonnet, F. Buyle-Bodin, J.M. Caussignac, B. Chau, J.L. Chazelas, B. Collin, C. Constantopoulos, P. Cote, D. Defer, X. Derobert, B. Fargeot, D. François, J.L. Garciaz, V. Garnier, B. Godart, C. Hugot, J.F. Lataste, D. Leroy, D. Marlot, C. Oglaza, G. Olivier, O. Paris, R. Poquet, V. Robert, P. Roenelle, C. Sirieix, F. Taillade and some others whose list would be too long but who have helped us during this three years project.