## 3 Experimental Results and Discussion
### Deformation Characteristics  
The typical stress-strain curves of concrete with two matrix strengths at different loading strain rates are shown in Figure 14. For the two kinds of plain concrete, the brittle characteristics of concrete became more obvious with increasing strain rate, which has been confirmed in the literature [27]. When the strain rate reaches \(10^{-2}\) s\({}^{-1}\), the failure modes have characteristics of brittle fracture, with the stress-strain curve showing a cliff-like landing. The addition of PVA can significantly improve the post-peak mechanical properties of concrete, showing a significant reduction in the drop rate per unit of time after undergoing peak stress. The stress and strain curves are smoother, indicating that the properties of the concrete change from brittle to ductile, which is of great significance in improving the seismic performance of concrete [12]. From the analysis of the typical stress and strain curves, it is found that the addition of PVA has little effect on the curve at the upward section, while showing a certain effect at the downward section. With the increase in the content of PVA, the descending rate gradually slows down at the downward section of the curve, indicating that PVA has a significant effect in improving toughness of fiber concrete [20; 21]. For all types of concrete, the slopes of the stress-strain curve at the downward section increase gradually with the increase of strain rate, showing  
Figure 12: Peak strain of FRC (C30).  
Figure 13: Peak strain of FRC (C40).  
that the bridging effect of the fiber on the concrete decreases. At this point, the loading rate plays a greater role in the failure behavior of the concrete.  
Figure 14: Typical stress–strain curve under different strain rates for (**a**) C30 PVA0, (**b**) C40 PVA0, (**c**) C30 PVA0.2, (**d**) C40 PVA0.2, (**e**) C30 PVA0.4, (**f**) C40 PVA0.4, (**g**) C30 PVA0.6, and (**h**) C40 PVA0.6.  
Another aspect of the change in the post-peak mechanical properties is the residual strength and the failure mode. As shown in Figures 15 and 16, the two matrix strengths of plain concrete appear to undergo the phenomena of fall-block and caving after destruction. When the concrete block is made into a bulk, the residual strength is nearly zero. However, when the fiber concrete is crushed, the evenly distributed fibers begin to bear the load. Due to the bridging effect of the fiber, the concrete structure can maintain a relatively complete form with a number of small cracks on the surface, and the residual strength remains at approximately 3-5 MPa without brittle damage, unlike the collapse of plain concrete.  
Therefore, the deformation characteristics of fiber concrete are different from those of plain concrete. The addition of fiber can improve the brittle properties of the concrete, including the fall-block and collapse of the concrete, the obvious decrease of the drop rate per unit time after undergoing peak stress, and the residual strength [27]. The stress-strain curves of concrete with different fiber contents are similar, and the deformation behavior of concrete at a high strain rate is more brittle than that at a low strain rate. The reinforcing effect of fiber on the mechanical properties of concrete is different for the two matrix strengths. The concrete material with low matrix strength is more ductile than the one with a higher matrix strength. The same trend is evident with the reinforcing effect of fiber improving the uniaxial compressive strength [18; 26].