Braided Hybrid Composites for Bridge Repair

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 worldwideMufti 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 loaddeformation 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. ! Philadelphia University, School of Textiles and Materials Technology " Drexel University, Department of Materials Engineering