Christiane Trela

Application of radar techniques to the verification of design plans and the detection of defects in concrete bridges

Non-destructive tests (NDT) are an essential tool used in special inspections to gather detailed information about the condition of a bridge. The inspection of bridge decks is a critical task, and, currently, can be successfully carried out using a wide range of NDT techniques. Nevertheless, some of these techniques are excessively expensive and time consuming. One of these techniques, the ground penetrating radar (GPR), has been used for some decades in the non-destructive inspection and diagnosis of concrete bridges. GPR is useful to find general information about the true position of reinforcement and tendon ducts, and check the quality of the construction and materials. A significant number of reinforced and prestressed concrete bridges are deteriorating at a rapid rate and need to be repaired and strengthened. During these rehabilitation processes, designers are often faced with a lack of original design plans and unawareness of the real position of reinforcement and tendon ducts. In this paper, three case studies of the use of GPR techniques for the inspection of concrete bridges are presented and analysed. The main aim of this research is to show the strong need and usefulness of these techniques, which can provide non-visible information about structural geometry and integrity required for strengthening and rehabilitation purposes.

Detecting voids in masonry by cooperatively inverting P-wave and georadar traveltimes

Collecting different geophysical data sets at the same object and site offers the opportunity to reduce uncertainties and ambiguities in data analysis and interpretation. To be effective, the different available data sets should be linked during the model-generation process, e.g. by cooperative inversion. In this study, we apply a recently developed zonal cooperative inversion approach based on fuzzy c-means cluster analysis to a non-destructive testing experiment. After briefly reviewing the fundamentals of the inversion strategy, we present a synthetic study investigating the potential of the method to detect air-filled voids in masonry by using ultrasonic and georadar traveltime data. Then, we present and discuss laboratory experiments including the results of cooperatively inverted ultrasonic and georadar traveltimes collected at a masonry test specimen. The geometry of the specimen is known and is thus an ideal test object for a first-time real application of the novel zonal cooperative inversion procedure. Compared to the results of separate inversions of ultrasonic and georadar traveltimes, the zonal cooperative inversion allows for an improved delineation of the size and position of the cavities. The P-wave and georadar velocities determined for the model regions corresponding to the cavities are also improved.

Multi-tool inspection and numerical analysis of an old masonry arch bridge

Complex special inspection of an old masonry arch bridge according to the Guideline on Inspection and Condition Assessment of Railway Bridges and numerical analysis of the structure are presented. The guideline summarises recommendations for the step-by-step investigation of railway bridges applying enhanced methods developed during the EU-funded project Sustainable Bridges. For the investigation of the arch barrel, the ballast parameters and the inner structure of the backfill behind the arch barrel a number of various advanced non-destructive and minor-destructive testing methods were applied. Deformation of the structure during load tests was measured using three independent measuring systems: laser vibrometer, LVDT and microwave radar. Results of calculations performed with 2D and 3D models based on FEM are compared with the field load tests. Sensitivity of the ultimate load of the structure to investigated parameters is studied in FE model. Some general conclusions according to methods of testing and modelling of masonry arch bridges are presented and discussed.

Positioning accuracy of an automatic scanning system for GPR measurements on concrete structures

Most GPR measurements use hand-held survey wheels for positioning, something not feasible in some scenarios, such as measuring a tall retaining wall, tunnels or if tedious and repetitive measurements are required. For situations like these an automated positioning system is required and was therefore developed at BAM. Such a system was tested to calibrate the positioning of a GPR grid measurement which was performed in regular one-direction traverse mode and in a meandric mode. The preliminary results show a systematic error in the positioning, which depends on the traveling speed of the scanning system, because there is an electronic delay of the trigger signal. These errors are duplicated in meandric mode compared to the errors occurring in one-direction traverse mode. This may be insignificant when larger objects are investigated, but can be important for the identification and location of small objects which show weaker reflections, like e.g. compacting faults with a size of a honeycomb.

Use of Complex Resistivity Tomography for Moisture Monitoring in a Flooded Masonry Specimen

Moisture ingress is one of the major deteriorating factors for building materials. Today, the only approved way to assess such damage is the gravimetric Darr method, which is essentially destructive. Substantial progress has been made using the geophysical complex-resistivity method, which can be applied non-destructively and provides spatial information along two-dimensional sections, rather than punctual along one borehole. Considerable advantages of complex resistivity are its sensitivity to textural properties, as well as the pore-fluid chemistry of wet, porous media. In a comprehensive laboratory study, and later in field scale experiments, it could be shown that complex resistivity may even be able to distinguish between salt content and saturation degree in a single measurement. A comparison with complementary nondestructive testing techniques points to the benefit and further research to be explored in multimethodical approaches.

Detection of granite landmarks in soil with GPR

In Germany plots of land were often marked with landmarks. They are typically made from granite stones in a defined shape and have been buried closely below the surface. The suitability of GPR for the detection of landmarks has been tested. GPR laboratory experiments in a sandbox under ideal conditions as well as field trials have been carried out. The influence of the water content of soil on the GPR measurements was investigated in laboratory. In these experiments suitable characteristic parameters have been identified to distinguish granite landmarks from other reflectors.