# Development of a New Rubber Buckling-Restrained Brace System for Structures
## 3 Details of Material Implemented in the Design of RBRB
### Concrete Component Design  
As previously mentioned, this study proposes a new design for the BRB system to enhance its vibration dissipation capacity under severe vibrations by incorporating concrete filler as a primary load-bearing component. Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) was selected as the construction material for the RBRB due to its superior strength and flexibility. Numerous analytical and experimental research works have been conducted, and various design equations have been proposed since the early stages of development [84].  
UHPFRC Mix Design and Material Properties  
The mix design used in this research was UHPFRC premix as a dry concrete. This advanced Ultra-High-Performance Concrete material is suitable for use as precast elements. Notably, UHPFRC does not contain coarse aggregates, allowing it to achieve compressive strengths beyond 150 MPa (Figure 11). Its unique blend of steel fibers and cementitious binders, combined with low water content, provides exceptional characteristics, including high compressive and flexural strengths, ductility, and durability.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Rubber Sample** & **Max Load (N)** & **Max Dis (mm)** & **Compressive Strength (Mpa)** \\ \hline
91 \(\times\) 91 \(\times\) 150 mm & 98,000 & 16 & 39.20 \\ \hline
91 \(\times\) 91 \(\times\) 150 mm & 98,000 & 17.5 & 39.20 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Rubber samples’ compression test result.  
Figure 11: Dried concrete in the mixer.  
The UHPFRC mixture is a homogeneous cementitious composite that primarily contains fine aggregates, enabling it to achieve compressive strengths exceeding 150 MPa. The proportions of the materials used in the mix are shown in Table 3.  
In this study, copper-coated micro straight steel fibers were used as a composite material in UHPFRC at a volume of 1.0%. The copper-coated hooked-end steel fibers have a tensile strength of more than 2500 MPa, with a diameter of 0.3 \(\pm\) 0.05 mm and a length of 25 \(\pm\) 1 mm. Figure 12 shows the fiber material used in the UHPFRC matrix.  
Each cubic meter of UHPFRC contains at least 120 kg of steel fibers, which is equivalent to 480 km of steel wire and 24 million pieces of discrete fibers.  
Compressive strength tests were conducted on cubes in accordance with BS EN 12390 Part 3 (British Standards Institute, 2009 c) [85]. The curing periods of 7, 14, 21, and 28 days were considered for the UHPFRC specimens to evaluate their compressive strength. Figure 13 shows the compressive stress testing setup and results for cube specimens. The compressive strength of UHPFRC was significantly higher compared to grade-40 concrete, reaching a maximum strength of 167 MPa in 28 days. The average strength of the samples at 28 days was 151 MPa, confirming the design strength of the UHPFRC concrete used. Throughout the curing periods, the UHPFRC specimens exhibited elastic behavior up to 90-95% of their compressive strength, followed by strain hardening to the peak strength.  
Figure 12: Fiber material used in the UHPFRC matrix.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Material** & **UHPFRC with Steel** \\  & **Fibers (T1) (kg/m\({}^{3}\))** \\ \hline Cement & 850 \\ \hline Densified Silica Fume (SF90) & 200 \\ \hline Dry Silica Fine Sand 30/100 PB & 695 \\ \hline Dry Silica Coarse Sand 16/30 PB & 295 \\ \hline Silica VC2644 & 40 \\ \hline Steel Fiber & 22.5 \\ \hline Free Water & 140 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Material properties of UHPFRC.