A new design concept for a hybrid FRP-High strength concrete beam for infrastructure applications

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1 Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland A new design concept for a hybrid FRP-High strength concrete beam for infrastructure applications A. Khennane UNSW at the Australian Defense Force Academy, Canberra, Australia ABSTRACT: A new design concept for a hybrid FRP-HPC (Fibre Reinforced Polymer High Performance Concrete) is presented. To reduce the initial cost of the beam, an off the shelf pultruded FRP profile is combined with high performance concrete in an innovative approach. The HPC concrete, known as a brittle material, is confined with an FRP laminate to increase not only its strength but most importantly its ductility. Before proceeding with manufacturing, the behaviour of the beam under four point bending is simulated using a parametric nonlinear finite element analysis. The obtained results are very encouraging. It was found that the confinement of the concrete not only increased the stiffness of the beam, its load carrying ability, but also its energy dissipating ability, which is extremely important for civil engineering applications. 1 INTRODUCTION The use of composite materials in the field of highway structures is predicated on performance attributes, better enviro-mechanical durability, as well as ease of transportation and construction. Despite these advantages over conventional materials, composites are making limited progress in this field of the construction industry. The reason is to be found in their high initial cost compared to concrete, timber and steel. Manufacture of composite materials by large volume automated processes such as pultrusion can help considerably in reducing this initial cost. According to Gabriele (1995), the current pultrusion technology has not only the potential to make composites price-competitive with traditional material like steel and concrete, but also to make composites less expensive to manufacture and assemble. However, pultruded profiles on their own, particularly those made from GFRP, suffer from a lack of stiffness, secondary failure modes such as web and flange buckling, as well as catastrophic failure without warnings (Banks & Rhodes 1993; Holmes & Just 1983). Even, when combined with a low cost material such as concrete to alleviate top flange compressive buckling, and a carbon laminate to increase the stiffness of the beam, the desired failure mode, CFRP failure-concrete crushing, synonymous of pseudo-ductile behaviour, is hardly achievable with off the shelf pultruded profiles (Deskovic & Triantafillou 1995). In addition, large pultruded profiles suitable for civil engineering applications are difficult to resource in the current market unless custom made, which, most of the time, is not possible, since the manufacturers generally require large commands to offset the cost of the die and of the set-up of the pultrusion process. As a result, most of the demonstration projects using the concept hybrid FRP-concrete beam have relied on in-house technology to build FRP boxes (Deskovic & Triantafillou 1995; Canning et al. 1999; van Erp et al. 2005; Springolo et al. 2006)

2 2 NEW DESIGN CONCEPT 2.1 Description The proposed new beam design is shown schematically on Figure 1. The section is composed of a GFRP box profile, an outer GFRP laminate with fibres oriented at ±45 o, a high performance concrete (HPC) block, and a CFRP laminate. Figure 1: Proposed design for a hybrid PFR-HPC beam To minimize the cost of the beam, a pultruded GFRP box profile, available off the shelf, is chosen. Thin walled box sections are very efficient for beams, and they have a high resistance to lateral torsional buckling. The biggest size available in this profile is mm, which somehow limits the span that can be tested. The use of HPC (High Performance Concrete) instead of normal concrete will result in an overall increase in the stiffness of the beam, in a reduction in its depth, and increased load carrying ability. In practice, this will translate into longer spans, increased beam spacing, and shallower sections. The other major advantage of using HPC is its improved permeability. With time, moisture will ingress through the FRP casing and eventually reach the concrete. With a low permeability coefficient, the moisture profile in the concrete will be less significant than what would result with normal concrete. As a consequence, there will be a few amounts of hydroxyl ions to react with the glass fibres in the GFRP, thus resulting in an improved durability of the beam overall. Figure 2: Stress-strain behaviour of confined concrete The addition of the outer laminate serves three purposes. Its principal role is to confine the concrete as to achieve a ductile behaviour. Indeed, a major problem with HPC is its relatively - 2 -

3 low ultimate strain in compression. But, it is also well known that confined concrete behaves in a ductile manner as shown on Figure 2 (Spoelstra, 1999; Razvi et al. 1999). Confining the concrete will also ensure a composite behaviour between the concrete and the pultruded profile, thus eliminating the risk of debonding failure. The lack of composite action between the concrete and the laminates was reported as a serious shortcoming of these sections (Canning et al. 1999). Finally, with fibres oriented at ±45 o, its third role is to improve the shear strength of the pultruded profile. In addition to the use of high strength concrete, the new design differs substantially from previous approaches, schematically represented on Figure 2-a (Deskovic & Triantafillou 1995), Figure 2-b (Canning et al. 1999), and Figure 2-c (van Erp et al. 2005). Indeed, all these previous designs use in-house technology to built the GFRP box, which has the potential of increasing the initial cost of the beam. Additionally, the concrete is not fully confined as to take advantage of its increased ductility and strength under confining stresses. Figure 3: Existing designs 2.2 Section design The presence of the confining laminate introduces Poisson s effect in the concrete, which cannot be captured with a section strain compatibility analysis as used by Deskovic & Triantafillou (1995) to design their beam. Instead, a parametric three dimensional nonlinear finite element analysis will be used to optimize the section in order to achieve the desired failure sequence CFRP laminate failure concrete crushing. As a result of the restrictions imposed by the size of the pultruded profile, a beam spanning 1.6 m is chosen for the analysis. The loading set-up under four point bending is shown on Figure 4. Figure 4: Beam dimensions and loading set-up - 3 -

4 2.3 Finite element modelling Due to symmetry, only a quarter of the beam model is analysed. Using ABAQUS (Simulia 2006), the pultruded profile, concrete bloc, CFRP laminate, and the outer laminate are meshed as independent parts and then assembled using multi-point-constraints to form the whole model as shown on figure 5. Figure 5: Finite element model The outer GFRP and CFRP laminates are meshed using conventional shell elements, while the relatively thick pultruded profile is meshed with continuum shell elements. The concrete is meshed with 8-noded brick elements. The parametric input capability of Abaqus is used to create the input file. The thicknesses of the concrete part, C, and the CFRP laminate, t C, are used as parameters. To reduce the number of parameters, the thickness of the GFRP laminate, whose effect on the bahviour of the section is considered somehow less important that those of the concrete or CFRP, is considered constant and equal to 2 mm; that is: 2 plies each 1 mm thick placed at ±45 o. The two parameters are then evaluated according to their definitions and substituted as input quantities before the analysis is run. In addition, the availability of a scripting interface within Abaqus makes it easy to automate all of these tasks, as well as submitting the jobs, and accessing the results. This allows a greater flexibility in building and manipulating of the models. 2.4 Material models The high performance concrete is modelled using the CONCRETE DAMAGED PLASTICITY option in Abaqus with a dilatation angle of 15 o. The elastic modulus is taken equal to 45 GPa, which is typical of high strength concrete, and the Poisson s ratio is taken as The inelastic behaviour in compression is modelled using the option CONCRETE COMPRESSION HARD- ENING with an elastic limit of 65. MPa, and an ultimate stress of 100 MPa, corresponding to an ultimate strain of The tensile behaviour outside the elastic range is defined using the CONCRETE TENSION STIFFENING, and the CONCRETE TENSION DAMAGE, type DIS- PLACEMENT options. The data for these options is derived from Wittmann (2002). This concrete model employs non-associated flow potential using the Drucker Prager hyperbolic function for the flow potential. Since this is a preliminary design to show that the new concept is feasible, the elastic properties of the pultruded profile, CFRP and GFRP laminates are not determined from tests, but typical values, as reported by manufactures and in the literature, are used in this study, and shown - 4 -

5 on Table 1. The linear elastic behaviour is defined using the option ELASTIC type LAMINA. The inelastic behaviours are defined using the damage model in Abaqus primarily intended for fibre reinforced materials. The descriptions of onset of damage, its evolution, and satbilization are defined using the options DAMAGE INITITIATION, DAMAGE EVOLUTION, and DAMAGE STABILIZATION. Table 1: FRP elastic properties E L (MPa) E T (MPa) υ LT (MPa) G LT (MPa) Pultruded box CFRP GFRP RESULTS The load versus mid-span deflection for different values of concrete and CFRP thicknesses are shown on Figure 6 and 7 respectively for C = 48 mm and C = 55 mm Figure 6: Load-displacement curves for different CFRP laminate thicknesses for C = 48 mm Figure 7: Load-displacement curves for different CFRP laminate thicknesses for C = 55 mm - 5 -

6 It can be seen that the desired failure sequence, CFRP failure concrete crushing, synonymous of pseudo-ductile behaviour is achieved for all the CFRP thicknesses considered. Once the laminate fails, the increased compression capacity of the confined concrete ensure that a pseudo-ductile behaviour is obtained. The combined capacity of the confined concrete and that of the pultruded profile enables the beam to carry significant additional load after the CFRP laminate fails. Most importantly, these simulations show that the increased load carrying ability of the confined concrete ensures that thick CFRP laminates can be used without jeopardising the pseudo-ductile behaviour of the beam, thus increasing its stiffness. Increasing the concrete thickness results in a slight increase in both the stiffness of the beam, and the failure load of the CFRP laminate 4 CONCLUSION A parametric finite element analysis of an innovative hybrid composite beam design has demonstrated that high strength concrete, known for its brittleness, can be combined with a pultruded FRP composites in a very efficient way to produce a beam with improved stiffness, ductility and strength suitable for infrastructure application. The innovative aspects of the design are: the confinement of the high strength concrete increases not only its ductility but also its strength; the possibility of increasing the thickness of the CFRP laminates to improve the stiffness and the load carrying ability of the beam without jeopardising its ductility; and, finally, the use of an of the shelf pultruded profile, which not only offers dimensional stability but also cost effectiveness when compared to in-house manufactured PFR box sections. Encouraged by theses results, an experimental program is underway. The results, discussions, and comparisons will be reported in subsequent publications. 5 PREFERENCES Banks W.M. and Rhodes J The instability of composite channel sections. Composite Structures 2. Proc. 2 nd Int. Conf. on Composite Struct. Can. Soc. for Civ. Engrg. Montreal, Canning, L, Hollaway, L. and Thorne A.M Manufacture, testing and numerical analysis of an innovative polymer composite/concrete structural unit. Proc. Instn. Civ. Engrs. Structs.&Bldgs, 134, Deskovic N and Trinatafillou, T.C Innovative design of FRP combined with concrete: Short term behaviour. ASCE Jou. Struct. Engrg. Vol. 121(7), Gabriele M.C. Pultrusion s promise Plastics Technology, Vol 41(3), p. 36 Holmes M. and Just D.J GRP in structural engineering. Applied Science Publishers Ltd, London, England Simulia, Abaqus Version 6.6, Razvi S. and Saatcioglu M Confinement model for high strength concrete. Journal of Structural Engineering, Vol 125(3), Spoelstra M.R and Monti G FRP-Confined concrete. ASCE J. Comp. Constr. Vol 3(3), Springolo, M., Van Erp G, and Khennane, A Design and analysis of a composite beam for infrastructure applications - Part I: Preliminary investigation in bending. Int. J. Materials and Product Technology, Vol. 25, No. 4, Van Erp, G., Cattell C. and Ayers S The Australian approach to composites in civil engineering. Reinforced Plastics Volume 49(6), Wittmann, F.H Crack formation and fracture energy of normal and high strength concrete. Sadhana, Vol. 27(4), Printed in India - 6 -