## 6 Results and Discussion
### Hardened Tests  
#### 6.2.1 Electrical Resistivity  
Figure 12 displays the results of electrical resistivity for various UHPC mixtures. Increasing SF to more than 15% in series 1 mixtures led to a rise in electrical resistance. Similarly, in series 2 and 3 mixtures, increasing SF enhanced electrical resistance. However, it is noteworthy that the increasing electrical resistance trend contradicts compressive resistance. Consequently, the authors undertook a more thorough investigation of this issue, which was previously unexplored. The conflict between compressive resistance and electrical resistivity, attributed to SF, may stem from two factors (to be elaborated on in the subsequent text): (1) SF filling C-S-H layers and (2) the inherent structure of SF.  
The optimal dose of SF in UHPC is generally considered to be around 15-20%, according to previous studies. Exceeding this optimal dose results in a decrease in the expected C-S-H value of the mixture. In UHPC, some of the cement exists as crystalline particles, with only a small portion of real cement being hydrated to form C-S-H. Therefore, using SF at higher doses increases the distance between particles, posing a challenge for C-S-H formation and ultimately weakening resistance. Although using SF at a high dose reduces resistance in UHPC, it also increases electrical resistance. The C-S-H structure consists of layers with capillary paths between them that conduct and transmit a small electric current. These capillary paths facilitate 3D spatial communication. Increasing SF fills these capillary paths, reducing the distance between layers, disrupting the spatial communication, and decreasing compressive strength due to reduced layer adhesion (Figure 13). As a result, the concrete becomes more brittle, with a glass-like fracture pattern. Conversely, lower SF percentages allow for more flexible layers in the UHPC structure, preventing glass fractures. On the other hand, the filling between the layers increases the electrical resistance due to  
Figure 11: Load–displacement curves for the squeeze flow test—checking the effect of SNF: (**a**) Series 1; (**b**) Series 2  
closed capillary paths and greatly increased density. However, this increase in density does not necessarily strengthen the structure.  
Figure 12 reveals that increasing the w/b ratio from 0.18 to 0.22 (i.e., increasing water content in the mixture) leads to a decrease in electrical resistance (consistent with the compressive resistance trend). A higher w/b ratio results in more water in the mixture and increases the volume of pores in the concrete after hydration. These pores serve as small pathways for ion movement, aiding electricity flow and decreasing resistance [89; 90; 91; 92; 93].  
The increase in SNF from 0.7% to 0.9% signifies a rise in density and an enhancement in electrical resistance (Figure 12), aligning with the trend of compressive strength. When superplasticizers are incorporated into concrete, they enhance workability by dispersing cement particles, thereby decreasing the thickness of the water layer surrounding them. This decrease boosts the packing density of the cement particles, resulting in a denser microstructure. Consequently, the route of electric current within the concrete becomes more circuitous, elevating electrical resistance. Furthermore, these additives can alter  
Figure 12: Results of electrical resistivity for series 1, 2, and 3 mixtures.  
Figure 13: Mechanism of high-dose SF in UHPC.  
concrete's pore structure, diminishing pore network connectivity and further elevating electrical resistance [83; 94; 95].