Keywords

1 Introduction

The integrated concrete building is a development direction and a hot research topic in the current construction field, but its quality control and detection technology is relatively lagging behind. Especially for the grouting quality at the joint of the component, there is no good method that is low cost and can effectively detect the filling degree of the grouting material. In the past, ultrasonic technology has been widely used in concrete defect detection. Using ultrasonic to detect the filling degree of grouting material in prefabricated shear wall is helpful to promote the innovation and development of detection technology, which can improve the quality of assembled integrated concrete building construction and improve the relevant quality detection technology system. The technology is suitable for the ultrasonic detection of filling degree of grouting material in the construction joint of prefabricated concrete prefabricated shear wall. The research results can provide an effective means for the quality control of assembled monolithic concrete buildings and the establishment of testing technology system [1].

2 Principle of Ultrasonic Nondestructive Testing Technology

The principle of ultrasonic non-destructive testing technology is to use the characteristics of ultrasonic propagation in the material to detect the internal structure and state of the material [2]. In ultrasonic non-destructive testing, ultrasonic probes are usually used to emit ultrasound to the inside of the material, and then by receiving the reflected ultrasonic signal, judge the defects, cracks, pores and other problems inside the material. When the ultrasonic wave propagates in the material, it will be affected by the internal structure, defects, cracks and other factors of the material, resulting in physical phenomena such as reflection, refraction and scattering. By measuring the characteristics of these phenomena, information such as the location, size and nature of defects, cracks, pores and other problems inside the material can be inferred. Therefore, ultrasonic nondestructive testing technology is an efficient and accurate detection method, which is widely used in the detection of various materials [3].

In this paper, the normal distribution is used as the statistical basis for judging distance. As shown in Fig. 1, it can be seen from the classical normal distribution that the probability of \(\left[\mu -\sigma ,\mu +\sigma \right]\) is 68.26%, the probability of \(\left[\mu -2\sigma ,\mu +2\sigma \right]\) is 95.44%, and the probability of \(\left[\mu -3\sigma ,\mu +3\sigma \right]\) is 99.74%.

Fig. 1.
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Classic normal distribution

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The probability judgment defect is related to the value of its standard deviation and the number of measurement points. When the standard deviation of the judgment defect is 3 times, the defect will not be misjudged, but it is easy to miss the judgment. When the defect is judged with 2 times the standard deviation, the defect is not easy to miss the judgment, but it will be misjudged. The multiple of standard deviation should be related to the number of measurement points. If judging by sound velocity V, amplitude A, or frequency F, the detection data can be arranged from largest to smallest, and the mean value \(M_{V}\), \(M_{A}\), \(M_{F}\) and corresponding standard deviation \(S_{V}\), \(S_{A}\), \(S_{F}\), of the detection data can be calculated respectively, using the formula:

$$ X_{0} = M_{X} - \lambda S_{X} $$
(1)

The X in the formula can respectively represent the sound speed V, amplitude A, or frequency F, λ according to the need to query the standard table, calculate the critical value to judge the suspicious value [4, 5].When the detection result data is outside the critical value of the measurement point, judged as abnormal. In addition, the received signals in the ultrasonic detection of the horizontal joint at the bottom of the prefabricated concrete structure and the grouting and anchor joint grouting material defects are comprehensively analyzed and evaluated according to three parameters: sound time, first wave amplitude and waveform [6]. By comparing the normal ultrasonic detection point waveform with the defect ultrasonic detection point waveform, the defect point can be intuitively judged.

In the transmitting and receiving acoustic channels, the difference of time generated in the direct propagation of sound waves or the propagation process of diffraction due to defects leads to the difference of sound velocity in the same ranging. Sound waves in different acoustic interfaces, due to the different acoustic impedance of the medium, resulting in differences in sound energy reflection or transmission, due to the attenuation of sound energy defects lead to a decline in the amplitude of the first wave. Due to the defects of concrete, the sound wave propagation produces reflection, refraction and diffraction, and the superposition results in the phase change of waveform or the distortion of the first wave of the received waveform [7].

3 Detection of Horizontal Joint Slurry at the Bottom of Shear Wall

3.1 Transducer

Although the detection of the quality of grouting material of prefabricated concrete structure can refer to the principle of ultrasonic detection technology of concrete, it is not completely equivalent to the detection of concrete defects. The existing ultrasonic equipment can meet the detection of concrete defects, but it is not fully applicable to the detection of the defect quality of grouting material of prefabricated concrete structure. The detection of the defect quality of the prefabricated concrete structure grouting material has its particularity.

The detection of the filling degree of grouting material has its particularity, such as the equivalent of the defect, the defect in the grouting material is much smaller than the defect in the concrete, but the advantage is that the ultrasonic ranging at the anchor node of the prefabricated shear wall is usually shorter. In view of the short ranging of shear wall and the needs of detecting small and medium-sized defects of grouting material, the suitable transducer is considered to be selected for matching. The transducer with high frequency and small diameter radiating end face is the technical means that must be adopted.

At present, the frequency of the transducer commonly used in engineering testing is 50 kHz, and the wave speed v of the ultrasonic wave is the product of the frequency f and the wavelength λ. When the sound speed of the grout is 4.40 km/s, such as the transducer with a frequency of 50 kHz, its wavelength λ is 88 mm. Because the wavelength is long, the ultrasonic wave is easy to bypass the defect propagation, the sensitivity of the 50 kHz transducer to detect the defect is very low, so the selection of the frequency of the transducer must consider the size of the defect that needs to be detected, usually set the minimum defect that can be measured by ultrasonic detection is approximately the wave length. If the transducer of grout is used in the detection of 500 kHz, its wavelength λ is less than 9 mm, that is, the use of high-frequency transducer can greatly improve the sensitivity of defect detection. In addition, at present, many domestic ultrasonic instruments factory generally configured 50 kHz transducer diameter of about 40 mm, foreign Switzerland PROCEQ ultrasonic instrument configuration 50 kHz transducer diameter is 50 mm, because the size of the transducer diameter is too large, used for structural grout grouting, ultrasonic detection is more difficult to find less than the transducer diameter defects. Therefore, the domestic first developed for prefabricated concrete structure grouting material grouting quality ultrasonic detection of high-frequency small diameter radial end transducer.

3.2 Simulation Test

Ultrasonic measuring points are arranged at the bottom of the shear wall connecting the horizontal joint pulp. First, a set of symmetric starting points are determined on both sides of the horizontal joint connected at the bottom of the component, and then the positive and negative sides of the component are drawn along the horizontal joint connected at the same direction with 100 mm equal spacing, and the measuring points are numbered according to 1, 2, 3, and ··N in turn. Ultrasonic detection will first transmit and receive transducers are placed at the symmetric starting point number 1 on the bottom horizontal joint on both sides of the positive and negative sides, and then move the transducer in the same direction according to the corresponding number on both sides of the test. After the end of the component detection, you can view the ultrasonic instrument real-time display of the detection data and its statistical summary analysis results, if there is a suspicious test point with a * prompt, you can retest the suspicious test point according to experience or increase the test point around it, judge the scope of the defect, assess the grouting material fullness or whether there is a local cavity.

According to the actual engineering situation of the horizontal joint slurry at the bottom of the shear wall, A simulated shear wall grouting layer defect specimen A was made, and 5 defects were arranged in the specimen: a large defect of 120 mm × 150 mm was set in the center position; Two small rectangular defects with a size of 20 mm × 50 mm are arranged on the left and right sides of this large defect, among which the two defects on the left side are horizontally located at 1/4 and 1/2 of the width of the specimen (perpendicular to the direction of the ultrasonic channel); The two defects on the right are vertically located at 1/4 and 1/2 of the width of the specimen respectively (parallel to the direction of the ultrasonic acoustic path), as shown in Fig. 2.

Fig. 2.
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Photos of the simulated defect position of specimen A and the number of ultrasonic measuring point

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Table 1. Test data of 18 ultrasonic test points of specimen A
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Ultrasonic measuring points: a pair of measuring points are arranged at each of the 4 small defects on the left and right sides of the specimen, and three pairs of measuring points are arranged at the big defect in the middle of the specimen, that is, a total of 7 pairs of ultrasonic measuring points at the defect. In addition, according to the work experience that the number of measuring points without defects is 1.6 times greater than the number of defect measuring points, 11 pairs of ultrasonic measuring points without defects are arranged, that is, A total of 18 pairs of ultrasonic measuring points on the simulated defect specimen A. The test data according to the ultrasonic detector are shown in Table 1.

3.3 Data Analysis

The 18 ultrasonic measuring points in Table 1 are classified, with an average sound velocity of 4.091 km/s and an average amplitude of 69.94 dB. According to the ultrasonic theory, the defect points are discriminated and the sound velocity or amplitude of the abnormal points is less than the average value, such as A-02, A-05, A-08, A-09, A-10 and A-13 in the table. The result of testing data is consistent with the reality.

In addition, the wave train view of all ultrasonic measuring points corresponding to the serial number of measuring points in Table 1 is shown in Fig. 3. The defect points are judged by observing the ultrasonic wave train view.

Fig. 3.
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Wave train view of ultrasonic measuring point

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In the wave train view of Fig. 3, the ultrasonic first wave starting point of 7 measuring points (2, 5, 8, 9, 10, 13, 16) is obviously backward, indicating that their sound time is too long in the same distance measurement. In Table 1, the (sound velocity, amplitude) data of these measuring points are obviously small. According to the acoustic parameters such as “sound velocity, amplitude, waveform” as the principle of judging the defects of grout, the defects can be clearly judged here. Figures 4, 5 and 6 illustrates the ultrasonic waveforms of A-05, A-09, A-16, and A-04 measuring points (Fig. 7) for comparison, which are typically poor.

Fig. 4.
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Defective waveforms at measuring point No. 05

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Fig. 5.
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First wave loss defect waveform at measuring point No. 09

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Fig. 6.
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Defective waveform at measuring point No. 16

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Fig. 7.
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Comparison waveform without defect at measuring point No. 04

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The judgment result printed by the ultrasonic detector is clear and correct and consistent with the actual defect situation. The defect size of the measuring point No. 16 is equivalent to that of the three measuring points No. 03, No. 05 and No. 13; However, the measuring point only shows the sound velocity value is small, and the amplitude dB value is not too small, because the direction (vertical) of the long size defect of the measuring point is in the same direction as the acoustic path of the transmitting transducer and the receiving transducer during the detection, and the ultrasonic wave is easy to diffract. The test results show that when the flat shape defect transmits and receives the acoustic path of the transducer transversely, ultrasonic detection is easy to find, on the contrary, the vertical defect may be missed.

4 Conclusions

In the field of assembling integral concrete building quality inspection technology, aiming at the grouting quality of the horizontal joint connected at the bottom of shear wall of prefabricated concrete building, the ultrasonic test research shows that:

  1. (1)

    The transducer with high frequency and small diameter radiating end face is the technical means that must be taken for the detection of grouting material fullness. The transducer should have the technical characteristics of short wavelength and high flaw detection sensitivity.

  2. (2)

    Ultrasonic detection of grout quality according to the “sound velocity, amplitude, waveform” and other acoustic parameters of the principle of discrimination, each ultrasonic measurement point

The wave train view can effectively judge the defects and degree of horizontal joints connected at the bottom of shear walls of prefabricated buildings.

  1. (3)

    Ultrasonic test results can be more correct to determine whether there are defects, but the lack of quantitative description of defects, can not provide a more specific engineering quantity for the subsequent repair of defects.