Ch. Maierhofer

Detection of shallow voids in concrete structures with impulse thermography and radar

Abstract New thermography systems enable the application of active investigation techniques, i.e. the observation of the cooling down process after heating the surface of a structure under investigation. Defects like voids in concrete having a different thermal diffusivity in comparison to the bulk material are visualised by different surface temperatures. The differences between temperature transient curves above sound regions and above inhomogeneities are expected to include information about the defect parameters. Experimental investigations with impulse thermography and radar on a concrete test specimen containing voids were analysed. Radar was used to determine the depth of the voids. The influence of size and depth of the voids on the temperature transient curves was studied for different heating times. The transient curves were also fitted with a simple semi-empirical model.

Transient thermography for structural investigation of concrete and composites in the near surface region

The cooling down process of building structures after heating up with an external radiation source was analysed to detect voids inside and below the surface of a concrete test specimen containing voids with different sizes at various depth or covered with carbon fibre reinforcing laminates. These experimental investigations were compared to the results of simulations performed with a finite difference program.

Investigation of dielectric properties of brick materials as a function of moisture and salt content using a microwave impulse technique at very high frequencies

A pulsed microwave method using a frequency range of about 7 GHz has been developed based on the principle of the pulsed radar method. This method is employed for the non-destructive quantitative determination of the moisture content and distribution in brickwork. The measurements are more precise than previous measurements that have used frequencies below 1 GHz. Owing to the shorter wavelength and the higher absorption coefficient of water in this frequency region, both real and imaginary parts of the complex dielectric properties are measured with a higher accuracy. The expected non-linear relationship between the dielectric properties of several building materials and their moisture content and salt concentration (NaCl) requires calibration measurements. Therefore, systematic measurements on brick material, like sand-lime brick, solid brick, hollow brick and porous concrete, have been carried out. The results are demonstrated and compared with actual theoretical models.

Combination of NDT techniques for site investigation of non-ballasted railway tracks

Abstract Todays civil engineering NDT needs are for quantitative, fast to apply and easy to interpret methods. They are required on site both for evaluation of existing structures/infrastructure and for quality control in new construction. With this purpose, BAM has carried out in-situ feasibility studies, using radar, impact-echo and ultrasonic-echo methods. Two examples are given of their combined application on different railway slab–track constructions, using high frequency radar, scanning impact-echo and ultrasonic array methods. The aim was to evaluate limitations and advantages, comparability and complementarity of these methods. The on-site measurements demonstrated the possibility to investigate, with good accuracy via NDT, the bonding conditions between sleepers and slab, to locate voids and measure layer thickness. The acoustic and electromagnetic methods used, were found to be complementary.

Non-destructive investigation of sluices using radar and ultrasonic impulse echo

Radar and ultrasonic impulse echo have been successfully applied for the assessment of two almost 100 year old sluices. The interior condition of the side walls has been investigated with radar enabling the detection of detachments of the faced brickwork as well as the determination of moisture content. With ultrasonic impulse echo working joints were found in the concrete slab of the sluice heads. The results of these investigations gathered comparative data to reduce the number of cores which had to be taken.

Investigating historic masonry structures with a combination of active thermography and 3D laser scanner

Methods for the combination (i. e. comparison and overlay) and data fusion (i. e. integration of all data in one data set, replacement of data) of active thermography and 3D laser scanner (light section method) have been developed. Systematic investigations for quantification of damage in historic structures are presented using both techniques. A case study shows that reproducible investigations at regular time intervals are very well suited for structural monitoring.

Non-destructive investigation of structural elements of the “Altes Museum” in Berlin

Strategies and methodologies for the assessment of historic masonry structures based on non-destructive and minor-destructive testing methods were developed in the European Research Project ONSITEFORMASONRY and were successfully applied to assess the structure and material properties of selected structural elements in the Altes Museum in Berlin-Mitte. For the planning of a broad reconstruction within the framework of a master plan concerning the whole of Museum Island in Berlin, several questions arose which are also typical for other historic structures in general, thus the Altes Museum was chosen as a pilot site for the validation of non-destructive testing methods.

INFLUENCE OF SURFACE PROPERTIES ON THE DETECTION AND QUANTIFICATION OF VOIDS IN CONCRETE STRUCTURES USING ACTIVE THERMOGRAPHY

Impulse thermography is an active method for quantitative investigations of the near surface region of various structures. It has recently been applied and optimized to applications in civil engineering. For quantitative analysis of data recorded on the building site, the problems are manifold. Here, the influence of the different surface properties on the detection and characterization of voids and honeycombing simulated by polystyrene cubes and cubes prepared of porous concrete are demonstrated.