2.4.2. Ultrasonic Impulse Method

After the specified number of sulfate attack, each test piece was removed. An NA-M4 nonmetal ultrasonic detector (shown in Figure 3) was used to measure the ultrasound velocity in the 100-mm cube of concrete. The transducers that were used were 50 mm thick. The ultrasonic method was used to measure the longitudinal wave velocity passing through the concrete samples. The path length between the sensors was 100 mm, the length of the dimensions of a concrete sample. Five pairs of measurement points (shown in Figure 4) were used to measure the ultrasound wave velocity and the average was taken. To ensure proper coupling of the transducer to the surface being measured, Vaseline was evenly applied to the transducer probe. For the entire test process, the ultrasonic frequency was 50 kHz, the transmission voltage was set to 500 V, and the sampling period was 0.4 μs.

**Figure 3.** NA-M4 nonmetal ultrasonic detector.

**Figure 4.** Measuring point positions of the detector.

The longitudinal wave velocity was calculated using Equations (2) and (3).

$$v = l/t\tag{2}$$

$$t = (t\_1 + t\_2 + t\_3 + t\_4 + t\_5) / 5\tag{3}$$

where *v* is the longitudinal wave velocity (m/s); *l* is the path length (mm); *t* is the average ultrasound time (μs); *t*1, *t*2, *t*3, *t*4, and *t*<sup>5</sup> are the ultrasound time values of 5 pairs of measuring points, respectively.

According to the relationship between the dynamic elastic modulus of the material and the ultrasonic sound velocity, the relative dynamic elastic modulus *Er*(*n*) according to the GB/T 50082-2009 standard for both test methods, for long-term performance and durability of ordinary concrete [36], can be expressed as Equation (4).

$$E\_r(n) = E\_n / E\_0 = v\_n^2 / v\_0^2 \tag{4}$$

where *Er*(*n*), *En*, *E*<sup>0</sup> are the relative dynamic elastic modulus of the concrete specimen, the dynamic elastic modulus after erosion time *n*, and the initial dynamic elastic modulus, respectively; *vn*, *v*<sup>0</sup> are the longitudinal wave velocity (m/s) of the concrete specimen after erosion time *n* and the initial longitudinal wave velocity (m/s).

## 2.4.3. Mechanical Test

When each set of sulfate erosion times was reached, the concrete sample was taken out of the solution and dried at room temperature. According to the GB/T 50081-2019 standard for test methods for the physical and mechanical properties of concrete [39], a CSS-YAN3000 press, produced by the Changchun Testing Machine Institute (Changchun, China), was used for uniaxial compressive strength tests of concrete (shown in Figure 5a) as well as split tensile tests (shown in Figure 5b); the loading rates were 3 mm/min and 1 mm/min, respectively. There were 3 parallel concrete samples in each group during the strength tests, and the results are averaged. The loading mode is shown in Figure 5. The corrosion resistance of concrete was reflected by the corrosion coefficient indexes of the compressive and tensile strengths of concrete, before and after being eroded by the sulfate solution. The calculation formulas are shown below (Equations (5) and (6)).

$$k\_{\mathcal{L}} = f\_{\mathcal{cm}} / f\_{\mathcal{c}0} \tag{5}$$

$$k\_t = f\_{\rm tu} / f\_{t0} \tag{6}$$

where *kc*, *fcn*, *fc*<sup>0</sup> are the corrosion resistance coefficient of the compressive strength of a concrete specimen, the compressive strength (MPa) after erosion time *n*, and the uncorroded compressive strength (MPa), respectively; *kt*, *ftn*, *ft*<sup>0</sup> are the corrosion resistance coefficient of the splitting tensile strength of a concrete specimen, the split tensile strength (MPa) after erosion time *n* and the split tensile strength (MPa) without sulfate attack, respectively.

**Figure 5.** Static compression and splitting tensile tests of specimens. (**a**) Static compression test and (**b**) splitting tensile test.
