# Buckling-Restrained Bracing System with Ultra-High-Performance Fiber Concrete
## 2 Implementing UHPFRC Concrete in BRB Device
### Material Properties
#### 2.2.2 Concrete Material Properties  
In this section, a comprehensive comparison between the behavior of the BRB infilled with conventional concrete and UHPFRC was conducted by investigating the material properties of both concrete types. The objective was to analyze and evaluate the differences in behavior and performance between these two materials when employed as infill material in a BRB device. In addition to the compressive and tensile stress-strain behavior, many other parameters have been defined for material properties, including: dilation angle, eccentricity, \(\mathrm{f_{bo}}/\mathrm{f_{co}}\), K and viscosity parameter, which are required to be used as material properties. Two parameters of \(\mathrm{f_{bo}}/\mathrm{f_{co}}\) and K modify the yield surface. \(\mathrm{f_{bo}}/\mathrm{f_{co}}\) is the ratio of biaxial compressive strength to uniaxial compressive strength, which influences the yield surface in a plane stress state. The K parameter is used to define the shape of the failure surface in the deviatoric plane, which is the ratio between distances measured from the hydrostatic axis to tensile and compressive meridians. The other two parameters are the dilation angle and eccentricity, which modify the non-associated potential flow. The dilation angle describes the angle of inclination of the failure surface towards the hydrostatic axis measured in the meridional plane. Eccentricity controls the deviation of the hyperbolic plastic potential from its asymptote. The viscosity parameter is used for the visco-plastic regularization of the concrete constitutive equations. Ultra-High-Performance Fiber-Reinforced Concrete  
The UHPFRC premix, which is suitable to use for precast elements, was used as dry concrete in this research. UHPFRC does not have any coarse aggregates to reach compressive strengths beyond 150 MPa. Its unique blend of steel fibers and cementitious binders caused by low water content provide the extraordinary characteristics of high compressive flexural strengths, ductility and durability. In this study, copper-coated micro straight steel fibers have been used as a composite material in UHPFEC at volume of 1.0%, the cooper steel fiber (C-GSF0325) has a tensile strength of more than 2500 MPa at diameter of 0.3 \(\pm\) 0.05 mm and length of 25 \(\pm\) 1 mm. Figure 6 shows the fiber material used in the UHPFRC matrix. Table 2 shows the UHPFRC mixture composition used in this study. Figure 5: Steel yield stress–plastic strain. Compressive strength tests were conducted on cubes in accordance with BS EN 12,390 Part 3 (British Standards Institute, 2009 c). The curing period of 7, 14, 21 and 28 days was considered for the UHPFRC specimens to check their compressive strength. Figure 7 shows the compressive stress testing setup and result for cubes specimens. Results illustrate a ductile failure and concrete surfaces were observed to remain plain even at total strength loss. As observed in Figure 7, the compressive strength was higher compared to a grade-40 concrete; it reached maximum strength to 167 MPa in 28 days. The average strength of the samples in 28 days was 151 MPa. This comparison highlights the potential superiority of UHPFRC in terms of strength performance compared to conventional concrete in this study. \begin{table}
\begin{tabular}{l l} \hline \hline
**Material** &
\begin{tabular}{l} **UHPFRC with Steel** \\ **Fibers (T1) (kg/m\({}^{3}\))** \\ \end{tabular} \\ \hline Cement & 850 \\ Densified Silica Fume (SF90) & 200 \\ Dry Silica Fine Sand 30/100 PB & 695 \\ Dry Silica Coarse Sand 16/30 PB & 295 \\ Silica VC2644 & 40 \\ Steel Fiber & 22.5 \\ Free Water & 140 \\ 3\% moisture & 30.93 \\ Total air voids & - \\ Total & 2408.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: UHPFRC Mixture composition. Figure 6: Fiber material used in the UHPFRC matrix. Figure 7: UHPFRC testing setup and result (**a**) compressive strength cube test, (**b**) cube compressive strength result.