PROGRESSIVE FAILURE OF FRP COMPOSITES FOR CONSTRUCTION

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1 PROGRESSIVE FAILURE OF FRP COMPOSITES FOR CONSTRUCTION Lawrence C. Bank Associate Provost for Research and Professor of Civil The City College of New York 160 Convent Avenue, New York, NY 10031, USA Abstract The purpose of this paper is to provide a review of and observations on progressive failure of Fiber-Reinforced-Polymer (FRP) composites of interest to civil and infrastructure construction applications. The primary reason for this is that although FRP composites have, over the last 25 years, successfully penetrated niche markets in civil engineering applications one of the most frequently heard concerns from designers is their discomfort with the ductility of these composites and the structures built or reinforced with them. If we wish to expand the market for FRP applications in construction we must as a community address this issue in greater depth. One approach is to use system-wide, structural, progressive failure behavior of the composite material itself to dissipate internal strain energy in-lieu of the elasto-plastic behavior seen in metallic materials. Specific applications of FRP composites in construction where progressive failure mechanisms have been considered are reviewed. These include FRP profiles, FRP reinforcing bars, externally bonded FRP or mechanically fastened FRP strengthening strips, and FRP column wraps. Keywords: Crashworthiness, Ductility, Energy Dissipation, Progressive Failure, Pultruded Profiles, Rebars, Strengthening, Wrapping. 1 Introduction Fiber Reinforced Polymer (FRP) composite materials typically used in civil engineering consist of brittle high strength fibers (e.g., glass, carbon) embedded in a thermosetting polymer matrix material (e.g., epoxy, polyester, vinylester). While some thermosetting polymer resins have a relative large strain to failure ( f ~5%) they are nevertheless also brittle materials like the fibers ( f < 1.5%). Neither components are ductile, that is, they do not deform plastically under tensile stress hence dissipating internal strain energy, like many metallic materials. The inability to dissipate strain energy is a significant impediment to structural applications of composite materials, especially in civil structures. Large structural deformations and significant load-carrying capacity prior to ultimate failure, typically seen in ductile-material structures, are critical in civil structures where sudden failure and especially the lack of warning of this sudden and generally catastrophic failure is unacceptable. Since the material constituents themselves are not ductile and do not deform plastically designers of FRP composite structures must look for alternative means to dissipate internal strain energy and to cause a structure to undergo large deformations while at the same time carrying the design loads. This can be achieved by judicious design of the FRP composite material and the composite structure to fail in a controlled progressive manner and hence dissipate the internal strain energy. In the aerospace and automotive industries the use of energy dissipation via the progressive, gradual and controlled failure of brittle fiber composites subjected to impact or dynamic loading has been exploited successfully in a number of commercial applications. Research in this field, also known as crashworthiness, has grown Page 1 of 10

2 considerably in recent years with the advent of high speed test methods and explicit finite element methods. In what follows, the experimental and numerical work on the quasi-static and dynamic progressive failure of composite material FRP profiles (typically tubes) subjected to axial compression is briefly reviewed. Thereafter, attempts to exploit stable, controlled progressive failure in FRP material products developed for use in civil structures are discussed. These include FRP reinforcing bars, externally bonded FRP or mechanically fastened FRP strengthening strips, and FRP column wraps. 2 FRP Profiles Subjected to Axial Compression The emphasis of this work over the past three decades has been to develop crashworthy composite material structures and components for automotive [1],[2] and rotorcraft structures [3]. Early work conducted in this area has been reviewed by Hull [4]. The work has focused on the ability of tubular thin-walled composite (circular or polygonal) tubes to absorb energy due to stable crushing when loaded axially. The objective of this work has been to develop a fundamental understanding of the physical phenomenon of, and the parameters that control, the crushing of composite tubes. The design objective has been to obtain a stable crushing behavior or a sustained crushing load (or stress) [1],[5] as shown in Fig. 1. Figure 1 Quasi static crushing of an FRP tube [1] Schematic of load-deflection curve of composite tube specimen [5] The global load-deflection behavior seen in Fig. 1 is due to progressive compressive failure on the local level. It can be termed pseudo-ductile since the global load-deflection behavior resembles the uniaxial stress-strain diagram of an elasto-plastic ductile material. Throughout the 1990s and 2000s experimental work continued much of it focused on understanding the morphology of the failure mechanisms and the effect of different hybrid fiber architectures and geometric parameters [6],[7]. Since the late 1990s, as simulation tools have improved, significant work has been conducted on the numerical simulation of tubular crushing using explicit finite element codes such as LS-DYNA [8],[9] as shown in Fig. 2. Page 2 of 10

3 Figure 2 Comparisons between numerical simulation and experimental results [8] A key feature of axial crushing of FFP tubes is that the tube has no elastic recovery following the axial splaying and fragmentation and hence all the energy dissipated is equal to the work done by the loading head as it crushes the tube (i.e., the area under the load displacement curve seen in Fig. 1). The energy dissipated is equal to the sustained crushing load multiplied by the crush distance. Although, applications of axially loaded tubular composites as energy dissipaters in the automotive, aircraft and to a limited extend in highway guardrail endterminals are now routine they have not been explored in civil structural applications to-date. 3 FRP Reinforcements for Concrete 3.1 FRP Reinforcing Bars and Grids for Beams Glass fiber reinforced polyester (and later vinylester) reinforcing bars for concrete ( rebars ) were first developed about 40 years ago and are currently available from a number of manufacturers [10]. Carbon, aramid and basalt fiber rebars have also been produced. These rebars are brittle and fail at ultimate load with little warning and no with ductility. Attempts have been made over the years to improve the ductility and energy absorption capability of FRP rebars by a using combinations of longitudinal fibers with different failure strains ( hybrid rebars ) and/or by using alternative manufacturing techniques (e.g., braiding) [11],[12],[13]. By hybridizing the fiber types a pseudo ductile stress-strain behavior in the bar and a pseudo ductile load-deflection response in a concrete beam reinforced with such bars has been obtained as shown in Fig. 3a. In a different method to develop progressive failure in the reinforcing bars themselves others [14],[15] have suggested 3-dimensional reinforcement cages constructed from pultruded FRP members to reinforce concrete beams,. The cage members fail progressively by local bearing failure at the junctions of the FRP cage as seen in Fig. 3b. Figure 3 Load-deflection responses in a concrete beams reinforced with hybrid FRP rebars [11] and FRP pultruded cage and progressive failure more [13] Page 3 of 10

4 The most commonly promoted method to develop some measure of ductility in FRP reinforced RC beams is to over-reinforce the beams such that they fail by concrete crushing in compression which is purported to be less brittle than the failure of the tensile FRP rebars. The load deflection characteristics of over-reinforced FRP reinforced concrete beams and under-reinforced steel reinforced concrete beams is not really similar, even though the deflection at failure in the FRP reinforced beams is reasonably large. In addition, since the unloading curves are not reported in most studies it is not possible to determine the energy absorbed by the FRP reinforced beams prior to ultimate failure. In a proposed new method to increase the ductility of over-reinforced beams [16] perforated SIFCON blocks have been inserted in the compression zone to create a compression yielding mechanism to develop stable crushing similar to that seen in axially crushed pultruded tubes. The response of these beams shows a plateau similar to that seen in under-reinforced steel RC beams as seen in Fig 4a. In related work [17] have used a combination of FRP rebars and Fiber Reinforced Concrete (FRC) [18] to develop a more ductile compression failure mode in over-reinforced beams as shown in Fig. 4b. Also, significant in this work is the attention to the unloading curves and the calculation of the ductility index based on dissipated and elastic energy recovered proposed in [19] using unloading data. Comparisons with the values of a proposed deformability index [20] are also reported. Figure 4 Compression yielding response in RFP bar reinforced beams [16] Comparison between plain and FRC reinforced beams with FRP rebars [17] 3.2 FRP Strengthening Systems for Reinforced Concrete Epoxy Bonded (EB) Systems The strengthening of reinforced concrete members with externally bonded (EB) FRP dry fabrics, dry fiber sheets and pre-cured pultruded FRP strips (or near surface mounted (NSM) bars ) is now well-accepted in structural engineering. The method is used to increase the flexural strength, the shear strength and to a limited extent the flexural stiffness of existing conventionally steel reinforced and pre-stressed concrete members [21],[10]. It is well know that the addition of FRP external reinforcement decreases the deformability (i.e., deflection and curvature at failure of the FRP strengthening system) of the strengthened member and that the failure is brittle. Nevertheless, since the original steel reinforced concrete member is typically designed to fail in a ductile manner by yielding of the internal steel the member can usually return to its pre-strengthened (and ductile) behavior following failure of the FRP strengthening system. Consequently, the use of FRP strengthened concrete is much less of a concern to structural engineers than the use of FRP in new structures. In addition, a common design philosophy is to design the strengthened beam to become an over-reinforced beam such that the ultimate failure of the strengthened beam will be due to concrete crushing and Page 4 of 10

5 not due to FRP rupture or debonding. However, as in the case of FRP rebar reinforced beams discussed above the ductility enhancement provided by this failure mode is not entirely convincing. The reason for the decrease in deformability and ductility in FRP strengthened members is the linear elastic and relatively low strain to failure of ( 1.5%) in the fiber-sheets and precured laminates currently available on the market. For design purposes the ultimate failure strain of these products needs to be reduced even further (to about 0.8 % for flexural and 0.4% for shear strengthening) to prevent premature debonding of the FRP strengthening system, which also occurs in a sudden and brittle fashion. In order to prevent plate (or strip) end and interior debonding failures, anchorage devices (typically bolted steel plates, FRP U- wraps, or fiber anchors) have been proposed and are frequently used in practice [22], even though design guidance for such anchorages is not codified as yet. The deformability of these anchored FRP strengthened members is generally improved by the use of such devices [22]. Anchorage devices can delay debonding failures. However, the ultimate failure is still sudden and may be due to FRP rupture, concrete compression failure or anchor debonding. As demonstrated in [23] and [24] the use of anchors can significantly increase the efficacy (and in some cases) the ductility of the strengthened beam as shown in Fig. 5a and 5b. The use of small U-shaped anchors distributed along the entire length of the beam has been shown to be very effective in increasing the efficacy of beams strengthened with multiple FRP strips [25]. Figure 5 Comparison of control and FRP anchored beams [23] [24] Hybrid fabrics and sheets have also been explored in order to achieve a gradual progressive failure in the FRP strengthening system which can lead to a ductile failure of the reinforced beam itself. This has been achieved by either using a triaxially braided hybrid carbon/glass fabric [26] or by using alternating plies of high modulus and high strength fiber sheets [27]. These techniques can provide some measure of a pseudo ductile response that is analogous to yielding seen in under-reinforced steel reinforced beams Mechanically-Fastened (MF) Systems An alternative method for attaching FRP precured plates to concrete members is known as the Mechanically-Fastened FRP (MF-FRP) method [28], [29],[30]. In this method the strip is attached with metallic fasteners (powder actuated pins (nails), concrete screws or concrete expansion anchors) and the load is transferred to the strip at the anchor points by bearing on Page 5 of 10

6 the specially designed FRP strip having high bearing strength. The method is particularly useful when the concrete substrate is poor or rapid installation is required. Strain compatibility between the strip and the concrete are intentionally not maintained. A review [30] of the experimental investigations conducted using the MF-FRP method indicates that equivalent strengthening to the epoxy bonded method can be obtained with the MF-FRP method and that the failure modes are extremely ductile due to a sustained bearing failure mode in the FRP strips at the fastener locations. Typical load-displacement curves for a single fastener and strip are shown in Fig 6a and load-deflection of MF-FRP strengthened beams are shown in Fig 6b. The ability of the strengthened beam to maintain a significant portioned of its strengthened capacity after the peak load can be seen. Numerical modelling of the behavior of MF-FRP strengthened beam shows very good agreement between experiment and theory [31],[32]. Figure 6 Single fastener bearing behavior response of MF-FRP strengthened beams [29]. 3.3 FRP Wraps for Ductility Enhancement FRP wrapping (or jacketing) of reinforced concrete columns is primarily performed to increase the lateral deformation capacity of the column to improve its seismic resistance [33],[34]. By wrapping FRP materials around the outside of concrete columns the internal concrete is better confined and increases in shear strength, plastic hinge zone strength and lap-splice bar pullout strength is obtained [35]. This allows the column to absorb energy due to crushing of the confined concrete and yielding of the primary steel reinforcement. The ability of a column to sustain its axial load with increased lateral displacement is enhanced and the ductility of the entire system is enhanced. The displacement ductility, µ, of a column is typically defined as the ratio of the maximum lateral displacement divided by the displacement at tensile yielding of the longitudinal steel bars. Fig. 7a shows a typical cyclic load-lateral deflection trace of a glass fiber wrapped column [36] which Fig. 7b shows a typical load-displacement envelope comparing the response of columns with different wrapping materials [35]. It can be seen that the ductility ratios of the confined column is greater than that of the unconfined column and similar to that of a steel jacketed column. Also, and perhaps more relevant to his paper is the fact that all tests conducted to examine ductility enhancement present the entire loading-unloading and reloading history over many quasi-static cycles from which the energy absorption can be calculated. Nevertheless, little work has been done to determine the precise nature of the progressive failure mechanisms occurring during these dissipation cycles and ways in which is control them. It is however important to note that the progressive failure mechanisms that dissipate the energy are not Page 6 of 10

7 due to the FRP failing progressively but rather due to the concrete failing progressively or the reinforcing steel yielding. Since most wraps are applied in the hoop direction only, it is generally assumed that the FRP wrap is linear elastic to failure. The failure strain of the fiber (glass, carbon or aramid) is usually limited to 0.4% by design codes, which is well below the failure strain of most fibers. This believed to be necessary to ensure aggregate interlock in the concrete [35]. The use of fiber wraps with off axis orientations would provide much larger strain to failure but the integrity of the column may be compromised. Figure 7 cyclic load-lateral deflection trace of a glass fiber wrapped column [36] load-displacement envelope comparing the response of columns with different wrapping materials [35]. The use of PET fibers with very high failure strains has been considered [37] as an alternative to using low strain to failure fibers. It has been shown that the ductility ratios of such systems can be very large (µ~10-15) and that the integrity of the concrete and the load carrying capacity can be partially maintained. The PET jackets reach a strain of 3% at maximum lateral load and 12% at rupture at the maximum lateral displacement [38]. Nevertheless it does not appear that the fibers themselves contribute to the energy absorption of the systems through a progressive failure mechanism. 4 Summary and Discussion Progressive failure in fiber reinforced polymer (FRP) composites and the implications that it has on the behaviour of FRP composites for construction have been reviewed in the paper. It has been shown that their remains a potential to exploit progressive failure of the composites themselves and the systems that they are used to reinforce or strengthen or confine in order to better address the concerns that designers have with the lack of ductility in FRP materials and structures. The first part of the paper demonstrates how FRP tubes have been used in the automotive and aerospace industries by exploiting the crashworthiness of tubular composite structures. Perhaps lessons can be learned from these applications that can be used in composites for construction. The second part of the paper reviews the different ways in which the notion of ductility is interpreted in FRP composites used in concrete reinforcement, strengthening and confining systems. It is shown that hybrid systems appear to hold some promise but that generally there has been insufficient attention to exploiting and enhancing progressive failure and energy absorption in the FRP composites themselves. Rather, the current philosophy appears to be to continue to use linear elastic composite materials in their on-axis and unidirectional architectures which have no ductility to speak of and attempt to provide some system energy absorption via non-frp mechanisms, such as concrete crushing, Page 7 of 10

8 debonding, or in the case of wrapping by enhancing the energy absorption ability of the existing non-frp materials. In FRP-only structures this is not possible and therefore more attention has been paid to the ways in which the composites themselves can dissipate energy through progressive failure. Perhaps it is time to adopt this philosophy in the concrete related applications of FRP composites for construction. 5 References [1] THORNTON, P. H., JERYAN, R. A., Crash Energy Management in Composite Automotive Structures, International Journal of Impact, Vol. 7, No. 2, 1988, pp [2] LIN K. H., MASE, G. T., An Assessment of Add-on Energy Absorbing Devices for Vehicle Crashworthiness, Journal of Materials and Technology, Vol. 112, 1990, pp [3] SEN, J. K., Designing for a Crashworthy All-Composite Helicopter Fuselage, Journal of the American Helicopter Society, April 1987, pp [4] HULL, D., A Unified Approach to Progressive Crushing of Fibre-Reinforced Composite Tubes, Composites Science and Technology, Vol. 40, 1991, pp [5] FARLEY, G. L., Effect of Specimen Geometry on the Energy Absorption Capability of Composite Materials, Journal of Composite Materials, Vol. 11, 1986, pp [6] MAMALIS, A.G., MANOLAKOS, D.E., IOANNIDIS, M.B., PAPAPOSTOLOU, D.P., Crashworthy Characteristics of Axially Statically Compressed Thin-Walled Square CFRP Composite Tubes: Experimental, Composite Structures, Vol. 63, 2004, pp [7] BISAGNI, C., Experimental Investigation of the Collapse Modes and Energy Absorption Characteristics of Composite Tubes, International Journal of Crashworthiness, Vol. 14, No. 4, August 2009, pp [8] MAMALIS, A.G., MANOLAKOS, D.E., IOANNIDIS, M.B., PAPAPOSTOLOU, D.P., The Static and Dynamic Axial Collapse of CFRP Square Tubes: Finite Element Modeling, Composite Structures, Vol. 74, 2006, pp [9] MCGREGOR, C. J., VAZIRI, R., POURSARTIP, A., XIAO, X., Simulation of Progressive Damage Development in Braided Composite Tubes under Axial Compression, Composites: Part A, Vol. 38, 2007, pp [10] BANK, L.C., Composites for Construction: Structural Design with FRP Materials, John Wiley & Sons, [11] BELARBI, A., CHANDRASHEKHARA, K., WATKINS, S. E., Smart Composite Rebars with Enhanced Ductility, Mechanics (eds. Y. K. Lin and T. C. Su) Proceedings of the 11th Conference, Ft. Lauderdale, Florida, ASCE, May 19-22, 1996, pp [12] HARRIS, H. G., SOMBOONSONG, W., KO, F. K., New Ductile Hybrid FRP Reinforcement Bar for Concrete Structures. Journal of Composites for Construction, Vol. 2, No. 1, 1998, pp Page 8 of 10

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