With the initial formation of the top-level system design, assembled buildings are flourishing everywhere, but it should be clearly seen that there are still many problems in assembled buildings at this stage. Reinforcement sleeve grout connection is the main connection method of assembled concrete structural members, so when there is an uncompacted grout, the axial force transmission of reinforcement will receive influence, which in turn greatly affects the safety of the structure. At present, there is a lack of practical technical methods for non-destructive testing of grout compactness of reinforcement sleeve grouting joints in assembled concrete structures at home and abroad. The article provides an in-depth discussion of a variety of NDT methods including those based on radiation, ultrasound, and shock echo, which are applied in the engineering community.

1. Radiation-based inspection techniques.

Radiation (X-ray, gamma rays, neutrons, iridium 192, etc.) has strong penetration and linearity, can be based on its attenuation, absorption and regenerative scattering law in the process of propagation, will be subject to different degrees of absorption of the rays projected on the X-ray film, after the development of different images corresponding to the material structure or defects, so as to determine the type, size, number and distribution of defects, so as to determine the defect The hazard and quality level of the defects. Due to the advantages of radiographic display and high accuracy of detection, radiographic inspection technology has been tried early in the field of concrete defect detection.

At present, high-power X-ray technology can achieve nondestructive testing of reinforcement sleeves in various arrangements in the laboratory. However, because the detection equipment is too large, and the radioactivity is very high and harmful to human beings, it is impossible to achieve the detection of engineering sites at this stage. Although the low-power portable X-ray machine has the advantages of smaller quality, less radioactivity, can be applied to the site, but subject to the equipment tube voltage, tube current and exposure time and other factors, the sleeve centered or plum-shaped arrangement of 200mm thick precast shear wall sleeve can see the sleeve shape, reinforcement form, joint parts and grouting dense and non-dense area; but the double row of symmetrically arranged sleeve, the inner leaf wall arrangement The prefabricated sandwich insulated shear wall sleeve cannot be imaged effectively. Large-scale promotion in the field also needs to address the degree of equipment radioactivity, equipment affected by factors such as tube voltage, tube current, etc.

2. Ultrasonic-based detection technology.

Ultrasonic detection technology is currently the most commonly used non-destructive testing technology for concrete defects, and the ultrasonic method is now widely used in non-destructive testing of concrete structures. Using the method of pair measurement, the transmitting and receiving 2 probes are placed on the 2 opposite sides of the specimen, the use of ultrasonic pulse wave propagation in the concrete acoustic time (or sound speed), wave amplitude and frequency and other relative changes in acoustic parameters to analyze the defect situation.

In the steel connection sleeve detection, when the ultrasonic waves pass through the rebar coupler of grout with debonding defects, the ultrasonic waves will propagate along the outer wall of the rebar coupler; when the ultrasonic waves pass through the rebar coupler of dense grout, the propagation path is radial propagation inside the sleeve. Therefore, the propagation of ultrasonic waves inside the rebar coupler can be used to detect the compactness of the grout inside the rebar coupler. Using the ultrasonic method, the compactness of the grout in the reinforcement sleeve connected to the transverse reinforcement can be detected before the concrete is poured at the assembly node. The measured ultrasonic sound velocity and with the help of the amplitude value, the compactness of the reinforcement sleeve pre-buried in the concrete connected to the vertical reinforcement can be determined.

Because the method needs to be measured from the 2 opposite sides of the specimen, the relative position of the transmitting and receiving probe and the contact surface requirements are strict, the test is time-consuming, and is not suitable for receiving the probe difficult to access the components, but also by the sensor coupling state, concrete moisture content and iron bellows and rebar and other factors of interference, making the method significantly reduce the detection efficiency, poor operation, difficult to practical.

3. Detection techniques based on the impact echo.

Shock echo method is used compared to ultrasound with greater energy, greater penetration, superior frequency distribution, more suitable for spectral analysis of shock waves for detection. When there is a defect in the grout, the excited elastic wave will produce an early reflection at the defect, while the elastic wave reflects back around the defect will also produce a hysteresis reflection, and the time used for the hysteresis reflection of the elastic wave is longer than that of the grout dense [5]. After the receiver receives the reflected shock echo, it uses spectral analysis techniques to convert the time domain data into frequency domain data, and then determines the peak frequency of the echo.

The impact echo detection technique is mainly used to detect internal defects and thickness of concrete, which can overcome all the defects of other detection techniques, and can test both concrete defects and concrete thickness, and the signal is intuitive, fast and accurate. Its most important feature is that it can both quickly qualitative testing, but also to achieve defect location, thus achieving the most optimal testing efficiency and accuracy, with a variety of other methods can not replace the significant advantages. In addition, the impact echo method is a single-sided detection method, which is very convenient to use and is widely used in the detection of the thickness of concrete structures and internal defects of structures.

At this stage in the laboratory, the impact echo method can qualitatively determine the compactness of grouting for a single row of reinforcement sleeves, although there are still errors in the detection accuracy; for a double-row arrangement of reinforcement sleeves, the test results are difficult to qualitatively determine the compact and non-compact zones of grouting for reinforcement sleeves. The realization of field testing still needs further research in testing methods, analysis methods, etc. to improve the accuracy and reliability, and the key technologies to improve the subsequent testing technology system include.

(1) The sleeve material of assembled concrete structural rebar coupler grouting connection is cast iron (or alloy steel), which will result in the phenomenon that the reflection of the sleeve and the reflection of the defect cancel each other out, thus affecting the discriminatory effect. Therefore, the detection principle of the existing impact echo method needs to be improved and perfected to ensure the correct defect discrimination rate.

(2) The assembled concrete structure rebar coupler connection is generally multi-row sleeve, and the detection method and steps for the compactness of multi-row grouting holes need to be proposed.

(3) To formulate quality evaluation criteria for grout compactness of assembled concrete structural rebar coupler connections and propose corresponding measures for handling grout compactness defects.

The above is the discussion about the rebar coupler connection grout compactness testing technology, if you need more detailed information, welcome to contact us!