# Influence of Cracking on the Durability of Reinforced Concrete with Carbon Nanotubes
## 3 Results and Discussion
### Carbonation Resistance
The \(x_{c,28d}\) in Equation (6) is the carbonation depth after 28 days (\(t_{28}\)) that is higher than Lc. At the end, \(K_{c,CR}\) is evaluated through Equation (7). \[t_{c}=\left(\sqrt{t_{28}}-\ \frac{x_{c,28d}-L_{c}}{K_{c}}\right)^{2} \tag{6}\]  
\[K_{c,CR}=\frac{L_{c}}{\sqrt{t_{c}}} \tag{7}\]  
The carbonation resistance in the uncracked region of artificially cracked concrete was improved with the incorporation of CNT. However, this improvement (10%) was lower than that found in uncracked concrete (18%). Moreover, compared to uncracked specimens, the Kc in the uncracked region of cracked concrete was higher (Table 6), especially in CNT concrete, with no obvious justification for this trend, besides the possible different moisture conditions. In addition, the Kc,CR was similar in concrete with and without CNT. This means that the contribution of CNT in cracks of about 0.05 mm width was not significant. In fact, the carbonation front propagation in this area essentially depends on the CO2 diffusion through the crack, not involving the region around. Therefore, for the same crack width, the behavior in different types of concrete tends to be identical. In naturally cracked specimens, the carbonation depth was lower than the crack depth, which allowed the direct calculation of the carbonation coefficient in the cracked area, \(K_{c,\text{CR}}\). The crack width and the carbonation depth at 7 and 28 days, for each tested concrete, as well as the respective carbonation coefficients, are presented in Table 6. The average crack width of CNT-reinforced concrete was 33% higher and 10% lower than that of reference concrete, for the targets 0.05 and 0.1 mm, respectively (Table 5). As expected, the carbonation rate was significantly higher in the cracked area than in the uncracked region. Moreover, the Kc,CR increased in concrete with wider cracks. However, as for the capillary absorption, the carbonation rate was not greatly affected by the crack width. Therefore, the carbonation behavior should not be significantly affected by the crack width variation between concrete specimens. The carbonation coefficient in the uncracked area was similar to that obtained in reference uncracked concrete. In addition, the Kc,CR was of the same order of that found in artificially cracked specimens, which suggests a little influence of the type of cracking in this mechanism. However, the Kc,CR was slightly reduced (9-10%) in concrete with CNT, regardless of the target crack width. As mentioned, the carbonation front under natural cracking occurred within the crack depth, which allowed the better contribution of the participation of CNT around the crack. In fact, the carbonation front was measured in the surrounding concrete around the crack instead of in the area over the crack tip, as occurred in artificially cracked concrete. Nevertheless, these results suggest the participation of CNT in the retention of microcrack propagation in naturally cracked concrete. Contrary to artificial cracks, the natural cracks intercept the CNT reinforced matrix. Moreover, as the natural cracks are mechanically induced, they introduce greater disturbance in the region around the crack. On the other hand, the artificial cracks are molded without relevant disturbance in the crack vicinity. Therefore, it is expected that the CNT reinforcement can be more effective in the region of natural cracks than in artificial cracks of equivalent width. As found for capillary absorption, the contribution of CNT tended to be higher for lower target crack widths, despite the lower average crack width of PC. It is possible that more small cracks around these cracks are benefited by the CNT bridging effect.