# Dispersion and Pressure Sensitivity of Carbon Nanofiber-Reinforced Polyurethane Cement
## Results and Discussions
### Relationship between the Coefficient of Variation and Sensitivity of Resistance  
In theory, the sensitivity of polymer-based materials will be obviously changed owing to change in the degree of fiber dispersion. The addition of the silica fume can effectively reduce the coefficient of variation of CNFPUC, increasing its sensitivity coefficient and making the piezoresistive effect of CNFPUC more pronounced in the elastic stage. For the case in which the content of CNF is 4%, the relationship between the ratio of aggregate to cement, coefficient of variation and sensitivity are shown in Figures 6 and 7. It can be determined that:  
1. When the ratio of polymer to cement is 1:1:1.2, the sensitivity coefficient increases with the addition of silica fume. 2. When the ratio of polymer to cement is 1:1:1.3 or 1:1:1.4, the sensitivity coefficient increases first and then decreases with the addition of silica fume. The minimum value of \(C_{\text{p}}\) is attained and the sensitivity coefficient reaches its maximum value. The maximum uniform dispersion of CNF in CNFPUC occurs and the sensitivity of stress perception is the highest. 3. When the ratio of polymer to cement is higher than 1:1:1.5, the sensitivity coefficient decreases significantly with the addition of the silica fume. 4. When the content of silica fume is free, the sensitivity coefficient first increases and then decreases with increase in the polymer ash ratio. When the ratio of the polymer to cement is 1:1:1.5, the sensitivity coefficient reaches its maximum value. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Type** & **CP1.7** & **CP1.6** & **CP1.5** & **CP1.5-5** & **CP1.4** & **CP1.4-5** & **CP1.4-8** & **CP1.4-12** \\ \hline Hexahedron (S\({}_{\text{h}}\)) & 87.4 & 171.9 & 519.1 & 371.7 & 359.8 & 697.2 & 605.3 & 503.7 \\ CNFPUC Mixture (Sp) & 77.2 & 153.3 & 490.4 & 346.7 & 334.8 & 680.8 & 584.4 & 474.9 \\ (S\({}_{\text{h}}\)-Sp)/S\({}_{\text{h}}\) (\%) & 11.72 & 10.84 & 5.52 & 6.73 & 6.95 & 2.35 & 3.46 & 5.72 \\ \hline
**Type** & **CP1.3** & **CP1.3-5** & **CP1.3-8** & **CP1.3-12** & **CP1.2** & **CP1.2-5** & **CP1.2-8** & **CP1.2-12** \\ \hline Hexahedron (S\({}_{\text{h}}\)) & 270.7 & 580.9 & 688.2 & 560.1 & 225.3 & 286.7 & 428 & 530 \\ CNFPUC Mixture (Sp) & 246.2 & 558 & 668 & 536.2 & 203.3 & 262.3 & 402.1 & 503.7 \\ (S\({}_{\text{h}}\)-Sp)/S\({}_{\text{h}}\) (\%) & 9.04 & 3.95 & 2.93 & 4.27 & 9.77 & 8.52 & 6.04 & 4.96 \\ \hline \end{tabular}
\end{table} 
Table 7: Sensitivity error between sampling hexahedron and corresponding mixture. Figure 6: Relationship curve for resistance coefficient of variation and poly-ash ratio in CNFPUC hexahedron. The sensitivity coefficient and resistivity variation coefficient of the CNFPUC specimen exhibit an obvious one-to-one correspondence. When the content of CNF is in the percolation region and constant, the coefficient of variation of CNFPUC resistivity calculated using the hexahedron is smaller, a higher the sensitivity coefficient corresponds to a pronounced effect of self-induction in the elastic compression stage. The coefficient of variation of resistivity of the hexahedron can reflect the sensitivity of the hexahedron after compression. The dispersion of CNF is closely related to the ratio of polymer to cement and the quantity of silica fume (SF) used in place of the cement, which directly affects the sensitivity of the CNFPUC. The silica fume particles are tiny glass spheres, with an average particle size of 0.1-0.2 \(\upmu\)m, which is 1/100-1/50 of the cement particle size [38; 39]. Under the same poly-ash ratio and a replacement of some cement particles with SF, the particle size of the SF and CNF are similar. The Vander Waals force between the CNF can be reduced and the dispersion of CNF can be increased owing to the friction between the SF aggregate and CNF [40]. In addition, according to the double seepage theory, the SF aggregates are added to the CNFPUC.