International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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ONSITEFORMASONRY - A European Research Project: On-site investigation techniques for the structural evaluation of historic masonryChristiane Maierhofer, Astrid Ziebolz, Christian Köpp
Federal Institute for Materials Research and Testing (BAM), Berlin, Germany
ONSITEFORMASONRY is a research project funded by the European Commission under the 5. Framework Programme 1998-2002 in the Thematic Programme: Energy, Environment and Sustainable Development and the Key Action: The City of Tomorrow and Cultural Heritage.
The main objective of the project is the development and improvement of methodologies for the evaluation of the structure of historic masonry Cultural Heritages. For effective restoration and conservation of historic buildings, a detailed assessment of the structural safety and physical damages of the masonry structure is required. Therefore, typical masonry damages and the most frequent pathologies in each region have been identified and summarised in a catalogue of problems and damages. Selected non-destructive (NDT) and minor-destructive (MDT) techniques are performed by the partners including the development of software packages for fast and automated data analysis. The strategy for an effective and useful combination of different NDT and MDT methods will be worked out considering the results of case studies and taking into account the experiences of the consortium members. Recommendations and guidelines for the application of the integrated methodologies will be developed in close co-operation with end-users.
Buildings, structures and especially Cultural Heritages are major riches of any society, but their maintenance, repair and rehabilitation is very costly and time consuming. The quality of life in any urban environment is strongly affected by the safety and the functionality of its infrastructure. Any restriction in the access to public areas has a far reaching effect to the life of the community, to tourism or public opinion.
The current approach normally used to assess structural safety and/or physical damages on historic masonry consists of visual inspection and local investigations by laboratory measurements on cored samples and load carrying tests. But the experience of the last decades has shown that as much as masonry appears homogeneous at the surface, the less significant and cost effective are these techniques. Thus, these techniques make it impossible to adopt desired standards for the quality and frequency of assessment. This step can only be carried out in a non-destructive and volume covering way in order not only to preserve the building, but also to avoid disturbing the state of stress and strain of the structure.
Therefore, in the project methodologies will be developed for the evaluation of historic masonry. The main objectives of the research project is an improvement of cost/benefit ratio for investigation and diagnosis and can be described as follows:
Project partnersThe consortium is well balanced bringing together all the needed expertise and including partners with complementary roles (full members see below): equipment manufacturers (BAM, MALA), developers of NDT methodologies (BAM, POLIMI, UNIPI), NDT users (GC, UNIPD, ZAG, UCLM, IRDI, PROVR, ITAM), experts for structural models (UCLM, UNIPI) and owners of Cultural Properties (SLG, RT, JCM, PROVR, IRMA) which play an important role as potential users of the methodology.
BAM (Co-ordinator, Germany):
The project is funded over a period of 36 months, starting was November 2001. Because of the large number of partners in the project working in the field of NDT/MDT, a multitude of applicable investigation methods together with the respective know-how are at the project's disposal. Beside the NDT methods like radar, ultrasonic, impact-echo, acoustic emission etc. some MDT techniques like endoscopy, flat-jack, core-drilling are performed.
Simultaneously a catalogue of damages and structural typologies has been worked out in close collaboration with the end-users involved in the project. The requirements towards the investigation methods and their technical equipment, derived from this catalogue, have been evaluated and summarised. With this synopsis a pre-selection of methods and a promising combination of methods for certain typical damages has been developed.
The combination of several investigation techniques like ultrasonics, impact-echo, sonics, radar and flat-jack can give more reliability for the interpretation of results and for the detection of irregularities like voids, cracks, presence of moisture and/or salt. etc. If a void is detected by all three methods, the presence of the irregularity can be regarded with a high level of reliability. Furthermore, it can help to clarify the morphology of the structure investigated, to give information about the presence of weakened areas and about the state of stress in masonry structures. The far reaching experiences, especially with the complex material masonry, available within the consortium make it possible to choose combinations of techniques well directed. The integration of several end-users in the development of methodologies allows to take architectural and historic characteristics into account at an early state of assessment.
Calibration activities through laboratory tests on masonry test specimen and specifically the correlation of the data of double flat-jack tests and sonic velocities will enhance the accuracy of quantified results.
Further reliability of measurements will be gained by comparing the results with structural models, especially developed for the project. These models, simulating the behaviour of masonry can give valuable hints for the origin and evolution of damages.
The developed methodology will be demonstrated and documented by case studies performed on selected pilot sites. These demonstrations will deeply involve the participation of end-users in the attending European countries.
The greatest challenge of the project is to focus the existing expert-know-how and at the same time to make it transparent. The methodologies developed in the project shall be seen as a tool for the non-scientist or the technical less experienced end-user, which enables them to make a first assessment of historic masonry. At the present time, there are no regulations, standards or codes of practice on the described field of work. Thus, the partners are collecting contributions to the best practice definition of the methods and methodologies. Contact to national and European end-users is established to obtain their needs and requirements of standards in conservation and restoration of Cultural Heritages. For standard dissemination, national and European workshops are organised.
Classification of damages and testing problems
To achieve a wide base of knowledge for the investigations ONSITEFORMASONRY is aiming at, in a first essential step it had to be determined which kind of damage typically affect historic masonry.
All project partners contributed to this state of the art by providing their case studies of former or ongoing investigations concerning historic masonry. The characterisation of the structural element, a description of the damage, the building materials, the probable origin of the damage and a significant picture form the wanted poster of the particular case study, completed by a proposition of applicable NDT-methods and relevant measurement parameters. When collecting the case studies, it appeared rather quickly that the term Damage Catalogue would have to be widened, since a lot of the investigations dealt with questions about the recognition of structures (one- or multiple-leaf walls, thickness of walls,presence of voids, etc.) which is a major aspect in restoration and preservation of monuments and historic buildings and which is as important as the survey of damages.
The collection of case studies is now called Standard Damage Catalogue and List of Structural Typologies and Related Requirements and comprises currently more then 100 examples, still being extended in the course of the project.
The document developed under supervision of the Spanish partner GEOCISA represents thus far more then just a collection of damages. It is a comprehensive classification of problems, questions and damages related to historic masonry.
The crucial points of the overview are the definition of structural typologies and the classification of damages together with a compilation of measurement parameters needed for their characterisation. The structural typologies comprise among others a systematic structuring of:
For an all-embracing assessment of the structures the chemical, physical and mechanical properties of the components have to be determined. The measurement parameters necessary for characterisation (e.g. mineralogical composition, mechanical properties, dynamical properties, dimension, morphology, etc.) have been compiled and described in detail.
The damages typical for historic masonry have been divided into two main groups:
For the determination of damages measurement parameters (e.g. salt and moisture content, shrinkage-swelling cycles, dynamic actions, presence of pollutants, etc.) have been compiled as well.
The Standard Damage Catalogue and List of Structural Typologies and Related Requirements offers the user the opportunity to make a first estimation of the extent, the cause and the kind of damages and indicates, which of the available investigation methods can be used for an evaluation. Relevant measurement parameters are described in detail. Additionally the Catalogue visualises typical types of damages and thus helps to identify problems occurring at historic masonry.
Development of NDT methods
In the following, the NDT methods to be developed and optimised during the project for the application on masonry structures are described.
Radar (ground penetrating radar, GPR)
In this project, the main modifications and optimisations of the radar techniques will be performed by MALA in the course of the development of a new light high frequency antenna with separated transmitter and receiver enabling impulse-echo and tomographic measurements with enhanced spatial resolution. Efficient strategies for data acquisition for 3D and tomographic investigations will be developed together with a new positioning sensor. Additionally, software will be processed for 3D and tomographic data inversion and for automatic data analysis.
Tomographic application of ultrasonics is widely used for metals in the frequency range higher than 1 MHz [Ref 17]. Few tomographic investigations with low frequency transducers (20 kHz and 70 kHz) have already been successfully performed for masonry structures [Ref 18, Ref 19].
For adapting ultrasonic impulse-echo and tomography to masonry in ONSITEFORMASONRY, investigations have already been performed by BAM to develop a transducer array taking into account coupling with and without agent. For the selected transducers, the optimised electric impulse will be chosen. Tomographic investigations will be performed focusing on the visualisation of areas having different ultrasonic velocities. For this, time of flight measurements are required and an algorithm for automatic determination of impulse arrival will be developed.
In the project, measurements with different impactors and sensors have been obtained by BAM at a masonry test specimen with a thickness of 50 cm containing voids with sizes of 24 cm x 12 cm at a depth of 12 cm. The results obtained along a line with a new impact-echo system from Olson Instruments, USA, are shown in figure 1 right in the frequency-domain. This picture displays the amplitude spectrum as a function of the source/sensor location. This B-Scan-like representation in the frequency-domain is called impact-echogram. In the impact-echogram, a dominant frequency at about 2.5 kHz can be identified, corresponding to the reflection at the backside of the test specimen. From the known thickness, an effective P-wave velocity of propagation of approximately 2500 m/s can be calculated. At the position of the voids, this reflection is weaker due to shadowing. A direct reflection from the voids was not detectable.
Also measurements at real structures (historic church in Potsdam, built from 1850 to 1852) have been performed demonstrating that even at a wall thickness of 1 m the backside reflection can be detected. Further on-site investigations are planned.
The data obtained by this technique not only give a geometrical description of the masonry structure but also an important input for the static verification procedures, also if attention must be given to transfer the dynamic values to the static modelling. Several efforts have been performed on calibrating and data interpretation [Ref 24, Ref 25, Ref 26]. But for reaching reliable information, calibration activities in combination with the flat-jack method are planned in the project by UNIPI and POLIMI.
The main problem for the correct evaluation of the seismic velocities on a small scale is the necessity of measuring arrival times that are a couple of orders of magnitude shorter than usual in geophysics. To obtain a good resolution it is necessary to work with very high frequencies. But high frequencies lead to very high attenuation of the signal. Practically the useful range resulted in the high sonic frequencies up to 4000 to 5000 Hz. Acquisition time can be saved by using a multichannel equipment but the market does not offer dedicated systems at these frequencies. Thus, low cost transducers, wave generator and recording systems are explored in the project by UNIPI.
A further limit of the present technology for sonic tomography is that the received energy is affected by many factors besides absorption so that the conventional programs for attenuation tomography based on amplitude backprojection are not applicable. An alternative method based on the frequency downshift effect has been developed by other authors [Ref 27, Ref 28]. The applicability of this method to tomographic measurements on building elements is still unexplored but will be applied in ONSITEFORMASONRY.
Parallel to these investigations, UNIPI is also working on the new idea of the use of shear waves (S-waves) to obtain a higher resolution. Due to the lower velocities of propagation of S-waves, the wavelengths are smaller and thus the resolution increases. Measurements to this innovative method have already been performed by UNIPI at a damaged wall of the transept of the Duomo of Lucca (see figure 2 left), where also new sensors have been applied [Ref 29, Ref 30]. In figure 2 right, two velocity tomographies recorded with P-waves (left) and S-waves (right) are compared. In the velocity distribution of the P-waves, there are several areas with very low velocity; some high velocity spots correspond to rigid stone structures connected to the nearby corners and to the structures of the portal.In the S-wave tomography the general trend is similar, but the image includes much more details; due to the decreased wavelength, the distances between adjacent shots or geophones could be reduced to the half and less. he total number of measured travel times was 588 (instead of 144 for the P-waves tests). This allowed to divide the section in much smaller cells (7x20 cm) for the tomographic inversion.
The strategies for an evaluation of historic masonry, which were developed in the project, have to undergo comprehensive tests, before they can be incorporated in guidelines or recommendations. Previous experiences with measurements at real masonry have shown that very often unexpected difficulties occur because of the distinctive inhomogeneity of many brickwork structures.
A first step was therefore the construction of some masonry specimen, which, featuring a diversity of properties (material, thickness, presence of voids), represent several aspects or characteristics of real historic masonry. These specimens are used to perform measurements under specific conditions. If for example the exact position of voids is known in the specimen, it is particularly qualified for proving the effectiveness and reliability of the respective investigation method.
A historic test specimen has been built at BAM in close cooperation with Stiftung Luthergedenkstätten (Wittenberg/Germany), Institut für Diagnostik und Konservierung an Denkmalen in Sachsen und Sachsen-Anhalt e. V. (Halle/Germany) and Institut für angewandte Forschung im Bauwesen (Berlin/Germany).
This specimen with the dimensions 7 m x 3 m x 1.5 m has been planned and constructed in consideration of traditional manufacturing techniques, partly using historical materials, stemming from demolitioned buildings. It represents a large variety of problems and characteristics of real historic masonry (mixed masonry, multi-leafed walls, hidden inclusions, cracks, voids, etc.). Each of the characteristics or properties of this Historic Masonry Specimen, which has been erected by a very experienced building company specialised in the restoration of cultural heritage buildings, is known in detail. This specimen is in a way the link between usual masonry specimen and real historic masonry buildings and will enable the validation and calibration of the investigation techniques.
For the performance of on-site calibrations and testing of methodologies, a preliminary selection of pilot sites has been made considering the following aspects:
These sites should allow the evaluation of the reliability of the results through the comparison with a priori information and /or with coring or other destructive investigations. The assessment of the pilot sites include also structural modeling based on the measured parameters. This is the way of integrating the NDT techniques considered within the project focusing to the final aim of every end-user: to determine the actual state and the load carrying capacity by considering data obtained from NDT and MDT methods. The applications will be carried out at all partners' countries to optimize integration and co-operation and to compare the obtained results.
One of the selected sites include the investigation of the inner structure of the wall of the Western gable of the Luther House in Wittenberg, as shown in figure 3. Here, few damages at the wall (see figure 3 right) and some cores showed that in some areas the wall consists of two leafs and in other regions the wall is homogeneous.
Another site will be the Pisece castle in Slovenia (see figure 4) built at the first half of the 13th century as defence fortification against the Hungarian danger by archbishop of Salzburg. Here, structural investigations related to cracks (see figure 4 right), multiple leaf, inhomogeneous masonry and localisations of metallic inclusions are planned.
Apart from the really non-destructive testing methods, the complementary minor destructive methods (flat-jack, endoscopy, bore-hole extraction, etc.) can be performed here, which is often impossible at highly protected historic buildings.
The on-site investigations are planned to be performed partly with the involvement of an interested audience in the frame of workshops, seminars or demonstrations. It is one of the characteristics of the ONSITEFORMASONRY project to make project results accessible for the public and to involve especially the end-users into the project already at an early stage.
Guidelines and recommendations
Guidelines and recommendations will be developed for different users on three levels:
Summary and Outlook
One of the main results of the first half of ONSITEFORMASONRY is that although the structure of historic masonry is very inhomogeneous, it can be classified by a small number of structural elements. In most evaluations of historic buildings not the whole building complex but these elements are under consideration. The scientific and technical achievements give first guidelines to choose the most appropriate method to solve every problem related to each level of assessment. To give a rigid procedure is not possible due to large number of variations of single problems.
During the remaining period of the project, calibrations actions and various on-site investigations are planned. For the development of guidelines and recommendations, close co-operation with all stakeholders is essential. A public website (www.onsiteformasonry.bam.de) has been established to keep anyone interested in the project informed. Contributions and opinions from the reader are requested and can be forwarded to the partners by e-mail.
The project is funded by the European Commission under the 5. Framework Programme 1998-2002 in the Thematic Programme: Energy, Environment and Sustainable Development and the Key action: The City of Tomorrow and Cultural Heritage.
The contribution of all Partners to this paper is gratefully acknowledged.