## 3 Discussion and Results
### Field Results on GFRP Piles, Paramteric Study  
As mentioned before, the use of full-scale test piles results in a more reliable outcome for analyzing the responses of the GFRP composite piles [25; 34; 35]. The capacities of GFRP composite piles were measured under axial loading. Installed strain gauges and accelerometers measured the member's displacements for both longitudinal and lateral loadings. The lateral deflection and displacement of the pile, as well as the axial strain and displacement responses of the piles, were measured and analyzed. For further illustration of the effect of GFRP tubes on the axial and bending capacities of the piles, a parametric study is presented as follows:  
In this parametric research, the effect of different percentages of fibers on the axial and bending resistances of a test pile is investigated. Furthermore, variations of thickness of the GFRP composite tubes in both longitudinal and hoop directions on pile capacities are studied. In this research, A GFRP composite tube with 250 inner diameters and filled with 30 MPa concrete is constructed using E-glass epoxy with the yield tensile strength of 27,500 psi based on the ASTM D638 specifications. Three arrangements for the laminate structure of the GFRP tubes were selected. The implemented percentages of GFRP fiber tubes in the longitudinal and hoop directions are varied using 1:6, 1:1 and 6:1 ratios. The 1:6 ratio indicates that 60% of the GFRP fibers are orientated in the hoop directions to increase the confinement capacity of the tubes under axial compression loading. The 6:1 GFRP laminate has 60% of these fibers orientated in the longitudinal direction to enhance the bending capacity of the composite tubes. The 1:1 ratio defines an equivalent distribution of fiber percentages in both hoop and longitudinal directions. Each of these ratios in the structures of the laminates is tested with wall thicknesses (t) 5, 7.5 and 15 mm, which are in respect equal to the 8%, 12% and 24%, as pile reinforcement ratios (4t/D). An analytical model is used to develop the interaction diagram [25]. The combination of three different propionate percentages and three different thickness values, a total of 9 cases, were analyzed. Figure 6 shows the axial-moment load interaction curve diagrams based on the thickness and fiber percentage variables. The axial compression and bending moment capacities of the tubes are presented based on the diameter of the pile D and compressive strength of the concrete \(f_{c}\). In each diagram, three curves show the fiber proportionate ratios of the GFRP laminate structures based on the specified 4t/D ratios. The diagrams show that an improvement in longitudinal stiffness properties of the pile, for example from 1:6 to 6:1 fiber ratios, considerably adds to the moment capacity of the tubes in different thickness variations. In addition, increasing the GFRP tube thickness results in enhancing both axial and bending capacities. However, an increase in hoop stiffness, for instance from 6:1 to 1:6 fiber ratios is not necessarily increasing the pure axial capacity of the pile in compression for different thickness values. For example, for a 15 mm-thick GFRP tube, adding to the hoop stiffness is not increased the axial capacity of the pile. Although, for the tube with a thickness of 5 mm the increase of hoop stiffness results in enhancing the pure axial compression capacity. These outcomes for responses of the pile can be justified using the equations below [25]. In the Equations (1)-(3), \(P_{AX}\) is the ultimate axial capacity in interaction with the concrete \(P_{C}\) and the GFRP tube\(P_{\text{GFRP}}\). The cross-section area of the concrete is shown as \(A_{C}\) and \(f_{cc}\) represents the confined compressive strength of the concrete. \(E_{f}\) is the effective modulus elasticity and \(\varepsilon_{ff}^{\prime}\) is the axial strain capacity of the tube at both ends.