Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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Guidelines for NDT methods in civil engineering

Alexander Taffe, Christiane Maierhofer
Federal Institute for Materials Research (BAM), Berlin, Germany

ABSTRACT

The German Society for Non-destructive Testing (DGZfP) is a center of activity in research, development, application and dissemination concerning non-destructive testing (NDT) methods. The transfer of results from research to practical application is one of the main targets. Therefore the Technical Committee NDT in Civil Engineering has published ten guidelines providing information from scientifical background to practical application concerning NDT-methods like radar, ultrasonic and radiography. Considering radar for example the content and structure of these guidelines is described below. To contribute to harmonization of European regulations and standards these guidelines should be transferred into the English language area. They can serve as a basis for harmonized European standards.

1 Dissemination of guidelines concerning NDT-CE

Non-destructive testing in civil engineering (NDT-CE) involves researchers, developers, service providers and users. Every group gains their own expert knowledge. The exchange of that experience within these groups is essential for the further development and acceptance of NDT-CE. The Technical Committee NDT in Civil Engineering of the DGZfP consists of members from all four groups. Collecting and improving the expert knowledge the Technical Committee has published ten guidelines on NDT-methods providing information from scientifical background to practical application. A compilation of all ten guidelines is added in chapter 7 of this article.

To ensure the transfer of this expert knowledge in other European countries the guidelines of the DGZfP should be available in English language. This article will show the structure of these guidelines and their benefit with Guideline B 10 (radar-method) as example [1]. Providing these guidelines in English language will make them accessible to a larger number of researchers, developers and users. So experts in other European countries will benefit from the information given in these guidelines. Together with expert knowledge in these countries they can contribute to the harmonization of European standards. This kind of benefit from experience and regulations from other countries has been proved in other fields of standardization in civil engineering.

2 Fundamentals of radar-method

Physical fundamentals of the radar procedure necessary to understand the technique are provided. Tables containing dielectric constants and their relating detection ranges for subsoil as far as materials with different conditions are provided in chapter 2 from the guideline.

Vertical or depth resolution give information how thin a layer might be to separate upper and lower boundary reflections in a radargram. Equations to calculate the lateral resolution are outlined as well as recommendations for the feasible vertical resolution.

3 Measuring procedures

Chapter 3 of the guideline gives detailed information about applicable configuration like shown in Fig. 1. Equations for the determination of the velocity of propagation in an unknown structural member are provided together with illustrations of the required antenna positions.

Fig 1: Reflection configuration (left) and transmission configuration (right) of transmitter and receiver antennas.

4 Data processing and evaluation

Chapter 4 of the guideline describes the data processing procedure and data evaluation. Therefore illustrations with explanations of the commonly used display of the data are shown like:

  • Single trace (A-scan)
  • Radargram (B-scan), Fig. 2
  • Time slice (C-scan), Fig. 3
  • 3D-presentation of a data cube and other slices

Schematic illustrations of the investigated surface are shown together with results of practical measurements (Fig. 2, Fig. 3). Comments on the evaluation of the shown results are also given.

Fig 2: Section (left) displayed in a radargram of measured data along a line in the Wiggle mode (right) by investigating a reinforced concrete slab along a line.
Fig 3: Section (left) displayed in a time slice by investigating a structural member along a grid pattern. Time slice through a reinforced concrete slab (right).

Investigating the surface of a structural member in a grid pattern will create a three-dimensional data record (3D-cube). The cube consists of numerous radargrams. 3-D-data processing computer programs provide the presentation of the inner structure from the structural member in arbitrarily sections. Fig. 3 shows the presentation of a time slice (horizontal projection) through a reinforced concrete slab.

5 Case studies: brickwork, concrete, asphalt, subsoil

In chapter 6 of the guideline case studies of conducted investigations are presented for brickwork, concrete, asphalt and subsoil.

5.1 Brickwork
For the investigation of brickwork, radar can be applied to the following topics [2], [3]:

  • Determination of the dimension from structural members (e.g. wall or slab thickness)
  • Investigation of the inner structure of multi-layer systems, detection of delamination, cavities, defects, metallic or wooden fixtures (anchors, clips), construction unit joints
  • Quality control after crack or joint injection
  • Determination of the moisture distribution [4], [5]

As case study the adorned gable of a gothic cathedral is presented. It shows damages due material deterioration. For the restoration the construction of the pilaster strips had to be surveyed. The arrangement of the bricks and joint was investigated to verify the load capacity as well as the location of metal brackets.

Furthermore from the running time t of the signal and the well-known wall thickness d the dielectric constant of the medium was determined. The results are shown in Fig. 4. The delay of the signal due to a higher dielectric constant er shown in Fig 4 visualises the qualitative effect of rising moisture to the velocity of propagation. This is sufficient for the localization of moisture in a building. To determine moisture contents quantitatively calibration of the relation of dielectric constant and moisture content have to be performed [6]. The influence from dissolved salts has to be considered.

Fig 4: Radar testing with visualisation of rising moisture in brick work.

5.2 Concrete
Applications of radar testing in concrete structures are:

  • Detection and localisation of the reinforcement and tendons ducts [7]
  • Detection and localisation of dowels and anchors in concrete highways [8]
  • Measurement of thickness from structural members that are only accessible from one side
  • Detection of structural anomalies such as cavities or voids
  • Detection and investigation of boundaries (screed or repair mortar)
  • Qualitative determination of the moisture distribution

Fig 5: Schematic presentation of the localization of reinforcement and tendon ducts in a concrete slab.

Tendon ducts as well as several layers of reinforcement bars or mats can be detected with radar depending on boundary conditions. According to the state-of-the-art tendon ducts with a concrete cover from 30 cm to 40 cm can be located under two layers of reinforcement mesh with a grid size not less than 10 cm. The relative accuracy of the method is 5% to 10% for the concrete cover. The method is not applicable if the grid of the mesh is less than 5 cm.

Other case studies concerning radar testing of asphalt highways or investigation of the subsoil are described in the DGZfP-Guideline B 10 [1].

6 Documentation and qualification

A proper inspection report has to confirm that the radar procedure has been carried out in accordance with the requirements of the guideline B 10. To ensure that the recorded data and the gained information can be used in a construction or refurbishment process information about the used equipment, software, data analysis as well as information on construction plans and structure are necessary. A detailed list of all information in a inspection report can be found in chapter 7 of the guideline B 10.

For the successful application of the radar method well trained personnel familiar with the testing device and established knowledge in building materials and constructions is required. An appropriate study in these fields in combination with sufficient practical experience has proved to be reasonable. A close collaboration between the radar specialist and the engineer responsible is essential for the successful evaluation of radar data.

7 Other guidelines

The DGZfP (www.dgzfp.de ) has published other guidelines with a similar structure in German:

B 1Guideline for radiography testing of reinforced and prestressed concrete, 1990
B 2Guideline for detection of reinforcement and measuring concrete cover of reinforced and prestressed concrete, 1990
B 3Guideline for electrochemical potential field testing to detect corrosion of reinforcement steel in reinforced concrete buildings, 1990
B 4Guideline for ultrasonic method for non-destructive testing of mineral building materials and structural members, 1999
B 5Guideline for thermographic investigations of structural members and buildings, 1993
B 6Guideline for visual examination and endoscopy as optical methods for non-destructive testing in civil engineering, 1996
B 7Guideline for the application of digital image processing in non-destructive testing in civil engineering, 1996
B 8Guideline for seismic methods for soil explorations and investigation of characteristic values of the soil, 1996
B 9Guideline for automated long-term monitoring in construction engineering, 2000
B 10Guideline for radar method for non-destructive testing in civil engineering, 2001

Further guidelines (e.g. impact-echo) are in preparation.

8 Acknowledgement

The presented guideline is the result from the work of the Technical Committee NDT in Civil Engineering of the German Society for Non-destructive Testing (DGZfP). The translation work will be supported by members of the technical committee.

9 References

  1. Deutsche Gesellschaft für Zerstörungsfreie Prüfung, DGZfP-Fachausschuss füt Zerstörungsfreie Prüfung im Bauwesen (AB) Unterausschuss Radar, Merkblatt B 10
  2. Kahle, M. und B. Illich, Einsatz des Radarverfahrens zur Erkundung von Struktur und Zustand historischen Mauerwerks, in: Bautechnik 69, Nr. 7, 1992, S. 342 - 353
  3. Kahle, M., Illich, B. und F. Wenzel, Untersuchungen an den Pfeilern der St. Wolfgangskirche in Schneeberg, in: Bautechnik 70, Nr. 7, 1993, S. 416 - 424
  4. Binda, L., Colla, C. and M. C. Forde, Identification of moisture capillarity into masonry using digitally impulses radar, in: J. Construction and Building of material, volume. 8, No. 2.,1994, pp. 101-107
  5. Colla, C. and L. Binda, Bestimmung der Feuchteverteilung mit Radar Tomografie, in: Feuchtetag `99 "Umwelt, Messverfahren, Anwendungen" DGZfP-Berichtsband BB 69-CD, Berlin, 7./8. Oktober 1999
  6. Maierhofer, Ch., Leipold, S. and H. Wiggenhauser, Investigation of the influence of moisture and salt content on the dielectric properties of brick materials using radar, in: Proceedings of the 7th International Conference on Ground Penetrating Radar (GPR) in Lawrence, USA, 27.-30. May 1998, Kansas, USA: Radar Systems and Remote Sensing Laboratory, Vol. 2, 1998, pp. 477-484
  7. Flohrer, C. und M. Pöpel, Combination of a covermeter with a GPR-System, A tool for detecting prestressed bars in concrete structures, in: Proceedings of the 6th International Conference on Ground Penetrating Radar, 10/1996, Sendai/Japan, 1996, pp. 273 - 277
  8. Rösch, A. and A. Schaab, An in-situ NDT-method to detect incorrectly positioned dowel bars in carriage way slabs of concrete highways, in: Proc. of Int. Sym. NDT in Civil Eng., 9/1995, Berlin, pp. 269-276
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