Inspection device and inspection method for concrete structures

Abstract

This method allows for accurate, non-destructive investigation of voids in concrete structures. [Solution] The acquisition unit inputs a pulse wave to the investigation area of ​​the concrete structure that is the subject of the investigation to determine whether or not there are voids, and acquires the measured waveform of the reflected wave measured at the investigation area. The theoretical value calculation unit calculates the theoretical value of the frequency of the reflected wave of the pulse wave in the concrete structure. The correlation calculation unit finds the cross-correlation function between the measured waveform and the waveform having the frequency related to the theoretical value. The estimation unit estimates whether or not there are voids in the concrete structure based on the peak frequency of the cross-correlation function.

Description

Technical Field

[0005] , ,

[0001] The present invention relates to an inspection device and an inspection method for concrete structures.

Background Art

[0002] A post-tensioned prestressed concrete (PC) bridge is known in which a sheath for inserting PC steel bars is placed and then concrete is placed, and then the PC steel bars are inserted into the sheath and the PC steel bars are tensioned. In such a post-tensioned PC bridge, after the PC steel bars are tensioned, the sheath is filled with grout in order to prevent corrosion of the PC steel bars and ensure the integrity of the PC steel bars and the concrete. However, it has been found that some old post-tensioned PC bridges have insufficient grout filling. Since the unfilled part of the grout becomes a void, the PC steel bars are exposed to air and corroded, and due to corrosion or the like, the cross-sectional area of the PC steel bars decreases and stress concentration occurs, which poses a risk of becoming the starting point of fracture of the PC steel bars. Therefore, it is required to investigate the state of concrete structures such as post-tensioned PC bridges. Patent Document 1 discloses a technique for non-destructively investigating the grout filling state using ultrasonic waves.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide an inspection device and an inspection method for concrete structures capable of accurately investigating the presence or absence of voids in concrete structures non-destructively.

Means for Solving the Problems

[0005] According to one aspect of the present invention, a concrete structure inspection device includes: an acquisition unit that inputs a pulse wave to an inspection area of ​​a concrete structure to be investigated for the presence or absence of voids and acquires a measured waveform of the reflected wave measured at the inspection area; a theoretical value calculation unit that calculates a theoretical value of the frequency of the reflected wave of the pulse wave in the concrete structure; a correlation calculation unit that finds a cross-correlation function between the measured waveform and a waveform having a frequency related to the theoretical value; and an estimation unit that estimates the presence or absence of voids in the concrete structure by comparing the peak frequency of the cross-correlation function with the theoretical value of the frequency of the reflected wave. [Effects of the Invention]

[0006] According to the above embodiment, the concrete structure inspection device can accurately and non-destructively investigate the presence or absence of voids in the concrete structure. [Brief explanation of the drawing]

[0007] [Figure 1] This figure shows an example of a concrete structure inspection device according to the first embodiment. [Figure 2] This is a schematic block diagram showing the software configuration of a computer according to the first embodiment. [Figure 3] This figure shows an example of the arrangement of the ultrasonic sensor 20 for calculating the theoretical reflection frequency according to the first embodiment. [Figure 4] This is a flowchart showing the method for calculating the theoretical reflection frequency according to the first embodiment. [Figure 5] This is a flowchart showing the method for estimating voids according to the first embodiment. [Figure 6] This figure shows an example of a map image according to the first embodiment. [Figure 7] This diagram illustrates the principle of the investigation device 1 according to the first embodiment. [Figure 8] This figure shows an example of a map image according to the second embodiment. [Figure 9] This is a schematic block diagram showing the configuration of a computer according to at least one embodiment. [Modes for carrying out the invention]

[0008] <First Embodiment> 《Inspection device for concrete structures》 The embodiments will be described in detail below with reference to the drawings. Figure 1 shows an example of a concrete structure inspection device according to the first embodiment. The inspection device according to the first embodiment investigates the presence or absence of voids inside a concrete structure. The object of investigation according to the first embodiment is a bridge girder T of a PC post-tensioned bridge. The bridge girder T of the PC post-tensioned bridge is made of concrete. A hollow sheath T1 through which PC steel passes is arranged inside the bridge girder T. The sheath T1 is arranged longitudinally at least in the web portion of the bridge girder T. If the sheath T1 is properly filled with grout, there are no voids in the bridge girder T. On the other hand, if the grout filling is insufficient, voids may exist inside the sheath T1. The inspection device 1 according to the first embodiment investigates the presence or absence of voids due to the lack of grout filling inside the sheath T1.

[0009] The survey device 1 comprises a computer 10 and an ultrasonic sensor 20.

[0010] The ultrasonic sensor 20 comprises a transmitter 21 that emits ultrasonic waves and four receivers 22 that receive ultrasonic waves. The transmitter 21 and receivers 22 are configured to be installed independently. For example, the ultrasonic sensor 20 may have a jig that fixes the positional relationship between the transmitter 21 and the receivers 22, and the transmitter 21 and receivers 22 may be detachable from the jig. In the example shown in Figure 1, the four receivers 22 are provided above, below, to the right, and to the left of the transmitter 21. The receivers 22 may be accelerometers. The ultrasonic waves emitted by the ultrasonic sensor 20 are selected to have a frequency high enough to prevent multiple reflections in the unfilled portions of the grout.

[0011] Figure 2 is a schematic block diagram showing the software configuration of the computer according to the first embodiment. The computer 10 estimates the presence or absence of gaps in the bridge girder T based on the waveform data measured by the ultrasonic sensor 20. The computer 10 according to the first embodiment includes an acquisition unit 11, a theoretical value calculation unit 12, a correlation calculation unit 13, an estimation unit 14, an output unit 15, and a storage unit 16.

[0012] The acquisition unit 11 acquires measurement signals from the ultrasonic sensor 20 and the four receivers 22, and generates ultrasonic waveform data. The transmitter 21 and the four receivers 22 are time-synchronized, and the acquisition unit 11 can recognize the relationship between the timing of ultrasonic emission by the transmitter 21 and the timing of ultrasonic reception by the receivers 22. The theoretical value calculation unit 12 calculates the theoretical value of the reflected wave frequency (theoretical reflection frequency) when a pulse wave is input to the bridge girder T, based on the measurement signal acquired by the acquisition unit 11. The theoretical value calculation unit 12 records the calculated theoretical value in the storage unit 16. The correlation calculation unit 13 determines the cross-correlation function based on the measurement signal acquired by the acquisition unit 11 and the theoretical reflection frequency calculated by the theoretical value calculation unit 12. Hereinafter, the waveform obtained by the cross-correlation function will also be called the cross-correlation waveform. The estimation unit 14 estimates the presence or absence of gaps in the bridge girder T by comparing the cross-correlation waveform peak frequency with the theoretical reflection frequency obtained by the theoretical value calculation unit 12. The estimation unit 14 records the estimation result in the memory unit 16, associating it with the location of the surveyed area. The output unit 15 outputs an estimation screen to a display or the like, which shows the relationship between the location of the survey area estimated by the estimation unit 14 and the estimation result.

[0013] Method for estimating void space Figure 3 shows an example of the arrangement of the ultrasonic sensor 20 for calculating the theoretical reflection frequency according to the first embodiment. First, the user of the inspection device 1 refers to the design drawing of the bridge girder T or the like, and attaches the transmitter 21 to a portion (first reference site P21) on the side surface of the web of the bridge girder T where the sheath T1 does not exist inside. Further, the user attaches one receiver 22 to the second reference site P22 facing the first reference site P21. When the transmitter 21 and the receiver 22 are attached to the bridge girder T, the user operates the computer 10 to cause the computer 10 to execute a calculation process of the theoretical value of the frequency.

[0014] FIG. 4 is a flowchart showing a method for calculating the theoretical reflection frequency according to the first embodiment. When the computer 10 starts the calculation process of the theoretical value of the frequency, the theoretical value calculation unit 12 outputs a transmission instruction of a pulse wave to the transmitter 21 of the ultrasonic sensor 20 (step S1). Thereby, a pulse wave is output from the transmitter 21. The acquisition unit 11 acquires the transmission signal of the transmitter 21 and the measurement signal of the receiver 22 from the ultrasonic sensor 20 (step S2). Since the receiver 22 is provided to face the transmitter 21, the waveform of the measurement signal of the receiver 22 represents the waveform of the transmitted wave of the pulse wave output by the transmitter 21. The theoretical value calculation unit 12 obtains the time from the time when the transmitter 21 outputs the pulse wave to the time when the transmitted wave is detected by the receiver 22 as the propagation time of the transmitted wave (step S3). Since the propagation time of the reflected wave is twice the propagation time of the transmitted wave, the theoretical value calculation unit 12 obtains the reciprocal of twice the propagation time of the transmitted wave (the period of the reflected wave) as the theoretical reflection frequency (step S4). The theoretical value calculation unit 12 records the obtained theoretical reflection frequency in the storage unit 16.

[0015] Next, the user of the inspection device 1 refers to the design drawing of the bridge girder T or the like, and attaches the transmitter 21 to the center of the portion (inspection site P1) to be inspected for voids as shown in FIG. 1. Further, the user attaches four receivers 22 at equal intervals above, below, left, and right in the vicinity of the transmitter 21 so as to surround the transmitter 21. When the transmitter 21 and the receivers 22 are attached to the bridge girder T, the user operates the computer 10 to cause the computer 10 to execute an estimation process of the void.

[0016] Figure 5 is a flowchart showing the method for estimating voids according to the first embodiment. When the computer 10 starts the gap estimation process, the estimation unit 14 receives input of position information for each receiver 22 from the user (step S11). For example, the position of the receiver 22 may be represented in a Cartesian coordinate system that represents its position on the side surface of the web of the bridge girder T. For example, the Cartesian coordinate system may be represented by an X-axis extending in the longitudinal direction of the bridge girder T and a Y-axis extending in the height direction, and the user inputs the position information of the receiver 22 according to this Cartesian coordinate system. The position of the receiver 22 may be obtained by analyzing images taken of the bridge girder T by the computer 10, or by measurement using other sensors.

[0017] Next, the estimation unit 14 outputs a pulse wave transmission instruction to the transmitter 21 of the ultrasonic sensor 20 (step S12). As a result, a pulse wave is output from the transmitter 21. The acquisition unit 11 acquires the transmission signal from the transmitter 21 and the measurement signals from each receiver 22 from the ultrasonic sensor 20 (step S13). Since the receiver 22 is mounted on the same plane as the transmitter 21, the measurement signal from the receiver 22 represents the waveform of the reflected pulse wave output by the transmitter 21. The correlation calculation unit 13 reads the theoretical reflection frequency of the pulse wave from the storage unit 16 and generates a waveform for one period of the reflected wave based on the theoretical value (theoretical reflection waveform) (step S14). The correlation calculation unit 13 obtains a cross-correlation waveform by calculating the cross-correlation function between the measurement signal acquired in step S13 and the theoretical reflection waveform generated in step S14 (step S15). The cross-correlation waveform represents the relationship between time and the correlation coefficient.

[0018] The estimation unit 14 identifies the peak frequency of the cross-correlation waveform (step S16). For example, the estimation unit 14 converts the cross-correlation waveform into a power spectrum using FFT (Fast Fourier Transform) or the like to identify the frequency with the highest power. Next, the estimation unit 14 calculates the error rate between the peak frequency identified in step S16 and the theoretical reflection frequency obtained in step S4 (step S17). If an air gap exists in the survey area P1, the error rate between the peak frequency and the theoretical value will be large. From the error rate obtained in step S17, the estimation unit 14 calculates an air gap risk value representing the probability of the air gap's existence (step S18). The air gap risk value is a value that increases monotonically with respect to the error rate. The air gap risk value may be the same value as the error rate. The estimation unit 14 associates the position of the receiver 22 identified in step S11 with the air gap risk value calculated in step S18 and records it in the storage unit 16.

[0019] The estimation unit 14 receives input from the user indicating whether or not there is a next survey area P1 (step S19). If there is a next survey area P1 (step S19: YES), the process returns to step S11 and processing is performed for the next survey area P1. On the other hand, if there is no next survey area P1, i.e., if estimation processing has been performed for all survey areas P1 to be surveyed (step S19: NO), the output unit 15 generates a map image showing the relationship between the position of the bridge girder T and the void risk value, which is recorded in the storage unit 16, and displays it on the display (step S20).

[0020] Figure 6 shows an example of a map image according to the first embodiment. The map image in Figure 6 shows an example in which void risk values ​​were determined by moving a set of four receivers 22 over multiple survey areas P1. In the map image in Figure 6, each of the four receivers 22 at each survey area P1 is colored according to the void risk value. By referring to such a map image, users can identify sheaths T1 where voids exist, i.e., where grout has not been filled, and take appropriate action such as refilling the grout.

[0021] Action / Effect Figure 7 is a diagram illustrating the principle of the investigation device 1 according to the first embodiment. Here, we will explain why the presence or absence of a void can be estimated by the inspection device 1 according to the first embodiment. If there is no void B in the inspection area P1, the pulse wave input from the side surface (input surface) of the inspection area P1 is reflected by the opposing surface (opposing surface) and returns to the input surface. Since this reflected wave travels back and forth between the input surface and the opposing surface, it appears as a reflected wave W1 with a frequency corresponding to the thickness of the inspection area P1 and the propagation speed of the pulse wave. On the other hand, if there is a void B in the inspection area P1, the pulse wave input from the input surface of the inspection area P1 bypasses the void B, reaches the opposing surface, and appears as a reflected wave W21 that returns to the input surface. Alternatively, the pulse wave appears as a reflected wave W22 that is reflected by the void B and returns to the input surface. Therefore, if there is a void B in the inspection area P1, the correlation between the waveform of the reflected wave and the thickness of the inspection area P1 becomes small. That is, when the pulse wave bypasses the void, the frequency of the reflected wave becomes low, and when the pulse wave is reflected by the void, the frequency of the reflected wave becomes high.

[0022] As can be seen from Figure 7, when there is no gap B in the surveyed area P1, the period of the reflected wave is twice the time it takes for the pulse wave to travel from the input surface to the opposing surface. To confirm this, the inventors conducted the following first experiment. At seven observation points in a bridge girder T with a web thickness of 150 mm where there was no sheath T1, the inventors placed the transmitter 21 and receiver 22 opposite each other in the web thickness direction, as in Figure 3, and calculated the propagation speed of the transmitted wave. The inventors also placed the transmitter 21 and receiver 22 side by side on the input surface, as in Figure 1, and measured the peak frequency of the reflected wave. When the web thickness was calculated from the obtained propagation speed and peak frequency, the error rate compared to the measured value of the web thickness was generally less than 10%.

[0023] Thus, if there is no void B in the surveyed area P1, the reflected pulse wave will have a peak frequency corresponding to the web thickness. On the other hand, if there is a void B, the reflected pulse wave will contain many components that differ from the frequency corresponding to the web thickness due to reflection and bypassing of the void. Utilizing this, the survey device 1 according to the first embodiment estimates the possibility of a void existing based on the error rate between the peak frequency of the cross-correlation waveform between the measured waveform of the reflected pulse wave and the theoretical reflected waveform, and the theoretical reflected frequency.

[0024] <Second Embodiment> In the first embodiment, the inspection device 1 determines the void risk value for each of the four receivers 22 installed at the inspection site P1. In contrast, in the second embodiment, the inspection device 1 uses the measurement signals from the four receivers 22 to cancel out noise and determine a single void risk value for the inspection site P1.

[0025] Specifically, the estimation unit 14 according to the second embodiment superimposes the measurement signals received by the four receivers 22 (by calculating the sum or generating an average waveform) to obtain a single signal. This signal is obtained by superimposing the signals, canceling out noise and emphasizing the reflected wave component. The estimation unit 14 uses this signal to calculate one void risk value for the survey area P1 in steps S15 to S18. Figure 8 is a diagram showing an example of a map image according to the second embodiment. The map image shown in Figure 8 shows an example in which void risk values ​​are obtained for multiple survey areas P1 while moving a set of four receivers 22. In the map image of Figure 8, the center positions of the four receivers 22 are colored according to the void risk value at each survey area P1. As a result, the survey device 1 according to the second embodiment can suppress the influence of noise and obtain void risk values ​​with high accuracy.

[0026] <Third Embodiment> The investigation device 1 according to the first embodiment determines the air gap risk value from the error rate between the peak frequency of the cross-correlation waveform and the theoretical reflection frequency. In contrast, the investigation device 1 according to the third embodiment evaluates the presence or absence of air gaps using a binary value, rather than determining the air gap risk value. The estimation unit 14 according to the third embodiment determines the presence or absence of a void in the surveyed area P1 by comparing the error rate between the peak frequency and the theoretical value with a predetermined threshold. In other words, the estimation unit 14 determines that there is a void if the error rate between the peak frequency and the theoretical value exceeds the threshold, and determines that there is no void if the error rate between the peak frequency and the theoretical value does not exceed the threshold. From the above experiment, the error rate between the peak frequency and the theoretical value is generally less than 10% when there is no void, so the threshold for determining the presence or absence of a void can be set to, for example, 10%.

[0027] <Fourth Embodiment> The investigation device 1 according to the first embodiment determines the theoretical reflection frequency from the propagation time of the transmitted wave using the method shown in Figure 4, but is not limited to this. The investigation device 1 according to the fourth embodiment determines the theoretical reflection frequency without measuring the propagation time of the transmitted wave. The propagation speed of transmitted waves in concrete structures is defined in standards such as JIS. In other words, if a concrete structure is made according to the standard, the theoretical reflection frequency can be determined using the propagation speed defined in that standard. Specifically, the theoretical value calculation unit 12 can determine the theoretical reflection frequency by measuring the thickness of the bridge girder T and dividing the propagation speed by twice the thickness.

[0028] <Fifth Embodiment> In the first embodiment, the investigation device 1 determines one theoretical reflection frequency from the propagation time of the transmitted wave using the method shown in Figure 4, and calculates the error rate between this and the peak frequency of the cross-correlation waveform. On the other hand, because the concrete material is not homogeneous, the reflection frequency may differ depending on the location even in the same concrete structure. In the fifth embodiment, the void risk is calculated in consideration of the variation in reflection frequencies.

[0029] In the fifth embodiment, the theoretical value calculation unit 12 of the investigation device 1 determines the theoretical reflection frequency for multiple parts of the side surface of the web of the bridge girder T where the sheath T1 does not exist inside (first reference part P21) using the procedure shown in Figure 4. Based on this, the theoretical value calculation unit 12 estimates the range (upper and lower limits) of the theoretical reflection frequency.

[0030] In step S17 of Figure 5, the estimation unit 14 calculates the error rate between multiple theoretical reflection frequencies within the estimated range and the peak frequency of the cross-correlation waveform, and identifies the smallest error rate. If there are voids in the survey area, the peak frequency of the cross-correlation waveform is likely to exceed the range of variation in reflection frequencies. Also, the ultrasonic waves emitted by the ultrasonic sensor 20 are selected to have a frequency high enough to prevent multiple reflections in the unfilled parts of the grout. Therefore, according to the fifth embodiment, if there are voids in the survey area, the error rate will not be zero. As a result, for example, the estimation unit 14 according to the fifth embodiment can estimate that the possibility of voids being present is low for areas with an error rate of 1% or less, and that there is a possibility of voids being present for areas with an error rate exceeding 1%.

[0031] <Other Embodiments> Although one embodiment has been described in detail above with reference to the drawings, the specific configuration is not limited to that described above, and various design changes are possible. In other embodiments, the order of the above-described processes may be changed as appropriate. Also, some processes may be executed in parallel. The survey device 1 according to the above embodiment may be composed of a single computer 10, or the configuration of the survey device 1 may be divided among multiple computers 10, and the multiple computers 10 may cooperate with each other to function as the survey device 1.

[0032] The inspection device 1 according to the above embodiment determines the presence or absence of air gaps from the frequency error rate, but is not limited to this. For example, the thickness of the inspection area P1 may be determined from the peak frequency according to another embodiment, and the presence or absence of air gaps may be determined from the error rate between that thickness and the actual thickness of the inspection area P1. Similarly, the inspection device 1 may determine the presence or absence of air gaps by converting the peak frequency of the cross-correlation waveform to another value.

[0033] In the above-described embodiment, the concrete structure to be investigated is a bridge girder T, but in other embodiments, the investigation device 1 may be limited to this and may investigate the presence or absence of voids in other concrete structures.

[0034] The ultrasonic sensor 20 according to the above-described embodiment comprises four receivers 22, but is not limited to this in other embodiments. For example, the ultrasonic sensor 20 according to other embodiments may have one receiver 22, two receivers 22, three receivers 22, or five or more receivers 22.

[0035] <Computer Configuration> Figure 9 is a schematic block diagram showing the configuration of a computer according to at least one embodiment. Computer 10 includes a processor 91, main memory 92, storage 93, and interface 94. The operation of each processing unit of the computer 10 described above is stored in storage 93 in the form of a program. The processor 91 reads the program from storage 93, loads it into main memory 92, and executes the above processing according to the program. The processor 91 also allocates memory areas in main memory 92 corresponding to each of the above-mentioned storage units according to the program. Examples of the processor 91 include a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), and a microprocessor.

[0036] The program may be for implementing some of the functions that the computer 10 is to perform. For example, the program may perform functions in combination with other programs already stored in storage, or in combination with other programs implemented on other devices. In other embodiments, the computer 10 may include a custom LSI (Large Scale Integrated Circuit) such as a PLD (Programmable Logic Device) in addition to, or instead of, the above configuration. Examples of PLDs include PAL (Programmable Array Logic), GAL (Generic Array Logic), CPLD (Complex Programmable Logic Device), and FPGA (Field Programmable Gate Array). In this case, some or all of the functions implemented by the processor 91 may be implemented by the integrated circuit. Such an integrated circuit is also included as an example of a processor. In other embodiments, the computer 10 may be virtualized on one or more computers.

[0037] Examples of storage 93 include magnetic disks, magneto-optical disks, optical disks, and semiconductor memory. Storage 93 may be an internal medium directly connected to the bus of the computer 10, or it may be an external medium connected to the computer 10 via an interface 94 or a communication line. Furthermore, if this program is delivered to the computer 10 via a communication line, the computer 10 that receives the delivery may load the program into the main memory 92 and execute the above processing. In at least one embodiment, storage 93 is a tangible storage medium that is not temporary.

[0038] Furthermore, the program may be intended to implement some of the functions described above. In addition, the program may be a so-called differential file (differential program) that implements the functions described above in combination with other programs already stored in storage 93. [Explanation of Symbols]

[0039] T...Bridge girder T1...Sheath 1...Inspection device 10...Computer 20...Ultrasonic sensor 21...Transmitter 22...Receiver 11...Acquisition unit 12...Theoretical value calculation unit 13...Correlation calculation unit 14...Estimation unit 15...Output unit 16...Memory unit P1...Inspection area P21...First reference area P22...Second reference area B...Gap

Claims

1. An acquisition unit that inputs a pulse wave to an inspection area of ​​a concrete structure that is subject to investigation for the presence or absence of voids, and acquires the measured waveform of the reflected wave measured at the inspection area, A theoretical value calculation unit that calculates the theoretical value of the frequency of the reflected wave of the pulse wave in the concrete structure, A correlation calculation unit that determines the cross-correlation function between the measured waveform and the waveform having the frequency related to the theoretical value, An estimation unit that estimates the presence or absence of voids in the concrete structure based on the peak frequency of the cross-correlation function, A device for inspecting concrete structures, equipped with the following features.

2. The estimation unit estimates the presence or absence of voids in the concrete structure by comparing the peak frequency with the theoretical value of the reflected wave frequency. The inspection device for concrete structures according to claim 1.

3. The theoretical value calculation unit inputs a pulse wave to a first reference point in the concrete structure, which is known to be free of voids, and measures the arrival time until the transmitted pulse wave reaches a second reference point opposite the first reference point. The unit then determines the frequency corresponding to twice the arrival time as the theoretical value. The inspection device for concrete structures according to claim 1.

4. The theoretical value calculation unit determines the theoretical value based on the thickness of the concrete structure and the velocity of the pulse wave. The inspection device for concrete structures according to claim 1.

5. The estimation unit estimates that there are voids in the concrete structure if the difference between the peak frequency of the cross-correlation function and the theoretical value of the frequency of the reflected wave exceeds a determination threshold. The inspection device for concrete structures according to claim 1.

6. The acquisition unit acquires multiple measurement waveforms measured at multiple locations near the point where the pulse wave was input within the survey area. The correlation calculation unit determines the cross-correlation function for each of the multiple measurement waveforms. The inspection device for concrete structures according to claim 1.

7. The estimation unit estimates the presence or absence of voids in the concrete structure by comparing the peak frequency of the average waveform obtained by summing the cross-correlation functions corresponding to the multiple measured waveforms with the theoretical value of the frequency of the reflected wave. The inspection device for concrete structures according to claim 6.

8. The steps include: inputting a pulse wave to a section of a concrete structure that is to be investigated for the presence or absence of voids, and obtaining the measured waveform of the reflected wave measured at the said section; The steps include: calculating the theoretical value of the frequency of the reflected wave of the pulse wave in the concrete structure; The steps include determining the cross-correlation function between the measured waveform and the waveform having the frequency related to the theoretical value, The step of estimating the presence or absence of voids in the concrete structure by comparing the peak frequency of the cross-correlation function with the theoretical value of the frequency of the reflected wave, A method for inspecting concrete structures, comprising the following features.

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