# Bending Strength of Connection Joints of Prestressed Reinforced Concrete Pipe Piles

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## Abstract

**:**

## 1. Introduction

## 2. Specimens and Materials

#### 2.1. Specimen Design

_{0}) of 25 mm and end plate bevel dimensions as shown in Figure 2, with full and continuous welds. In particular, the inner edge of the end plate of the JT2 joint was also welded. The pile type parameters are shown in Table 1.

#### 2.2. Material Properties

## 3. Methods

#### 3.1. Loading Scheme of Tests

_{cr}and ultimate bending moment M

_{u}of the pipe pile shaft were calculated according to the theoretical equations in the literature [6]. Firstly, the specimen was loaded from zero to 80% of M

_{cr}at a grade difference of 10% of M

_{cr}; then, it was loaded to M

_{cr}at a grade difference of 5% of M

_{cr}, and loading was continued until the specimen was damaged at a grade difference of 5% of M

_{u}. The load was held for 3 min at each level. The loading was terminated when the concrete crack in the tensile zone of the pipe pile specimen reached 1.5 mm or the reinforcement fractured, and the concrete in the compressed zone was crushed or the joint connection failed.

#### 3.2. Numerical Simulation

#### 3.2.1. Finite Element Model

#### 3.2.2. Constitutive Models and Parameters

_{0}is the initial modulus of elasticity, d is the damage factor, and d

_{t}and d

_{c}are used to characterize the uniaxial tensile and compressive stiffness degradation, the calculation of which is detailed in the ABAQUS Technical Manual [38] and Code for Design of Concrete Structures GB 50010-2010 [28]. Accordingly, the plastic stress–strain and damage evolution curves of C80 concrete were obtained and are shown in Figure 5.

_{s}, E

_{s}is the modulus of elasticity of the reinforcement, and f

_{y}and ε

_{y}are the yield strength and strain of the reinforcement.

#### 3.2.3. Contact Properties and Boundary Conditions

_{s}) was used to calculate the temperature reduction required for prestressed reinforcement, where the steel expansion coefficient α is taken as 1.2 × 10

^{−5}. Loading calculation was by applying displacement at the point where the load was applied.

## 4. Results and Discussion

#### 4.1. Failure Characteristics of Test Specimens

#### 4.2. Bending Bearing Capacity

_{cr}and ultimate bending moment M

_{u}as well as the measured crack moment M’

_{cr}and ultimate bending moment M’

_{u}of each specimen and their comparison are given in Table 3. From Figure 8 and Table 3, it can be seen that:

_{cr}near the PRC pipe pile joint was 15–23% larger than the calculated pile crack moment M

_{cr}(mean value 20%), which was slightly larger than the measured value of ZS1 pile; with JT1 as the reference, the measured crack moment M’

_{cr}of the joint was only 4–7% higher when the comprehensive reinforcement ratio was increased by 1–2 times. When multiple sections of piles were involved in the design of pipe piles for foundation support, the strength at the joints should be considered for reduction [6]. The ratio between the measured ultimate bending moment of the joint and the calculated crack moment of the pile in this test was 1.63–2.30; therefore, it also further verified that under the national standard JGJ/T 406-2017 [6] it is safe and reasonable to adopt the crack moment of the pile without considering the effect of non-prestressed reinforcement as the design value of the joint bending moment of the pipe pile under the condition of the comprehensive subfactor of 1.25 for the load of the supporting structure.

_{u}of pile specimen ZS1 was slightly larger than its calculated value M

_{u}, but the measured ultimate bending moment M’

_{u}of the joint specimen was smaller than the calculated ultimate bending moment M

_{u}of the pile, which was only 58–87% of the calculated value, among which the measured ultimate bending moment M’

_{u}of joint JT1 was 81% of the measured value of pile ZS1. In one previous study [6], seven sets of bending tests carried out on PRC pipe pile joints had 500 mm diameter and 1000 mm wall thickness, and the bending strength of PRC pipe pile joints was improved by about 10% compared with PHC pipe pile joints, but the measured ultimate bending moment was only 64–81% of the measured value of the pile shaft, and similar results were also obtained in three sets of PHC pipe pile joint tests in another study [8]. It can be seen that it is difficult for the traditional welded joint to achieve the bending strength of the pipe pile shaft; the pipe pile can improve the bending bearing capacity of the pile shaft by compound reinforcement, but it does not improve the bending bearing capacity of the pipe pile joint to the same extent.

#### 4.3. Numerical Model Validation

#### 4.4. Mechanisms of Stress and Deformation of Connection Joints

#### 4.5. Influencing Factors on Bending Strength of Joints

#### 4.5.1. Concrete Strength

#### 4.5.2. Effective Precompression Stress of Concrete

_{con}of prestressed reinforcement in pipe piles was limited to 0.5–0.9 times the standard value of the tensile strength of steel reinforcement f

_{ptk}, which is generally taken as 0.7 times of f

_{ptk}. To analyze the effect of the effective precompression stress of concrete, three cases of σ

_{con}= 0.5f

_{ptk}, 0.7f

_{ptk}, and 0.9f

_{ptk}were considered, corresponding to 70, 100, and 130% of the reference value (σ

_{con}= 0.7f

_{ptk}) of prestress, respectively.

#### 4.5.3. Joint Connection Mode

## 5. Conclusions

- (1)
- The crack resistance of PRC pipe pile welded joints was comparable to that of the pipe pile shaft, and it is safe and reasonable to adopt the crack moment of the pile without considering the effect of non-prestressed reinforcement as the design value of the joint bending moment of the pipe pile.
- (2)
- The bending strength of welded joint of PRC pipe pile was lower than that of the pile shaft. The main failure mode was that the tensile yield of the end plate was in the shape of a drum, the pile hoop and end plate were obviously separated from the pipe pile, and the concrete on the upper edge of the pile hoop was crushed.
- (3)
- The bending strength of the joint with fully welded end plates and long pin connection was close to that of the pile shaft. Rachel reinforcement connection implanted into concrete and filled with high-strength structural bar glue between end plates can be used to maintain the continuity of the joint section and force and improve the bending strength of the joint.
- (4)
- The bending capacity of the joint can be improved by increasing the strength grade and reinforcement ratio of concrete at the same time, or by strengthening the precompression stress of concrete. However, the comprehensive reinforcement ratio of PRC pipe pile with a pile diameter of 1000 mm and a wall thickness of 130 mm should not be more than 3.4%, otherwise brittle failure will occur easily.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**Schematic diagram of reinforcement of PRC pipe piles: (

**A**) ZS1, JT1, and JT2; (

**B**) JT3; (

**C**) JT4; (

**D**) cross-sectional reinforcement and end plate connection (unit: mm).

**Figure 5.**Plastic stress–strain curves and damage evolution curves of C80 concrete: (

**A**) compression; (

**B**) tension.

**Figure 11.**Stress characteristics of pipe pile joint JT1 (unit: MPa): (

**A**) pile hoop and end plate; (

**B**) prestressed steel rods; (

**C**) non-prestressed reinforcements.

**Figure 12.**Deformation of pipe pile joint (display enlarged by 10 times, unit: mm): (

**A**) end plate; (

**B**) pile hoop.

**Figure 14.**Influence of precompression stress of concrete on the bending strength of joints: (

**A**) load-midspan deflection curves; (

**B**) tension damage; (

**C**) compression damage.

Test Sample | Pile Length (m) | Prestressed Reinforcement | Non-Prestressed Reinforcement | Hoop Reinforcement | Anchored Reinforcement | ${\mathit{\rho}}_{\mathit{s}}$ (%) |
---|---|---|---|---|---|---|

ZS1 | 40 | 44Φ12.6 | 22Φ10 | Φ8 | / | 2.0 |

JT1 | 26 + 26 | 44Φ12.6 | 22Φ10 | Φ8 | / | 2.0 |

JT2 | 26 + 26 | 44Φ12.6 | 22Φ25 | Φ8 | / | 4.6 |

JT3 | 20 + 20 | 44Φ12.6 | 22Φ25 | Φ8 | 22Φ12.6 | 2.3–5.4 |

JT4 | 20 + 20 | 44Φ12.6 | 22Φ25 | Φ8 | 22Φ25 | 7.6 |

_{s}is the comprehensive reinforcement ratio; that is, the ratio of the total area of prestressed reinforcement and non-prestressed reinforcement to the effective area of concrete.

Material | Density $\mathit{\rho}$ (kg/m ^{3}) | Elastic Modulus E (GPa) | Poisson’s Ratio $\mathit{\upsilon}$ | Yield Strength f _{y} (MPa) | Compression Strength f _{c} (MPa) | Tensile Strength f _{t} (MPa) |
---|---|---|---|---|---|---|

Prestressed steel rods | 7800 | 200 | 0.30 | 1280 | / | 1420 |

Non-prestressed reinforcements | 7800 | 200 | 0.30 | 400 | / | 540 |

End plate | 7800 | 200 | 0.30 | 225 | / | 400 |

Pile hoop | 7800 | 200 | 0.30 | 345 | / | 470 |

C60 concrete | 2400 | 38 | 0.20 | / | 50.2 | 3.11 |

C70 concrete | 2400 | 37 | 0.20 | / | 44.5 | 2.99 |

C80 concrete | 2400 | 36 | 0.20 | / | 38.5 | 2.85 |

Test Sample | M_{cr}(kN·m) | M_{u}(kN·m) | M’_{cr}(kN·m) | M’_{u}(kN·m) | M’_{cr}/M_{cr} | M’_{u}/M_{cr} | M’_{u}/M_{u} |
---|---|---|---|---|---|---|---|

ZS1 | 1285 | 2760 | 1503 | 2958 | 1.17 | 2.30 | 1.07 |

JT1 | 1285 | 2760 | 1478 | 2400 | 1.15 | 1.87 | 0.87 |

JT2 | 1285 | 3640 | 1542 | 2298 | 1.20 | 1.79 | 0.63 |

JT3 | 1285 | 3675 | 1565 | 2506 | 1.22 | 1.95 | 0.68 |

JT4 | 1285 | 3640 | 1576 | 2093 | 1.23 | 1.63 | 0.58 |

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## Share and Cite

**MDPI and ACS Style**

Tang, M.; Ling, Z.; Qi, Y.
Bending Strength of Connection Joints of Prestressed Reinforced Concrete Pipe Piles. *Buildings* **2023**, *13*, 119.
https://doi.org/10.3390/buildings13010119

**AMA Style**

Tang M, Ling Z, Qi Y.
Bending Strength of Connection Joints of Prestressed Reinforced Concrete Pipe Piles. *Buildings*. 2023; 13(1):119.
https://doi.org/10.3390/buildings13010119

**Chicago/Turabian Style**

Tang, Mengxiong, Zao Ling, and Yuliang Qi.
2023. "Bending Strength of Connection Joints of Prestressed Reinforced Concrete Pipe Piles" *Buildings* 13, no. 1: 119.
https://doi.org/10.3390/buildings13010119