# Static Compressive Properties of Polypropylene Fiber Foam Concrete with Concave Hexagonal Unit Cell
## 3 Results and Discussion
### Energy Absorption Capacity
The addition of polypropylene fiber can effectively provide an additional binding force and increase the tensile strength and enhance the material toughness. When the polypropylene fiber content is 0-1.5%, it can not only enhance the material toughness of the concrete, but also increase the porosity of the concrete. However, the impact on the compressive strength of the concrete is not significant. The energy absorbed by the deformation and fracture of the foam pore wall in the compression process is increased. At this point, the specific energy absorption of the specimen is enhanced relative to that of the sample without fibers. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **Quality/g** & **Average/g** & **Ea/J** & **Average/J** & **Esa/J\(\cdot\)kg\({}^{-1}\)** & **Average/J\(\cdot\)kg\({}^{-1}\)** \\ \hline A-0-1 & 133.6 & & 3.599 & & 26.943 & \\ A-0-2 & 135.4 & 135.1 & 3.664 & 3.736 & 27.060 & 27.644 \\ A-0-3 & 136.4 & & 3.946 & & 28.930 & \\ \hline A-0-5-1 & 135.3 & & 4.414 & & 32.623 & \\ A-0-5-2 & 133.2 & 134.3 & 4.226 & 4.221 & 31.727 & 31.431 \\ A-0-5-3 & 134.4 & & 4.024 & & 29.943 & \\ \hline A-1-0-1 & 132.2 & & 4.313 & & 32.625 & \\ A-1-0-2 & 132.6 & 131.8 & 3.743 & 3.881 & 28.230 & 29.440 \\ A-1-0-3 & 130.6 & & 3.587 & & 27.464 & \\ \hline A-1-5-1 & 131.6 & & 3.323 & & 25.253 & \\ A-1-5-2 & 130.2 & 130.4 & 3.731 & 3.580 & 28.654 & 27.471 \\ A-1-5-3 & 129.3 & & 3.686 & & 28.504 & \\ \hline A-2-0-1 & 126.3 & & 3.283 & & 25.994 & \\ A-2-0-2 & 124.5 & 124.5 & 2.992 & 3.191 & 24.0330 & 25.629 \\ A-2-0-3 & 122.8 & & 3.298 & & 26.860 & \\ \hline A-2-5-1 & 121.5 & & 2.961 & & 24.374 & \\ A-2-5-2 & 120.7 & 120.8 & 3.105 & 2.937 & 25.723 & 24.307 \\ A-2-5-3 & 120.3 & & 2.746 & & 22.826 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mass and energy absorption index of different concave hexagonal unit cells. Figure 11 shows the microscopic morphology of cracks in the concave hexagonal unit cell at low (0.5%) and high (2.5%) fiber content, respectively. The exposed polypropylene fibers at the high fiber content crack in Figure 11b are longer than those at the low fiber content crack in Figure 11a, which indicates that the polypropylene fibers break free from the foam concrete under force when the specimen is structurally damaged. With the increase in polypropylene fiber content, the number of pores in the foam concrete also increases, which leads to the thinning of the internal pore walls and a reduction in the contact surface between the polypropylene fibers and the concrete material. The increase in porosity leads to a reduction in the peak stress of the specimens and the inability of the polypropylene fibers to exhibit the inhibiting effect during the compression of the specimens. In higher stress conditions, polypropylene fibers have not yet played a restraining role; the foam concrete begins to rupture in advance, so the energy absorbed by the deformation and fracture of foam concrete is reduced. A small amount of polypropylene fiber content can effectively increase the energy absorption efficiency of the concave hexagonal unit cell. In the process of static compression of foam concrete, the internal pore wall will continuously produce brittle fracture damage,  
Figure 11: Microscopic morphology of cracks in the concave hexagonal unit cell: (**a**) low fiber content; (**b**) high fiber content. Figure 10: Effect of fiber content on energy absorption capacity of the concave hexagonal unit cell. thus exerting the energy absorption ability of the material. The addition of polypropylene fibers can provide a binding force for the internal pore walls and convert part of the energy into fracture energy of polypropylene fibers after structural damage, thus increasing the energy absorption of the concave hexagonal unit cell.