234-DUC : Design procedures for the use of composites in strengthening of reinforced concrete structures
Technical Committee 234-DUC
Deputy Chair: Dr. José Manuel SENA-CRUZ
Activity ending in: 2015
Composite materials, as FRP (Fiber-Reinforced Polymer), are currently used to repair and strengthen existing reinforced concrete (RC) and prestressed reinforced concrete (PRC) structures both with Externally Bonded (EBR) and, with less diffusion, Near Surface Mounted (NSM) techniques. The structural behaviour of FRP-strengthened RC elements and structures has been widely studied over the last decades and some studies have resulted in the first design guidelines for strengthened concrete. American ACI 440.2R-08 (2008), European fib T.G. 9.3 (2001), Italian Recommendations CNR DT 200 (2004), German, British, Japanese and Canadian documents, are examples of such guidelines (currently under revision due to the continuously increasing knowledge on the topic) but their predictions are sometimes contrasting and disagreeing with experimental results related to particular applications. In this context, the research on FRP strengthened RC elements (both EBR and NSM) is more advanced with respect to masonry, for which the TC MSC is already active, but there are a number of critical issues, typical of RC structures, which are not sufficiently clear. These aspects, synthetically described below, will be treated in the proposed TC.
Despite the number of experimental, analytical and numerical works available in literature aimed at investigating the mechanical performances of RC strengthened elements and structures, design rules for the intervention and procedures aimed at checking the efficiency of the strengthening technique are not well defined yet. In particular the efficiency of the intervention in relation to the existing mechanical and geometric characteristics of the RC structural element, particularly the existing steel reinforcement, does not seem deeply studied.
Particular problems/critical issues related to:
- bond between concrete and FRP,
- shear strengthening of flexural elements,
- confinement of columns,
are topics currently under investigation by the scientific community and code predictions related to these problems can lead to unconservative results and have to be improved.
Special problems, for which, at the moment, the knowledge is not sufficiently advanced to propose reliable recommendations e.g.:
- FRP prestressing,
- durability/environmental effects,
- long term behaviour,
- cyclic/seismic behaviour,
could be also treated by the TC which could be organized in some task groups related to the above particular and special problems.
The preliminary work of the TC will be the analysis of the current knowledge on structural behaviour of RC elements and structures strengthened with composite materials (experimental, analytical and numerical approaches for EBR and NSM), particularly in relation to the above topics, and the comparison of the predictions of the current available codes/recommendations/guidelines with selected experimental results. This basic work will contribute to show or confirm possible critical issues (discrepancies, lacunae, relevant parameters, test procedures, etc.) related to current code predictions or to evaluate their reliability, in order to develop more uniform methods and basic rules for design and control of FRP strengthened RC structures.
The proposed TC will therefore clarify the general problems/critical issues on the basis of the actual experiences, detect discrepancies in existing codes, lacunae in knowledge and, concerning these identified subjects, provide proposals for improvements. The TC will also point out possible developments and in-depth study of specific problems (e.g., serviceability limit states such as deflection and crack control, particular strengthening configurations, use of new binders, fatigue behaviour, performance of particular components, bridges, use of NDT-Non Destructive Test, fire resistance, etc.) which may promote activities of further TCs focused on those aspects.
The final aim is to develop a technical report on the use of composite materials for strengthening and repair of existing RC structures, including suitable and practical indications (design tools, limitation parameters/upper limits related to the efficiency, experimental procedures, etc.) and proposals for code improvements mainly focused on the critical issues, typical of RC structures, described above.
The desirable collaboration among institutions (Universities, Public Bodies, Research Agencies), service providers and users, during the subsequent dissemination and exploitation phase, will contribute to promote and consolidate a more qualified and conscious approach towards rehabilitation and strengthening existing RC structures with composites and their possible monitoring.
Terms of reference
The TC is proposed to work for about 4 years, planning regular meetings (about two-three per year), and additional opportunities to exchange knowledge (seminars, symposia), also in possible conjunction with the activities of the RILEM Weeks.
The TC will consist in a group of experts, well representing most of the main international institutions 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 reinforced concrete and composites, standardization groups, service providers, national and international groups who have contributed to the development of the current codes/recommendations/guidelines. In particular, since fib, ACI and other committees have been dealing with topics close to the proposed one (but they are working in separate groups), selected members of these committees will be engaged for this TC.
The work will imply bibliographical research on experimental, analytical and numerical studies, on standardized methods and code approaches for the definition of the State-of-the-art. Comparisons between predictions of various existing codes and selected experimental results will be developed in order to evaluate their reliability, develop more uniform methods and rules for design/control or improve actual predictions.
On these basis, possible critical issues will be individuated and new/improved models shared between the participants of the TC and validated on available experimental results will be proposed in order to solve conflicting results provided by some currents codes/models.
Detailed working programme
1st Year: State-of-the-art and evaluation of codes reliability
Significant experimental results regarding mechanical behaviour of strengthened RC structures (mainly related to the problems currently under investigation by the scientific community e.g. bond between concrete and FRP, shear strengthening of RC beams, confinement of RC columns, FRP prestressing, durability/environmental effects, long term behaviour, cyclic/seismic behaviour) will be collected in a database. Comparison with code predictions will be developed in order to contribute and show possible critical issues and lacunae related to these current codes or evaluate their reliability also in relation to the geometric and mechanical characteristics of the existing RC element.
2nd Year: Calibration of analytical and numerical methods and procedures
According to current experimental results regarding mechanical behaviour of strengthened RC structures, analytical and numerical methods and procedures can be calibrated in order to identify significant parameters, improve some current models and eventually propose new models related to those particular issues not completely solved yet.
Specific round-robin tests (experimental and/or numerical) will be planned, if necessary, to clarify these particular aspects typical of RC structures also in relation to the existing mechanical and geometric characteristics of the structural element.
3rd Year: Proposal of recommendations
A draft of proposal of a technical report on the use of composite materials for the strengthening and rehabilitation of existing RC constructions, including shared proposals for code improvements, will be developed on the basis of the previous phases. The document will mainly deal with those particular aspects investigated in the above phases for which a number of uncertainties and code contrasting predictions actually exist. Proposal of possible developments, in-depth study of specific problems (e.g., serviceability limit states such as deflection and crack control, particular strengthening configurations, use of new binders, fatigue behaviour, performance of particular components, bridges, use of NDT – Non Destructive Test, fire resistance, etc.) and new topics of research will be also included.
4th Year: Dissemination of knowledge
The last year will be devoted to the finalization of the previous phases and, in particular, of the document, by the possible discussion and calibration during workshops planned to be open to users, industrial and professional people and service providers. Dissemination of results will be performed by seminars to be organized in educational and professional institutions.
The proposed TC will contribute to clarify scientific aspects/critical issues and improve code predictions concerning the strengthening of existing RC structures with innovative materials focusing the activity on the typical aspects related to RC structures. Other active TCs correlated to the subject are: 223-MSC (Masonry strengthening with composite materials), 207-INR (Interpretation of NDT results and assessment of RC structures). TC 223-MSC is included in the RILEM cluster E (Bitumen, Masonry and Timber) whereas TC 207-INR is included in cluster C (Design and Service Life).
The proposed TC can also continue/update/integrate the work of the TC FRP (FRP-concrete bond in structural strengthening and rehabilitation) chaired by prof. Banthia which completed its task recently.
Moreover, the TC integrates and harmonizes the standardization and recommendation groups which have been or are still active in the field of the application of FRP and composite materials on RC structures, both at national (e.g. CNR – National Council of Researches group in Italy) and international level (e.g. fib - édération internationale du béton - and ACI - American Concrete Institute - groups).
The following results will be expected:
- State-of-the-art on experimental procedures, analytical and numerical models for the characterization of the mechanical behaviour of RC structures and components strengthened with EBR and NSM FRP materials including comparisons with actual codes predictions particularly in relation to critical issues, typical of RC structures, actually in discussion and not solved by the scientific community (discrepancies in existing codes, lacunae in knowledge, relevant parameters, test procedures, etc.).
- RILEM technical report on the use of composite materials for strengthening and rehabilitation of existing RC constructions mainly dealing with those particular aspects for which uncertainties and code contrasting predictions actually exist. The document will include, if necessary, proposals of harmonized experimental procedures and modelling methods to characterize material properties/structural behaviour, proposals for improvements of code predictions, possible future developments, in-depth study of specific problems and new topics for research.
- 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, authorities managing roads and railways, service providers.
Specific use of the results
Current recommendations regarding experimental testing, analytical and numerical modelling on existing RC structures strengthened with innovative materials need specific revision and harmonization, according to the significant scientific developments now available at international level and critical issues actually in discussion. In this context, rehabilitation, strengthening, retrofitting of strategic RC structures and infrastructures and upgrading structures including fatigue, particularly road and railway bridges, is a key issue in many countries due to increasing intense traffic loads, especially when seismic risk or other severe hazardous causes of damage are present. Efficient and aware intervention solutions on existing RC constructions maintain continuous functioning and reduced maintenance costs.