## 3 Discussion and Results
### Conclusions  
Following the experimental data analysis, the use of glass-fiber-reinforced polymer has many benefits for the design and implementation of pile structures in large-scale construction projects, such as bridge structures. The use of piles reinforced with GFRP material in bridge design results in increasing the durability of these deep foundations and decreasing their construction costs. The allocated expenditure for recovering and repairing pile structures in both North America and Europe exceeds $1.5 billion dollars. Therefore, maintenance cost is a significant factor in the design of cost-effective structures. Using recycled materials as fillers in reinforced GFRP tubes decreases the total cost of production. In the past decade, using steel-reinforcement methods increased the cost of construction; however, developing structural piles with hybrid GFRP materials resulted in reducing the use of steel materials as reinforcing members. In addition, hybrid FRP materials, such as GFRP, enhance the anticipated lifespan of pile structures, even in such hazardous environments as oceans, where corrosion is one of the main concerns for the durability of these structures. Finally, these hybrid materials increase the anticipated strength capacity of the structures significantly. The outcome of this research is based on a specific arrangement of GFRP layers, including their diameter, outer and inner thickness, and the degree of inclination from the longitudinal direction. Adjusting these specifications, as well as changing the material's properties--such as E\({}_{\text{Axial}}\), E\({}_{\text{Hoop}}\), and Poisson ratios--optimized the results. Based on the results from three full-scale samples, the deflection responses were linear in an axial loading condition before reaching the failure criteria. Since recycled material was used as filler inside the tubes, the long-term effect of cracking was negligible for high values of calculated strength, as well as under axial or lateral loadings. Some minor inconsistencies in such parameters as initial measurements or the calibration of samples could affect the outcomes. Analyzing the internal forces of the GFRP tubes filled with recycled material showed the semi-linear behavior of the pile members, which resulted in increasing the total strength and ductility of the reinforced piles. The lateral responses of both the GFRP piles and pre-stressed piles showed comparable results, in that the effect of cracking under axial and lateral loadings was negligible. A parametric study is conducted to investigate the effect of wall thickness and fiber percentages of the GFRP tubes on the axial and bending capacities of the piles. The structure of GFRP tubes, such as their thickness and ratios of the implemented GFRP in the pile considerably affects the axial-moment interaction curves of the piles. The T-2 type GFRP tube with higher hoop elastic modulus, 17.3 Gpa and relatively low Poisson's ratio 0.085 has more confinement strength compared with T-1 and T-3 tubes with lower hoop modulus of elasticity values. Furthermore, the axial capacity of the tubes is changed due to higher GFRP fiber ratios in the hoop direction for small thickness values. However, the bending capacity of the tubes is governed by both thin and thick wall structures tubes for higher GFRP fiber ratios in the longitudinal direction. The increase rate of axial capacity for the GFRP with more thickness values tubes is higher in the longitudinal direction compared to the hoop direction. Furthermore, the effect of axial capacity is higher than the confinement capacity in tubes with lower thickness values. The required axial and bending resistance capacities of piles in different ranges of eccentricities can be met using the combination of tube wall thickness and GFRP fiber percentages following the curve interaction diagrams.