International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Guidelines for NDT methods in civil engineeringAlexander Taffe, Christiane Maierhofer
Federal Institute for Materials Research (BAM), Berlin, Germany
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 . 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.
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:
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.
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.
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 . The influence from dissolved salts has to be considered.
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 .
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:
Further guidelines (e.g. impact-echo) are in preparation.
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.