What are the internal defects of concrete detected by ultrasonic wave?

At present, ultrasonic technology is widely used in quality inspection of various projects. Ultrasonic testing is an important aspect of concrete nondestructive testing technology, especially in detecting internal defects and uniformity of concrete. This paper expounds the principle and significance of ultrasonic detection of concrete cracks, introduces the main acoustic parameters and common methods involved in this method, and probes into the development trend of ultrasonic detection technology.

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Ultrasonic testing; Concrete structure; Cracks; construction quality

Due to various reasons, cracks generally exist in concrete structures. The appearance of cracks will reduce the impermeability of buildings and affect the use function of buildings. At the same time, it will also cause steel corrosion and concrete carbonization, reduce the durability of materials and affect the bearing capacity of buildings. Therefore, it is necessary to formulate a reasonable crack detection scheme, determine the nature of cracks, determine the harmfulness of cracks, and formulate corresponding remedial measures.

Ultrasonic detection of concrete cracks is one of the important methods for nondestructive testing of concrete structures. Ultrasonic testing is a nondestructive testing method developed in 1960s. It reflects the compressive strength of concrete by using the correlation between ultrasonic propagation speed and rebound value and the compressive strength of concrete, and calculates the crack depth and determines the position of internal cracks by using the changes of ultrasonic propagation time (acoustic time), amplitude value and frequency value in concrete. This method has the advantages of simple, rapid, accurate and low cost, and is widely used in concrete engineering.

Principle and method of 1 ultrasonic single-side flat plate measurement

1. 1 Basic principle of ultrasonic single-side flat measurement method

When the electroacoustic transducer is contacted with the concrete surface, the ultrasonic wave emitted by the transmitting transducer will be received by the receiving transducer. When ultrasonic waves encounter concrete cracks, diffraction, reflection and attenuation will occur. According to the change of acoustic time, amplitude and other parameters, the fracture depth can be judged and calculated through regression analysis.

1.2 ultrasonic single-side flat measurement method

When there is a measurable surface at the crack part of the structure, and the estimated crack depth is not more than 500 mm, the single-side leveling method can be used. During horizontal measurement, the measuring points should be arranged at the crack measuring parts at different distances according to whether they cross the joint or not. When arranging measuring points, the reinforced concrete radar locator should be used to determine the position of steel bars in the crack detection area to avoid the influence of steel bars. The detection steps are as follows:

1) Put the T and R transducers on the same side near the crack, and measure the inner edge spacing li '= 100 mm, 150 mm, 200 mm and 250 mm, respectively. Because the actual propagation distance of ultrasonic wave is larger than the distance between the inner edges of two transducers, it is difficult to determine directly. In order to obtain the ultrasonic propagation sound velocity value with the minimum error, the least square method should be used for linear regression to determine the more accurate actual propagation li distance of ultrasonic wave and the ultrasonic propagation sound velocity value in the concrete with seamless intersection, as shown in figure 1. The linear regression equation is as follows:

li = vti + a ( 1)

Where, V is the regression coefficient, that is, the value of sound velocity in concrete crossing without joints, km/s; A is a regression constant.

2) Place the T and R sensors on the symmetrical side with the crack as the axis (see Figure 2). The connecting line between the centers of the two transducers is perpendicular to the fracture direction. The acoustic wave values are read at li' = 100 mm, 150 mm, 200 mm, 250 mm and 300 mm respectively, and the phase change of the first wave is observed at the same time.

3) The calculated value of crack depth at each measuring point is calculated according to formula (2).

(2)

According to formula (3), calculate the average value of crack depth at the test site.

Where hci is the fracture depth; L is ultrasonic ranging; Ti is the sound wave time of concrete measured without cross joint; Concrete sound wave time measured across joints; V is the sound velocity of concrete measured without cross joints.

1.3 determination method of fracture depth

1) Three-point average method: When the first wave is found out to be out of phase in cross joint detection, hci is calculated by using the measured values of acoustic time difference of this ranging and its two adjacent ranging, and the average value of three hci is taken as the crack depth hc.

2) Average plus elimination method: When it is difficult to find the first wave inversion in cross-seam measurement, the average value (mhc) of each ranging depth (hci) can be obtained first. Then the ranging li' is compared with mhc. If the range li'