Christiane Maierhofer

Non-destructive evaluation of reinforced concrete structures

Part one discusses deterioration of reinforced concrete and testing problems with chapters on topics such as key issues in the non-destructive testing of concrete structures, when to use non-destructive testing of reinforced concrete structures, deterioration processes in reinforced concrete, modelling ageing and corrosion processes in reinforced concrete structures, components in concrete and their impact on quality, and predicting the service life of reinforced concrete structures. Part two reviews classical and standard testing methods including microscopic examination of deteriorated concrete, the analysis of solid components and their ratios in reinforced concrete structures, the determination of chlorides in concrete structures, and investigating the original water content of reinforced concrete structures.

COMPLEMENTARY APPLICATION OF RADAR, IMPACT-ECHO, AND ULTRASONICS FOR TESTING CONCRETE STRUCTURES AND METALLIC TENDON DUCTS

Nondestructive testing of concrete structures plays an increasing role in civil engineering, although until now the full potential of such techniques has not been tapped. For posttensioned structures, the investigation of tendon ducts is one of the most essential testing problems. The location of tendon ducts, the determination of concrete cover and, especially, the detection and quantification of ungrouted areas inside the ducts are the relevant questions. Recent developments and opportunities of radar, impact-echo, and ultrasonics for the investigation of tendon ducts are presented. Although the obtained results on positioning and concrete cover determination are sufficient, the location of ungrouted areas is still a matter of research. Thus, new approaches for this testing problem have to be considered. Additionally, the combined use of complementary techniques offers a high potential to increase the reliability of results. Data will be displayed on the combined application of acoustic and electromagnetic impulse-echo methods and of data fusion related to the investigation of tendon ducts.

Pulse phase thermography for characterising large historical building façades after solar heating and shadow cast – a case study

For assessing building façades using active thermography, in case of direct solar exposure the sun itself can be used as heat source. It is shown that active thermography can be applied to large areas successfully, if a shadow cast occurs. After performing a sequence reconstruction in order to correct the temporal behaviour of shadow movement, the data could be analysed by pulse phase thermography (PPT). Compared to raw thermograms, the obtained phase images display an improved image quality with a lot of details. The frequency of the phase images is related to the probing depth. The presented case study describes the thermographic investigation of an historical building façade, where an area of 17 m × 13 m has been investigated.

Assessment of Moisture and Salt Contents in Brick Masonry with Microwave Transmission, Spectral-Induced Polarization, and Laser-Induced Breakdown Spectroscopy

The determination of water and salt distribution in brickwork and stonework is a frequent problem in cultural heritage protection. Conventional but reliable methods are usually based on the investigation of core samples and provide only punctual information. To avoid the destruction of valuable building fabric, a non-destructive approach using a combination of investigation methods is needed. A case study, demonstrating the possibilities of almost non-destructive testing methods and their combination is presented in this article.

Efficient data evaluation for thermographic crack detection

We present an all-purpose crack detection algorithm for flying spot thermography which is directly applicable to a thermogram sequence without the need of any additional information about the experimental setup. A single image containing distinct crack signatures is derived in two steps. Firstly, the spatial derivative is calculated for each frame of the sequence and, secondly, the resulting data set is sorted pixel wise along the time axis. The feasibility of the proposed procedure is proven by testing a piece of rail that comprises roll contact fatigue cracks and by comparing the results with magnetic particle testing.

Integration of active thermography into the assessment of cultural heritage buildings

Applications of infrared thermography in civil engineering are not limited to the identification of heat losses in building envelopes. Active infrared thermography methods enable structural investigations of building elements with one-sided access up to a depth of about 10 cm. Masonry and especially historical masonry has a very heterogeneous structure containing several different materials (brick, stone, mortar, plaster, wood, metal, etc.) with various thermal properties. As many classes of damage originate from defects that are close to the surface, active thermography is in general very well suited to assessing different test problems in cultural heritage buildings. In this paper, the physical background, equipment, environmental influences and material properties are discussed. Several application results are presented. It is shown how active thermography can be integrated into a holistic approach for the assessment of historical structures.

DEFECT SIZING BY LOCAL EXCITATION THERMOGRAPHY

In this article, we present a measurement procedure to gain information about depth and angle of surface braking cracks. The method is based on a local excitation with, e.g., a laser. The resulting surface temperature is recorded with an infrared camera. Based on this data, crack-caused anisotropies in the lateral heat flow can be detected and exploited to characterise the cracks. The experimental set-up is based on a Nd:YAG laser for heating and a high-speed infrared camera (InSb FPA, 3 to 5 µm) providing a frame rate of up to 500 Hz. Up to now, only qualitative information was gained from measurements of this type, whereas the local transient behaviour of temperature distribution provides also quantitative information of the crack parameters. Using FEM simulation we can show herein that it is possible to simultaneously resolve the angle and depth and, in particular, the depth of non-perpendicular cracks.

Assessment of Structure Through Non-Destructive Tests (NDT) and Minor Destructive Tests (MDT) Investigation: Case Study of The Church at Carthusian Monastery at Žiče (SLOVENIA)

Increasing the knowledge of the structural behavior of existing masonry requires a multi-level approach, with proper application of diagnosis and assessment methodologies. The structural performance of a masonry wall structure can be understood provided that its geometry, the characteristics of its masonry texture and the general characteristics of masonry as a composite material are known. For the compilation of all required data, an effective on-site testing campaign is needed, including the application of different test methods by means of combination of minor destructive tests (MDT) and non-destructive tests (NDT). The case study presented herein, encompasses experimental work carried out at the Church of St. John the Baptist at the Carthusian monastery at Žiče, Slovenia. Within this project, on-site investigation was carried out in order to characterize wall cross-sections, to identify different types of masonry, and to collect the samples of mortars and stone particles for further laboratory tests. Methods that were applied for this case study were: crack pattern investigation, radar investigation of masonry walls and internal structure of the ground in the Church, thermography, coring, and boroscopy, as well as laboratory tests on cored samples.

Detection and Characterization of Defects in Isotropic and Anisotropic Structures Using Lockin Thermography

Lockin thermography is a well-suited method for the characterization of structures made of both metal and fiber reinforced plastic. In most cases, only phase images are analyzed, although the amplitude images might contain useful information as well. Thus, systematic studies of lockin thermography are presented, assessing amplitude and phase images for the detection and quantification of defects in isotropic (steel) and anisotropic (carbon fiber reinforced plastic) materials. Characterized defects are flat bottom holes with different diameters and various remaining wall thicknesses as well as crossed notches at different depths. The excitation frequency was varied while keeping the number of analyzed excitation periods nearly constant for each material. The data analysis was focused on the detectability of the defects both in the amplitude and phase images, including the determination of the signal-to-noise ratio and of the spatial resolution. As a result, the limits of defect detectability and spatial resolution are given for each material.

Comparison of quantitative defect characterization using pulse-phase and lock-in thermography

Using optical excitation sources for active thermography enables a contactless, remote, and non-destructive testing of materials and structures. Currently, two kinds of temporal excitation techniques have been established: pulse or flash excitation, using mostly flash lamps; and periodic or lock-in excitation, using halogen lamps, LED, or laser arrays. From the experimental point of view, both techniques have their advantages and disadvantages. Concerning the comparison of the testing results of both techniques, only very few studies have been performed in the past. In this contribution, the phase values obtained at flat bottom holes in steel and CFRP and the spatial resolution measured at crossed notches in steel using flash and lock-in excitation are compared quantitatively.