## 5 Conclusions  
Mechanical performances, thermal insulation properties, and microstructural analysis of rubberized concrete were executed under uniaxial compression tests, thermal insulation tests, and SEM. The following conclusions are made:  
1. The compressive strength of rubberized concrete was remarkably reduced after introducing rubber particles into concrete, which was negatively correlated with rubber content and positively correlated with rubber particle size. Moreover, when rubber content was high, the effect of rubber particle size on the compressive strength was no longer significant. In addition, by increasing rubber content or decreasing rubber particle size, the peak strain corresponding to the uniaxial compressive strength gradually increased.
2. Rubber can affect the failure modes of cubic specimens due to rubber's low brittleness and high toughness, inhibiting and delaying the appearance and extension of the cracks in specimens. The control specimens showed brittle failure under compression. By adding rubber particles into plain concrete, the number and size of surface cracks and penetrating cracks decreased as rubber content increased. In comparison to the rubber particles of three sizes, the failed specimens mixed with rubber particles of 3-5 mm and rubber content of 30% had the best morphological integrity. Considering that rubberized concrete has high crack resistance and low compressive strength, it can be applied to construction projects requiring crack resistance and lower strength such as base paving.
3. Increasing the rubber content or reducing the rubber particle size contributed to an increase in the thermal insulation performance of the specimens. Additionally, the rubber particle size was positively correlated with heat flux, and negatively correlated with heat resistivity, while the rubber content was the opposite. In particular, when the rubber content was less than 30%, the thermal insulation performance of the specimens increased at a high rate as the rubber content increased. Conversely, the increasing rate was gradually slowed down. Thus, when rubber particles of 40 mesh and 50% content were mixed into concrete, the best thermal insulation was obtained. Due to its good thermal insulation, rubberized concrete can be used to produce thermal insulation materials for buildings.
4. The SEM analysis revealed that rubberized concrete specimens contained many voids, and rubber, as a hydrophobic material, weakened the bond between rubber particles and other aggregates, which can explain the reduction of compressive strength of rubber modified specimens. Under SEM, only a few tiny cracks were found in the rubber modified samples. Moreover, EDS analysis showed that the addition of rubber changed the species and content of elements in concrete which can indirectly reflect the mixing amount of rubber.  
In order to make better use of waste rubber tires, our future work will investigate the effects of other factors such as high temperature treatment of rubberized concrete and the modification of rubber particles on the mechanical properties of rubberized concrete, attempting to prepare rubberized concrete with better performances.  
Conceptualization, [PERSON] and S.H.; methodology, [PERSON] and S.H.; validation, [PERSON]; investigation, [PERSON] and S.H.; data curation, [PERSON] and S.W.; writing--original draft preparation, [PERSON], [PERSON], and M.H.; writing--review and editing, [PERSON] and [PERSON]; supervision, [PERSON] and [PERSON]; funding acquisition, [PERSON].  
This research was funded by the Provincial Key Technological R& D Program of Henan of China (Grant No. 152102210318) and the National Natural Science Foundation of China (Grant No. 51778215).  
The authors greatly appreciate the financial supports from funding bodies, and would be grateful to the reviewers for their valuable comments and suggestions to improve the quality of the paper.  
The authors declare no conflict of interest.