# Influence of Cracking on the Durability of Reinforced Concrete with Carbon Nanotubes
## 4 Conclusions  
In this study, the durability of cracked concrete reinforced with CNT was analyzed by means of capillary absorption, carbonation, and chloride penetration tests.  
The incorporation of CNT was shown to be efficient in improving the mechanical and analyzed durability properties of uncracked concrete, with little influence on the concrete density and workability. Compared to reinforced reference concrete, the mechanical strength and chloride penetration resistance were only moderately improved (up to 10%), while the properties measured in concrete previously subjected to drying conditions (promoting microcrack formation), namely capillary absorption and carbonation resistance, were improved up to 18% and 27%, respectively. In cracked concrete, CNT may further participate in arresting the microcrack propagation near the macrocracks.  
As expected, it was found that the capillary absorption, carbonation, and chloride penetration may be significantly increased in cracked concrete. Moreover, the water absorption, carbonation, and chloride diffusion only slightly varied with the crack width, for the range considered in this study. Cracks over 0.05 mm offered lower resistance to water and gas percolation, regardless of the incorporation of CNT. For the crack width range considered in this study, the main contribution of CNT occurred in the crack vicinity.  
The CNT contributed in reducing the influence of cracking in the capillary absorption. This was confirmed by the 9% and over 40% reduction achieved in the new suggested crack coefficient, \(\beta_{\text{CR}}\), and in the estimated absorption coefficient of the cracked region, respectively. Moreover, an average 12% reduction of the rate of absorption was found in naturally cracked CNT-reinforced concrete.  
The carbonation coefficient through the artificial crack, K\({}_{\text{c,CR}}\), was not significantly altered with the incorporation of CNT. The measured carbonation depth was essentially controlled by the diffusion of CO\({}_{2}\) trough the crack, without involving the CNT-reinforced concrete in its vicinity. However, in naturally cracked concrete, the K\({}_{\text{c,CR}}\) was up to 10% lower in concrete with CNT than in plain concrete. In this case, the carbonation front occurred within the crack depth, allowing to show the more evident participation of CNT in the concrete region around the crack.  
The chloride penetration resistance was improved in both artificially and naturally cracked concrete with the CNT addition. The diffusion coefficient in the artificial crack region was 14% reduced, which led to a lower influence of cracking in the chloride penetration resistance. However, in naturally cracked concrete the CNT was shown to greatly contribute in the crack surrounding area, reducing the D\({}_{\text{cl}}\) in about one third.  
Conceptualization, J.A.B., H.H.A.; methodology, J.A.B., H.H.A.; formal analysis, J.A.B., H.H.A.; investigation, J.A.B., H.H.A. and T.D.; resources, J.A.B.; data curation, J.A.B., H.H.A.; writing--original draft preparation, J.A.B., H.H.A.; writing--review and editing, J.A.B., H.H.A.; funding acquisition, J.A.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors wish to thank Foundation for Science and Technology (FCT) for funding this research under project UIDB/ECI/04625/2020 (CERIS), as well as the companies SECIL and BASF for supplying the materials used in the experiments. The author also acknowledge the collaboration of [PERSON] on the experimental work.  
The authors declare no conflict of interest.