## 2 Experimental Tests
### Results and Discussion  
#### 2.2.1 Results  
Cracking  
The reference pile (PREF) was longitudinally reinforced with 1420 steel rods and spiraled with Q235 steel. The first four cracks were observed on the bottom of the mid-span, at a load of 27.6 kN. As the load increased to 39.2 kN, a few new cracks appeared on the specimen. As the load increased to 42.1 kN, the cracking propagated to the flexural-shear zone. The pile failed at a load of 71 kN with a loud sound. The cracking of the reference pile before and after failure is shown in Figure 6.  
For specimen PCFRP1000, which was longitudinally reinforced with CFRP rods with a 1000 MPa prestressing level, the first rack was observed at a load of 57 kN. The first crack was located at 100 mm from the mid-span. As the load increased to 62 kN, a second crack was observed symmetrical to the first one. As the load increased to 71 kN, one more crack was observed near the loading point. The concrete started to spa  
Figure 5: Test setup and instrumentation.  
Figure 6: Cracks before and after failure of the reference pile.  
and a loss of capacity occurred at a load of 145 kN. The cracking of specimen PCFRP1000 before and after failure is shown in Figure 7.  
Specimen PCFRP1500 started to crack at a load of 86.5 kN. The first crack was located at 110 mm from the mid-span. As the load increased to 90.8 kN, the second crack was observed on the other side of the pile, at a location of 300 mm from the mid-span. As the load increased to 107.7 kN, cracks were observed outside the range of the distributing beam. The concrete started to crush at a load of 174 kN, and the pile failed at a load of 180 kN. The cracking of specimen PCFRP1500 before and after failure is shown in Figure 8.  
The cracking load, ultimate load, deformation at failure and failure mode of the tested specimen are summarized in Table 2. The flexural capacity was calculated using Equation (1), which took the self-weight into consideration. Equation (1) is specified in Chinese code GB 13467-2023, Pretensioned Spun Concrete Piles [36]. The flexural capacity was the bending moment of the mid-span section at failure. As the steel reinforcements were replaced by CFRP rods, significant improvement was observed in terms of the cracking load and the ultimate load. In addition, the failure mode switched from rupture of the longitudinal reinforcement to crushing of the concrete.  
\[\text{M}=\frac{\text{P}}{4}(\frac{3\text{L}}{5}-2\text{a})+\frac{1}{40}\text{ WL} \tag{1}\]  
where M is the flexural capacity, P is the applied load, L is the length of the pile, W is the weight of the pile and a is the load span.  
Figure 8: Crack before and after failure of PCFRP1500.  
Figure 7: Cracking before and after failure of PCFRP1000.  
Load-Deflection Response  
A load-deflection diagram of the tested piles is shown in Figure 9. It can be seen that the stiffness was significantly improved by replacing the steel reinforcement with CFRP rods, especially after the yielding point. It should be noted that even though the CFRP is an elastic material, the yield of concrete still resulted in a significant stiffness reduction. Before the yielding point, the cracking of concrete led to a slight stiffness reduction as well.  
The three specimens exhibited similar deformation at failure. However, the pile with a higher prestressing level exhibited the highest capacity and smallest deformation at failure. All the specimens experienced a sudden drop in capacity at the final stage, regardless of whether the failure was caused by concrete crushing or rupturing of the reinforcement.  
The CFRP improved the stiffness of the pile, especially at a later stage. This was because the steel yielded soon after loading, and no more tensile resistance could be provided by the steel. However, the CFRP continued providing tensile resistance until failure. In addition, the pile with higher prestressing exhibited higher stiffness.