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Those involved in research and design activities in the field of strengthening with fibre-reinforced polymer composites (FRP) consider that Italy is gaining a worldwide reputation, both for the value of its contribution to improving the knowledge in this field and for its significant building heritage, [1-6] which includes buildings of historical and architectural relevance and more recent structures in masonry, reinforced concrete, prestressed concrete, or steel. Most of the latter structures need structural rehabilitation.
(Published on September 2007 – JEC Magazine #35)
L. ASCIONE & A. GIORDANO DEPARTMENT OF CIVIL ENGINEERING, UNIVERSITY OF SALERNO, ITALY
The Italian National Research Council (CNR) has been pushing for a series of guidelines on the structural use of innovative materials, and in particular for the design and construction of externally bonded FRP systems for reinforcing existing structures. Italy’s Advisory Committee on Technical Recommendations for Construction approved the first of these technical documents, No. 200/2004, as a draft version on July 13, 2004. Following incorporation of the modifications derived from the public hearing, the document was approved as a final version on April 26, 2005. Four additional documents were eventually licensed by the CNR.
Materials and basic concepts of FRP strengthening
Document CNR-DT 200/2004, or “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures – Materials, RC and PC Structures, Masonry Structures”, provides guidelines within the constraints of Italian regulations for the design and construction of externally bonded FRP systems for reinforcing existing structures [7-9]. It is important to highlight that a guideline is not a binding regulation, but merely an aid for practitioners interested in the use of composites.
The document deals with the materials and basic concepts used in FRP reinforcement, including a summary of the many advantages and some disadvantages of FRP. Specific guidelines are also provided for the strengthening of reinforced and prestressed concrete structures and for masonry structures for constructions subject to earthquakes, in accordance with the most recent national and international design codes (Figures 1 & 2). An appendix presenting notions on the mechanical characterisation of composite materials is also included.
FRPs in the rehabilitation of timber structures
Document CNR-DT 201/2005, or “Guidelines for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures – Preliminary Study. Timber Structures”, aims to inform the technicalprofessional community on the use of FRPs in rehabilitating timber structures and to identify safe, suitable reinforcement systems. This study is only a first step towards specific design guidelines, which should be forthcoming in the near future, once a more complete and universally accepted understanding of the subject is reached from ongoing theoretical and experimental studies world-wide.
The most widespread application of FRPs is the reinforcement (primarily in bending) of timber elements such as single beams, floor beams, or individual components of more complex structural elements such as trusses and frames (Figure 3). Reinforcement can consist of laminae or sheets of various materials, applied according to those criteria offering the best advantages in terms of strength, deformability and ductility. Another use is the reinforcement of structures subjected to in-plane forces. A typical application is timber or mixed floors. Such structures, quite widespread throughout many European countries, have good characteristics in terms of low weight, strength, acoustic insulation and compartmentalisation, but offer limited stiffness and only moderate effectiveness in the transmission of horizontal forces within their plane.
Another application is the strengthening of joints between timber elements. Composites with different microstructures can be used to reduce the failure risk induced by tensile stresses perpendicular to the fibre, and to increment the hysteretic-dissipation capabilities of the connections under cyclic loads.
FRPs in the rehabilitation of metallic structures
Like the previous document, document CNR-DT 202/2005, or “Guidelines for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures – Preliminary Study. Metallic structures”, constitutes a first step towards specific design guidelines. So far, only the English guidelines on the use of FRPs to reinforce metallic structures have been drawn up . The results of this study will be useful in identifying any unsolved problems, so that the scientific community can concentrate on them over the next few years.
Reinforcing with FRPs gives better results in a number of situations. The use of FRPs resolves several of the problems associated with the use of traditional techniques and materials. The strengthto- weight ratio of FRPs is much higher than steel. FRPs are also much more resistant to corrosion, if not completely unaffected, and extremely easy to handle. Their use in reinforcing metallic structures is not as developed as their concrete/masonry counterparts. Only some of the possibilities have been studied, including reinforcing riveted elements to counteract fatigue cracking, and reinforcing tensile elements or tensile sections of prestressed structural elements to reduce the strains in service conditions due to increased loads or to corrosion.
The use of FRPs also yields better results in reinforcing older metallic constructions, because the mechanical properties of FRPs integrate well with those of cast iron (Figure 4). The high strain resistance of FRPs in the fibre direction and the use of pre-stressing techniques compensate for the modest strain resistance of cast iron. The study deals with:
The document also includes two appendices, including several international examples of FRP structural strengthening projects, and bibliographic references.
Concrete structures reinforced with FRP bars
The last document is CNR-DT 203/2006 or “Guide for the Design and Construction of Concrete Structures Reinforced with Fibre-Reinforced Polymer Bars”. The use of fibrereinforced polymer composites as a substitute for steel in concrete structural elements is a widespread practice in many countries [11-13]. Because FRP materials are not vulnerable to corrosion, they are particularly suitable in a number of situations.
From a theoretical perspective, there are no conceptual differences in relation to the classical theory of steel-reinforced concrete elements. What does need to be taken into account is the different mechanical behaviour of FRP materials, whose constitutive law is fundamentally linear elastic up to failure.
There are several reasons that make FRP bars preferable to conventional steel bars. As mentioned above, these innovative bars are not susceptible to corrosion, which makes them suitable for marine structures and structures exposed to harsh environments. Furthermore, glass FRP composites (GFRP) are nonconductive and therefore can be used effectively when stray currents are an issue, as in the case of rail transportation structures (railway or subway lines), either outdoor or in tunnels. Even bridge decks used as railway overpasses can be affected by the same phenomenon (Figure 5).
This document focuses on structures reinforced with non-prestressed FRP bars. However, the possibility that elements of such structures have traditional steel reinforcement is also accounted for. Based on the experience and knowledge acquired, this Guide is restricted to the use of bars and grids made of fibrereinforced thermosetting resins with a fibre volume fraction greater than 50%. Considering the great variety of bars available on the market, including in terms of cross-section geometry, it is worth referring to an equivalent (or nominal) circular cross section, with both the diameter and area being defined properly. A procedure for evaluating the equivalent diameter and area is described in an appendix.
FRP bars used as reinforcement in concrete structures shall undergo inspections to ensure a satisfactory level of both mechanical and physical properties.
The following is required:
Product sampling and testing of the product samples shall be carried out regularly in the manufacturing plant. Both mechanical and physical qualification tests shall be carried out by a certified laboratory provided with the necessary equipment and experience in the characterization of composite materials. If the factory laboratory is either not available or not certified, the qualification tests shall be carried out by an external laboratory certified in the characterisation of composite materials. The mechanical characteristics reported in the technical data sheets of the products shall be obtained through a statistical analysis, including the characteristic values, percentile, sample mean, sample standard deviations, confidence intervals, and the number of samples tested. Suitable partial factors of the semi-probabilistic method should be employed for the characteristic values evaluated.
Test methods for characterising the FRPbar mechanical properties are reported in Appendix C.
Fibre-reinforced concrete structures
The document entitled “Guide for the Design and Construction of Fibre- Reinforced Concrete Structures” deals with the structural use of fibrereinforced concrete (FRC). FRC is a concrete material incorporating short steel fibres and polymeric, inorganic (carbon or glass) or natural fibres. Adding fibres to concrete makes it significantly more resistant to traction after cracking.
FRC applications of relevant interest include: load-bearing front panels, paving slabs, beams, structural joints and nodes, jacketing elements, structures subject to impacts and/or fatigue, railway sleepers, high tension poles, etc. (Figure 6).
The document deals with the following issues:
The document also has five appendices devoted to several theoretical aspects that are briefly discussed in the instructions and, due to their innovative nature, merit further mention with an informative aim.
Through the technical documents described in this article, the Italian National Research Council has contributed to better knowledge of composite materials within the Italian professional community, to the definition of design principles and application rules accepted by the Italian scientific community and to the standardisation of test procedures for the mechanical characterisation of FRP materials.