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Trends in the development of nondestructive testing methods to assess the actual state of structures

: Schmitz, V.

Organisation for Economic Co-operation and Development -OECD-; Organisation de Cooperation et de Developpement Econimiques -OCDE-:
OECD-NEA workshop on instrumentation and monitoring of concrete structures. Proceedings
Brussels, 2000
Session 3, Beitrag 5
Workshop on the instrumentation and monitoring of concrete structures -OECD-NEA- <2000, Brussels, Belgium>
Fraunhofer IZFP ()
Elastodynamic Finite Integration Technique (EFIT); one-sided access NMR (OSA NMR); nuclear magnetic resonance; Synthetic Aperture Focusing Technique; 2000

From the many non-destructive testing methods, this contribution covers main streams of research and development; these are identified to be in the field of mathematic-numerical modelling, possibilities to measure moisture, improvements in ultrasonic pulse echo testing using array probes and imaging techniques and the fusion of radar and ultrasonic data.

The EFIT-code (Elastodynamic Finite Integration Technique) was used to model the ultrasonic wave propagation in the concrete and the interaction with tendon ducts of good quality and the case of injection faults. The additives have been modelled by ellypsoids varying statistically in size and orientation. To identify the echo signals of the duct properly, the concrete has been simulated with and without porosity which may consist of air- or water filled pores. It has been shown that there is a strong dependency between the damping of the elastic wave and the volume fraction of air inclusions and that there is a need to model three-dimensionally.

An important field of research is the development and the advancement of new non-destructive methods for moisture measurements in civil structures. In masonry and concrete, moisture influences the physical properties and is responsible for the origin and the development of serious damage. Hence, the knowledge about the content, the distribution and the transport mechanisms of water in these building materials is of fundamental interest for durability and service life-time prediction. Higher frequency pulsed radar (7GHz) with horn antenna allows a rapid screening and detection of humidity by mapping the dielectric material properties. A "One-Sided Access Nuclear Magnetic Resonance (OSA-NMR)" is used to measure the one-dimensional water distribution (moisture profile) in building materials. With its aid, quantitative information about the water transport during capillary absorption and pressure driven permeation can be determined in-situ.

With respect to concrete structures, methods have been applied to reveal structure irregularities in prestressed or post-tensioned concrete structures by ultrasonic pulse echo methods using a combination of piezoceramic transducer and receiving laser vibrometer. The data have been processed by a three-dimensional tomographic algorithm for imaging with an improved signal-to noise-ratio and presentation of internal structures of the concrete.

Similar to ultrasonic imaging techniques, remote controlled scanners can carry RADAR antennas to transmit signals of 1 GHz or 2 GHZ; the received signals form - similar to ultrasound - hyperbolic shaped curves from reflectors within the concrete material. The future trend in the processing of these data is to use a 3D-tomographic algorithm for image processing and to compare and combine RADAR and ULTRASONIC IMAGES for the assessment of concrete material.