Skip to main content

On-site investigation techniques for the structural evaluation of historic masonry buildings

Deliverables

This result provides the end-users with guidelines for the correct use of diagnostics for the structural assessment of Historic Masonry buildings. Recommendations for end users will be published both in printed and CD format. This publication attempts to council a necessary scientific methodological approach in plane language, in order to stimulate end-users, not always well prepared in terms of knowledge of the NDT, MDT diagnostic approaches, to utilise them for an effective control and assessment of the building that they have in charge. This document is based on several tables where the end-users can find the correct approach to the solution of their problems in terms of applicable diagnostic techniques and descriptions of the main available methods. Documentation Type: Booklet and CD Details: Recommendations for End-Users � D11.3 � English and Italian Status: PU Current stage of development: Other: Text (already prepared) is going to be processed in order to provide navigation capability and will be integrated in a CD with all the deliverables of the projects and summarised in a printed booklet. Potential offered for further dissemination and use: This final deliverable and result synthesises the complex of the exploitation strategy, based on the growth of sensibility of the end users in the modern diagnostic and assessment methods, in order to force the creation of a standard and the relevant market for services. Profile of additional partner(s) for further dissemination and use "Intermediate" end users: designers (engineers or architects) which need the maximum of knowledge of the building to be restored in order to develop a good project without "surprises" during the works; companies or groups of professionals which intend to sell services in the market of diagnostics; "Final end users": Public institutions that are the "owners" of the Monuments or are charged of its management or conservation.
The result consists of a prototype software developed in a high level language to perform the tomographic inversion of sonic and/or radar data. The software is an extension of a previous prototype developed by POLIMI for tomography. Traveltime and Amplitude inversions can be performed to obtain velocity and attenuation maps on 2D sections of building elements with any kind of geometry. For amplitude tomography, radar data is processed by compensating the antenna directivity function. A new algorithm based on frequency analysis is also included for attenuation tomography on sonic data where amplitudes are not reliable being affected by many factors besides absorption.
The goal of the positioning system is mainly to provide a fast, simple and accurate positioning system within a small area of size 1-2 square metres. The absolute accuracy is expected to be below 5mm. Several possible solutions have been evaluated in the project. The most promising solution consists of a retractable string with a length encoder built together with an angular encoder. This solution has the benefit that it is easy to set up in the field, and it provides a sufficient accuracy. Furthermore, the measuring speed can be very high. A user interface is developed that allows the operator to view the measured path on-line on a screen, thus making it easier to make sure that the whole area is covered with sufficient dense measurements. The positioning system is quite unique in the sense that no commercial system with similar functionality exists. There are alternatives, but they are either more expensive by at least an order of magnitude, or they lacks accuracy. A prototype of the system is build and under testing. Furthermore, a theoretical investigation of error accumulation and precision has been performed. Also a theoretical investigation of the calibration routines has been completed. The possible use of the positioning system is multifarious. It can with only slight modifications are implemented in other applications in which an accuracy of a few millimetres is needed within a measuring area of some square metres.
The work package includes the development of a new high frequency radar antenna system with a center frequency around 1.6 GHz. This allows for high-resolution non-destructive mapping. The antenna system will be based on existing radar antenna technology from Malå Geoscience. The new high-frequency system will be flexible in the sense that it will allow for a separation of the transmitter and receiver units. Two identical units containing both receiver and transmitter antenna can be connected to the control unit, one acting as transmitter, the other acting as receiver. This flexibility makes the system useful both for reflection mode measurements and tomography measurements in which the transmitter and receiver are located on both sides of a wall, for example. The commercial value of the antenna will be high, due to the fact that there are multiple applications in which a high resolution is needed. This holds true not only for the actual project, but also for applications like measurement of asphalt thickness and the detection of fractures in concrete. Other applications, like the estimation of water content in small samples of different materials are possible with this new high-frequency antenna. Examples of such materials are seed, corn, crop and sawdust. Unfortunately certain restrictions in the use of high-frequency systems are foreseen. Today such restrictions for frequencies exceeding 960MHz are already implemented in the USA. The value of a high-frequency antenna lies in the fact that the resolution is improved compared to existing antennas. In all sorts of NDT (non-destructive testing) where the interesting scale is of the order decimeters, it is important that the wavelength is of the same order of magnitude. A 1GHz antenna will thus have a wavelength of about 10cm in most materials, and a 1.6GHz antenna will have a wavelength of about 6cm. This allows for detailed mapping of structures down to only a few centimetres. Furthermore, the bandwidth of the antenna extends beyond the centre frequency of 1.6 GHz, thus allowing for high-resolution mapping. The major benefit of the antenna is the combination with the data collection system and the positioning system. The whole system is designed in such a way that the start-up time is very short. By activating a push-button on the antenna, the operator can in a few seconds calibrate the positioning system for the actual measuring area, and while measuring the operator can control the data collection via the push-button. On the attached monitor (robust, tough and equipped with a highly visible display) the path of the antenna movement is visualized in real-time, together with the data collection. The antenna is very small (LxWxH = 135x73x54mm) with a weight of 600g (handle not included).
The result consists of a software prototype developed in a high level language to perform the 3D processing of echo-radar data. All the most important function normally applied to GPR data is included in the software. In addition the prototype includes new algorithms for specific tasks that are: - Pre-processing algorithms to remove the background preserving the horizontal reflections generated by masonry leafs and/or by leaf detachments and/or by fractures; - Algorithms for 3D multi-offset data focusing to produce interpretable pictures where reflecting or diffracting objects or voids appear with the true dimension and position.
The "Catalogue of typical problems and parameters to be measured of historic masonry" is oriented to give an overview about frequent structural typologies, damages and other problems affecting ancient masonry constructions including the parameters which are useful and the non-destructive (or minor destructive) techniques (NDTs, MDTs) that may be applied to characterise those damages or the structure itself for a structural assessment in the preservation of Cultural Heritage. The information was collected from partners from different European Countries to include possible regional characteristics and also from different disciplines and sectors of the Cultural Heritage with complementary roles (Equipment designers, developers of NDT methodologies, NDT users, experts for structural models and owners of Cultural Properties), in order to cover and integrate the different points of view. So, the information given by NDT users are mostly related to practical cases like, for example, the strategy to detect the presence of moisture in a certain historical building, and the owners of historical structures are mostly interested in the historical research of these structures. The catalogue includes 88 templates about problems affecting historic masonry containing a description and their possible origins illustrated with photos or figures, the parameters to be measured for its characterisation and the applicable NDTs in order to solve them. Therefore, it is a useful instrument for service providers, researchers on NDTs, cultural heritage managers, city authorities and also policy makers, to get a first impression about the aspects involved in the preservation of Cultural Heritage, that have to be taken into account in diagnosis or repairing projects, providing a baseline to have some criteria about different actual methods to assess historical structures. The catalogue is still in progress by collecting more templates with information to improve the catalogue till the end of the project.
The result consists of a number of laboratory masonry specimens produced to test and calibrate some NDT single or combined methods. The specimens are located at the facilities of the partners responsible for the preparation (BAM, Berlin; ITAM, Prague; GEOCISA, Madrid). Some of these specimens will remain available for a time after the end of the project. The specimens are built with stones and/or bricks. Different construction techniques and different problems are reproduced in the specimens (single and multiple leafs, fractures, detachments, voids, inclusions, humidity, etc).
Commercially available impact-echo systems are designed for concrete testing applications, where the collected data are single point readings for thickness determination or for high-resolution near-surface investigation. Historic masonry is often characterised by very inhomogeneous, thick, composite walls. This causes a strong attenuation of the acoustical waves and requires discrete line or area covering measurements. The commercial impact-echo tools producing the acoustical signal will be modified to have sufficient energy and frequency content to allow high-resolution investigation of thick, inhomogeneous structures (high penetration depth, reduced attenuation, improved resolution). The commercial receiving sensor has to be adapted to detect signals in a broad frequency range with optimised signal to noise ratio. A scanning system for automated impact-echo measurements with high positioning resolution will be made available to allow data collection along lines and for area coverage. This automated collection mode is aimed for 2D, 3D and eventually for tomographic data reconstruction. The scanning IE data acquisition software will be developed to account for new IE impact-echoers and receivers. The scanning IE data visualisation software for real time 2D data plots will be modified accordingly to the acquisition software changes.
For the validation of non-destructive methods and structural models for the structural assessment of historic masonry, it is necessary to carry out experimental tests on pilot sites. Due to the great number of different types of masonries and structural typologies, the studied methods have to be applied on several pilot sites to check the reliability and applicability according to different materials and typologies, deterioration mechanisms, environmental conditions, and types of constructions. Therefore, the reports on the evaluation on pilot sites will be useful information for manufactures of NDT equipment, service providers, cultural heritage managers, construction industry and also city authorities, to have information about appropriate methods for the structural assessment of a masonry buildings. The end-users have proposed 25 buildings of different typology and materials affected by some typical problems that have to be investigated by the application of NDTs. The strategies will be validated according to the different problems on the selected pilot sites by different partners (NDT-appliers). The measurement campaigns at the selected sites started in summer of 2003 and they will continue till march 2004. After analysing the data and combine the results the reports will be prepared.
Microseismic Tomographies (2-D or SST), as singular techniques, give two levels of information across the masonry structures: - A geometrical description of the structure, putting in evidence discontinuities, cavities, layering, changes in the materials; - Information on the elastic properties of the materials (elastic moduli) Several multi-method strategies can be proposed by combining the microseismic tomographies with other NDT techniques. The optimal strategies vary according the problems, but typical strategies could involve Radar tomography, which can give complementary information; Impact-Echo or Ultrasounds for a more detailed shallow information; flat jack and seismic surface profiling for calibration. The potential applications for the results of the research will be an increased know-how for the diagnostic study of historical buildings at different levels, from a preliminary screening to the design of restoration works. These NDT techniques can be used, with minimal technological adaptements, to study the vulnerability of modern buildings and in case of earthquakes. The end-users can be found in two categories: - Professional or companies that work on diagnostics and restoration of masonry buildings; - The owners or the responsible authorities of historical buildings. The innovative features of the proposed techniques provide new powerful instruments for the assessment of the actual condition of historical buildings and can constitute a factor of success for the people working in the sector of the conservation of buildings that belong to the Cultural Heritage.
The result consists of a software prototype developed in a high level language to perform the comparison and the integrated analysis of results from different NDT methods. The aim is to improve the interpretation reliability and to facilitate the end-user interpretation as much as possible. The software allows a direct and effective comparison of images produced by sonic tomography with images from thermography and/or radar tomography and/or focused sections extracted from 3D echo-radar data. Graphic options and processing tools help the user to explore the images and to correlate the features contained in the data images.
The ultrasound prototype developed at BAM was created for the transmission mode. In order to realise quick measurements on-site the principle of ultrasonic transducer array will be adapted for transmission measurements at masonry. 10 to 20 transducers should be used with an electronic switch (multiplexer). The actual commercially available ultrasonic array-systems (e.g. Spectrum A1220) are designed for echo mode operation. Historic masonries are often characterised by thick inhomogeneous walls, which are sometimes composite walls. Therefore ultrasound prototype have been adapted for signals in different frequency ranges and will use time-averaging for a good signal to noise ratio. The optimum frequency range and the type of sending pulse have been chosen. Special low frequency transducers have been tested only in a prototype version yet. The optimal processing for tomographic measurement-data has been selected. This processing is applicable for ultrasonic through transmission with tomographic evaluation. The measurement software harmonises with the analysis-software. The result is a 2D-data plot showing the detected failures or the holes in the investigated wall. The software is described by the partner POLIMI.
The method of sonic 2-D tomography is a small-scale variant of the cross-hole time-travel tomography used in geophysical high-resolution investigations. Both sides of the masonry structure must be accessible. A series of receivers (small geophones) is positioned on one side of the wall and connected to a multi-channel seismograph. On the other side of the wall a series of pulses are generated by hitting the surface along a profile in correspondence to the receivers line. For every shot the arrivals to all the receivers are recorded. The source-receiver rays cover a section across the wall. The interpretation consists of a tomographic reconstruction of the distribution of the sonic velocities across the section (which is divided in pixels). This velocity distribution is expressed, via graphical routines, in a grey-scale or in a colour scale. Another kind of seismic tomography is the so-called Tomographic Seismic Sounding (TSS). In this case the transducers are in a hole drilled in the wall, while the energising points are along straight profiles at different distances from the hole. A triangular section is covered by the seismic rays. The data treatment uses algorithms similar to the 2-D tomography. The result is a velocity profile along the axis of the hole. This method gives the variations of velocities with the highest resolution. A "prototype" for seismic tomography is not included in target of the Project as seismic tomography is a procedure, not an instrument. The research program foresees the technological development of several equipment items at different stages of the process: prototypes of new P-waves and S-waves transducers will be proposed. Also suitable positioning tools will be built. The software for the tomographic reconstruction has been improved. Instead of a "prototype", a "package" for the application of micro-seismic tomographies on masonry structures can be proposed. This can include a high-dynamic, high frequency seismograph; a set of specific tools (energy systems, receiving transducers, positioning tools, etc); one software package for data processing and imaging.
Numerical and analytical models for fracture and failure of the brick and stone masonry. Experimental results of brickwork masonry failure under normal/shear loading. Experimental results of regular and irregular stone masonry failure. In situ measurements on brickwork masonry and stone masonry. A better integration of the NDT and MDT with the structural analysis.