*3.1. Specimen Preparation*

Type I (ASTM C150) ordinary Portland cement (OPC) was used [28]. The fine aggregate was river sand with a fineness modulus of 2.6, and the coarse aggregate was crushed limestone with a maximum aggregate size of 20 mm. A commercial high-range water reducer (HRWR) was added to improve the workability and flowability of the concrete. The measured 28-day compressive strength of the concrete was 32.0 MPa on 150 mm cubes. Table 1 presents the mix proportion of the concrete. The longitudinal reinforcements in the concrete beam was a plain bar made of HPB235, with a nominal diameter of 10 mm, and the stirrups comprised a plain bar made of HPB235, with a nominal diameter of 6 mm. Then, eight RC column specimens of 100 mm × 100 mm × 400 mm were cast. A detailed configuration of the specimens is shown in Figure 1.

**Table 1.** Mix proportions of concrete.


The impressed current method was adopted to induce reinforcement corrosion in the

A total of eight RC column specimens were prepared and divided into four groups with different levels of reinforcement corrosion, namely, 0% (Z-0), noncorroded as a reference; 5% (Z-5); 10% (Z-10); and 15% (Z-15). According to Faraday's law, electrification duration of 30 days, 60 days and 90 days, respectively, represents the corrosion level of

tion apparatus is shown in Figure 2. The surfaces of the specimens were wrapped with sponge and further contained in a stainless-steel cage. In order to keep the RC specimens fully wet prior to inducing the impressed current, the specimens were soaked in a 3.5% NaCl solution for 72 h. During accelerated corrosion, the steel embedded in the specimen was connected to an anode of the direct current (DC)-regulated power supply, and the stainless-steel cage was connected to the cathode. A 3.5% NaCl solution was sprayed us-

*3.2. Corrosion Acceleration* 

ing a sponge every day.

5%, 10% and 15%.

**Figure 2.** Accelerated corrosion test.

**Figure 1.** Reinforcement configuration (units mm). **Figure 1.** Reinforcement configuration (units mm).

(**a**) (**b**)

### *3.2. Corrosion Acceleration* fully wet prior to inducing the impressed current, the specimens were soaked in a 3.5%

*3.2. Corrosion Acceleration* 

**Figure 1.** Reinforcement configuration (units mm).

The impressed current method was adopted to induce reinforcement corrosion in the RC specimens to reach the target level of corrosion. The layout of the corrosion acceleration apparatus is shown in Figure 2. The surfaces of the specimens were wrapped with sponge and further contained in a stainless-steel cage. In order to keep the RC specimens fully wet prior to inducing the impressed current, the specimens were soaked in a 3.5% NaCl solution for 72 h. During accelerated corrosion, the steel embedded in the specimen was connected to an anode of the direct current (DC)-regulated power supply, and the stainless-steel cage was connected to the cathode. A 3.5% NaCl solution was sprayed using a sponge every day. NaCl solution for 72 h. During accelerated corrosion, the steel embedded in the specimen was connected to an anode of the direct current (DC)-regulated power supply, and the stainless-steel cage was connected to the cathode. A 3.5% NaCl solution was sprayed using a sponge every day. A total of eight RC column specimens were prepared and divided into four groups with different levels of reinforcement corrosion, namely, 0% (Z-0), noncorroded as a reference; 5% (Z-5); 10% (Z-10); and 15% (Z-15). According to Faraday's law, electrification duration of 30 days, 60 days and 90 days, respectively, represents the corrosion level of 5%, 10% and 15%.

The impressed current method was adopted to induce reinforcement corrosion in the RC specimens to reach the target level of corrosion. The layout of the corrosion acceleration apparatus is shown in Figure 2. The surfaces of the specimens were wrapped with sponge and further contained in a stainless-steel cage. In order to keep the RC specimens

*Crystals* **2021**, *11*, x FOR PEER REVIEW 4 of 16

**Figure 2.** Accelerated corrosion test. **Figure 2.** Accelerated corrosion test.

A total of eight RC column specimens were prepared and divided into four groups with different levels of reinforcement corrosion, namely, 0% (Z-0), noncorroded as a reference; 5% (Z-5); 10% (Z-10); and 15% (Z-15). According to Faraday's law, electrification duration of 30 days, 60 days and 90 days, respectively, represents the corrosion level of 5%, 10% and 15%.

### *3.3. Test System*

The test system included the AE signal data acquisition system, the loading system and the load and displacement recording system. The DS2-AE acquisition system produced by Beijing Science and Technology Company was adopted in the test. Eight RS-35C sensors with a frequency of 150 kHz and a 40 dB preamplification were used in the AE system. The sampling frequency was set at 3 MHz. As shown in Figure 3, the eight sensors were arranged on the two sides of the columns. The characterization of the AE source was therefore measured and determined by analyzing the features of the waveform [29].

*3.3. Test System* 

ducted at a rate of 0.5 mm/min.

*3.3. Test System* 

**Figure 3.** Sensor location in the reinforced concrete. **Figure 3.** Sensor location in the reinforced concrete. 6: 10, 100, 50

The load and displacement acquisition system consisted of a load sensor, resistance strain gauge, displacement sensor, displacement transmitter and recorder. The details are shown in Figure 4. The loading device was a 100 T high-performance testing machine, and the static loading was executed in the test. According to the standard of the concrete structure test method (GB/T50152-2012) [30], loading in the displacement control was conducted at a rate of 0.5 mm/min. **Figure 3.** Sensor location in the reinforced concrete. 7: 10, 0, 350 8: 90, 0, 50 X Y 3 4 6 8

5: 90, 100, 350

The test system included the AE signal data acquisition system, the loading system and the load and displacement recording system. The DS2-AE acquisition system produced by Beijing Science and Technology Company was adopted in the test. Eight RS-35C sensors with a frequency of 150 kHz and a 40 dB preamplification were used in the AE system. The sampling frequency was set at 3 MHz. As shown in Figure 3, the eight sensors were arranged on the two sides of the columns. The characterization of the AE source was therefore measured and determined by analyzing the features of the waveform [29].

*Crystals* **2021**, *11*, x FOR PEER REVIEW 5 of 16

The load and displacement acquisition system consisted of a load sensor, resistance strain gauge, displacement sensor, displacement transmitter and recorder. The details are shown in Figure 4. The loading device was a 100 T high-performance testing machine, and the static loading was executed in the test. According to the standard of the concrete structure test method (GB/T50152-2012) [30], loading in the displacement control was con-

The test system included the AE signal data acquisition system, the loading system and the load and displacement recording system. The DS2-AE acquisition system produced by Beijing Science and Technology Company was adopted in the test. Eight RS-35C sensors with a frequency of 150 kHz and a 40 dB preamplification were used in the AE system. The sampling frequency was set at 3 MHz. As shown in Figure 3, the eight sensors were arranged on the two sides of the columns. The characterization of the AE source was therefore measured and determined by analyzing the features of the waveform [29].

**Figure 4.** Experimental system. **Figure 4.** Experimental system.

### **4. Results and Discussion**

*4.1. Accumulative AE Hit Number Analysis*

The relationship between the accumulative AE hit number and load levels of the RC columns is shown in Figure 5. The effect of corrosion rate on this relationship is depicted in Figure 5a–d.

**4. Results and Discussion** 

in Figure 5a–d.

*4.1. Accumulative AE Hit Number Analysis* 

The relationship between the accumulative AE hit number and load levels of the RC columns is shown in Figure 5. The effect of corrosion rate on this relationship is depicted

It can be seen from Figure 5 that the development of experimental curves in the four groups is similar. The AE accumulative hit number increases with the increase of the load level. However, there are also significant differences in the curves at different corrosion rates. The curve of Z-0 is similar to that of Z-5. Both curves in these groups show slow growth in the beginning, and a sharp increase occurs when the load level reaches 0.9. This indicates that, when the corrosion rate is below 5%, the internal damage accumulation is slower and lasts for a longer time during the loading process. Therefore, these two groups of specimens show a brittle characteristic, which means internal microcracks are rare before loading. It should be noted that although there is little variation between the two specimens for each group, the trends among different corrosion rates are significant.

Compared to the previous two groups' tests (Z-0, Z-5), the curves of Z-10 and Z-15 are significantly different, showing no obvious turning points. This reveals that the AE signals of these two groups increase in the whole loading process. The initial growth rate of the AE signals is stable and increases until specimen failure. It can be found that with the increase of the corrosion rate, the curve shape changes from steep growth to steady growth. This is because the corrosion of the steel causes a large number of corrosion cracks in the columns [31]. The greater the corrosion rate, the more the cracks occur. During the loading process, with the increase of the load level, the propagation speed of the cracks

increases, which leads to a significant increase in the acoustic emission signal.

**Figure 5.** Correlation curve of the acoustic emission (AE) accumulative hit number and load for corrosion in Z-10 and Z-15 is much more than Z-0 and Z-5, which strengthens the release **Figure 5.** Correlation curve of the acoustic emission (AE) accumulative hit number and load for different corrosion rates

different corrosion rates

energy [33].

*4.2. Analysis of AE Energy Distribution* 

mulative energy in three phases is calculated accordingly.

was calculated on the area under three ranges of the load level (0–0.3, 0.3–0.6 and 0.6–0.9), as shown in Figure 6. In order to reduce the influence of machine error and environmental noise on the critical failure, this paper focuses on the analysis of energy release within the load level of 0–0.9 [32]. The loading process is divided into three phases: preloading (0– 0.3), middle loading (0.3–0.6) and late loading (0.6–0.9). The proportion of the AE accu-

As shown in Figure 6, regardless of the degree of corrosion, the proportion of energy released in the late loading period is the largest but the smallest in the preloading period. The result shows that the internal damage degree of the RC column increases with the increase of the load level. Comparing among the four groups, the energy proportion of Z-0 is 5% and 6%. When the corrosion rate reaches 15% (Z-15), the preloading energy proportion only increases by 6%, which indicates that the corrosion of the reinforcement has little impact on internal damage in the preloading period. Although the internal crack increases with the increment of the corrosion degree on the reinforcement, the crack propagation is not obvious due to the low load level, which may explain the small increase in

In the middle loading period, the energy proportion of Z-0 is 6% and 8%. As the corrosion rate increases, the energy proportion in the middle loading period can increase to 26% and 23% at a growth rate of 20%. This indicates that the load in the middle loading period can enlarge the original microcracks, resulting in a large AE energy release. The

It can be seen from Figure 5 that the development of experimental curves in the four groups is similar. The AE accumulative hit number increases with the increase of the load level. However, there are also significant differences in the curves at different corrosion rates. The curve of Z-0 is similar to that of Z-5. Both curves in these groups show slow growth in the beginning, and a sharp increase occurs when the load level reaches 0.9. This indicates that, when the corrosion rate is below 5%, the internal damage accumulation is slower and lasts for a longer time during the loading process. Therefore, these two groups of specimens show a brittle characteristic, which means internal microcracks are rare before loading. It should be noted that although there is little variation between the two specimens for each group, the trends among different corrosion rates are significant.

Compared to the previous two groups' tests (Z-0, Z-5), the curves of Z-10 and Z-15 are significantly different, showing no obvious turning points. This reveals that the AE signals of these two groups increase in the whole loading process. The initial growth rate of the AE signals is stable and increases until specimen failure. It can be found that with the increase of the corrosion rate, the curve shape changes from steep growth to steady growth. This is because the corrosion of the steel causes a large number of corrosion cracks in the columns [31]. The greater the corrosion rate, the more the cracks occur. During the loading process, with the increase of the load level, the propagation speed of the cracks increases, which leads to a significant increase in the acoustic emission signal.
