Figure 1
Schematic diagram of the buckle-type mechanical connector. (a) Before connection, (b) after connection, (c) reinforced bar mechanical connection method, (d) internal structure of the connector.
Table 1
Main design parameters of specimens.
| Specimen | S1 | S2 | S3 | S4 | S5 | S6 |
|---|---|---|---|---|---|---|
| Axial compression ratio | 0.2 | 0.2 | 0.2 | 0.2 | 0.3 | 0.4 |
| Prestress | — | — | 0.4f ptk | 0.6f ptk | 0.6f ptk | 0.6f ptk |
| Concrete | C40 | C40 | C40 | C40 | C40 | C40 |
| Column reinforcement | 8φ20 | 8φ20 | 8φ20 | 8φ20 | 8φ20 | 8φ20 |
| Beam reinforcement | 4φ16 | 4φ16 | 4φ16 | 4φ16 | 4φ16 | 4φ16 |
| Stirrups | φ8 | φ8 | φ8 | φ8 | φ8 | φ8 |
| Remarks | Cast-in-place | Buckle-type connection | Buckle-type connection | Buckle-type connection | Buckle-type connection | Buckle-type connection |
Figure 2
Detailed diagrams of each specimen. (a) S1, (b) S2, (c) S3–S6, (d) column-end reinforcement (joint region), (e) column-end reinforcement (normal region), (f) beam-end reinforcement (dense zone), (g) beam-end reinforcement (normal zone), and (h) key grooves.
Figure 3
Specimen fabrication process. (a) Fabrication of column rebar cage, (b) fabrication of cast-in-place joint rebar cage, (c) attachment of strain gauges, (d) insertion of rebar cage into the mold, (e) concrete pouring, (f) second concrete pouring, (g) tensioning of steel strands, (h) strain gauge sensor, and (i) placement diagram of the strain gauge sensor.
Table 2
Concrete material properties.
| Batch | Cube strength (MPa) | Axial strength (MPa) | Tensile strength (MPa) | Elastic modulus E c (MPa) |
|---|---|---|---|---|
| First | 46.6 | 34.6 | 3.2 | 33,960 |
| Second | 41.7 | 31.5 | 3.1 | 32,980 |
Figure 4
Mechanical properties testing of steel reinforcements and mechanical connectors. (a) Tensile test of steel bar, (b) fracture of steel bar, (c) tensile test of mechanical connection steel reinforcement, (d) fracture of mechanical connection steel reinforcement, and (e) comparison of uniaxial tensile stress–strain curves for rebar and mechanical connector.
Table 3
Mechanical properties of steel materials.
| Material type | Diameter (mm) | Yield strength f y (MPa) | Ultimate strength f u (MPa) | Elastic modulus E s (MPa) |
|---|---|---|---|---|
| HRB400 | 8 | 422 | 541 | 2.00 × 105 |
| 18 | 442 | 618 | 2.00 × 105 | |
| 20 | 448 | 633 | 2.00 × 105 | |
| Buckle connection HRB400 | 18 | 427 | 611 | 2.00 × 105 |
| 20 | 436 | 633 | 2.00 × 105 | |
| φS15.2 steel strand | 15.2 | — | 1,930 | 1.95 × 105 |
Figure 5
Experimental setup and loading schematic. (a) Schematic of the experimental setup and (b) on-site image of the experimental setup.
Figure 6
Loading system.
Figure 7
Layout of measuring points and measurement methods. (a) Reinforcement strain measurement points, (b) speckle pattern in the core region, (c) DIC instrument, and (d) crack width measuring instrument.
Figure 8
Crack distribution and failure features of cast-in-place specimen S1. (a) Cracks at the left beam of specimen S1, (b) cracks at the right beam of specimen S1, (c) cracks at the joint core of specimen S1, (d) failure at the left beam end of specimen S1, and (e) concrete crushing in the right beam of specimen S1.
Figure 9
Crack development and failure characteristics of precast non-prestressed specimen S2. (a) Overall failure mode of specimen S2, (b) cracks in the left beam of specimen S2, (c) cracks in the right beam of specimen S2, and (d) cracks in the joint core of specimen S2.
Figure 10
Crack development and failure characteristics of precast prestressed specimen S3. (a) Cracks in the left beam of specimen S3, (b) cracks in the core joint of specimen S3, (c) cracks in the right beam of specimen S3, (d) overall failure mode of specimen S3, (e) reinforcement exposure in the left beam of specimen S3, and (f) connector exposure in specimen S3.
Figure 11
Crack propagation and beam-end damage in precast prestressed specimen S4. (a) Cracks in the left beam of specimen S4, (b) cracks in the joint core of specimen S4, (c) cracks in the right beam of specimen S4, (d) concrete spalling at the bottom of the beam in specimen S4, and (e) reinforcement exposure in the right beam of specimen S4.
Figure 12
Cracking, connector exposure, and spalling in precast prestressed specimen S5. (a) Cracks in the left beam of specimen S5, (b) cracks in the joint core of specimen S5, (c) cracks in the right beam of specimen S5, (d) overall failure mode of specimen S5, (e) concrete spalling at the bottom of the beam in specimen S5, (f) connector exposure at the bottom of the beam in specimen S5, and (g) concrete crushing at the upper end of the beam in specimen S5.
Figure 13
Severe cracking and reinforcement exposure in precast prestressed specimen S6. (a) Cracks in the left beam of specimen S6, (b) cracks in the joint core of specimen S6, (c) cracks in the right beam of specimen S6, (d) overall failure mode of specimen S6, (e) concrete crushing at the upper part of the left beam in specimen S6, (f) reinforcement exposure in the right beam of specimen S6, and (g) exposure of connectors and stirrups at the bottom of the beam in specimen S6.
Figure 14
Comparison of the hysteresis curves. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, and (f) S6.
Figure 15
Comparison of the skeleton curves.
Table 4
Comparison of the characteristic parameters.
| Specimen number | Loading direction | Yield load P y (kN) | Peak load P max (kN) | P max/P y | Average ratio |
|---|---|---|---|---|---|
| S1 | Positive | 78 | 83.61 | 1.07 | 1.10 |
| Negative | 73.24 | 82.62 | 1.13 | ||
| S2 | Positive | 74.54 | 86.52 | 1.16 | 1.10 |
| Negative | 77.98 | 80.66 | 1.03 | ||
| S3 | Positive | 78.39 | 87.43 | 1.12 | 1.15 |
| Negative | 76.84 | 90.75 | 1.18 | ||
| S4 | Positive | 80.09 | 91.8 | 1.15 | 1.13 |
| Negative | 83.01 | 29.53 | 1.12 | ||
| S5 | Positive | 79.76 | 91.93 | 1.15 | 1.13 |
| Negative | 93.85 | 103.04 | 1.10 | ||
| S6 | Positive | 87.46 | 110.45 | 1.26 | 1.26 |
| Negative | 76.65 | 97.09 | 1.27 |
Figure 16
Comparison of the strength degradation. (a) Strength degradation of S1, (b) strength degradation of S2, (c) strength degradation of S3, (d) strength degradation of S4, (e) strength degradation of S5, and (f) strength degradation of S6.
Figure 17
Comparison of the stiffness degradation.
Figure 18
Equivalent elastic-plastic energy method.
Table 5
Comparison of the characteristic displacements.
| Specimen | Loading direction | Δ y (mm) | Δ u (mm) | μ | Average coefficient |
|---|---|---|---|---|---|
| S1 | Forward | 37.64 | 89.90 | 2.39 | 3.52 |
| Reverse | 19.35 | 90.11 | 4.66 | ||
| S2 | Forward | 28.1 | 72.03 | 2.56 | 2.37 |
| Reverse | 32.92 | 71.94 | 2.06 | ||
| S3 | Forward | 29.28 | 72.05 | 2.38 | 2.32 |
| Reverse | 32.97 | 71.97 | 2.06 | ||
| S4 | Forward | 27.65 | 67.87 | 2.45 | 2.20 |
| Reverse | 34.91 | 68.12 | 1.95 | ||
| S5 | Forward | 26.48 | 68.10 | 2.57 | 2.29 |
| Reverse | 33.96 | 67.92 | 2.00 | ||
| S6 | Forward | 26.66 | 75.03 | 2.81 | 2.63 |
| Reverse | 30.73 | 74.90 | 2.44 |
Figure 19
Energy dissipation coefficient calculation diagram.
Table 6
Comparison of energy dissipation coefficient (E) and equivalent viscous damping coefficient (h e).
| Specimens | S1 | S2 | S3 | S4 | S5 | S6 |
|---|---|---|---|---|---|---|
| E | 1.53 | 1.12 | 1.2 | 1.33 | 1.38 | 1.63 |
| h e | 0.25 | 0.18 | 0.19 | 0.22 | 0.22 | 0.26 |
Figure 20
Comparison of energy dissipation coefficient.
Figure 21
Comparison of beam longitudinal reinforcement strains of specimens S2 and S6.
Figure 22
Comparison of column longitudinal reinforcement strain of specimens S2 and S6.
Figure 23
Comparison of the stirrup strain distribution of specimens S2 and S6.
Figure 24
DIC strain contour plots of specimen S2.
Table 7
Comparison of residual deformation rate.
| Specimen | Loading direction | Δ r (mm) | Δ u (mm) | μ r (%) | Average value (%) |
|---|---|---|---|---|---|
| S1 | Reverse | 78.08 | 90.11 | 0.87 | 0.87 |
| Forward | 77.65 | 89.9 | 0.86 | ||
| S2 | Reverse | 50.03 | 71.94 | 0.70 | 0.75 |
| Forward | 58.06 | 72.03 | 0.81 | ||
| S3 | Reverse | 49.58 | 72.17 | 0.69 | 0.73 |
| Forward | 55.21 | 72.05 | 0.77 | ||
| S4 | Reverse | 42.87 | 68.12 | 0.63 | 0.67 |
| Forward | 48.79 | 67.87 | 0.72 | ||
| S5 | Reverse | 47.66 | 67.92 | 0.70 | 0.76 |
| Forward | 55.48 | 68.1 | 0.81 | ||
| S6 | Reverse | 60.38 | 74.9 | 0.81 | 0.83 |
| Forward | 64.14 | 75.03 | 0.85 |