The architectural preservation process is generally based on a sequence of anamnesis and analysis, diagnosis, therapy, control and prognosis. The importance of structural monitoring within this process is generally accepted. Ongoing research at the Building Materials and Building Technology Division of the Civil Engineering department of K.U.Leuven focuses on 3 different non-destructive monitoring techniques, namely: 3D laser scanning, acoustic emission and geo-electrical measurements:
• 3D-laserscanning: The stability of masonry vaults very much depends on its overall geometry and the analysis results strongly depend on the accuracy in which this geometry can be measured in practice. To obtain an accurate estimate of the geometry of the vaults, 3D-laserscanning was performed. Based on the 3D point cloud, a 3D-model of the vaults allows determining the lines of thrust in the structure as well as the reaction forces (application point and magnitude) at the abutments. This contribution gives a critical appraisal of the process used in the preliminary research phase: gathering the point-clouds, 3D-modelling, structural analysis calculating thrust lines and the consolidation requirements that are obtained. The focus is on the added value of the methodology, its applicability, cost-effectiveness, overall advantages and drawbacks;
• The interest in acoustic emission (AE) as a monitoring technique to assess the damage in masonry structures is gaining field recently. The AE technique, detects transient energy waves emitted by a material as a result of stress redistributions. Therefore, the technique requires an external load to be imposed on the structure, such as a mechanical, thermal or chemical loading. In this paper, the focus is on long term monitoring and the use of AE-monitoring to assess damage evolution under sustained stresses, such as damage accumulation during creep phenomena in masonry. Additional diffulties are encountered that come from the complex masonry layout and the monitoring time frame. In order to gain more insight in the use of AE-monitoring in this field, a laboratory test campaign is set up. Several long term tests are performed on different types of masonry at different stress levels. The latter are increased at specific time intervals. In addition to stress-strain and crack monitoring, AE-monitoring is performed at discrete moments in time. Both sources of data are used to validate the AE-monitoring technique to assess the damage accumulation. Hereby, both damage initiation due to load increment and the damage accumulation due to time-dependent plastic deformation under sustained loading were assessed. Focus is both on the added value and limitations of the AE for damage assessment, with an appraisal towards its applicability for on site NDT monitoring;
• geo-electrical measurements: A geophysical resistivity measuring device was modified to perform automatic monitoring of historical masonry structures before, during and after grout injection for consolidation purposes. The obtained image is called a geo-electrical tomography. The technique was used to evaluate the deteriorated masonry of the recently partly collapsed Maagdentoren in Zichem, Belgium. The results of these measurements are dis-cussed. An important improvement is the adaptation of the “cross-borehole” measurement technique for masonry. With this technique, a constant resolution in depth is achieved. Laboratory tests, using an adaptation of the “cross-borehole” technique, enabled to show the resistivity distribution changing during the injection of a test specimen with a hydraulic grout.
Focus is on how these structural monitoring techniques are a tool to help in the structural understanding of often complex historical buildings.