Application Cases

Experiment Name: Principles of Ultrasonic and Smart Aggregate Damage Monitoring

Research Direction: Nondestructive testing, damage localization

Experimental Principle: High-frequency signals generated by a transducer propagate through a medium. When encountering defects such as cracks or voids, phenomena like reflection, refraction, and diffraction occur, causing significant attenuation by the time they reach the receiving end. Consequently, certain acoustic parameters change. Based on these changes, the relationship with the mechanical properties of concrete can be established, allowing determination of the degree of structural damage.

Testing Equipment: ATG-2031 power signal source, data acquisition card, control panel

Figure: Principles of Ultrasonic and Smart Aggregate Damage Monitoring

Figure: Ultrasonic Testing System

Experimental Procedure:
The distribution of PZT piezoelectric smart aggregates in steel fiber-reinforced high-strength concrete is shown in Figure 3. To prevent the piezoelectric signals applied to the smart aggregates from propagating directly through the steel bars, which could distort the test signals, specialized cable ties were used to suspend the smart aggregates between two adjacent steel bars. They were labeled SA1, SA2, and SA3. The test slab was divided into two zones (Zone and Zone). After the model was poured, the BNC connectors of the SA  lead wires were protected with sealed plastic bags to prevent oxidative corrosion caused by environmental factors. A multifunctional data acquisition and control system (DACS) was used to monitor the piezoelectric smart aggregates. The smart aggregates acting as sensors were connected to the AD conversion ports of the DACS, while those acting as actuators were connected to the D/A conversion ports of the DACS via a power amplifier. A laptop was used to connect to the system and record data. During testing, the pre-amplification excitation voltage of the D/A converter was 3 V. A frequency-swept sinusoidal wave was used to repeatedly excite the smart aggregates. The sinusoidal sweep signal linearly increased from 100 Hz to 25 kHz within 1 second. The sampling frequency of the data acquisition and control system was 250 kHz. The amplifier provided a fixed gain of 10 and an operating bandwidth of 0.1–250 kHz.

Figure 3: Smart Aggregate Monitoring System

Experimental Results:
The aforementioned acoustic velocity tests indicate that as the volume content of steel fibers increases, the acoustic wave propagation velocity in concrete increases, and the wave impedance is enhanced. The acoustic velocity in SFRC3 (steel fiber-reinforced concrete with 1.8% volume content) is 1.62 times that of plain SFRC. The reason for this increase is that steel fibers have a relatively large specific surface area. Adding a certain volume content of steel fibers to plain concrete increases the bonding thickness between the matrix and the fibers, reduces internal porosity, enhances matrix compactness, and thereby increases acoustic velocity. From the acoustic velocity changes and damage indices measured after explosions, it can be seen that under the first three loading cycles, the differences in acoustic wave velocity across different parts of the slabs were small. The impact of the explosive shock wave on the structures was not significant, with damage indices (D) below 0.05. Under explosive charges of 80 g and 160 g, the acoustic velocity decrease of the SFRC0 slab was more pronounced compared to other SFRC slabs. At the L/2 location, the damage index D of the SFRC0 slab increased from 0 to above 0.9, while the maximum damage index of the slab with 1.8% steel fiber content was 0.4. This further demonstrates that the addition of steel fibers significantly enhances the local impact resistance and shear capacity of the structure. Comparing the damage index variations at L/2 and L/4, the damage index at L/2 was greater than that at L/4 for all slabs, indicating more severe internal structural damage in the direct blast impact zone. However, the damage index at L/2 for the SFRC0 slab also exceeded 0.88. Therefore, during structural repair and reinforcement, special attention should be paid to the parts of the structure directly subjected to impact. For plain reinforced concrete slabs, damage near the supports should also be examined.

Figure: Specifications of  ATG-2000 Series Power Signal Source