## 4 Materials and Experimental Methodology
### Experimental Setup and Procedure  
The dynamic spalling experiments were carried out using a large-diameter Split Hopkinson pressure bar (SHPB) system, with bars of 74 mm in diameter. Figure 14 depicts the schematic arrangement of the one-dimensional stress spalling experiment. The projectile, also measuring 74 mm in diameter and 100 mm in length, was launched by a gas gun. A 2 mm thick annealed copper disk was used as a pulse shaper, placed on the impact face of the projectile to tailor the incident stress wave, typically by elongating its rise time and promoting a smoother profile. This helps in achieving a more uniform stress state within the specimen before fracture and satisfying the conditions for one-dimensional wave analysis. The compressive incident wave generated upon impact is directed into the concrete or UDRC specimen, leading to spall fracture near its free end. To ensure good mechanical coupling and wave transmission, a thin layer of Vaseline was applied to the impact interface between the specimen and the end of the incident bar. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Density (kg/m\({}^{3}\))** & **Young’s Modulus (GPa)** & **Poisson’s Ratio** & **Compressive Strength (MPa)** & **Tensile Strength (MPa)** \\ \hline
2379 & 32.0 & 0.20 & 60.0 & 4.6 \\ \hline \hline \end{tabular}  
* Properties were determined as the average of three replicate specimens for each test type.  
\end{table} 
Table 4: Quasi-static mechanical properties of the C40 concrete *. \begin{table}
\begin{tabular}{l l} \hline \hline
**Component** & **Specification/Type** \\ \hline Cement & Haichang P.O 52.5 Portland cement \\ Admixture & CL-19 polycarboxylate superplasticizer \\ Supplementary Material & Class F Grade II fly ash \\ Fine Aggregate & Zone II medium sand \\ Coarse Aggregate & Continuously graded crushed stone (10 mm max. size) \\ Water-to-Cement Ratio & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mix design of the C40 grade concrete. A Kirana-05M ultra-high-speed camera system (Specialised Imaging, Tring, UK) was employed to capture the full-field deformation and spalling process of the specimen surface. The camera was typically focused on the region 600 to 900 mm from the impact end of the specimen, where multiple spall fractures were anticipated. This setup provided adequate spatial resolution (approximately 0.334 mm per pixel) for subsequent digital image correlation (DIC) analysis. The imaging system operated at a frame rate of 500,000 frames per second (fps), corresponding to a 2 us interframe sampling interval. For complementary strain measurements, two pairs of electrical resistance strain gauges were symmetrically positioned on the specimen's surface at distances of 300 mm and 600 mm from the impact end. These gauges were used to record the axial strain time history at discrete locations, providing reference data on wave propagation and general strain levels within the specimen. Furthermore, a HSV-E-100 laser Doppler ((Polytec GmbH, Waldbronn, Germany)) vibrometer was utilized to precisely record the particle velocity history at the center of the specimen's free (rear) end. This infrared velocity meter has a spot diameter of approximately 2 mm, a velocity measurement range of 0 to 24 m s\({}^{-1}\), and a high temporal resolution with a sampling capability of up to 4 data points per microsecond. Figure 15 shows representative data obtained from a test, including the strain history recorded by the strain gauges and the velocity history of the specimen's free end recorded by the laser vibrometer. Analysis of such strain histories indicates that the incident stress pulse typically had a duration of approximately 180 us. Given the concrete's wave speed, this corresponds to a wavelength of roughly 1 m within the specimen.