Keywords

1 Introduction

In recent years, the prefabricated concrete structure with its unique advantages rapidly spread across the country, under the guidance of the government, all places have encouraged and guided its development, the prefabricated building has begun to show results. There are a large number of connection nodes in the prefabricated concrete structure, and the nodes are mostly connected by reinforced grouting sleeve [1]. The connection quality of the grouting sleeve and the construction quality of the installation joint are particularly critical, which directly affects the safety and service life of the whole building. The main factors affecting the connection quality of the grouting sleeve are the filling degree of the grouting, the depth length of the reinforcement and the strength of the grouting material. The main research in this paper is the detection of the grout filling degree and the detection of the insert length of the steel bar in the grout connection of the reinforcement sleeve [2].

Sleeve filling degree refers to the degree to which the concrete injected into the sleeve fills the space of the sleeve. The degree of fullness is critical to the stability and safety of the structure, because if the grouting is not full, it will lead to gaps inside the sleeve, which will reduce the compressive strength of the concrete and the carrying capacity of the structure. In addition, incomplete grouting will also cause quality problems such as water seepage and cracks, which seriously affect the service life of the building. As shown in Fig. 1, the grouting in a practical project is not full, resulting in deformation of the connection part.

Fig. 1.
figure 1

The reinforcement sleeve is not full grouting

Full size image

At present, the common detection methods of reinforcement sleeve grouting fullness include endoscope method, X-ray method, impact echo method, embedded wire drawing method and embedded sensor method. This article mainly introduces the first two quantitative detection methods, that is, the actual detection of the fullness of the sleeve grouting [3, 4].

The insertion length of steel bar directly affects the anchorage of steel bar and concrete, and determines the load transfer of sleeve grouting. At present, the steel bar insertion length detection adopts the function of showing the distance between the shooting end face of the measurement lens and the selected point, and directly measures the vertical distance between the end end of the connecting steel bar and the shooting end face of the lens, and then considers the vertical gap between the shooting end face of the lens and the center of the grout port, the insertion length of the connecting steel bar can be calculated [5,6,7].

2 Grout Fullness

2.1 Endoscopic Method

In the case of incomplete filling or leakage of grouting in the grouting connection of the sleeve, due to the action of gravity, the liquid level of the mobile grouting material will be lower than the grouting outlet of the sleeve. According to this feature, endoscopy detection holes are prepared at the height of the grouting outlet of the sleeve. An endoscope with dimension measurement function can be used to quantitatively determine the incomplete range and then calculate the grouting fullness. The endoscopy method is divided into preforming endoscopy method, grouting hole drilling endoscopy method and sleeve cylinder wall drilling endoscopy method, which can be selected according to the shape of the grouting hole. When the pulping hole is straight, the preforming endoscopy method or the pulping hole drilling endoscopy method can be used according to the actual situation, and the sleeve barrel wall drilling endoscopy method can also be used if necessary. When the outlet hole is not linear, the cylindrical wall drilling endoscopy method can be used.

The testing instruments of endoscopy mainly include endoscope, inner snooping head, front looking measuring lens, side looking measuring lens, rigid sleeve and rubber plug. The endoscope is a new type of measuring instrument, which uses a miniature camera and sensor to monitor and measure the internal condition of the building structure in real time, as shown in Fig. 2. The focal length of the inner snoop head is 5 mm–150 mm, the resolution is 1080 mm, and the effective visual distance is 15 cm. The front view measuring lens and side view measuring lens are optical acquisition lenses, respectively, to observe and measure the measurement structure in the front and side. The rigid casing and rubber stopper are used for auxiliary equipment and for the stable line of measurement process.

Fig. 2.
figure 2

Endoscope

Full size image
  1. 1.

    Detection principle of pre-perforated endoscope method: the pre-perforated combination plug is designed. During grout construction, the straw in the film is removed after the slurry is uniformly discharged from the grout outlet, and the film is left to form a detection hole before detection.

    The preparation of the hole must be completed before the detection of the pre-perforated endoscope. The preparation process of the hole: the first step is to connect the straw wrapped with a film to the rubber plug; The second step is to insert the combined plug into the outlet pipe from the surface of the prefabricated member; The third step is to block the rubber plug; The fourth step is to test the hole.

    Fig. 3.
    figure 3

    Test result of preformed hole endoscopic method

    Full size image

    Detection steps: the first step is to prick the film; The second step use the front view lens to observe the cavity inside the punctured place; The third step is to extend the side-looking three-dimensional measurement lens into the gap between the inner wall of the sleeve and the connecting steel bar and observe it down; The fourth step is to measure the vertical distance between the upper surface of the grout material and the end face of the side-looking 3D measuring lens, and the grout fullness can be obtained according to the conversion of the vertical distance, as shown in Fig. 3.

  2. 2.

    The endoscopic detection procedure of the grouting hole drilling is shown in Fig. 4: the first drilling depth is 20 mm–30 mm, and the grouting material of the whole section of the grouting hole is crushed and cleaned, so that the rubber plug in the probe positioning device can be installed during the detection. Drill holes along the outlet hole, and clean up the grout debris and powder every 20 mm–30 mm forward; When the distance from the sleeve outlet is smaller than 20 mm, every 5 mm forward, clean up the grout debris and powder, observe the drilling condition, until the detection hole penetration; Use the front view lens to observe, if not full and then use the side view measurement lens to measure the length of the grout defect area.

    Fig. 4.
    figure 4

    Endoscopic method of grout hole drilling

    Full size image

    As shown in Fig. 5, the grouting of No. 1 and No. 3 sleeves is full, while No. 2 and No. 4 sleeves are not full. The measured data are 23.59 mm and 60.09 mm respectively by direct distance measurement. According to the measured data, the filling degree of sleeve grouting can be directly calculated.

    Fig. 5.
    figure 5

    Test result of endoscope method of hole drilling

    Full size image
  3. 3.

    Tube wall drilling endoscopic method of detection steps: use the steel bar scanner to determine the position of the sleeve; Partial removal of the concrete protective layer outside the corresponding position of the pulp outlet height of the sleeve, exposing the outer wall of the sleeve; The drilling equipment is first equipped with a metal drill bit to make a hole in the wall of the sleeve, and then replaced with a stone drill bit to continue drilling into the inner cavity of the sleeve 4 mm–6 mm. Use the front view lens for observation. If it is not full, use the side view measurement lens to measure the length of the grout defect area.

2.2 X-ray Method

When an X-ray penetrates the object under test, the intensity of the ray decreases as it passes through the object. The degree of ray attenuation is not only related to the energy of the ray, but also directly related to the property, thickness and density of the measured object, etc. A certain detector (such as: film, IP board, DR Flat panel detector, etc.) is used to record the intensity of the transmitted ray, and the projected image inside the sleeve can be obtained. Through professional processing software, the full degree of sleeve grouting can be quantitatively detected.

Application conditions of X-ray method: the thickness of the structural layer in the inspected area should not be greater than 200 mm, the sleeve is arranged in a single row or “plum blossom shape”, the personnel must withdraw to the radiation safety area, and the X-ray local damage method can be used if necessary.

The equipment of X-ray method is portable X-ray device. The X-ray machine system is mainly composed of a transmitter tube, a console, a laser aligner, a control cable, a power line, etc. The traditional imaging method is industrial film imaging. In recent years, CR (Computed Radiography) and DR (Digital Radiography) techniques are gradually emerging.

The 16# building of a project is a prefabricated shear wall structure with a wall thickness of 200 mm. X-ray method is used to detect the construction quality of the sleeve joints of the 6 storey shear wall components of the 16# building, in order to verify the reliability of the X-ray method in practical engineering applications.

Fig. 6.
figure 6

X-ray detection

Full size image

The detection procedure is shown in Fig. 6: According to the selected perspective focal length 700 mm, the X-ray machine and imaging device are placed at both ends of the measured wall panel. After taking good radiation protection measures, set the voltage at the console to 280 kV and preheat the machine for 37 min. After the preheating is completed, input the selected perspective parameters in the console: tube voltage 250 kV, tube current 2.0 mA, DR Exposure time (2 × 5) s, film exposure time 10 min. Turn on the ray for perspective lighting and turn off the ray machine after completion.

X-ray method can effectively identify two indexes of grout sleeve - grout fullness and steel bar insertion length, and this method can meet the actual testing requirements.

3 Detection of Steel bar Insertion Length in the Sleeve-Ray Method

After the field splicing of the prefabricated member is completed and before the grouting construction of the sleeve, the insertion length of the connecting steel bar is fixed, and it is convenient for endoscopic detection. The detection principle is to use the characteristics of high dimensional accuracy of the sleeve, the measurement of the insertion length of the connecting steel bar into the measurement of the relative distance between the end of the insertion section of the connecting steel bar and the known reference point in the sleeve, the three-dimensional measurement endoscope accurately measures the relative distance, and the insertion length of the connecting steel bar is calculated.

3.1 Semi-grout Sleeve Detection Method

Taking the center of the grout outlet of the sleeve as the reference point, auxiliary tools are used to pound horizontally from the bottom of the grout hole before testing, quickly determine the relative position of the end of the connecting steel bar and the grout outlet of the sleeve, select the corresponding measurement lens, and combine the later observation results can be divided into the following three working conditions:

The first working condition is that the connecting steel bar is higher than the pulp outlet. It is judged that the end of the steel bar is higher than the bottom of the pulp outlet through the introduction of auxiliary tools; Using the forward looking measuring lens for observation, the end of the steel bar is higher than the top of the slurry outlet of the sleeve; After investigation, the common brand and conventional model of semi-grouting sleeve on the market at present, the end of the connected steel bar under the design anchoring length is located in the range of 5 mm above and below the bottom of the grouting port of the sleeve, and the upper cavity of the grouting port is used as the adjustment margin, so the insertion length of the steel bar can be directly determined to meet the requirements.

The second condition is that the end of the connecting steel bar is at the position of the pulp outlet. Rigid bushings and rubber plugs are used to assist the positioning of the forward-looking measuring lens; The appropriate position is selected in the outlet channel to make the imaging clear and get the 3D image. The point-to-surface measurement function is adopted, in which the surface is formed by the leftmost end point on the outlet, the rightmost end point and any point in the outlet hole on the same horizontal plane. The point is positioned at the end of the connecting steel bar. The relative distance between the end of the connecting steel bar and the horizontal section of the center of the outlet hole is measured, and the insertion length of the connecting steel bar is calculated.

Fig. 7.
figure 7

The end of the connecting steel bar is lower than the outlet

Full size image

The third condition is that the end of the connected steel bar is lower than the pulp outlet. As shown in Fig. 7, it is judged that the end of the steel bar is lower than the bottom of the pulp outlet through the introduction of auxiliary tools; The side-looking measuring lens is selected, and the rigid sleeve and rubber plug are used to assist the positioning of the side-looking measuring lens; Adjust the insertion depth of the rigid sleeve so that the side view measuring lens is located in the inner cavity of the sleeve to observe and shoot 3D images inside the sleeve; Using the function of displaying the distance between the shooting end face of the measuring lens and the selected point, the vertical distance between the end of the connecting steel bar and the shooting end face of the lens is directly measured, and the vertical deviation between the shooting end face of the lens and the center of the grout port is considered to calculate the insertion length of the connecting steel bar.

3.2 Full Grout Sleeve Test Method

The detection method of full grouting sleeve takes the upper surface of the limit stop card in the middle of the sleeve as the known reference point, extends the inner snooping head equipped with a forward looking measuring lens into the inner cavity of the sleeve, and captures the 3D image of the middle of the sleeve. The relative distance between the end of the connecting steel bar at the installation end and the upper surface of the limit gear card can be measured by the measurement function of the endoscope point to surface, and then the insertion length of the steel bar can be calculated.

The probe should be extended down the gap to the limit stop card in the detection of the full grout sleeve, so that the camera can take a clear picture of the middle of the sleeve and measure the distance. In order to meet the requirements of detection, the lens measuring range should not be less than 80 mm, taking into account factors such as the Angle of view of the front view measuring lens, the thickness of the limit stop card, the adjustment space reserved for the steel bar and the insufficient insertion length.

4 Conclusions

In this paper, combined with the actual engineering sleeve grouting connection method, the detection technology of grouting fullness and steel bar insertion length is explored, the main results are as follows:

  1. (1)

    The endoscope method can effectively detect the filling and insertion length, and the X-ray method can quantitatively detect the filling. The two test results are in agreement with the actual project.

  2. (2)

    The endoscope method has the advantages of simple structure, low cost and wide applicability, and can be widely used in practical engineering. X-ray method has higher requirements for equipment and certain technical requirements for operators, which need to be improved and promoted.

  3. (3)

    The endoscopic method and X-ray method cannot achieve dynamic observation during construction, and the subsequent research direction can extend the post-construction detection to the construction, so as to effectively control the construction quality of the sleeve grouting connection.