# Influence of Cracking on the Durability of Reinforced Concrete with Carbon Nanotubes
## 3 Results and Discussion
### Capillary Absorption  
The incorporation of CNT can essentially reduce the capillary absorption of concrete in two ways: by filler and nucleation effect; by bridging effect. As mentioned, the filler and nucleation effect refine the microstructure of concrete, namely in the paste and at the aggregate-paste interfacial transition zone. The bridging effect contributes to retain the formation and propagation of microcracks, reducing the pore connectivity. In general, both compositions showed absorption curves with similar shape (Figure 3). Contrary to what could be expected, the absorption did not vary linearly with the root of time. This is essentially related to the non-uniform moisture distribution in the concrete depth. In fact, absorption curves are characterized by a higher rate of absorption in the first three hours, followed by a significant decrease in the rate of absorption for later periods. Therefore, for uncracked specimens, each phase was analyzed separately. Two absorption coefficients were calculated, one for the initial period up to 3 h, \(\mathrm{C_{abs,0-3h}}\), and other up to 72 h, \(\mathrm{C_{abs,6-72h}}\). Both coefficients, as well as the water absorption at 72 h, are displayed in Table 5. A high R\({}^{2}\) was obtained in each phase, suggesting a good correlation between the water absorption and the square root of time. There was a significant reduction from \(\mathrm{C_{abs,0-3h}}\) to \(\mathrm{C_{abs,6-72h}}\). At later stages, the higher water content inside the specimens reduces the long-term water absorption and this phenomenon tends to be controlled by the progressive water uptake of less accessible and smaller pores near concrete surface [46]. As expected, the incorporation of CNT enabled the reduction of both coefficients of absorption, indicating the refinement of the concrete microstructure. However, contrary to what was found in a previous study [16], the highest reduction, of around 27%, was obtained for \(\mathrm{C_{abs,6-72h}}\). In fact, a greater contribution of CNT at early age would be expected, involving the water absorption of drier and more cracked surface concrete [30]. As mentioned earlier, this may be related to the higher contribution of the less accessible and smaller pores, where CNT may better participate through their nucleation and filler effects. On the other hand, the rapid water absorption during the first few minutes is affected by the variable surface characteristics of concrete specimens. Nevertheless, the overall reduction of the absorption coefficient was similar to that reported by [PERSON] et al. [24], of about 25%. The absorption at 72 h registered a reduction of 12.3% in CNTRC, which was lower than that found for \(\mathrm{C_{abs,6-72h}}\). This means that CNT contributed to the refinement of the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Fresh Density**} & \multicolumn{2}{c}{**Slump**} & \multicolumn{2}{c}{**Hardened Density**} & \multicolumn{2}{c}{**Compressive Strength**} \\ \hline \multirow{2}{*}{**Mix**} & \(\rho_{\mathrm{f}}\) & \(\Delta\) & & \(\rho_{\mathrm{28d}}\) & \(\Delta\) & \(\mathrm{f_{cm,28d}}\) & \(\Delta\) \\  & **(kg/m\({}^{3}\))** & **(\%)** & **(cm)** & **(kg/m\({}^{3}\))** & **(\%)** & **(MPa)** & **(\%)** \\ \hline PC & 2290 & - & 17.5 & 2354 & - & 45.6 & - \\ CNTRC & 2303 & 0.6 & 18.0 & 2364 & 0.4 & 50.0 & 9.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fresh density (\(\rho_{\mathrm{f}}\)), 28 days hardened density (\(\rho_{\mathrm{28d}}\)), and 28 days compressive strength (\(\mathrm{f_{cm,28d}}\)) of concrete. porous structure, but not that much to the reduction of total porosity. The same was also reported by [30]. In Table 5, the absorption coefficients of artificially cracked specimens are indicated, as well as the average crack depth (I\({}_{c}\)) measured in each concrete composition. As expected, the cracked specimens showed higher absorption coefficients than reference uncracked concrete. This is a result of the higher initial absorption promoted by the crack itself.