290-IMC : Durability of Inorganic Matrix Composites used for Strengthening of Masonry Constructions
Technical Committee 290-IMC
Deputy Chair: Dr. Catherine PAPANICOLAOU
In the recent decades’ fiber-reinforced materials in the form of composites comprising a polymeric matrix and continuous fiber sheets (FRP) have been extensively studied and used in structural strengthening of civil constructions. When their application was extended to historical masonry construction, the presence of a poor substrate (masonry) led to a low degree of exploitation of epoxy-bonded fibrous reinforcements. In this perspective, a new generation of fiber-reinforced materials was studied and proposed to the market, by substituting the polymeric matrix with a cement-or-lime based mortar and the continuous fiber sheets with fiber grids (meshes). These new materials, typically known as FRCM (Fabric-Reinforced Cementitious Matrix Systems), or TRM (Textile reinforced Mortars), can be bonded on the masonry substrate in the same way as a traditional plaster. In the context of heritage buildings, these materials appear to be more compatible with the historical masonry substrates, due to a set of favorable properties that include but are not limited to breathability, mechanical characteristics comparable to those of the substrate and acceptable degree of reversibility. Moreover, the use of inorganic matrices renders the strengthening layer with a higher resistance against elevated temperatures in regards to polymer-based (i.e. organic) systems. According to these information, it appears that the use of FRCM/TRM materials may be a possible and sustainable solution not only for ordinary existing concrete and masonry constructions but also for construction problems related to the architectural heritage, whenever structural upgrade, or retrofit, or seismic protection are needed.
Comparing with the large number of experimental data, guidelines, design provisions and technical recommendations available for FRP materials, the relevant information on FRCM/TRM ones is of considerably lesser volume. At the moment, only the American Concrete Institute published a document which provides operative instructions to practitioners (ACI 549.4R-13 - Guide to Design and Construction of Externally Bonded Fabric-Reinforced Cementitious Matrix (FRCM) Systems for Repair and Strengthening Concrete and Masonry Structures); however, the ACI document is mostly addressed to external strengthening of concrete structures or masonry structures which does not typically fall into the field of European historical masonry. Recently, in Italy, the technical document DT215/18- Guidelines for Design, Execution and Control of Strengthening Interventions by means of Inorganic Matrix Composites - has been developed within the framework of the Italian Research Council (CNR). In this scenario the new committee would be considered as a natural continuation and evolution of the efforts and objectives addressed by the past TC 250, which succeeded in defining the experimental set-up for bond tests. An important Rilem Recommendation document has been produced in this field. The large experimental activity driven by the TC 250 allowed to conclude a Round Robin Test for the mechanical characterization of different TRM/FRCM systems. Moreover, the activities of TC 250 allowed to coordinate a synergic effort between RILEM and ACI (RILEM TC 250-CSM & ACI 549) aimed to assess a relevant technical document - Guide to Design and Construction of Externally Bonded Fabric-Reinforced Cementitious Matrix (FRCM) and Steel Reinforced Grout (SRG) Systems for Repair and Strengthening Masonry Structures - sharing the design models and safety levels even maintaining the general American and European design rules. The two last documents are still unpublished.
In the last decade, a significant amount of research work has been produced both in the field of textile reinforced mortars and in that of masonry strengthening with fibre-reinforced materials thanks to the activities driven by the past RILEM committees: TC 201-TRC (Textile Reinforced Concrete), TC TDT (Test methods and design of textile reinforced concrete), TC 223-MSC (Masonry strengthening with composite materials) and TC 250-CSM (Composites for sustainable strengthening of masonry). Within the framework of the afore-mentioned Technical Committees selected mechanical properties of FRCM/TRM materials were experimentally investigated and new test methods for the assessment of the tensile strength and the critical shear bond behaviour parameters when dealing with concrete or masonry substrates were proposed. In essence, the new committee is proposed as a follow up of the TC-250 which - apart from contributing to the existing knowledge regarding the structural aspect of strengthening of masonry structures by use of inorganic matrix composites - also succeeded in bringing together a critical mass of experts in the field working interactively for round robin testing and publication activities.
At the moment, research programs are under way to further study the behaviour of FRCM/TRM-strengthened structural members at a larger scale. External FRCM-strengthening provided by confinement of compressed RC/masonry columns has been studied at its early stages. A larger amount of data is available in the field of strengthening of masonry panels subjected to either in-plane or out-of-plane loads. Diagonal shear, compression-shear tests and flexural tests provided the means for the quantification of the effectiveness of this new technique as a function of different experimental variables. These variables include the type of masonry, the geometry of the walls, the type of reinforcement (mortar and fibers), the presence of anchorages, etc.
To complete the background of recent applications, a hybrid system has been studied and used in forms of Composite Reinforced Mortar (CRM) in the field of masonry strengthening. This consists of a cementitious/lime matrix in which a FRP pre-cured grid is immersed, as same as in the past it was done by using steel welded grids. The advantage of substituting the steel mesh consists of a smaller thickness of the plaster needed (no concrete cover is required against corrosion), ease of installation, high durability and structural efficiency.
All information available at the moment on these new materials and strengthening systems/techniques are pertinent to their short-term behaviour (exception being some pioneering studies). Thus, it appears necessary to experimentally assess the long-term structural efficiency of FRCM/TRM and CRM strengthened masonry structures under various environmental exposure conditions and combinations thereof. Testing should focus on constituent materials, standalone inorganic composites, masonry elements and composite/substrate interfaces. According to this scientific target, research groups all over the world, involved in this research field, aim at focusing their efforts toward the following tasks:
1. A state-of-the-art study regarding the durability of individual FRCM/TRM and CRM materials and the durability of masonry assemblages strengthened with such materials. The study would produce a detailed "gap analysis", according to which an experimental program will be further detailed, based on the following task description.
2. Experimental and analytical study. An experimental study which would bring to a set of standardized tests, to be used for the mechanical characterization of FRCM/TRM/CRM, subjected to detrimental environmental actions. The scaling up tests will be conducted on individual materials (matrices and textiles), composites (FRCM/TRM/CRM), substrates (masonry of selected configurations) and on the combined system (masonry + FRCM/TRM/CRM overlays); these tests are essential to assess the structural efficiency of the whole structural system following its exposure to aggressive environments. Two main problems will be studied in terms of exposure conditions: (i) the presence of alkaline ions in the matrix with respect to the known vulnerability of glass and basalt fibers to these; (ii) the effect of challenging and extreme moisture/temperature scenarios. Different textiles (fabrics) and mortars, according to the availability of manufacturing companies, would be tested since the systems that are actually proposed and used may strongly differ in terms of constituent materials. Different masonry substrates may be chosen to study the possible changes in the interface properties between the reinforcement and its support.
The analytical study is aimed to formulate preliminary degradation laws expressed as a function of the investigated variables. At the same time, the effect of accelerated tests will also be correlated to natural conditions, for the investigated periods.
3. Proposal of guidelines for testing and simulation of environmental factors, to be suggested for the preparation or implementation of future design guidelines for inorganic mortar-based systems for the strengthening of existing masonry structures.
4. Dissemination of the results and interaction between multidisciplinary competences from academia, institutions, producers, contractors and engineering practitioners.
In terms of work organization, the collaboration among the involved institutions (Universities, Public Bodies, Research Agencies), manufacturing companies, service providers and users will contribute to promote and consolidate a more qualified and conscious approach towards existing masonry constructions, to be also diffused during the following dissemination and exploitation phase.
Terms of reference
The TC is proposed to work for 4 years, planning regular meetings (two per year) and additional opportunities to exchange knowledge (workshops, seminars, symposia), possibly in conjunction with the activities of the RILEM sponsored or co-organized conferences. The committee will start as a relatively small group of experts, well-representing most of the main international centres working on the subject, with collaborative and fruitful purposes. Members will be recruited from international academic and research institutions, other RILEM TCs members working on masonry, standardization groups, Cultural Heritage administration and management bodies, service providers, manufacturing companies and individual connections.
Detailed working programme
The work of the present TC is organized in four Work Packages (WP):
WP 1. State-of-the-Art
Assuming that uncertainties related to the long-term properties on FRCM/TRM/CRM make it difficult for the practicing civil engineer and designer to use these materials on a routine basis, a scientific effort is needed. Starting from the work carried out by the previous TCs (TC223- MSC and TC250-CSM), the knowledge of the state-of-the-art on the durability of mortar-based fibrous reinforcements, namely FRCM, TRM and CRM will be studied and extended. According to a detailed study of scientific literature, a complete background would be available in order to individuate the gaps of knowledge in the field. At the moment, lack or inaccessibility of data, related to the durability of these materials is proving to be one of the major challenges that needs to be addressed prior to the widespread acceptance and implementation of these materials in masonry construction. A gap analysis will be produced, able to pinpoint the specific lacks that need to be filled for the production of a comprehensive database. As part of the overall attempt by the new TC to provide pertinent information to the end user in a format that is easily accessible and usable, the durability gap analysis would not only comprise a document related to durability issues in the earlier identified high priorities. It would also provide a road map for further data collection, assessment, and research to be conducted to fill in the gaps identified in the study.
WP 2. Experimental and analytical research
2a. Experimental investigation and development of testing procedures
The experimental variables that need to be investigated at the material level are: accelerated conditioning protocols; type of fibrous reinforcement; type of mortar/matrices; type of masonry substrates. The experimental tests will include an extensive test plan of mechanical tests and material characterization measurements.
The tests on materials will be performed by considering their susceptibility in respect to harsh environments of the single constituent materials: fibers and mortar for FRCM/TRM systems; fibers and resins for CRM systems. The following tests are previewed:
- Durability tests on the textiles (fabrics) (used in FRCM/TRM systems)
Target: Chemical/physical vulnerability of the textiles (fabrics) - Properties retention of aged textiles (fabrics)
- Durability tests on FRP reinforcement (used in CRM systems)
Target: Chemical/physical vulnerability of the pre-cured fibrous reinforcement - Properties retention of aged pre-cured grids
- Durability tests on mortar matrices
Target: Properties retention of aged inorganic mortars
- Durability tests on polymeric matrix (used in CRM systems)
Target: Chemical/physical vulnerability of the resin matrix
- Durability tests on FRCM/TRM systems
Target: Properties retention of aged FRCM/TRM Systems
- Durability bond tests
Target: Bond properties of aged strengthening systems
Conditioning of fabrics will include immersion in alkaline solutions, by studying the effects of different chemical agents (Ca+, Na+, K+) and different pH levels. This because the aggressive aqueous solution, used for accelerated testing, should be able to simulate lime-based and cement-based mortars. The solution recommended by ASTM D7705 will be used in addition to two other alkaline solutions, taken from scientific literature. The fibers in forms of strands/grids or FRP strips (cut form a pre-cured FRP grid) will be immersed at different temperatures (23°C, 40°C and 70°C) and different periods (1000 hrs, 20000 hrs, 3000 hrs). Similarly, conditioning procedures will be planned to investigate the effects of extreme moisture and high service temperature, as well as freeze and thaw cycles. The same conditioning will be applied to those polymeric resins used as composite matrix in FRP pre-cured reinforcements. In addition, the polymeric resins will be subjected to absorption tests ASTM D570 after different exposure times. The possible changes in glass transition temperature would be measured through Differential Scanning Calorimetry (DSC) according to ASTM D3418.
The changes in mechanical properties will be measured by performing tensile tests before and after conditioning on: fabrics, FRP specimens, polymeric resins (ASTM D638).
Tests to be performed on mortar matrices include: flexural and compression test (EN 998-2) and DSC, before and after the same conditionings reported before.
The durability tests on FRCM/TRM and CRM specimens will include tensile tests before and after conditioning.
Single lap shear tests, as defined within the preceding TC (TC-250), will also be performed before and after conditioning. In addition, single-yarn pull-out test from the mortar matrix will be planned, considering the relevant effect of the interface behavior matrix/fabric, on the whole mechanical performance.
2b. Analytical study
The analytical study will be addressed to formulate relationships between the conditioning effects of tests accelerated by the temperature and ageing protocols performed at 23°C. This would be very useful in order to calibrate ageing factors for accelerated tests to be performed in laboratory, in place of long-term tests to be performed in the field.
Moreover an analytical calibration of the possible damage mechanisms that could bring to a decay of the macro mechanical properties of the FRCM/TRM/CRM materials and their bond to masonry substrate is a main target of this research phase.
WP 3. Proposal of recommendations
The results of the experimental and analytical study will be collected and analyzed since the main purpose of the TC is to provide usable and friendly information to be suggested for the implementation of future guidelines.
WP 4. Dissemination of knowledge
Dissemination of results will be performed by seminars to be organized in educational and professional institutions. Discussion workshops will be planned to be open to users, industrial and professional people and service providers. Training courses will also be provided to practicing engineers, PhD students and young researchers.
The proposed TC will contribute to clarify scientific aspects concerning the strengthening of existing masonry structures with innovative materials, particularly focusing on those aspects related to durability and structural reliability in the long-term.
Other active TCs correlated to the subject are:
- 269-IAM : Damage Assessment in Consideration of Repair/ Retrofit-Recovery in Concrete and Masonry Structures by Means of Innovative NDT
- 250-CSM : Composites for sustainable strengthening of masonry
- 271-ASC : Accelerated laboratory test for the assessment of the durability of materials with respect to salt crystallization
- 277-LHS : Specifications for testing and evaluation of lime-based repair materials for historic Structures
- 275-HDB : Hygrothermal behaviour and Durability of Bio-aggregate based building materials
- 247-DTA : Durability testing of alkali-activated materials
- FTC : Durability and Service Life of Concrete under the Influence of Freeze-Thaw Cycles combined with Chloride Penetration
Moreover, the TC integrates the standardization and recommendation groups which have been or are still active in the field of the application of composite materials on structures both at national and international level.
The following results will be pursued:
- State-of-the-Art and gap analysis on the durability of inorganic matrix fibre reinforced materials used for strengthening of masonry.
- Experimental data on aged FRCM/TRM/CRM materials and strengthening systems based on the use of inorganic matrices.
- Harmonized experimental procedures and modelling methods to characterize material properties and structural behaviour in the long-term.
- Proposal for implemented design guidelines of FRCM/TRM/CRM materials with particular focus on durability aspects and long-term structural reliability.
- Workshops/symposia to integrate contribution from users, industrial and professional people, and service providers
- Seminars and training courses.
Group of users
Testing laboratories, academics, industrial and professional people, contractors, managers of Cultural Heritage, service providers and general users of masonry structures and Cultural Heritage buildings.
Specific use of the results
The results may be used primarily by technical institutions involved in the editing of design guidelines, such as the National Research Council (Italian CNR) and American Concrete Institute. Moreover all the technical Institutions of Civil Engineers and Architectural Heritage Committees all over the world may find a strong scientific decisional support by consulting the results of the present TC. Manufacturing firms and service providers involved in the structural repair/retrofitting of existing masonry structures are also strongly considered as beneficiary stakeholders of the proposed research.