## 1 Introduction
Developing the application of GFRP hybrid materials for use in construction enhances the ductility of the assembled structural members, as well as their flexural strength. Using epoxy resin for externally bonding GFRP sheets increases the strength of members during flexural loading. An advantage of using hybrid FRP laminates, such as GFRP sheets, is their nonlinear stress-strain behavior [9]. This behavior results in enhancing both the ductility and strength of the members [10; 11; 12]. On the other hand, unlike steel, non-hybrid FRP sheets demonstrate linear stress behavior; in contrast, FRP adds to the flexural capacity of the member but reduces the ductility of the member significantly [8]. Despite increasing demands for the use of FRP materials, as an alternative method to replace traditional reinforcement techniques, the behavior of such hybrid materials as GFRP sheets have not been subject to extensive amounts of research. Moreover, the design and implementation methods of hybrid materials have not been sufficiently established in the guidelines such as EUROCOMP, CUR 96, and BD90/05 when compared to FRP materials. This study examined the behavior of hybrid fiberglass tubes filled with recycled material and concrete for use in deep foundations (piles). Many research studies have been conducted on the structural responses of recycled concrete material [13; 14; 15]. In this study, fiberglass tubes were filled with concrete containing recycled materials, such as shredded and used tires, to determine if the use of recovered materials resulted in lowering construction costs in an environmentally friendly manner. In this way, seemingly worthless materials could be used as part of the construction process, without sacrificing natural resources or producing hazardous materials during the construction process. Significant factors should be considered in the design of structural members, such as composite piles which are reinforced with FRP layers. Using the appropriate hybrid material--for example, GFRP, hybrid carbon, and glass-reinforced polymers (HCG), or carbon fiber reinforced polymers (CFRP)--is one of these important factors. As another reinforced hybrid option, using basalt fiber-reinforced polymer (BFRP) sheets is almost 15% less expensive than using GFRP materials, and with the same compressive strength as GFRP [16]. Moreover, the arrangement, thickness, and width, along with the number of the designated layers, and compressive strength of the hybrid material affect the experimental outcomes. Another important parameter is the shape of the reinforced materials, in that various circular or squared arrangements of the specimens may result in different outcomes. Some experimental research works have been performed on the performance of circular tubes using FRP material [17; 18; 19; 20; 21; 22]. Hybrid FRP materials such as GFRP are composed of either bonded or embedded fibers in a matrix arrangement. This specific matrix pattern transfers structural loads within mediums and reinforces the fibers against environmental hazards. With this arrangement, the fibers mainly are designed to sustain structural loads [23]. Moreover, the direction of applied loads and the stiffness of a pile affect the results considerably. Based on the axial or lateral loading design criteria, the structure of the FRP tubes can be modified to enhance the strength capacities. For example, the tubes can be filled with concrete to increase their strength and stiffness. The filled tubes function as a permanent formwork that confines and protects the concrete. In addition, the tubes serve as flexural reinforcement for concrete by preventing local buckling of the tubes. Therefore, the considerable axial capacity and lateral strength of the FRP composite piles make them a reliable alternative to replace traditional piles.