## 4 Test Results and Discussions
### Dynamic Compression Tests
#### 4.2.3 Dynamic Increase Factor and Energy Absorption  
The strain rate versus the dynamic peak stress relationships of the FCHC specimens are given in Figure 10. Note that in Figures 10-12, each curve is given a label in the form of D-XN in which 'D' refers to 'dynamic', 'X' is an alphabet representing the type of FRP jacket ('A' for 'AFRP', 'B' for 'BFRP', and 'C' for 'CFRP'), and 'N' is a number indicating the number of FRP layer. It can be determined from Figure 10 that the dynamic compressive strength of FCHC is obviously higher than its static compressive strength and the dynamic peak stress of FCHC increases with an increase in the strain  
Figure 9: Stress-Strain curves of FCHC under dynamic compression. (**a**) Unconfined specimens; (**b**) One-layer BFRP-confined specimens; (**c**) Two-layer BFRP-confined specimens (**d**) Three-layer BFRP-confined specimens; (**e**) One-layer AFRP-confined specimens; (**f**) Two-layer AFRP-confined specimens; (**g**) Three-layer AFRP-confined specimens; (**h**) One-layer CFRP-confined specimens; (**i**) Two-layer CFRP-confined specimens; and (**j**) Three-layer CFRP-confined specimens. rate and the thickness. This demonstrates that the existence of FRP confinement can greatly increase the dynamic peak stress of high-strength concrete. With the same FRP layer number, the dynamic peak stress of the CFRP-confined concrete is the largest, while that of the BFRP-confined concrete is the smallest. This is similar to the conclusion drawn from the quasi-static compression tests on FCHC. Figure 10: Relationship between the FCHC peak stress and the strain rate. (**a**) CFRP-confined specimens; (**b**) AFRP-confined specimens; and (**c**) CFRP-confined specimens. Figure 11: Relationship between the DIF and the logarithm of strain rate. (**a**) BFRP-confined specimens; (**b**) AFRP-confined specimens; and (**c**) CFRP-confined specimens. The dynamic increase factor (DIF) is defined as the ratio between the dynamic compressive strength and the quasi-static compressive strength of concrete, which is used to describe the strength enhancement of the concrete material with respect to the strain rate. As seen from Figure 11, the DIF increases with an increase in the strain rate. This finding is slightly different from the conclusion found by [PERSON] et al. [32], in which the strength of CFRP confined concrete is found not significantly sensitive to the strain rate. Figure 12: Relationship between the energy absorption and the strain rate. (**a**) BFRP-confined specimens; (**b**) AFRP-confined specimens; and (**c**) CFRP-confined specimens. to strain rate. This is probably due to the effect of the specimen size. Given a specimen with a same number of FRP layers, the DIF of the AFRP-confined high-strength concrete is the largest, indicating that AFRP-confined high-strength concrete has a favorable impact resistance. This is most likely due to the larger FRP hopo strain efficiency for the AFRP, as seen in Table 3. For FCHC with the same type of FRP jacket, the DIF of the three-layer FRP-confined specimen is the smallest, and the DIF is the largest for the one-layer FRP-confined specimen. This is due to the confinement lag effect in the confined concrete under dynamic loadings. Namely, the dynamic compression leads to failure of the concrete while the failure is not necessarily developed in the FRP timely. Therefore, the DIF is not proportional to the number of FRP layers. The absorbed energy, \(W\), is often defined as the area surrounded by the stress-strain curve of the material. The following equation can be used:  
\[W=\int\varepsilon d\sigma, \tag{7}\]  
where \(\sigma\) is axial dynamic stresses and \(\varepsilon\) is axial dynamic strains. Figure 12 shows the energy absorption versus the strain rate relationships for confined concrete with different FRP types and thicknesses. From Figure 12, it can be seen that the energy absorption increases with an increase in the strain rate, provided that the other parameters (i.e., FRP type and thickness) are identical.