## 4 Result and Discussion
### Compressive Strength of Cubic Specimens
#### 4.1.2 Effect of Rubber on Failure Modes of Cubic Specimens  
Failure modes were studied by randomly selecting one specimen from three cubic specimens produced from each mixture (10 samples in total). Figure 10 illustrates that specimens with rubber  
Figure 9: Effects of rubber content and rubber particle size on compressive strength. (**a**) Effect of rubber content on compressive strength and (**b**) effect of rubber particle size on compressive strength. particles kept better morphological integrity. As seen in Figure 10a, the control specimen was completely destroyed and cracks appeared throughout the entire surface, showing brittle failure. Cracks appeared around the specimen, which ran through the whole specimen, and a large area collapsing phenomenon was also observed in the specimens. When mixing the rubber particles, the number and size of cracks decreased as the rubber content was increased. For any rubber particle size (3-5 mm, 1-3 mm, or 40 mesh), with an increase of rubber content, the integrity of the specimen was better, and the number and size of surface cracks and penetrating cracks gradually decreased. In particular, when the rubber particle size was 3-5 mm, the integrity of specimens was the best, with only a few fine surface cracks (Figure 10b-d), and only when the rubber content was 10% (Figure 10b), a narrow penetrating crack appeared. However, when the rubber content was 20% or 30%, no penetrating crack appeared. In addition, when respectively mixing 1-3 mm and 40 mesh rubber particles, the crushing degree of specimens was higher than specimens with 3-5 mm rubber particles, and only few materials were peeled off, mainly manifested as tensile damage. When the rubber content was low, it was manifested as shearing and tensile composite damage, conversely, few surface cracks and penetrating crack appeared in the specimens. In conclusion, the ideal cracking resistance was obtained by incorporating 3-5 mm rubber particles with a rubber content of 30%. Rubber has low elastic modulus and high hydrophobicity. Low elastic modulus endows the specimens with high elastic deformation capacity. Table 6 lists the average elastic modulus of each concrete mixture calculated based on the stress-strain curves shown in Figure 8. Since three cubic specimens were prepared for each mixture, the elastic modulus of the three specimens was averagedin order to ensure the reliability of the results. It can be seen from Table 6 that the elastic modulus of concrete decreased sharply by adding rubber particles, and decreased gradually with a decline in rubber particle size or an increase of rubber content. High hydrophobicity weakens the bond between rubber particles and cementitious materials, and stress concentration occurs easily at the location of rubber particles, which reduces the strength of the specimens. When rubber content is low, rubber particles are sparsely distributed in cementitious mixtures, having little effect on the properties of concrete. When rubber particles are large and content is high, the voids between partial coarse aggregates are filled by the rubber particles. The voids between rubber particles are filled with sand, and therefore form a relatively stable spatial structure, which results in few surface cracks and penetrating cracks after failure of the cubic specimen. Although the addition of waste rubber particles into concrete reduced its compressive strength, its properties of crack resistance were improved. Therefore, rubberized concrete can be applied to projects requiring crack resistance, such as building of light fillers for the base layer of pavement and bridge supports. In addition, the incorporation of rubber particles can affect concrete's failure path. Many cracks were distributed in the entire reference specimen, and the evolution, propagation, and coalescence of the cracks eventually lead to its large-scale brittle fracture.