Method and principle of ultrasonic sensor for detecting concrete

- Jun 12, 2019-


Ultrasonic probe transmission method

Ultrasonic transmission method is employed. The ultrasonic transmitting probe and the receiving probe are respectively inserted into different steel pipes which are pre-buried in the pile body and filled with water, and the ultrasonic waves generated by the transmitting probe pass through the water coupling to penetrate the concrete of the pile to reach the receiving probe in the other steel pipe, and the receiving probe will receive The incoming information is transmitted to the instrument, and the quality of the received concrete in the concrete is analyzed by comprehensively analyzing the characteristics of the parameters such as the speed of sound, the amplitude of the sound, the frequency and the waveform, and the quality of the concrete of the pile is evaluated. Its basic basis is that when there are defects in the concrete, the ultrasonic sound velocity, sound amplitude, frequency and waveform parameters are reflected. The first is that when there is a defect inside the concrete, a discontinuous medium is formed on the ultrasonic wave-receiving path, and the low-frequency ultrasonic wave will propagate forward around the defect. Within the detection distance, the diffraction reaches the desired "sound time" than the ultrasonic wave. The "sound time" required for direct propagation in defect-free concrete reflects the reduction in the speed of sound of the ultrasonic waves. Secondly, due to the lack of defects, the acoustic energy attenuation increases when the ultrasonic wave propagates in the concrete, and the amplitude of the first wave of the received signal decreases. The third is that the high frequency component disappears faster than the low frequency component due to defects in the concrete, and the frequency of the received signal is always lower than the frequency received by the defect free concrete of the same distance measurement. Later, due to the complex reflection and refraction of the ultrasonic wave at the defect interface, the phase of the sound wave propagation is different, and the superimposed result causes the waveform of the received signal to be distorted. Based on this, the quality of the interior of the concrete can be judged.


Ultrasonic transmission method


Detection points


1. Pre-buried detection tube:

(1) Double pipe should be buried in the pile diameter of 0.6-0.8m; three pipes should be buried in the pile diameter of 0.8-2.0m; four pipes should be buried in the pile diameter of 2.0m or more. According to the specific conditions of Chifeng Bridge, three pipes can be buried in the pile foundation. .
(2) The sound wave detecting tube should be made of steel pipe, plastic pipe or steel bellows, and its inner diameter should be 50-60mm. The connection of the test tube should be smooth transition, the nozzle should be higher than the top of the pile by more than 100mm, and the height of each test tube should be the same. The lower end of the tube should be closed, the upper end should be covered, there should be no foreign objects in the tube, and the tube body should not be damaged.
(3) The test tube can be welded or bundled on the inside of the steel bar, and the test tubes should be parallel to each other.
(4) The test tube should be filled with clean water.

2. On-site inspection


(1) Determine the delay time t of the sound wave monitor transmitted to the receiving system before the on-site inspection. And calculate the sound time correction value.
(2) The transmitting and receiving probes should be lifted at the same elevation or at a fixed height difference during measurement.
(3) The measuring point distance is 40cm, and then encrypted to 20cm when an abnormality is found.
(4) Select the appropriate transmit voltage and amplifier gain, and remain unchanged during the test.
(5) Display and record the time history curve of the received signal in real time.
(6) Combine multiple sound tubes with two detection profiles to complete the detection of multiple profiles.
(7) After each group of test tubes is tested, the test points should be randomly repeated 10%, and the relative standard deviation of the sound should not exceed 5%; the relative standard deviation of the amplitude should not exceed 10%.


3, data sorting


Implementation of "Technical Specifications for Building Pile Foundation Testing" (JGJ106-2003J256-2003)

4, the determination of the integrity of the pile


According to the table below
Sound wave transmission method pile body integrity judgment table


Category


I There are no abnormalities in the acoustic parameters of each detection section, and no sound speed is lower than the low limit.


II An acoustic parameter of an individual measuring point of an inspection section is abnormal, and the soundless speed is lower than the low limit abnormality.


III. Acoustic parameter anomalies of multiple measuring points appear continuously in a certain detection section; the acoustic parameters of two or more detection sections at the same depth point are abnormal; the local concrete sound velocity is lower than the low limit abnormality


IV. Acoustic parameters of multiple measuring points appearing continuously in a certain inspection section are obviously abnormal; the acoustic parameters of two or more detection sections at the same depth point are obviously abnormal; the concrete speed of the pile body is generally lower than the low limit abnormality or Unable to detect severe distortion of the first or first wave received signal.


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