THE DESIGN OF EXTERNALLY BONDED REINFORCEMENT (EBR) FOR REINFORCED CONCRETE STRUCTURES BY MEANS OF FIBRE REINFORCED POLYMERS (FRP) Introduction Dott. Ing. Giovanni Cerretini Studio Technica (studio@technica.net) Via del Caravaggio, 43 50143 Firenze, Italia Today there are a lot of applications concerning fibre reinforced polymer (FRP) in civil engineering. The aim of this report is to study the externally bonded FRP reinforcement for reinforced concrete structures applied to the flexural strengthening. FRP reinforcements are joined to the structure by means of adhesives and they work therefore with a resistant mechanism based on the adhesion between the materials. Different fabrics and fibres are used to obtain FRP with the following properties: high resistance of the materials; low specific weight that not modify the total weight of the structure; small thickness that not modify the volumes; the possibility to design a reinforcing material with specific mechanical property, optimised for its particular stress distribution. This paper presents a software that has been developed on the basis of the FIB Bulletin 14 [1] Specifications (based on Eurocode 2 [2]). This software is intended as a tool for the design of the reinforced section and a tool to evaluate which kind of FRP system is more convenient. FRP reinforcement systems It's possible to choose between several systems of reinforcement that differ for the type of fibre, for the resins and also for the application techniques. The fibres used in civil engineering are carbon fibres (CFRP), glass fibres (GFRP) and aramid fibres (AFRP). The systems available are the following: - Wet Lay-Up Systems composed by dry multidirectional or unidirectional fibre sheets, cured in-situ; - Pre-cured Systems composed by cured strips, shells, jackets or angles, installed through the use of adhesives; - Prepreg Systems composed by unidirectional or multidirectional fibres sheets preimpregnated with a "Saturating Resin" but not still cured. In the future will be possible to use fabrics designed for specific applications. The basic FRP strengthening technique, which is most widely applied, involves the manual application of Wet Lay-Up Systems (see Fig. 1). The application involves the following steps:
1) preparation of concrete substrate with "Putty-Filler" resins; 2) application of "Primer" resins on concrete substrate; 3) application of "Saturating" resins; 4) application of fabric sheet; 5) application of another layer of "Saturating" resins; 6) application of "Protective Coatings" resins. Fig. 1 - Installation of FRP reinforcement on RC floor Flexural Strengthening Reinforced concrete elements, such as beams and columns, may be strengthened in flexure through the use of FRP composites bonded to their tension zones, with the direction of fibres parallel to that of high tensile stresses. A lot of studies shows that the greater contribution brought from FRP reinforcement is given at the ultimate limit state. It is possible to obtain a great increment in the bending moment capacity (see Fig. 2). 2
Fig. 2 - Increment in the bending moment capacity with n layers of FRP reinforcement in a section 25x50 cm for various mechanical reinforcement ratios. Betontex GV 330 U-HT, is an High Tenacity CFRP reinforcement produced by Betontex (Italy). The failure modes of a reinforced concrete element may be divided into two classes: those where the full composite action of concrete and FRP is maintained until the concrete reaches crushing in compression or the FRP fails in tension and those where composite action is lost due to peeling-off of the FRP. Bond failure in the case of EBR implies the complete loss of composite action between the concrete and the FRP. When localised debonding propagates and composite action is lost, this failure is called peeling-off. Bond failure may occur at different interfaces between the concrete and the FRP reinforcement. Bending moment capacity at the ultimate limit state with full composite behaviour The following assumptions are made in calculating the flexural resistance of a section strengthened with an externally applied FRP system: - design calculations are based on the actual dimensions, internal reinforcing steel arrangement, and material properties of the existing member being strengthened; - the strains in the reinforcement and concrete are directly proportional to the distance from the neutral axis, that is, a plane section before loading remains plane after loading; - there is no relative slip between external FRP reinforcement and the concrete; - the shear deformation within the adhesives layer is neglected since the adhesives layer is very thin with slight variations in its thickness; - the tensile strength of concrete is neglected. When FRP reinforcement is being used to increase the flexural strength of a member, it is 3
important to verify that the member will be capable of resisting the shear forces associated with the increased flexural strength. If additional strength is required, FRP laminates oriented transversely to the section can be used to resist shear forces. Unless all loads on a member, including self-weight and any prestressing forces, are moved before installation of FRP reinforcement, the substrate to which the FRP is applied will be strained. These strains should be considered as initial strains and should be excluded from the strain in the FRP. The initial strain level on the bonded substrate can be determined from an elastic analysis of the existing member based on cracked section properties. To evaluate the bending moment capacity at the ultimate limit state, the failure mode and the position of the neutral axis must be known. In the software developed the FIB Bulletin Specifications are used, and those specifications has been extended to T cross sections. To solve the problem a "Trial and Error " procedure has been used (see Appendix 1). The software With the software presented in this paper it is possible to calculate the bending moment capacity at the ultimate limit state for rectangular and T cross sections. The software is platform independent and it is available on Internet. Everyone who has an internet connection can use it and the upgrades are ready for the use as soon as they are published on Internet. The input data are the geometry of the section, the mechanical properties of the materials, the concrete strain and the used model code. Data are inserted in a HTML form and the result are presented in a HTML page. Input Data Requested data are grouped in the following sections: Geometry of the section - In this section the user must specify the kind of cross section, the arrangement of the steel reinforcement, the section of one layer of FRP reinforcement and the number of layers. Concrete strain - In this section the user must specify the concrete strain, a positive value means to stretching and a negative value means shrinkage. Model code - In this section the user must specify the used model code. The model code is used to choose the material's model and to obtain all the values omitted by the user. γm coefficient - In this section the user must specify the value of γm coefficient to reduce the contribution of FRP to the bending moment capacity of the reinforced section. Material properties - In this section the user must specify the material properties and the safety material coefficient at the ultimate limit state. The software uses the values specified by the user or it uses the default values of the chosen model code. Results The software give the following results: a graphical view of the reinforced section and of the no-reinforced section where it's possible to view the number of FRP layers, the position of the neutral axis and the bending moment capacity (see Fig. 3); a summary of the geometry 4
data and material properties data; a table with the stress of the materials (see Fig. 4). Fig. 3 - Graphical view of the software results. Fig. 4 - Results table with stress and strain data. 5
Appendix 1 Bending moment capacity To evaluate the resistant moment at the ultimate limit state, it is necessary to know the failure mode, the position of the neutral axis and the material's stresses. The forces acting on the section are illustrated in the following figure (see Fig. 5). Fig. 5 - Stress and strain distribution on the reinforced section. To evaluate the bending moment capacity at the ultimate limit state, the stress-strain model and associated material safety factor given in this section can be assumed (see Fig. 6). For the concrete a parabolic-rectangular stress block can be assumed, as provided by Eurocode 2 [2]. For the steel reinforcement, a bilinear stress-strain relationship is considered, as provided by Eurocode 2 [2]. For the FRP reinforcement, an elastic linear response can be assumed. Fig. 6 - Design stress-strain curve of constituent materials at the ultimate limit state. It's important to place the attention on the fact that, for the calculation of the ultimate limit 6
state in bending, is not necessary to know the exact distribution of the compression on the concrete, but is sufficient to know its resultant (R) and the distance (c) between its point of application and the compressed edge. The software extend the FIB Bulletin 14 [1] Specifications to the T cross sections. From the equilibrium of forces and strain compatibility the depth of the neutral axis is obtained from the following: To evaluate X a "Trial and Error" procedure must be utilised. An X value must be chosen and used in the previous equation: if the result is zero the chosen value is the right one, otherwise a new value must be chosen and the procedure must be iterated until the result of the equation is zero. When the position of the neutral axis has been evaluated the resistant moment is obtained from the following: γm coefficient is introduced in ACI 440 [3] to reduce the contribution of FRP reinforcement to the bending moment capacity. References [1] Task Group 9.3, Externally bonded FRP reinforcement for RC structures, fib CEB-FIP Bulletin 14, Switzerland, 2001 [2] CEN, Eurocode 2 - Design of concrete structures ENV1992-1-1, Brussels - Belgium, 1991 [3] ACI Committee 440, ACI 440.2R-02 Guide for the Design and Construction of Externally Bonded FRP systems for Strengthening Concrete Structures, USA, 2002 7