# Material Characterization for Sustainable Concrete Paving Blocks
## 4 Conclusions  
According to the experiment results in the properties of concrete paving blocks mixed with different types of recycled wastes, the following conclusions can be drawn:  
* Using RCCA as coarse aggregate for concrete paving blocks can lead to a significant decrease of both compressive strength and tensile splitting strength, and also increase the water absorption of the concrete paving blocks. The abrasion resistance test results indicate that more than 80% RCCAreplacing the natural coarse aggregate can have a negative impact on the abrasion resistance. In order to meet the standard requirements, the replacement level of RCCA shall be less than 60%.
* As a recycled waste used to replace the fine aggregate in the blocks, RCFA has an adverse effect on the compressive strength and tensile splitting strength of concrete blocks. Plus, a higher percentage of RCFA also leads to higher water absorption of concrete paving blocks. Nevertheless, adding RCFA in the blocks do not have a negative effect on the block's abrasion resistance. To sum up, in order to meet the standard requirements, the replacement level of RCFA shall be less than 20%.
* Replacing the coarse aggregate with crushed glass (CG) can enhance the strength of concrete paving blocks. With the replacement level ranging from 10% to 40%, all concrete paving blocks mixed with crushed glass had higher strength and lower water absorption than the blocks mixed with natural aggregates. When the replacement level of crushed glass is at 20%, the concrete paving blocks have the highest compressive strength and lowest water absorption. The tensile splitting strength of concrete blocks mixed with crushed glass reached the maximum value when the replacement level was 40%. Furthermore, the highest performance of the block's abrasion resistance was observed when the replacement level of CG was 30%.
* With an increasing amount of crumb rubber in the concrete paving blocks, there is not only a sharp decrease of compressive strength and tensile splitting strength, but also a rapid increase of the water absorption. Adding crumb rubber can also lead a slightly reduction of the block's slip resistance. Although the value of groove length keeps decreasing while the content of CR increasing, the average value of groove length is still very high. To sum up, the crumb rubber is not recommended to replace the fine aggregates of concrete paving blocks.
* Replacing cement with a certain content of GGBS is proved to have a positive effect on the block's properties. Considering the variation of compressive strength, tensile splitting strength and water absorption, 30% and 50% replacement levels of GGBS have an almost equal positive impact on those properties. The blocks with 30% GGBS that replaced cement have a higher performance of abrasion resistance than the one with 50% GGBS. Furthermore, using GGBS to replace 70% cement in the concrete mixture turns out to be harmful to the block's five properties that were tested in this experiment. In general, the replacement level of GGBS shall be equal to or less than 50%.
* This series of experiments and test results provide a large amount of reference data for the further investigation about the concrete paving block mixed with multiple recycled materials. In order to ensure the block mixed with multiple recycled materials could meet the standard requirements, the replacement levels of each recycled materials can be considered as: RCCA 0% to 60%; RCFA: 0% to 20%; CG: 0% to 40%; GGBS: 0% to 50%.  
Conceptualization, C.S.C.; methodology, X.W.; investigation, X.W.; writing--original draft preparation, X.W.; writing--review and editing, C.S.C., J.X.; supervision, C.S.C., J.X.  
This research was funded by Xi'an Jiaotong-Liverpool University.  
The authors gratefully acknowledge the technical support received from Civil Engineering Department of Xi'an Jiaotong-Liverpool University.  
The authors declare no conflict of interest.