## 3 Test Results and Discussions  
### Effect of CN on Compressive Strength and Tensile Properties of ECC  
Nanomaterials have been shown to significantly enhance the properties of composite materials. However, they are susceptible to agglomeration, necessitating an investigation into the optimal inclusion level of CNs to enhance the performance of ECCs. The compressive strength data for the ECC samples at 28 d are presented in Figure 3. As observed, the compressive strength of the control sample (without CNs) was 75.2 MPa. The compressive strengths of the CN-ECC samples with 0.05%, 0.10%, 0.15%, and 0.20% CN were 77.3, 81.9, 84.2, and 82.1 MPa, respectively. These values represent increases of 2.79%, 8.91%, 12.0%, and 9.18% compared to the control group. This enhancement is likely due to the nanoscale CN filling the internal pores of the cement matrix and improving the interfacial bonding between the PE and the matrix [35], which enables the sample to withstand higher loads. However, when the CN content reached 0.20%, the compressive strength slightly decreased compared to the 0.15% CN sample. At higher CN dosages, the reinforcing effect on the ECC compressive strength appears to diminish. This could be attributed to the formation of hydrogen bonds between the hydrophilic groups on the surface of the CN, leading to the agglomeration of the CN chains, which prevents effective dispersion within the cement matrix and limits its reinforcing potential [36].  
Figure 3: The compressive strength results of the ECC.  
Figure 2: The sample and instrument for the uniaxial tensile test.  
The uniaxial tensile test results for the ECC samples aged for 28 d are presented in Figure 4. Three specimens were tested per sample group, and the average value was taken as the representative result. The strain rate of the control group (without a CN) was 3.62%. For CN-ECC samples incorporating 0.05%, 0.10%, 0.15%, and 0.20% CN, the strain rates were 4.01%, 4.53%, 5.61%, and 5.42%, respectively. The incorporation of CNs enhances the strain rate of ECC samples, with the effect progressively increasing as the CN content rises, though a slight decrease in strain rate is observed at 0.20% CN content. A hydrophilic CN not only fills the internal pores but also interacts with PE fibers and C-S-H, thereby improving the interfacial bond between the PE fibers and the matrix [35]. This interaction, facilitated by the CN, enhances the bridging effect of the PE fibers within the matrix, distributing the applied load more uniformly and mitigating brittle cracking in cement-based materials, ultimately leading to an improvement in the strain rate of ECCs. The tensile strengths of the five ECC variants are 6.21 MPa, 6.52 MPa, 6.82 MPa, 6.82 MPa, and 6.81 MPa, respectively. The addition of CNs effectively fills the internal pores within the cement matrix, enhances the compactness of the ECC structure, and consequently improves its tensile strength.  
In conclusion, a CN content of 0.05% does not significantly enhance ECC performance, while the improvement observed with 0.20% CN is less pronounced than that achieved with the 0.15% sample group. Taking into account compressive strength, tensile properties, and economic considerations, it is reasonable to incorporate CNs at concentrations of 0.10% and 0.15% into the ECC.