## 4 Conclusions  
The main objective of this study was to investigate the moisture susceptibility (by ITSR values) and the ageing effect (TEAGE procedure) of nanoclay-modified AC mixtures containing flakes of plastic film collected as urban waste. Three AC mixtures were produced: (1) a control one, without plastic and nanoclay; (2) a mixture containing only flakes of plastic film; and, (3) a mixture with both flakes of plastic film and nanoclay. In addition, aged and unaged specimens were considered.  
After testing the AC mixtures and comparing the results, the following conclusions can be drawn:  
* In general, the conditioning for moisture sensitivity caused a decrease in the ITS of the AC mixtures, and the ageing conditioning caused an increase in the ITS of the AC mixtures.
* The use of plastic to modify the AC mixture caused an increase of the ITS values in both ages (unaged and aged) and moisture conditioning (dry and wet), except in the case of the aged dry group where the ITS values were similar. However, the trends observed regarding moisture susceptibility and effect of ageing were not clear.
* The plastic and nanoclay-modified AC mixture presented higher ITS values, lower moisture susceptibility, and enhanced ageing resistance. Regarding moisture susceptibility, the mixture  
Figure 8: Values of ageing index for the tested mixtures and dry/wet specimens.  
presented similar ITS values for the dry and wet specimens, and consequently high ITSR values, indicating that the moisture conditioning had no effect on the properties of the mixture. Regarding ageing, if compared with the control mixture, the plastic and nanoclay-modified AC mixture presented slightly lower ageing index. The improvements obtained can be considered as relatively small, although, it worth mention that the plastic and nanoclay-modified AC mixture had an air void content higher than the control AC mixture, thus theoretically more vulnerable to moisture and ageing effect. Thus, the results obtained from the side of the modified AC mixture must be considered as conservative.
* The reduced air voids content of the AC mixtures tested certainly influenced the results. Lower compaction energy would have led to a higher air voids content and consequently, the moisture susceptibility, as well as the ageing effect, would be higher.  
Therefore, the modification of AC mixtures with flakes of plastic and nanoclay hydrophilic bentonite might be a viable solution for the recycling of plastic film collected as urban waste. From the environmental point of view, it is an eco-friendly alternative to plastic waste disposal in landfills.  
Regarding economic aspects, nanoclay modification is an expensive way to improve AC mixture performance. In fact, the construction cost can increase up to five times in relation to a conventional AC mixture [42]. Furthermore, in spite of plastic film and nanoclay together do not constitute today an industrial alternative to other types of modified mixtures in order to increase performance, this paper underlines that it is a trend with potential. Nanoclay plus plastic film could improve the performance and reduce the environmental burden, using an attractive mixture, even on the life-cycle cost point of view, especially if all the environmental costs are considered.  
Although the reported results give a good indication of the potential of the modifications in evaluation, future research is recommended to see what happens in an AC mixture with a higher air voids content as well as to enhance the effect of ageing and moisture on plastic-modified AC mixtures.  
Conceptualization, A.A., J.C., C.A. and L.P.S.; methodology, A.A., J.C., C.A. and L.P.S.; validation, A.A., J.C., C.A. and L.P.S.; investigation, A.A., J.C. and C.A.; resources, A.A., J.C. and C.A.; writing--original draft preparation, A.A. and C.A.; writing--review and editing, A.A., J.C. and L.P.S.; supervision, L.P.S.; project administration, L.P.S.  
This research received no external funding.  
The authors declare no conflict of interest.