## 4 Result and Discussion
### Thermal Insulation Properties of Specimens
#### 4.2.2 Heat Flux and Heat Resistivity  
In this study, heat flux and heat resistivity were utilized to evaluate the thermal insulation performance of the specimens. [PERSON]'s study revealed the thermal conductivity values of rubberized concrete were in the range of 0.241\(\sim\)0.443 W/m\(\cdot\)K [24]. Considering that the rubber particles and mix ratios of rubberized concrete used in [PERSON]'s research were similar to that in this study, the same test was not carried out and the thermal conductivity values (\(k\)) obtained from the results of [PERSON]'s research were adopted. The variations of the thermal conductivity are shown in Figure 13. Obviously, the rubber particle size was positively correlated with the thermal conductivity, while the rubber content was the opposite.  
According to the one-dimensional steady heat transfer theory, the heat flux (\(q\)) and heat resistivity (\(r_{k}\)) of a single-layer flat wall are calculated separately by the following two formulas:  
\[q=\frac{k\Delta T}{d}, \tag{1}\]  
\[r_{k}=\frac{d}{k}, \tag{2}\]  
where, \(q\) = heat flux (W/m\({}^{2}\)), \(r_{k}\) = heat resistivity (m\({}^{2}\cdot\)K/W), \(k\) = thermal conductivity (W/m\(\cdot\)K), \(\Delta T\) = temperature difference between thermocouples at the bottom and top of each specimen (\({}^{\circ}\)C), and \(d\) = specimen thickness.  
Figure 12: Thermocouple no-temperature curve of each specimen: (**a**) Rubber content, 15%; (**b**) rubber content, 30%; (**c**) rubber content, 40%; and (**d**) rubber content, 50%.  
Figure 14 shows the variation of heat flux and heat resistivity of specimens. Obviously, decreasing rubber particle size or increasing rubber content reduced heat flux, while inversely increasing heat resistivity. It is noteworthy that, the heat flux was directly proportional to the rubber particle size and inversely proportional to the rubber content. In addition, the heat resistivity is inversely proportional to the rubber particle size and directly proportional to the rubber content. In conclusion, when rubber particles of 40 mesh and 50% content were incorporated into concrete, the best thermal insulation performance was obtained. In view of its good thermal insulation, rubberized concrete can be applied to building insulation.  
Two reasons may account for the results of the thermal insulation tests. First, rubber is a poor conductor of heat, which results in low thermal conductivity of rubberized concrete, and the larger the rubber content, the lower the thermal conductivity. Additionally, the smaller the rubber particles, the better the thermal insulation performance of specimens. A possible explanation is that numerous smaller particles can form denser three-dimensional \"thermal insulation network\" which can effectively block the heat transmission. Secondly, tiny bubbles enter the mixtures with the addition of rubber particles during the mixing process. Air in the tiny bubbles is a poor conductor of heat, which can improve the overall heat insulation capacity, thereby indirectly improving the thermal insulation performance of rubberized concrete. We know that the smaller the particle size of a substance,  
Figure 14: Variation of heat flux and heat resistivity of each specimen: (**a**) Variation of heat flux each specimen and (**b**) variation of heat resistivity of each specimen.  
Figure 13: Thermal conductivity of each specimen.  
the larger its specific surface area, and therefore more air can be adsorbed on the surface of the substance. Therefore, the smaller the rubber particle size, the greater the amount of air brought into concrete by rubber particles during concrete mixing, which may contribute to the improvement of the thermal insulation performance of rubberized concrete.