257-DHM : Design and application of hydraulic grouts for repair and strengthening of historic masonry structures
Technical Committee 257-DHM
Deputy Chair: Prof. Elizabeth VINTZILEOU
Activity ending in: 2016
General presentation of the subject matter
1) Grouting constitutes one of the most common techniques applied for the repair and strengthening of masonry structures or fissured architectural members, when interconnected voids are present. This technique can also be suitable for the in situ consolidation of detached and cracked mosaics, wall paintings or other valuable decorative renderings.
Although grouting is a non-reversible technique, it is well accepted even in monuments of high historical and architectural value, since it has the capacity to retrieve the continuity, cohesion and strength of the damaged structures, without altering their morphology, geometry and load-bearing system. To this end the grout as well as the method of its application to historic structures must satisfy a series of performance requirements, comprising that of re-treatability. Consequently, grouting may be a durable and mechanically efficient intervention technique, only under the condition that the grout is suitably designed and the injections are correctly implemented.
To this end, the design of the grout composition (selection of appropriate raw materials, mix proportions, mixing method, etc), as well as the procedure and method of its application to historic structures, should satisfy a series of requirements (involving injectability, strength and durability aspects) set on the basis of an overall consideration of the structure to be repaired:
(a) the characteristics of masonry to be strengthened (e.g. constituent materials, way of construction of the interior of masonry, presence of decorative elements on the façades, etc.),
(b) the nominal minimum width of fissures and voids to be filled and the roughly estimated percentage of internal voids
(c) the observed pathology and the actions masonry is expected to be subjected to (including environmental actions),
(d) the desirable compressive and tensile strength "fwc,s, fwt,s" of the masonry as a whole (after strengthening), as dictated by the structural behaviour envisaged,
(e) the durability demands after intervention and f) the materials and equipment availability to be used in situ for the
application of the intervention.
Moreover, it should be noted that the aforementioned requirements to be fulfilled by a grout namely,
- high injectability (to fill internal cracks and voids under low pressure),
- adequate tensile strength and bonding capacity (to improve the mechanical behaviour) and
- sufficient durability of the grout and the injected structure (to ensure re-treatability with minimized side effects),
are quite often contradicting each other.
Thus, the design has to be based on a specific methodology and rational criteria, which could lead to an optimization of the grout composition for each specific case. Such a rational holistic approach of the entire grouting intervention should offer the possibility to proceed by steps. The actions taken within each step should be based on the data from previous steps, whereas intermediate decisions and formulation of alternative solutions should be documented and justified.
Due to the large number and the variety of key parameters encountered in each individual restoration case (e.g. construction types and combination of materials, pathology, environment, in situ available restoration material etc.), the need for such a holistic and rational approach for the selection and application of a suitable grout has to be emphasized. The use of recipies suitable for all cases, seems to be an easy and quick solution; it may, however, lead to an inefficient intervention or, in the worst case, it may result to non-reversible negative effects on the structure to be preserved.
2) There is a great variety of raw materials that can be used for grouting. Based on the results of extended research and experience of in situ applications, the use of polymer-based grouts is not recommended for injecting masonry structures.
Hydraulic binders associated in the most of the cases to ultrafine materials were proved to be adequate, when designed in a rational way. In fact, hydraulic lime or cement is used as the basic binder; binary or ternary compositions are usually designed by combining these basic binders with ultrafine materials such as pozzolanic materials (natural or artificial), hydrated lime, etc. In some cases, the addition of fine aggregates (like sand, usually dmax ~1mm) can be envisaged, in order to prepare a lower injectability grout to be applied locally, in order either to fill known large voids, or to block the entrance of high injectability grout to a specific part of the construction (for example the foundation walls). Finally various admixtures are used, when necessary, mainly in order to reduce the water content.
3) On the other hand, the appropriate design of a grout composition itself, cannot ensure the successful completion of the grouting intervention. Evidently, particular care has to be taken for the adequate execution of the entire intervention in situ.
Experience acquainted during in situ applications revealed a series of problems encountered in practice, and highlighted the need for elaborating detailed practical guidelines for achieving a satisfactory application of the technique. Such guidelines should comprise all relevant matters such as: (i) The main performance requirements regarding the raw materials, the grout composition and its application. It has to be reminded that the acceptance criteria should be formulated during the design of grout composition, based on the structural restoration study of each specific construction, (ii) General information regarding the preparation of the masonry or architectural member or other decorative element and the installation of grout entrance and exit tubes, (iii) The main requirements for the in situ grouting equipment, (iv) Basic information concerning the preparation of the grout in situ, the in situ controls of injectability characteristics of the grout, as well as the in-time evolution of strength and porosity or other characteristics, if needed, (v) The injection procedure itself, (vi) The data that have to be recorded during execution of injections and their evaluation for the continuous quality control of the intervention, (vii) Post intervention assessment of grouting efficiency using NDT techniques.
Available knowledge and need for interdisciplinary action
4. Since grouting constitutes a very promising technique, a lot of research has been done the last three decades, regarding both the development of appropriate testing methods and the elaboration of rational design criteria for ensuring high injectability (penetrability, fluidity and stability of the suspension), which is the governing property for successful grouting. Moreover, research has been undertaken concerning the study of the mechanical characteristics of various types of grouts, and the mechanical efficiency of grouting intervention (tensile and shear bonding capacity of grout to stones or other support materials, impact of grouting on the mechanical characteristics of infill material, as well as of large scale wallets and even of entire structures). Less extended research was related to durability aspects of the grouts and the injected structures.
Furthermore, some important efforts have been realized towards the elaboration of guidelines for the holistic design of grout, necessary for the determination of the optimum composition and for the methodology of application and the quality control during intervention. Notable efforts have been done also for the control of grouting efficiency by means of NDT’s, MDT’s and monitoring (sonic tests, radar, endoscopy, seismic monitoring, etc).
5. On the other hand, important experience has been gained from some successful in situ applications, as well as from some problematic cases, where difficulties have been encountered, concerning mainly the injectability capacity leading to an unsatisfactory filling of voids.
Furthermore, failures observed in the behavior of real structures repaired in the past with grouting which have been damaged again due to recent earthquakes, revealed that inadequate design and application leads to an unsuccessful intervention and to new damage, that can be detrimental. In fact, the owner has the impression that the structure has been repaired or even strengthened with grouting, but this goal has never really been achieved: The injectability of grout was not adequate and measures for controlling the intervention during and after the execution of works were not taken.
Moreover lessons related to durability problems were also learned by some problematic grouting interventions. In fact there were cases in which major damage due to undesirable chemical reactions of hydraulic grouts with mortars containing gypsum was occurred. The analysis of existing materials was not carried out during the design of the grout, thus durability requirements were not fulfilled.
A first comparison between the data of some examples of in situ applications reveals that the rational holistic design and the meticulous application, following all necessary in situ protocols and controls are the basic ways for satisfactory injectability and filling of voids, as well as for ensuring strength and durability.
6. To summarize, there is an urgent need for a synthetic review of relative knowledge and exchange of experiences in an international level, in order to clarify possibly misunderstandings, to determine the matters for which further research is urgently necessary, to prepare international recommendations for all the matters related to grouting design and application and to disseminate the knowledge worldwide. This is necessary, as the technique is used very often for repairing various constructions of cultural heritage, especially in seismic areas, and therefore its cultural, economic and social impact is not negligible.
7. The approach of the new RILEM TC will be the collection of in-depth knowledge gained by research institutions, service providers and users. Their experience will be combined, assessed and will be applied in a rational way, in order to solve specific grouting problems of historic masonry structures (stones, mosaics, etc). This requires multidisciplinary work between engineers (Structural, Chemical, Electronic,…), Materials scientists, Architects, Contractors and Technical personnel, and owners of Cultural Heritage. In case of masonry structures decorated with frescoes or other decorative elements the cooperation of conservators, art historians and archaeologists is necessary, even for the repair of the masonry itself, as the masonry grouting will undoubtly affect the decorative elements. This multidisciplinary approach is very important whereas it is actually performed only sporadically. Such an approach was performed in the case of important world heritage monuments (Acropolis Parthenon, Pisa Tower, Dafni Monastery, …), where important research and application experience has been gained.
Scope and means
8. Despite the ample accessibility to various raw materials, and despite the number of experimental and analytical works available by now in literature, it has to be recognized that well accepted design rules of the grout formulation and their application, feasibility recommendations and procedures aimed at checking the effectiveness of the technique, are not yet well developed.
That is why there is an urgent need:
i) to compare the relevant testing procedures and test apparatuses, as well as the, sometimes contradictory, results from laboratory and field investigations,
ii) to exchange experience and try to reach a common understanding of the fundamental parameters and phenomena influencing the behaviour of a grout, especially in its liquid state (Injectability= penetrability + fluidity+ stability), before and during injection,
iii) better highlight the major properties and mechanisms which influence the impact of grouting on the mechanical behaviour of the structure,
iv) to point out the limits of our knowledge, and propose specific research matters to be further developed, and
v) to draft detailed international recommendations concerning all related matters, from laboratory testing to the in situ application and quality control.
9. To this end, the first step of work of the proposed TC will be the systematization of current knowledge on grouts (concerning all the main matters, i.e. injectability, strength and durability of the grout itself), on the structural behaviour and the durability of masonry structures and architectural elements repaired and/ or strengthened with grouting, as well as on the equipment used in situ and grouting procedure followed. The latter is very important; thus, the support of lessons learnt from case studies and practical successful or problematic experiences will be considerably taken into account.
Furthermore, the contextual examination and comparison of existing current design and application recommendations available at national and international level, the checking of their evolution and innovation, will contribute to evidence possible existing discordance between them or to confirm their reliability, in order to develop more uniform methods and basic rules for design and control.
10. The final scope will be to draft international recommendations on the design and application methodology of hydraulic grouts for the strengthening and repair of existing masonry constructions with or without decorative elements, for a justified cautious approach on historic constructions.
The work of the TC will focus on the following matters:
- types of structures-damages and related performances of grouting intervention, requirements for grout design based on the existing and on the expected performances of the structure,
- fundamental properties of grouts and basic parameters influencing them (in liquid state, during solidification and in solid state),
- testing methods for studying those properties and the role of the parameters influencing them
- impact of grouting to the mechanical behavior and durability of the repaired structure and main parameters that influence it, in relation with the type of grout and structure
- rational design methodology to fulfill the target requirements, and optimization criteria - both quantitative and qualitative-, in order to handle the often contradictory influencing parameters that may jeopardize grouting efficiency,
- methodologies for in situ application, depending on the type of structure and expected grout performances,
- adequate equipment and machinery, application procedures, worksite organization and technical staff qualifications,
- tests and simple experimental procedures, that can be applied in the field for the necessary quality control of critical properties during application
- data to be collected and methodology of their use, for the evaluation of the quality of the injection and the grout volume consumption per volume of grouted structure or element.
- NDT’s and monitoring possibilities to control grouting efficiency
11. The proposed TC will also point out possible developments and in-depth study of specific problems (e.g., durability, further study of the role of superplasticizers and other admixtures, use of NDT’s, etc..) which may promote activities of other TCs focused on those aspects.
The subject will include different types of masonry structures (two or three-leaves stone masonry, brick masonry, adobe, etc), as well as various types of architectural members (damaged stone elements that are part of an ancient temple, etc) to be repaired, as well as their possible combination, which is sometimes present in real cases. The desirable collaboration among institutions (Universities, Public Bodies, Research Agencies), contractors or other service providers, equipment manufacturers, raw materials producers and users, during the subsequent dissemination and exploitation phase, will contribute to promote and consolidate a more qualified consideration of grouting intervention in existing cultural heritage constructions.
The possible extension of the work of the TC on grouting in case of mosaics, frescoes or other decorative renderings is proposed to be examined in a second phase of the work of the TC. The clarification of main grouting aspects, both concerning the design and application during the work for masonry structures, constitute a prerequisite for dealing with the most delicate subject of mosaics and frescoes grouting.
Terms of reference
The TC is proposed to work for about 4 years, planning regular meetings (two-three per year), and additional opportunities to exchange knowledge via seminars and symposia), also in possible conjunction with the activities of the RILEM Weeks.
The committee will start as a relatively small group of experts, well representing most of the main international centres working on the subject, in a spirit of collaboration and efficiency. The whole mat Members will be recruited from international academic and research institutions, other RILEM TCs members working on mortars, NDT’s and masonry, standardization groups, Cultural Heritage administration and management, service providers and individuals.
The work will be divided into 3-4 working groups which will be asked to work in a parallel way. For the definition of the State-of-the-Art, the work will strongly imply bibliographical research on experimental, analytical and numerical studies, on standardized methods and approaches, as well as the collection of significant cases studies. Special care will be taken for surveying past in situ applications, both successful and problematic ones, in order to better connect the theory with the practice and highlight the difficulties usually encountered, as well as the means to overcome them.
Based on the systematization of experimental test results and methods, specific actions supported by possible round-robin tests will be planned, in order to select and optimize the significant parameters for the calibration of suitable testing procedures and quality controls.
Particular attention will be paid to the validation and/or the set-up of simple data collection methodology and experimental procedures, able to characterize the quality of grouts and injections, during and after interventions, easy to perform in situ in a daily basis.
Detailed working programme
1st Task: State-of-the-ArtData concerning research relative to all various grouting aspects, available equipment and materials, real in situ grouting applications and existing recommendations available at national and international level, will be selected and thoroughly examined in a comparative and synthetic way.
Thus a systematization of current knowledge will be achieved, including the following main matters:
i) types of structures that may be repaired by grouting and their characteristics influencing the grout design,
ii) main grout properties injectability, strength and durability and the testing procedures for their study,
iii) mix design procedures, via an optimization of (occasionally) contradictory design parameters
iv) impact of grouting on the structural behaviour of masonry constructions, architectural members and decorative elements repaired and/ or strengthened with grouting,
v) durability matters of the grout and the grouted structure,
vi) methodologies for grouting application,
vii) equipment and procedures to be used in situ and
viii) quality control before, during and after intervention.
This first task of the work of the proposed TC will lead to the generation of "A state of the art report". All results obtained from bibliographic survey, together with running and finished research projects will be evaluated. The state- of-the-art report will be generated in the first year and will be updated during the duration of the TC’s activities.
2nd Task: Calibration of methods and procedures proposals for new tests
The state-of the art report will contribute either to evidence possible existing discordance between the collected data concerning all the aforementioned matters, or to confirm their reliability, in order to develop more uniform methods, tests, optimization criteria methodologies for design and application, and basic rules for quality control.
Specific round-robin tests (experimental and/or numerical) will be planned, if necessary, to clarify particular aspects related to the testing procedures, design criteria, equipment adequacy etc. Besides the collaboration between the research centers working on the relevant matters, in some cases such tests will need the collaboration with materials and equipment industry or cultural authorities concerning real in situ applications.
3rd Task: Proposal of recommendations
A draft of proposal of international recommendations on the design of grouts and on the methodology of their application for the repair and strengthening of existing constructions (masonry, architectural elements, decorative elements) taking into account the basic performance requirements set for each specific case), will be developed on the basis of the results of the previous tasks. Guidelines should give detailed information on all relevant matters regarding the design and application of grouting in different constructions (various types of masonry, architectural and decorative elements), as well as the control of the execution of the injections during application and their effectiveness through non-destructive tests.
Strategies for intervention choices in the field of historical constructions with innovative materials will also be included in the document.
4th Task: Dissemination of knowledge
The last year will be devoted to the finalization of the previous phases and, in particular, of the recommendations document, by means of possible discussion and calibration during workshops planned to be open to users, service providers, contractors, materials and equipment industry. Dissemination of results will be carried out by seminars to be organized in educational and professional institutions. Training courses will be also provided, taking care to demonstrate effective limits and potentials of the various applications and of the quality control methods.
The proposed TC will contribute to clarify scientific aspects concerning the repair and strengthening of existing constructions (masonry structures, architectural and decorative elements) with hydraulic grouts, particularly in the field of cultural heritage.
Other active TCs correlated to the subject are: RHM, (repair mortars for historic masonry, chaired by G. Groot), SAM (strategies for the assessment of historic masonry structures with NDT, chaired by L. Binda), and MSC Masonry strengthening with composite materials, chaired by M. R. Valluzzi.
All the mentioned TCs are included in the RILEM cluster E (Special construction materials and components, whose convener is L. Binda).
The idea of implementing a new TC concerning the Design and application of hydraulic grouts for masonry structures and decorative elements, is based on the fact that an injectable grout is a particular building material that cannot be understood via the general knowledge concerning building mortars: Its liquid condition imposes an emphasis on penetrability, fluidity and intime stability that are hardly examined in the case of mortars; besides, control methods of the final result of grouting are drastically different. Thus, with the completion of the work of the RHM TC, there is an urgent need for establishing a new TC as grouting is a technique applied very often in repair and strengthening of various types of constructions.
The following results will be pursued:
- State-of-the-Art Report dedicated to:
i) types of structures that may or may not be repaired by grouting and their characteristics influencing the grout design.
ii) types of grouts and classification according to their basic properties
iii) main grout properties injectability, strength and durability and the testing procedures for their study,
iv) mix design procedures, via an optimization of (occasionally) contradictory design parameters,
v) the impact of grouting on the mechanical characteristics and behaviour of masonry constructions, architectural members and decorative elements repaired and/ or strengthened with grouting,
vi) the durability matters of the grout and the grouted structure and related test procedures,
vii) the methodologies for the preparation of masonry (or other construction element) for grouting and for the application of injections,
viii) the equipment and procedures to be used in situ for preparing the grout and executing the injections and
ix) the testing methods for quality control before, during and after intervention.
- Data-base of the different hydraulic grouts (based on hydraulic lime or on cement, combined with pozzolanic materials and/or hydrated lime or other innovative materials) and their basic properties, of types of structures where grouting can be applied, of successful and problematic case studies, etc.
- Validation reports as a result of round robin tests concerning particular aspects related to the testing methods for grout characterization, the grout design criteria, the equipment adequacy and the quality control.
- Harmonized experimental procedures, testing methods and modelling to characterize material properties and structural behaviour
- RILEM guideline recommendations for the holistic design of hydraulic grouts, their application on existing structures, including specific aspects for historic constructions and quality control (design, execution and control by NDT)
- Workshops/symposia to integrate contribution from users, industrial and professional people, and service providers.
- Dissemination of knowledge, via seminars (1-2 days) and training courses.
Group of users
Testing laboratories, academics, architects and engineers specialized in the repair of existing structures, industrial and other professional people developing innovative materials and equipment, specialized contractors and other service providers. Preservation, restoration and conservation Departments of Ministries of Culture or other Funds or State Services or Archaeological Schools, involved in the elaboration of studies and in the execution of repair and strengthening interventions.
Owners and users of masonry structures or Cultural Heritage buildings, as well as managers of Cultural Heritage.
Specific use of the results
Preservation, restoration and management of Cultural Heritage is a subject of high importance in many countries, especially when seismic risk or other severe hazardous causes of damage are present. Efficient intervention solutions on existing constructions maintain continuous functioning and use and reduced maintenance costs. The use of grouting in a rational, fully controlled, and effective way is presenting a great interest for historic masonry structures, architectural elements of ancient monuments and decorative elements like mosaics and frescoes. Systematization of existing knowledge in the light of the important research and application results obtained in the last decades will be of great scientific, social and economic importance at international level. Furthermore the elaboration of international guidelines and recommendations will be the basis for the best possible design and application of the technique, thus ensuring a durable and efficient intervention.