# Electrical Characteristics of Ultra-High-Performance Concrete Containing Carbon-Based Materials
## 3 Results and Discussions  
### Compressive Strength  
Figure 4 shows the compressive strengths at the age of 3 days. The specimens with 0.5% carbon content exhibited negligible differences in compressive strength; thus, 0.5% carbon material content did not affect the strength of concrete. The CNT dispersion did not influence the compressive strength of concrete; however, the compressive strength of the specimens containing chopped, milled fibers, and graphite decreased on increasing their content. The compressive strength difference between the 0.5% and 2.0% CF specimens was 17.1 MPa, between MF specimens it was 15.5 MPa, and between the 0.5% and 1.5% GP specimens it was 40.8 MPa. The 1.5% GP specimen exhibited the lowest compressive strength (32.7 MPa).  
The CNT particles (nanometer size) did not significantly influence the cement, which has micrometer-sized particles, because of their comparatively smaller size. However, CF and MF, with micrometer and millimeter sizes, respectively, could affect the microstructure of UHPC. Furthermore, carbon fibers influence the UHPC process, forming a dense structure with a high compressive strength [31; 32]. In UHPC, therefore, mixing a carbon material of a size greater than a micrometer could adversely affect its compressive strength.  
Despite the nanometer size of expanded graphite, a significant decrease was observed in the compressive strength of the GP specimens. Expanded graphite contains numerous pores and generates a loosely formed concrete microstructure [33; 34]. Moreover, the expanded graphite absorbed water through its pores, reducing the water available for concrete hydration, thereby decreasing its compressive strength. Therefore, increasing the mixing content of graphite significantly lowered the compressive strength of concrete.  
Figure 3: The complex impedance plane plot.  
Figure 5 shows the compressive strengths for each variable at the age of 28 days; the compressive strengths exhibited similar trends at the ages of 3 and 28 days. For each variable, the compressive strength decreased as the mixing content of the carbon material increased; additionally, the strength decreased significantly when using expanded graphite.  
According to the literature, the compressive strength of concrete increases by increasing the amount of carbon material [35; 36]. However, normal strength concrete has a relatively loose structure compared to UHPC, and the presence of nanomaterials could increase the microstructure density. The 120 MPa UHPC exhibited negative effects on compressive strength enhancement due to its density characteristics. Furthermore, carbon nanomaterials absorb a significant amount of water over time [37]; therefore, the water availability for concrete hydration decreases with increasing carbon content, finally becoming insufficient. Consequently, despite the high performance of carbon materials, their addition compromises the mechanical performance of concrete. These phenomena could significantly affect UHPC due to its low W/B ratio.  
Figure 4: Compressive strengths at the age of 3 days.  
Figure 5: Compressive strengths at the age of 28 days.