2.1. Specimen Design
In order to investigate the connection performance of the novel type of corrugated pipe-restrained grout anchor joint, 17 specimens were designed for pull-out experiments. The cross-sectional dimensions of the design components are 200 × 200 × 700 mm, and the protective layer’s thickness is 25 mm.
The main body of each specimen is composed of C30 ordinary concrete, and the corrugated pipe is filled with Wei Lan JW-E1 A, B, and C grouting materials, respectively. The compressive strengths of these three grouting materials are equivalent to those of C40, C50, and C60 ordinary concrete. The grouting materials produced by Zhengzhou Jingwei Building Materials Technology Co., Ltd. (Zhengzhou, China). Exhibit characteristics such as self-compacting and non-shrinking hardening, meeting the requirements of the experiment. A centrally located steel plate is embedded within each specimen to mitigate eccentricity to ensure that the clamping end steel bars and the grout anchor steel bars are aligned on the same straight line. The vertical steel bars in the main body of the specimen are 4C12, where 4 indicates the number of bars, C indicates that the steel bar is grade 3, 12 indicates the diameter of the bar, and the stirrups are C8@200, where C indicates that the steel bar is grade 3, 12 indicates the diameter of the bar, @200 indicates the spacing of the stirrups. The explanation below is the same. The metal corrugated pipe has a diameter of 40 mm and a height of 600 mm. The anchorage lengths of the grout anchor steel bars are adjusted according to the experimental design, ensuring alignment with the center of the corrugated pipe before injecting the grouting material.
Based on the Chinese code GB 50010-2010 (the Code for Design of Concrete Structures), the basic anchorage length of the steel bar is calculated using the formula Lab = α·fy/ft·d, where the shape coeffic ient α for ribbed steel bar is 0.14, fy is the tensile design strength of the steel bar, ft is the design tensile strength of concrete, and d is the diameter of the steel bar. The anchorage length is determined by La = k·Lab, with k is the anchorage length coefficient. For this experiment, values of 0.4, 0.6, 0.8, 1.0, and 1.2 were selected for k. In order to assess the impact of U-shaped stirrups on the joint performance of grout anchor structures, 15 specimens were designed without U-shaped stirrups, while the remaining two included U-shaped stirrups for comparative analysis.
Figure 2 illustrates the construction and dimensions of the specimen. The clamping and grout anchor steel bars are aligned on the same straight line. Displacement gauges W1 and W2 are positioned adjacent to the clamping steel bar, while W3 and W4 are symmetrically arranged on the concrete surface near the grout anchor steel bar. Gauge W5 is placed on the welded steel bar at the interface between the grout anchor steel bar and the concrete. The structural schematic diagram of the center positioning steel plate is shown in
Figure 3. The center-positioning steel plate has a thickness of 2 mm, and the steel bars are symmetrically placed on both sides at a spacing of 100 mm. HRB400 C14 steel is employed for the grout anchor steel bars, clamping steel bars, positioning steel plates, and distributed steel bars.
The specific parameters of the specimens in Group A are listed in
Table 1. The specimen numbering format in the table employs an “A-B” scheme, where “A” denotes the concrete strength grade corresponding to the grouting material, and “B” signifies the influence coefficient of the basic anchorage length of the grout anchor steel bar. For instance, “50-0.8” is a specimen with grouting material corresponding to concrete strength grade C50 and a basic anchorage length coefficient of 0.8 L
ab.
Tests for Group B were designed to explore the impact of U-shaped stirrups on the connection performance of the grout anchor structure using the Group A experiments as a control. The specimens in Group B include U-shaped stirrups, which are uniformly distributed from the bottom to the top on the exterior of the corrugated pipe to provide confinement. The construction and dimensional diagrams of the supplementary specimens in Group B are illustrated in
Figure 4. When the spacing of the U-shaped stirrups is either 200 or 400 mm, three or two strain gauges are installed, respectively.
Table 2 details the parameters for each specimen in Group B. The specimen code adopts an “A-B-C” format, where “A” and “B” have the same implications as in the Group A experiments, and “C” indicates the spacing of the U-shaped stirrups. For example, “50-0.8-400” describes a specimen with grouting material corresponding to concrete strength grade C50, a basic anchorage length coefficient of 0.8 L
ab, and a U-shaped stirrup spacing of 400 mm.
2.4. Loading Method and Measurement Contents
Strain gauges are placed at four equidistant points along the inserted section of the bars to quantify the strain variation of the grout anchor steel bars during the loading process. After the installation of these gauges, epoxy resin is applied to their outer surfaces for protection. The layout of the strain gauges and the data acquisition system is depicted in
Figure 6. Data collected during the experimental loading process primarily include load values, strain values of the grout anchor steel bars, and displacement values obtained from the displacement sensors.
Based on the standard for test methods of concrete structures (GB 50152-2012) [
18] and considering the specific conditions of the loading device, the step loading method is selected. Each stage is incremented by 10 kN, and the load is sustained for 30 s at each step until the grout anchor steel bar is extracted, the steel bar experiences tensile failure, or the specimen itself fails, at which point the test concludes. The loading device comprises a 50-T through-heart hydraulic jack and a specialized steel frame. The 50-T through-heart hydraulic jack incorporates a hydraulic jack, operating lever, and digital pressure gauge. The specialized steel frame comprises four threaded steel columns, pressure-bearing steel plates, and bolts.
Figure 7 depicts the loading device.