F98-P01 Braided Hybrid Composites

NTC Research Project F98-P01

Frank K. Ko

Braided Hybrid Composites for Bridge Repair
Christopher M. PastoreF and Frank K. Kog

Goal: The goal of this research activity is to develop an understanding of the mechanical behavior of braided hybrid composite bars and correlate this behavior to the textile processing conditions.

Abstract: Mechanical properties of hybrid braided composite rebars are evaluated and compared to theoretical predictions which account for specific processing parameters including yarn size, braiding machine setup, and fiber type. Using a combination of high modulus carbon (Thornel P-55S) and aramid (Kevlar 49) fibers bars have been produced of up to 10 mm diameter with a Young's modulus of 200 GPa, a bi-linear stress-strain tensile curve with a definite yield, an ultimate strength higher than yield and an ultimate failure at between 2% and 3% strain. Excellent bond characteristics were obtained by integrating ribs into the braided jacket to increase the mechanical interaction at the bar to concrete interface. The ductility indices were based on definitions of ductility according to displacement, rotation and energy considerations.

Introduction: Replacement of the steel reinforcement in concrete structures with more corrosion resistant substitutes such as composites is rapidly becoming a more economical option for construction facilities worldwide- Mufti et al., 1991, Iyer and Sen, 1991, Nanni and Dolan, 1993, Basham 1994, Saadatmanesh and Ehsani, 1996, 1998. Composites can be used in new or repaired reinforced concrete structures. In general, composites have high strength, a range of moduli and low ultimate tensile strains compared to steel. The stress-strain behavior of all of these fiber systems is linear up to failure, which makes it impossible to have significant hysteretic behavior. In spite of their superior light weight, corrosion resistance and non-magnetic properties, the lack of material ductility and energy absorbing capabilities is a severe limitation of all these fiber systems if they are to be considered for earthquake resistant applications.

Design Concept: In order to achieve ductility in reinforced concrete structures without using conventional steel rebar, a new design methodology was introduced to identify suitable composite materials that mimic the stress-strain characteristics of steel [Somboonsong et al., 1998]. The technology of braiding, as detailed by Ko and Pastore [1989], is a well established technology which intertwines three or more strands of yarns to form a tubular structure with various combinations of linear or twisted core materials. By judicious selection of fiber materials and fiber architecture for the braid sleeve and the core structure, the load-deformation behavior of the braided fibrous assembly can be tailored. For this research the sleeve is a tough aramid (Kevlar) and the core structure is a high modulus carbon to provide the initial resistance to deformation. The rib effect is built into the sleeve structure during the braiding process.
A 24 carrier braiding machine was employed to form the structure. The braiding yarns were 3 ply 1,240 denier Kevlar 49, except one of the bobbins was loaded with a 15 ply 1,240 denier Kevlar 49. This large bundle is used to create a rib in the braid for mechanical bonding between the resulting composite rebar and the concrete. The design of this rib is to be similar in concept to the current steel rebars. A regular modulus carbon (T300) and aramid (Kevlar 49) fibers in a vinylester matrix was used for the 3 mm bar, whereas a high modulus carbon (Thornel P-55S) and aramid (Kevlar 49) fibers in an Epon 828 epoxy matrix was used for the 5 mm and 10mm bars.
The process takes the braided fabric through a forming ring, and runs the braid through an infusion zone wherein epoxy resin is dripped onto the fell of the cloth. The wet fabric is then run through a heated chamber to cure the resin. The fabric has a 30 minute dwell time before being collected. A resin system from Shell Chemical, consisting of EPON Resin 9310/EPI-CURE Curing Agent 9360/EPI-CURE Curing Agent Accelerator 537 is used for consolidation. The process, called "Braidtrusion" is schematically illustrated in Figure 1.

Figure 1. Schematic Illustration of Braidtrusion process

A typical rebar is shown in Figure 2. The rib yarns running in opposite helices can be clearly seen. The core carbon yarns are completely encased by the aramid sheath yarns.

Figure 2. Micrograph of typical hybrid rebar showing rib yarns on surface.

The effect of the large braiding yarns on creating a rib structure is shown in Figure 3. These very large yarns create substantial distortions in the braided structure which provide the mechanical connection to the concrete system, but provide additional difficulties in modelling.

Figure 3. Effect of rib yarns on braided fabric geometry.

Considering the effect of core yarns on the rebar diameter a geometric model was constructed accounting for the core yarn size, number of aramid braid carriers, and rib effect. Figure 4 shows basic processing consideration effects.

Figure 4. Effect of core yarn ply count on the nominal braided rebar OD and the size of the rib protrusion for bonding with concrete.

Tensile Response: Tensile stress-strain characteristics have been obtained for all three sizes of rebars that have been produced in the laboratory. The monotonic stress-strain behavior of the 5 mm D-H-FRP bar is compared with theoretical predictions in Figure 5. Note that the 5 mm. hybrid bar achieved high initial modulus as well as a ductile failure mode characterized by a bi-linear stress-strain curve. The definite yield strength is achieved by the hybridization process and it's a manifestation of the fracture of the fibers with the lowest failure strain. The controlled nature of the failure process has been explained by Somboonsong et al. [1999]. The apparent ductility is associated with resin cracking at the interlacing points of the aramid braiding yarns and some continuing reinforcement effect of the failed carbon yarns behaving like short fiber composites. This is not yet accounted in the predictive model.

Figure 5. Comparison of Experimental and Theoretical stress-strain response for 5 mm hybrid braided rebars.

The theroretical model of stress-strain behavior illustrated in Figure 5 is based on the process modelling of the braid structure and use of a stiffness averaging predictive model. Failure of the core yarn is predicted by strain energy criteria. The braided yarns reorient themselves during loading, resulting in the slight non-linearity of the curve. The next step of modelling is to include the resin cracking at the interlacing points and the residual contribution of the fractured carbon yarns in the core.
Considering the effect of yarn scale on the rebars, it is shown in Figure 6 that predictive models of the 3mm, 5mm and 10mm bars do not show identical behavior. This is due to different braider yarn orientations necessary to maintain proper coverage and the various percentages of core yarns compared to braider yarns realised in the process.

Figure 6. Comparison of theoretical stress-strain predictions of hybrid rebars formed at various diameters with constant braid yarn and carrier count configuration.

Bond and Cracking Characteristics: One of the shortcomings of presently available composite rebars is their low bond strength to concrete. The approach taken in this research is to build a set of ribs during the braiding operation. This greatly increases the bond strength through a mechanical interlock between the composite rebar and the surrounding concrete. This improved bond strength has been demonstrated in 5 mm and 10mm bars that have been produced by the proposed method and tested in pull-out specimens. Average bond strength of the bars is comparable to deformed steel bars of similar diameter.
A series of simple beams of 50 mm x 100 mm cross-section and 1.2 m long were tested in four point bending. The beams were designed to have the same ultimate moment in all beams irrespective of type of reinforcement based on nominal strength properties. Steel wire stirrups were used in both steel and FRP reinforced beams, which were cast simultaneously using a properly sized model concrete [Harris and Sabnis, 1999]. One steel reinforced and three braid reinforced beams were cast at the same time.
A comparison of the load-deflection behavior of the steel reinforced and the ductile hybrid FRP reinforced beams is shown in Fig. 7a. Note from Fig. 7a that the D-H-FRP reinforced beams had very repeatable behavior with a high initial stiffness (identical to the companion steel reinforced beam) up to the cracking load. The precracking behavior of all three D-H-FRP beams was identical to that of the steel reinforced beam. The post cracking behavior of all three D-H-FRP reinforced beams was very similar and all had a bilinear load-deflection curve up to the yield point.

Figure 7a. Load versus Deflection for 5 mm bars

A maximum of five load/unload cycles were performed on each of the reinforced beams in the cracked and post yield ranges to study the nature of their inelastic behavior. As can be seen from Figure 7a, the new braided hybrid composite has significant energy absorbing capabilities very similar to those of steel reinforcing bars. This behavior is not found to this extent in any existing composite that has linearly elastic characteristics up to failure but is only possible to materials such as the hybrid braided bar, which has a definite yield point. Unloading from the yielded condition produced significant permanent deformation (and hence warning) as seen in Figure 7. The steadily increasing load carrying capacity of the beam shown in Figure 7 is a direct result of an ultimate strength in the new FRP bar that is higher than its yield (Fig. 5).
Moment-curvature relationships of all three D-H-FRP reinforced beams were computed numerically from the equally spaced deflection measurements at the mid-span and are shown in Figure 7b. As can be seen, a ductile behavior was obtained for all three D-H-FRP reinforced beams with good reproducibility. The predicted moment-curvature relation, based on the bilinear lower bound stress-strain curve is plotted in Figure 7b and shows very close agreement with the experimental results. It should be noted that the bi-linear moment-curvature behavior of the new D-H-FRP is made possible only by the fact that, through its special design, it possesses a definite yield point, an equivalent bilinear stress-strain curve, and an ultimate strength higher than the yield.

Figure 7 b. Moment versus Curvature for beams with 5 mm diameter bars

Ductility indexes for the three reinforced beams, were in the range from 4.6 to 5.4 and compare well with the 6.1 of the companion steel reinforced beam. Ductility indexes based on measured curvature ranged from 5.7 to 6.3 for the braided beams as compared to 12 for the companion steel reinforced beam. Ductility indexes computed on the basis of energy considerations as given by Naaman and Jeong [1995] ranged from 3.4 to 3.8 for the D-H-FRP beams and compare to 4.3 for the companion steel reinforced beam.

Cyclic Response: The test specimen was anchored to a very substantial reinforced concrete base, oriented in a horizontal position and loaded at its end by the loading head of a 44.5 kN universal testing machine. The model beams were designed to be approximate 1/12 scale models of a hypothetical prototype beam and their dimensions were greatly simplified (Figure 8).

Figure 8: Dimensions of beam specimens under reverse cyclic loading.

The base foundation was designed to so that cracking would be minimal. This was accomplished by providing the equivalent of #10 bars for the prototype and 2.65 mm deformed steel bars for the model. In addition, the base foundation was externally post-tensioned to the supporting base with steel clamps.
Figure 9 shows the column reinforcement cage prior to casting. First the footing was cast in an open wooden mold (Fig.9b) and then the column form was attached to the footing mold. The column cage was held in place as the model concrete was introduced into the mold using a vibrating table and hand rodding. The model and its control specimens were removed 24 hours after casting and introduced into a moist room for curing until testing after 28 days.

Figure 9: Fabricated cage of cyclic specimen and casting procedure for footing

The testing set-up is shown schematically in Fig. 10. As shown in Fig. 10a, the beam is tested in a horizontal position with the cyclic loading applied by the cross-head of the Tinius-Olsen testing machine (Fig. 10b)
The hysteretic load-displacement is of primary importance in the evaluation of the D-H-FRP reinforcement since it gives an overall basis for evaluating the beam specimen, including degradation rates and energy absorption, and with less emphasis on local response characteristics such as cracking and bond-slip. A summary of the test results is given in Table 1. Typical hysteretic load-deflection results for specimen C-1 are shown in Fig. 10a. This specimen showed significant ductility and energy absorption capability. The final loading to failure of specimen C-1 achieved a ductility factor of 10.9.
The moment-rotation response of the D-H-FRP reinforcement was also plotted for each specimen. The moment-rotation is influenced by local behavior of the model and the contribution of cracking and bond-slip in the hinging region. As can be seen by the response of specimen C-1 shown in Fig. 10b, no apparent deterioration was evident in this specimen until the higher ductility ratios were reached.

Figure 10: (a.) Load-Deflection and (b.) Moment-Rotation for beam specimen under reverse cyclic loading.

CONCLUSIONS
Predictive models of the hybrid braided composite show promise. Processing conditions and initial failure can be predicted well. Post-yield ductile response requires additional modelling. The effects of yarn size and process parameters on the tensile response have been demonstrated. Remaining to be done is post-yield behavior and cyclic response.
Results have been presented of the cyclic flexural behavior of a new ductile hybrid braidtrusion reinforcing bar for earthquake resistant concrete structures. Load-deflection and moment-curvature relations from small beams show that the D-H-FRP rebar can achieve a ductile behavior with ductility indexes similar to those of mild steel reinforcement.

REFERENCES
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El-Badry, M. M. (1996). Editor, Advanced composite materials in bridges and structures, The Canadian Society of Civil Engineering, Montreal, Canada.
Harris, H. G., Somboonsong, W. and Ko, F. K. (1997), "A new ductile hybrid fiber reinforced polymer (FRP) reinforcement for concrete structures," Proceedings of the 1997 International Conference on Engineering Materials, 8-11 June, Ottawa, Canada, Vol. I, pp 593-604.
Harris, H. G. and Sabnis, G. M. (1999), Structural Modeling and Experimental Techniques, Second Edition, CRC Press, Boca Raton, FL.
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Ko, F.K., (1989), "Three dimensional Fabrics for composites," Textile Structural Composites: Series 3, Chou, T.W. and Ko, F.K., Editors, Elsevier, New York.
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Naaman, A. E. and Jeong, S. M. (1995)."Structural ductility of concrete beams prestressed with FRP tendons", Non-metallic (FRP) Reinforcement for Concrete Structures," Edited by L. Taewere, RILEM, Published by E & F N Spon, 2-6 Boundry Row, London.
Nanni, A. and Dolan C. W. (1993). Editors, "Fiber-reinforced-plastic reinforcement for concrete structures", Proceedings of International Symposium, Vancouver, ACI SP-138.
Saadatmanesh, H. and Ehsani, M. R. (1996). Editors, "Fiber composites in infrastructure", Proc. First International Conf. on Composites in Infrastructure, Tucson, Arizona, Jan. 15-17, Dept. of Civil Eng. and Eng. Mech., University of Arizona.
Somboonsong, W. (1997). "Development of ductile hybrid fiber reinforced polymer (D-H-FRP) for concrete structures", Ph.D.Thesis, Department of Civil and Architectural Engineering, Drexel University, Philadelphia, PA, December 1997.
Somboonsong, W., Ko, F. K., and Harris, H. G. (1998). "Ductile hybrid fiber reinforced plastic (FRP) rebar for concrete structures: design methodology," ACI Materials Journal, V. 95, No. 6, Nov.-Dec., pp. 655-666.

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