## 3 Discussion and Results
### Response Evaluation of GFRP and Pre-Stressed Piles  
As mentioned before, the full-scale composite GFRP piles are instrumented using strain gauges to measure the deflection at the middle of the height of the eccentrically loaded column specimen. The strain gauges are also used to record the strains of the extreme fibers under tension and compression, based on the applied constant bending moments on the specimens. The relative slip range of the GFRP tube in contact with the recycled material is measured at both top and bottom of the columns. The load application is conducted by controlling strokes and appropriate care of cushion. The average rate of the loading was 1.5 mm/min. Figure 5 depicts the curvature-moment responses of the GFRP piles for the three geometrically similar eccentrically loaded column specimens. The crack moment, 43 kNm, is considerably small compared to the ultimate bending capacity of the specimens. An immediate drop in the bending capacity of the columns is recorded which is the result of the stroke control test after occurring the crack. An analytical comparison is conducted between the responses of the GFRP composite piles and pre-stressed piles filled with concrete. A prediction of curvature-moment response of the pre-stressed piles was calculated following the strain compatibility and equilibrium theory. Details of such an analytical model is available in the previous research studies [27]. The Ramberg-Osgood equation was used to simulate the relationship between strain and strain responses of the pre-stressed strands. To simulate the nonlinear stress-strain behavior of the concrete in the pile reinforced with these pre-stressed steel material, a generalized expression model by [PERSON] [28] is used. This predicted curvature-moment response of the pre-stressed concrete filled pile is shown in Figure 5 for analytical comparison purposes. ACI 318M-19 Standard and AASHTO LRFD bridge design specification limited the ultimate useable compressive strain to 0.003, for the conventional plain concreter with limited ductility [29; 30]. Following these standards, the same ultimate maximum strain of the concrete is used in the prediction of the responses of the pre-stressed piles filled with concrete material [31]. The behavior of pile members in different loading scenarios is governed by type of the materials used for the construction of the piles. As a result, various composite materials considered in this research such as GFRP tubes, recycled and concrete materials provide the pile members with different strength and pile-deflection responses. Full-scale specimens provide this study with the opportunity to examine the response of such some composite pile members. Due to the various strengths of these materials, bridge design specifications recommend several types of design methods to deal with different reinforcement systems of pile members [32]. Based on the responses, analyses of the pile members were performed by comparing moment-curvature graphs [33]. First, the geometric characteristics of the GFRP filled with recycled materials were analyzed. A filled GFRP tube was stripped longitudinally and sampled. From the stress-strain data gleaned from the experimental test, which was semi-linear, the internal forces acting on the filled materials and the GFRP were evaluated by conducting a numerical analysis, as well as using regression on the resultant laboratory data, as is shown in Figure 5. Second, in order to develop the moment-curvature diagram, the calculated internal forces were used to anticipate the internal moments in various locations following the estimated strain data. Three tests were conducted to compare the lateral responses of the GFRP piles. As is shown in Figure 5, when the internal moments reached the failure point, cracking occurred. At the cracking failure point, the moment capacity of the GFRP is decreased significantly following a non-linear response. Before the cracking, the stiffness capacity of the GFRP tubes and conventional pile were almost identical.