# Influence of Cracking on the Durability of Reinforced Concrete with Carbon Nanotubes
## 3 Results and Discussion
### Chloride Ion Penetration  
The chloride penetration coefficient, D\({}_{\text{cl}}\), and the crack width and depth are presented in Table 7. A modest decrease of the chloride ion penetration was reached with the addition of CNT (7%) (Figure 5a). The same level of reduction is reported in [24] for the same type and content of CNT. This is attributed to the fact that specimens were wet cured until testing, without pre-loading or drying shrinkage, preventing the development of microcracking. Moreover, contrary to the previous tests, since the chloride migration test involves all the specimen thickness, the contribution of CNT upon surface microcracking becomes less relevant. Therefore, the contribution of CNT is essentially limited to the nucleation and filler effects. In fact, the percentage improvement attained in these tests was close to that found in the long-term 72 h capillary absorption (Section 3.2). The coefficients of diffusion in the uncracked (D\({}_{\text{cl}}\)) and cracked region (D\({}_{\text{cl,CR}}\)) of artificially cracked concrete are presented in Table 7 and Figure 5a. The values of Dcl in uncracked region were similar to those in uncracked concrete, showing that specimens were produced and tested in identical conditions. The incorporation of CNT led to only a 5% reduction of Dcl in the uncracked region (similar to what was found in uncracked specimens), but to as much as 14% in the cracked region. Moreover, the crack depth was slightly higher in CNT concrete (7%, Table 7). Therefore, despite the low microcracking induced around artificial cracks (Section 3.3), the positive contribution of CNT in reducing the D\({}_{\text{CLCR}}\) is confirmed. As expected, the chloride coefficient of the cracked region was significantly higher than that of the uncracked area. However, this increase was lower in concrete with CNT (58%) than in plain concrete (71%). This also suggests a lower influence of cracking in the chloride penetration resistance of concrete with CNT. Note that it is not only the crack alignment that is affected by the chloride penetration, but also the surrounding area, as shown in Figure 5b. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Cracking Condition**} & \multirow{2}{*}{**Mix**} & \multirow{2}{*}{**Crack Width 1**} & \multirow{2}{*}{**Crack Depth, L\({}_{\text{cl}}\)**} & \multicolumn{2}{c}{**Chloride Diffusion Coefficient**} \\ \cline{5-6}  & & & **(mm)** & & **D\({}_{\text{cl}}\)** & **D\({}_{\text{cl},\text{CR}}\)** \\ \cline{5-6}  & & & & **(\(\times\)10\({}^{-12}\) m\({}^{2}\)/s)** & **(\(\times\)10\({}^{-12}\) m\({}^{2}\)/s)** \\ \hline \multirow{2}{*}{Uncracked} & PC & - & - & 15.81 & - \\  & CNTRC & - & - & 14.74 & - \\ \hline \multirow{2}{*}{Artificially cracked} & PC & 0.05 & 20.1 & 15.7 & 26.83 \\  & CNTRC & 0.05 & 18.8 & 14.86 & 23.01 \\ \hline \multirow{3}{*}{Naturally cracked} & PC & 0.05 (0.11) & 49.3 & 58.53 & 123.15 \\  & CNTRC & 0.05 (0.07) & 48.5 & 37.17 & 113.01 \\ \cline{1-1} \cline{2-6}  & PC & 0.1 (0.16) & 49.4 & 66.44 & 129.31 \\ \cline{1-1}  & CNTRC & 0.1 (0.18) & 48.6 & 43.07 & 115.19 \\ \hline \hline \end{tabular}  
* target crack width (measured average crack width).  
\end{table} 
Table 7: Chloride penetration resistance for concrete under different cracking conditions. Figure 5: (a) Chloride coefficients in artificially cracked concrete. (b) Artificially cracked specimens after chloride attack. In naturally cracked concrete, the D\({}_{\text{cl,CR}}\) is independent of the crack length, because it involves all the specimen thickness. This allows a more accurate comparison between different concretes. There was a substantial increase of D\({}_{\text{cl}}\) in natural cracked specimens, even in the uncracked region. This is mainly explained by the different testing conditions, in which the crack crossing the specimen's thickness sped up the chloride penetration (Figure 6). This phenomenon is more relevant in wider cracks.