International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Location of Reinforcement in Structures by Different Methods of Gamma-RadiographyB. Redmer, F. Weise,U. Ewert
BAM Berlin (GER)
Institute of Theoretical and Applied Mechanics of Russian Academy of Sciences (RUS)
The quantitative measurement of kind, positions and diameters of steel reinforce ments in building constructions is a typical inspection problem in civil engineering. A classical solution is the combination of film radiography with the multi angle technique and graphical backprojection. New digital detectors reduce the exposure time and allow the application of stereo radiography and computed laminography as routine applications.
The computed laminography allows fast and effectively the determination of the depth of reinforcements in concrete or masonry. A selected area of the steel re-bar structure is penetrated by gamma-rays from different angles of incidence (multi angle-technique) with a Co-60-source. Instead of a radiographic film phosphor imaging plates were used as a new digital medium for filmless radiography. A 3D-image of the measured region of interest is reconstructed from the digitised projections of a test block. The reconstruction is based on special tomosynthesis algorithms which need few projections only.
Radiographic techniques were uses in completion to other techniques like radar, electromagnetic alternating field inspection to examine the structure of different buildings. Experiences of inspections of historical buildings, ancient marble structures and a bridge girder are presented where different levels of radiography were applied.
The non-destructive determination of the kind, position and geometry of re-bars and other metallic fittings in building elements is important for the evaluation of the load bearing capacity and the usage property within the scope of building condition and damage analysis. In this case different non-destructive testing methods can be applied.
Devices, based on the electromagnetic alternating field method, can locate ferromagnetic reinforcement in a maximum depth of approx. 12 cm. The radar method allows to locate the reinforcing steel in a larger depth. However, this method is not suitable to determine the cross section geometry quantitatively. In contrast to this methods an exact determination of cross section geometry and depth position of the reinforcing steel can be realised with the radiography. Thereby, depending on the complexity of the test problem, classical and modern laminographical methods can be applied.
2 Radiographic Methods
2.1 Classical Radiography
The classical inspection arrangement consists of a radiation source at one side of the object to inspect and an X-ray film at the other side. There exist also radiation methods for one-side inspections, using the backscatter effect . These methods need long inspection times and are limited to single layers of reinforcement near the surface.
The selection of the source depends on the wall thickness, which shall be inspected. Fig. 1 shows the wall thickness range for the different sources available.
Typical X-ray films are covered with lead screens for high spatial resolution. These films need about ten times higher exposure times than films with fluorescent screens. The latter ones are usually chosen if no requirements for a spatial resolution better than 0.2 mm exist.
Recently phosphor imaging plates are used for concrete inspection (computed radiography or CR). Its sensitivity depends on the thickness of its phosphor layer. With a typical layer thickness of about 0.3 mm (medical plates) they can be used for digital radiography of concrete . The required exposure time is comparable with the fluorescent screen film systems. For better image quality higher exposure time is recommended. Also flat panel detectors based on amorphous silicon photodiode arrays with fluorescence screen are available for high energy applications. Due to its high price and limited working temperature range, mobile applications are very limited.
Especially for high wall thickness inspections all detectors provide only sufficient image quality if intermediate filters are used. For film radiography above 40 cm of concrete an intermediate sandwich filter, consisting of 2 mm lead and 2 mm tin or copper or 2 mm steel shall be used. Good experiences were made with lead, tin, copper sandwiches. Copper or steel shall be at the film side. Phosphor imaging plates are more sensitive against scattered radiation than film systems. Therefore, 4 mm lead (instead of 2 mm for film) was used for Co-60-exposures above 50cm concrete for CR.
2.2 Stereo Technique
2.3 Computed Laminography
Computed laminography (fig. 4) can be applied if the radiation source is moved step by step above the object and parallel to the detector. For each source position a digital radiograph is required. The movement of the detector is not required anymore, since it can be done by image processing (image translation). The reconstruction is based on several methods, which are described in . The average method  provides the most stable results. Other methods like filtered backprojection, planar tomography or extreme value method may be also applied . The computed laminography based on the coplanar translational laminography was applied for inspection of steel reinforcement with known direction.
3.1 Investigations on historical masonry by classical radiography
The aim of the investigations was the localisation of the tie member and the verification of its position, size and anchorage construction in the corner pillar by non-destructive methods. Therefore, the electromagnetic alternating field method, the radar method and classical radiography were applied.
The radiographical inspection was carried out with the g-source Co-60 and phosphor imaging plates with an intermediate filter of a lead-tin sandwich in front of the detector. A 0.5 mm steel filter was used in contact with the imaging plate in the cassette. Due to the arrangement of the g-source and detector the set up of the imaging plate was especially important in order to be able to project all details on the detector. The exposure time amounted to 10 to 35 min. depending on the penetrated thickness.
As a result it can be stated, that due to the large depth, the tie member could be located only with the radar method and radiographical method. Additionally the radiography allows an exact determination of the cross-sectional dimensions of the tie member and the verification of its anchorage in the corner pillar (fig. 6). Fig. 6e shows a typical arrangement of this anchorage. It could be observed that the flat bars are arranged vertically staggered in the corner area above the vault. For the anchorage the flat bars are bent around a vertically situated round bar.
These results were very helpful for assessment of the structure by the structural engineer.
3.2 Localisation of metallic fittings in antique monuments
The aim of the investigations was the non-destructive determination of the kind, position and size of metallic fittings (e.g. steel dowels) in the architrave of the column group. This concerned in particular the material interface of a original marble part and stone substitute material, for example mortar. The investigations were carried out in two phases: preliminary investigations with the radar method and stereoradiography at selected positions.
As a result of the preliminary investigations the global arrangement of the metallic fittings could be localised. However, further points remained open, which refer to the size and position of the metallic dowels and to the existence of a continuous steel girder.
The stereoradiography was used to solve these problems. Fig. 8 shows a measuring set up for verification of the position of the metallic dowels. A Co-60-source was applied as radiation source using a long guide tube for the radiation source, which was positioned in a bore hole with a diameter of 14 mm above the architrave.
This special arrangement was necessary to reduce the penetrated material thickness and the corresponding exposure time per radiograph.
The positioning of the imaging plates below the architrave is described in fig. 8. The enlargement of the cross section is presented in fig. 8a and the detail view in fig. 8b. In the digital images the position of the metallic dowels is clearly recognizable (fig. 9c). They are embedded in mortar in the original marble part and located in the transition area to the stone substitute material. They are bent upwards. This is also recognizable in another radiograph, which was taken at the back side of the architrave (fig. 9). Remarkable is here, that a gap appears in the inner area between the original marble part and the stone substitute material. In this gap we have found the upward bent parts of the metallic dowels. This means, they cannot fulfil its static function. Obviously this static function is performed only by a L-shaped continuous steel section.
3.3 Inspection of steel reinforced girders
The multi angle radiography requires the inspection from different viewing angles. A bridge girder was inspected with Co-60 and imaging plates as well as with films, which were digitised later (see fig. 10). For reduction of the penetrated wall thickness a hole was drilled above the re-bars. The Co-60 source was positioned at 9 different positions in the hole and 9 corresponding projections were acquired from the same detector location below the girder. The source detector distance amounted to 47 cm and the penetrated concrete thickness was measured angle dependent, between 42 and 56 cm.
The reconstruction can be done graphically or computer based. Fig. 11 shows the complicated structure of a graphical back projection, which was performed for a two layer system of steel reinforcement. This inspection was carried out for a gird of the bridge in Havelberg, Germany. The operator needs some intuition to find the right crossing points of the edge projection lines.
The numeric reconstruction using the principle of computed laminography  was developed and carried out at a test block with steel reinforcement. The block was 30cm thick. Co-60 was used as source again and the computed radiography system from Agfa-NDT, ACR2000, was used with two combined imaging plates obtaining a detector length of 80 cm. Fig. 12 shows the result of the inspection. Different steel bars can be visualised in different layers of different depth in the concrete block. This technology shall be applied for the inspection of marble structures of historical constructions as well as for bridge and building structures.
The authors thank the ARGE Pfanner for the cooperation and kindly permission of publication of investigation results at the column yoke of Priene in the Pergamon Museum in Berlin. Furthermore, the authors are grateful to those co-workers of BAM VIII.31 and VII.1, who took part in the inspections.