## 3 Test Methodologies
### Test Procedure  
All quasi-static tests were accomplished by a compression loading machine (Matest, Treviolo, Italy) at the Guangdong University of Technology and the data were recorded using a TDS-530 (TML, Tokyo, Japan) acquisition instrument. All specimens tested under quasi-static compression were leveled with high-strength gypsum at both ends and were preloaded before the loading test to check the physical centering. The quasi-static compression loading rate of the unconfined specimens was 0.18 mm/min and that of the confined ones was 0.50 mm/min. Generally, an impact test can be divided into two types of impact loading modes according to the impact velocity, as follows: Low-speed hammer impact and high-speed projectile impact. There are two reasons for using an SHPB device in this test. First, the stresses and strains in the concrete can be measured by the elastic compression bars, which is more accurate than the direct measurements using dynamic SGs and LVDTs. Second, the SHPB device covers a wider strain rate range (10\({}^{1}\)-10\({}^{4}\) s\({}^{-1}\)) than the hammer impact device. Note that only the launch pressure of the bullet could be controlled during the impact tests. A unified launch pressure was adopted for each set of specimens, aiming to obtain an identical strain rate in the specimens. However, the actual strain rates in the specimens were based on the strains recorded from the pressure bars using the stress wave theory and were not strictly identical, even though the launch pressures were identical. This is probably due to the friction between the bullet and the barrel. The lengths of the incident bar, transmission bar, and buffer bar of the adopted SHPB device were 5500, 3500, and 1000 mm, respectively. The bar was made from 60 Si2 Mn, whose modulus of elasticity, Poisson's ratio, mass density, shear modulus, and yield strength were \(2.06\times 10^{11}\) N/m\({}^{2}\), \(0.29\), \(7.74\times 10^{3}\) kg/m\({}^{3}\), \(7.99\times 10^{10}\) N/m\({}^{2}\), and \(1.18\times 10^{9}\) N/m\({}^{2}\), respectively. The SHPB experimental technique is based on (1) a one-dimensional stress wave. The transmission of the stress wave and the deformation of the compression bar are only along the longitudinal axis. The premise of this hypothesis is that the compression bar is an elastic material and its diameter is much smaller than the wavelength of the stress wave. (2) The stresses and strains of the short specimen were distributed uniformly along its length direction. Four measurements were adopted to reduce the experimental error in the present study, as follows: (1) The roughness of the contact surface of the specimens was limited to 0.02 mm using an MY259 grinder [37], (2) the contact friction was reduced by applying Vasculine gel at both ends of each specimen and the adjacent rods to decrease the stress wave that developed in the lateral direction in the specimens, (3) the reflection wave without specimen loading was minimized by continuous debugging before the test, with the typical waveform shown in Figure 2, and (4) a small specimen length-diameter ratio and soft material gaskets installed at both ends of the specimens were adopted to minimize the effect of the nonuniformity of concrete on reducing the wave velocity. Given the above assumptions, the axial strains of the specimens can be deduced [43]. The steps are as follows: The average stress, \(\sigma_{s}(t)\), average strain rate, \(\dot{\varepsilon}_{s}(t)\), and average strain, \(\varepsilon_{s}(t)\), of the specimens are calculated using Equation (1) as follows:  
\[\left\{ \begin{array}{l}\sigma_{s}(t)=\frac{1}{2A_{s}}[F_{1}(t)+F_{2}(t)]\\ \dot{\varepsilon}_{s}(t)=\frac{1}{L}[V_{1}(t)-V_{2}(t)]\\ \varepsilon_{s}(t)=\int_{0}^{t}\varepsilon_{s}(t)dt,\end{array} \right.