# Influence of Cracking on the Durability of Reinforced Concrete with Carbon Nanotubes
## 2 Materials and Methods
### Concrete Production  
Prior to their incorporation in concrete mixes, the CNT were dispersed by a combination of physical procedures, involving sonication and magnetic stirring of the aqueous suspension. The dispersion process was defined according to a previous study of the authors [17]. The procedure involved 45 min of sonication followed by 60 min of magnetic stirring with 40% of the concrete mixing water. This procedure was selected to obtain maximum level of dispersion without severely damaging the CNT.  
Two concrete mixes were produced with the same water to cement ratios (w/c of 0.55); one CNT-reinforced concrete (CNTRC) and one plain concrete (PC) without CNT, for comparison purposes. The concrete compositions are presented in Table 3. The CNT content, of 0.1% by weight of cement, was defined considering the results of previous studies [20; 22; 23] regarding the mechanical characterization of cement pastes and mortars with 0.015-0.1% of the same type of CNT.  
Concrete mixes were produced in a vertical shaft mixer with bottom discharge. The mixing procedure began with the addition of all the aggregates in descending order of its size and mixed for 3 min with 60% of the total mixing water. After this stage, the mixer was stopped for 1 min. The cement and the remaining 40% of water, with or without dispersed CNT, were then added to the mixture, which was mixed for further 4 min. In total, the mixing procedure lasted approximately 8 min. After mixing, the slump and fresh density were measured according to EN 12350-2 [32] and EN 12350-6 [33], respectively.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Purity** & **Outer Diameter** & **Inner Diameter** & **Length** & **Specific Surface Area** & **Density** \\ \hline \textgreater{}98\% & 20–80 nm & 5–15 nm & 10–20 μm & \textgreater{}60 m\({}^{2}\)/g & \textasci{-}2.1 g/cm\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of carbon nanotubes (CNT) provided by the supplier.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & \multicolumn{2}{c}{**Fine Sand**} & \multicolumn{2}{c}{**Coarse Aggregates**} \\ \cline{2-5}  & **Fine Sand** & **Coarse Sand** & **Fine Gravel** & **Coarse Gravel** \\ \hline Water absorption at 24 h (\%) & 0.16 & 0.38 & 0.63 & 0.37 \\ Density of the dried particles (kg/m\({}^{3}\)) & 2577 & 2595 & 2591 & 2504 \\ Density of impermeable particles (kg/m\({}^{3}\)) & 2588 & 2621 & 2634 & 2528 \\ Density of saturated surface dried & 2581 & 2605 & 2608 & 2513 \\ particles (kg/m\({}^{3}\)) & 1573 & 1512 & 1313 & 1355 \\ Granulometric fraction (d\({}_{15}\)/D\({}_{90}\)) & 0.125/1 & 0.5/4 & 2/6.3 & 5.6/11.2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical characteristics of aggregates.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Mix** & **w/c** & **Cement** & **Water** & **Fine Sand** & **Coarse Sand** & **Fine Gravel** & **Coarse Gravel** & **CNT** \\  & **(kg/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **(g/m\({}^{3}\))** \\ \hline PC & 0.55 & 380 & 214 & 303 & 454 & 242 & 710 & \\ CNTRC & 0.55 & 380 & 210 & 303 & 454 & 242 & 710 & 4222 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Concrete compositions.