**4. Conclusions**

The effect of crumb rubber on the strength and penetration of chloride ion in recycled aggregate concrete was investigated using the surface resistivity test method. Different concentrations (5%,10%, 15% and 20%) of crumb rubber in recycled aggregate concrete were considered in the study. The results clearly shown that addition of crumb rubber reduces the compressive strength of the recycled aggregate concrete moderately when its concentration is limited to 5%. The study also observes the resistivity of chloride ions for recycled aggregate concrete with 5% crumb rubber concentration. Based on the ultrasonic pulse velocity results, the use of recycled aggregate concrete with crumb rubber content beyond 5% is questionable.

The results of the surface resistivity tests shows that the resistivity of the recycled aggregate concrete to chloride ion penetration is moderate under the air dried curing technique. However, using the water bath technique, the resistivity was found to be low based on the specifications listed in Table 7. Hence, the study suggests the application of recycled aggregate concrete with crumb rubber in the construction industry where the exposure conditions are not extreme. The rubber recycled aggregate concrete could be useful, for example, in making concrete wall panels, concrete lintels and road kerbs where the criteria for exposure conditions for durability are not so critical.

Furthermore, the ductility of recycled concrete with crumb rubber is found to be greater than that of recycled concrete without crumb rubber. As a result, using rubberized concrete for structural members subject to seismic loads is a promising potential application. Rubberized recycled concrete is an alternative solution for meeting sustainability targets and reducing embodied carbon in the construction industry. However, its limitations are the reduction in strength and the willingness of contractors to use the concrete product on new projects due to the previously mentioned associated drawbacks.

#### *Further Research*

Further research on the durability performance of the recycled concrete with crumb rubber concentration using different mix ratios is needed. Pre-treatment of the crumb rubber particles could also be considered when assessing the durability of the recycled concrete.

**Author Contributions:** R.B.A. and Y.C.W. proposed the study concept and designed the experiments. R.B.A. conducted the tests and analyzed the data. The first draught was authored by R.B.A. and Y.C.W. helped with the proofreading. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to convey their heartfelt gratitude and appreciation to Paul Nedwell and John Mason for their assistance with the laboratory experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
