Els Verstrynge

A novel technique for acoustic emission monitoring in civil structures with global fiber optic sensors

The application of acoustic emission (AE)-based damage detection is gaining interest in the field of civil structural health monitoring. Damage progress can be detected and located in real time and the recorded AEs hold information on the fracture process which produced them. One of the drawbacks for on-site application in large-scale concrete and masonry structures is the relatively high attenuation of the ultrasonic signal, which limits the detection range of the AE sensors. Consequently, a large number of point sensors are required to cover a certain area. To tackle this issue, a global damage detection system, based on AE detection with a polarization-modulated, single mode fiber optic sensor (FOS), has been developed. The sensing principle, data acquisition and analysis in time and frequency domain are presented. During experimental investigations, this AE-FOS is applied for the first time as a global sensor for the detection of crack-induced AEs in a full-scale concrete beam. Damage progress is monitored during a cyclic four-point bending test and the AE activity, detected with the FOS, is related to the subsequent stages of damage progress in the concrete element. The results obtained with the AE-FOS are successfully linked to the mechanical behavior of the concrete beam and a qualitative correspondence is found with AE data obtained by a commercial system.

Debonding damage analysis in composite-masonry strengthening systems with polymer- and mortar-based matrix by means of the acoustic emission technique

Different types of strengthening systems, based on fiber reinforced materials, are under investigation for external strengthening of historic masonry structures. A full characterization of the bond behavior and of the short- and long-term failure mechanisms is crucial to ensure effective design, compatibility with the historic substrate and durability of the strengthening solution. Therein, non-destructive techniques are essential for bond characterization, durability assessment and on-site condition monitoring. In this paper, the acoustic emission (AE) technique is evaluated for debonding characterization and localization on fiber reinforced polymer (FRP) and steel reinforced grout-strengthened clay bricks. Both types of strengthening systems are subjected to accelerated ageing tests under thermal cycles and to single-lap shear bond tests. During the reported experimental campaign, AE data from the accelerated ageing tests demonstrated the thermal incompatibility between brick and epoxy-bonded FRP composites, and debonding damage was successfully detected, characterized and located. In addition, a qualitative comparison is made with digital image correlation and infrared thermography, in view of efficient on-site debonding detection.

Implementation of Scan-to-BIM and FEM for the Documentation and Analysis of Heritage Timber Roof Structures

Current heritage analysis applications and documentation techniques for timber roof structures rely on manual measurements to provide the spatial data. Major simplifications are made to document these structures efficiently. However, these simplified geometric models provide less reliable results. Therefore, the need exists for more realistic models. Additionally, the exchangeability of information between varying parties is paramount. Hence, the construction elements should be defined in a Building Information Model (BIM). This allows users to reuse the model, allowing the distribution of information throughout the project. The goal of our research is to create a realistic BIM model of a complex heritage roof structure employing dense point clouds. The comparison of our complex geometric model to a traditional wire-frame model proves that our approach provides more reliable results in terms of geometry and structural behaviour. Our work covers the acquisition, the modelling and the structural analysis of timber roof structures.

Predicting the Time to Failure in Heavily Loaded Masonry Specimens with the Acoustic Emission Technique

This paper presents the results of a research project in which the knowledge on testing of creep damage in masonry and acoustic emission (AE) monitoring are combined. Results from different types of creep tests are combined to investigate whether AE monitoring could predict the failure time of the masonry specimens. In previous work, it was observed that the AE event rate is related to the time to failure of the specimen. Processing of the results of new tests enables to update the previously found relation between AE event rate and failure time and to indicate a confidence interval for predictions made with this model. Additionally, the question can be raised whether temporary monitoring could detect unstable damage accumulation and predict failure. Therefore, the results of long-term creep tests are analysed and compared with data from strain monitoring. The results indicate that in most cases, the failure can be predicted.

Applications of CT for Non-destructive Testing and Materials Characterization

Several important technological and economic trends are shaping the research on non-destructive testing techniques. X-ray computed tomography is also a NDT product of these ongoing developments and has become a very important tool for doctors, material scientists, geologists, biologists, civil engineers, bio-engineers, dentists, quality engineers, etc., all dealing with materials of which the fine internal structure or the changes within the material are of outmost importance to understand the behaviour of the material or to have insights in the processes going on. X-ray CT with micron- and submicron resolution is now well accepted in those disciplines. In the next paragraphs a wide variety of application fields will be addressed in order to show how X-ray CT can be applied as an NDT technique for quality control, the study of the material behaviour and its functional properties under specified environmental conditions, and for production and material optimization.

Multi-scale approaches for the assessment of time-dependent mechanical damage in masonry. (keynote)

Although salt crystallization damage is a widespread damage process in the porous materials of our built cultural heritage, no definite solution yet exists to improve the durability of materials with respect to salt crystallization. Most research focuses on improving material properties, whereas only few studies concentrate on changing the crystallization process in order to make it less harmful.Within this last trend, recently the use of salt crystallization modifiers has been considered. Crystallization modifiers are ions or molecules that promote or inhibit crystal growth and/or change the crystal habit, thereby possibly reducing salt damage.We give in this paper experimental evidence that borax is a promising crystallization modifier for sodium sulfate and that it might mitigate salt crystallization damage when mixed in lime-based mortars. Lime-based mortars are chosen as a model system, since they are especially prone to salt crystallization damage due to their limited mechanical strength and bi-modal pore size distribution.

Experimental study of failure mechanisms in brittle construction materials by means of X-ray microfocus computed tomography

X-ray microfocus computed tomography (CT) is a powerful tool in the 3D visualization of fracture initiation and propagation in brittle materials, based on the attenuation of X-rays. Aim of this paper is to present the experience obtained at the Building Materials and Building Technology research group in using CT for the experimental study of failure mechanisms in brittle construction materials during three test programs, focusing on following issues: (i) triaxial behaviour of mortar joints, (ii) effect of pore saturation on micro fracture in sandstone and (iii) influence of flow distance on fibre distribution and orientation in fibre reinforced concrete. The resolution limit of the applied system is in the order of micro meters (microCT). Common goal of the presented experimental programs was to analyse the material’s inner structure and fracture propagation at microscale, in support of studying local failure mechanisms and developing numerical models. For each test program, setup optimization and resulting failure modes will be discussed. Several types of in-house-made loading stages were applied for observation of step-wise induced failure mechanisms. As a general conclusion, most test results supported the theoretical framework relevant to the effects of varying conditions (relative stiffness, pore saturation, viscosity) on the observed failure modes. However, sample size / image resolution balance remains an important focus point. Additionally, future research is briefly discussed, in relation to the experience gained in the described test programs.

Quality management of interventions on historic buildings

The evaluation of the initial quality and long-term durability of an intervention on a historic building proves to be complex. On the one hand, technical and functional requirements can be defined which will lead to recipes and intervention methods. On the other hand, the heritage repair principles such as reversibility, compatibility and retreatability, should be respected. How are these aspects taken into account when defining an intervention in current practice? And how are they translated by the craftsmen into an actual intervention that meets their skills? The quality management approach presented in this paper was developed specifically for the cultural heritage sector and is based on a literature study and a study of current practice within Europe. Both literature and practice demonstrate that preservation of cultural heritage can be obtained by applying a planned preventive conservation approach and through a positive collaboration of all stakeholders. The proposed scheme will be confronted with the restoration of the fifteenth century enclosure of the Abbey of Tongerlo in order to understand its applicability in practice as a means for measuring the initial quality and long-term durability of interventions.

The stratified significance of a historic facade as a basis for a more durable conservation approach

Abstract In heritage conservation, a gap is often observed between the theory of conservation as a durable process that aims at the preservation of a historic building and the practice of restoration as a single intervention that aims at a fast and convincing result. This paper describes the proposed approach for the conservation of the main façade of the Shoemakers Chapel (in Dutch: Schoenmakerskapel) in Antwerp (Belgium), a listed monument since 1976. It serves as an example of how to develop a durable and realistic approach for the conservation of a sixteenth century façade. The basis for the conservation approach is the understanding that each intervention should take the stratified significance of the historic façade into account. In this paper, it will be shown how to combine a study of the façade from a technical point of view with an analysis of the façade as a carrier of cultural significance.

Controlled Intervention: Monitoring the Dismantlement and Reconstruction of the Flying Buttresses of Two Gothic Churches

This study on controlled intervention focuses on the interest of monitoring during structural interventions on historical constructions. In addition to being a mechanism for control or validation of the intervention, in some cases, an adequate monitoring system can be a prerequisite for the execution of complex structural interventions. In this contribution, the methodology of controlled intervention is applied on the dismantlement and reconstruction of flying buttresses and illustrated with two case studies. In one case, the St. James Church in Leuven, the monitoring process governs the intervention. For the other case, the Church of Our Lady in Laken, monitoring data are used to control and validate the design of the intervention measures. In both cases, thanks to the monitoring systems, the dismantlement was controlled in real-time and the disturbance to the structure could be kept to a minimum.