# Static Compressive Properties of Polypropylene Fiber Foam Concrete with Concave Hexagonal Unit Cell
## 3 Results and Discussion
### Destruction Process  
In the static compression test of the concave hexagonal unit cells, a difference in the process of destruction of the concave hexagonal unit cells with different fiber contents could be observed. Three types of hexagonal unit cells with different fiber contents were classified as no fiber addition, low fiber addition, and high fiber addition. Figure 8 shows the damage process of three different types of concave hexagonal unit cells in the static compression test. For the concave hexagonal unit cell with 0% fiber content, the damage process is shown in Figure 8a. During the compression process, the cell wall first showed an oblique crack on the left and right sides along the concave edge of the hexagon, basically in the  
Figure 8: Failure process of different concave hexagonal cell specimens: (**a**) A-0; (**b**) A-0.5; (**c**) A-2.5. form of \"upper left and lower right\" or \"lower left and upper right\" distribution. With the increase in pressure, multiple secondary cracks rapidly extended around the oblique cracks and extended to the upper and lower pressure-bearing surfaces of the cell wall. The cracks penetrated each other to cause the cell wall to break up, and there were pieces of debris spalling down continuously. The specimen was completely destroyed; the concave parts of the left and right sides of the cell wall were more thoroughly damaged, and the supporting surface was also damaged by crack penetration. Since the damage process of the specimen developed rapidly and the fracture surface was rough, it can be judged that the damage process of the unit cell specimen with 0% fiber content belonged to the category of brittle damage. For concave hexagonal unit cells containing a small amount of polypropylene fiber, a sample with 0.5% fiber content was taken as an example, and its failure process is shown in Figure 8b. Under pressure, oblique cracks appeared on the left and right sides of the cell wall in a \"upper left and lower right\" or \"lower left and upper right\" distribution, and the oblique cracks continued to increase under load, resulting in the destruction of the cell wall structure through the front and back of the specimen. The oblique crack through the specimen was divided into upper and lower parts. The upper part slid downward along the oblique crack under the continuously increasing load, and extruded the lower part. The concave surfaces in the cell wall produce secondary cracks during the extrusion process, causing the part to continue to crack. After the compression of the specimen, it could be observed that the concave parts on the left and right sides of the cell wall broke into several pieces, while there were only a few cracks distributed on the pressure-bearing surface. Although the specimen maintained good integrity after the damage, the connection between the fragments was not tight. With the increase in fiber content, the effect of fiber on concrete material was more obvious. For concave hexagonal unit cells with high polypropylene fiber content, a sample with 2.5% fiber content was taken as an example, and its failure process is shown in Figure 8c. The specimen was penetrated by oblique cracks distributed on the left and right sides of the cell wall, showing \"upper left and lower right\" or \"lower left and upper right\" distribution, and was then divided into upper and lower parts. The upper part slowly slid downward along the oblique crack under the action of a continuously increasing load, and squeezed the lower part. In this process, a few secondary cracks were generated around the oblique crack. After the compression of the specimen, the concave portions on the left and right sides of the cell wall broke into fewer pieces. There were more concrete particles at the crack, and there was no obvious crack on the pressure-bearing surface of the cell wall. Concave hexagonal unit cells with high polypropylene fiber content had better integrity after destruction, and the connection between fragments was tighter than in cells with low polypropylene fiber content.