This paper presents an easily installed improved perfobond connector (PBL) designed to reduce the shear concentration of PBL. The improvement of PBL lies in changing the straight penetrating rebar to the Z-type penetrating rebar. To study the shear performance of improved PBL, two PBL test specimens which contain straight penetrating rebar and six improved PBL test specimens which contain Z-type penetrating rebars were designed and fabricated, and push-out tests of these eight test specimens were carried out to investigate and compare the shear behavior of PBL. Additionally, Finite Element Analysis (FEA) models of the PBL specimens were established and validated against the test results. Through FEA, the effects of concrete grade, perforated plate’s aperture, Z-type penetrating rebar’s diameter, Z-type penetrating rebar’s bending angle, and bending length on shear behaviors were discussed. The results indicate that (1) Compared with PBL specimens with straight penetrating rebars, Z-type penetrating rebar can significantly improve the shear resistance and shear stiffness of the specimens. This enhanced performance can be mainly attributed to the increased adhesion of the transverse rebar. (2) By comparing the load-slip curve, the slip of PBL test specimens which contain straight penetrating rebar increases rapidly and the bearing capacity decreases rapidly after concrete craking, while the bearing capacity of Z-type penetrating rebar specimens decreases first and then increases gradually, showing better ductility. (3) The stress of the PBL shear connector with Z-type penetrating rebar is more uniform than the PBL shear connector with straight penetrating rebar, and the overall deformation is more uniform. (4) The higher the concrete grade, the higher the shear bearing capacity and the better ductility of the new PBL. Increasing the aperture of the perforated plate or the diameter of the rebar has a very limited effect on the improvement of the shear capacity of PBL. Through the systematic analysis of the mechanical properties of Z-type penetrating rebar PBL specimen, the experimental reference is provided for improving the structure and design of new type PBL.

Due to large shear stiffness, high ultimate shear capacity, good ductility, excellent fatigue resistance, and simple configuration, PBL (Perfobond Leiste) shear connectors have been widely used in composite girder bridges [

The above improved methods for PBL shear connectors greatly increase the shear bearing capacity and stiffness of PBL shear connectors. However, for Y-type shear bond, T-type shear bond, V-type steel plate shear bond, and carbon brazing material enhanced shear bond, their failure modes all stem from the failure of the concrete mortise. Changing the type of shear bond yields only limited improvement on the specimen. Although employing ultra-high-performance concrete significantly enhances the shear bond performance, implementing aforementioned novel shear bonds with ultra-high-performance concrete is costly and needs endeavors. Consequently, adopting these aforementioned improved methods for PBL shear connectors would inevitably escalate construction costs and complexities.

To this end, an improved PBL shear connector with Z-type penetrating rebar is proposed in this paper. It only requires placing the Z-type penetrating rebar into the perforated plate, which is simple and convenient for construction.

Compared with ordinary strain gauges, the Fiber Bragg grating (FBG) sensors have better corrosion resistance, anti-electromagnetic interference, high sensitivity and transmission capacity, and can realize multi-point distributed measurement [

To investigate the shear performance of the improved PBL with Z-type penetrating rebar, this paper carries out push-out tests and finite element analysis. By push-out tests, the load slip curve, concrete failure mode, ultimate bearing capacity and shear stiffness of ordinary PBL test specimens and improved PBL test specimens are obtained. The differences of the two types of test specimens in the above four responses are obtained. The strain values of straight penetrating rebar and Z-type penetrating rebar are measured by using fiber Bragg grating sensor in the entire push-out test process, and their results are compared and analysed. Further, PBL shear connectors with different parameters are established through the validated finite element analysis model, including the diameter of the penetrating rebar, the aperture of the perforated plate, the strength of concrete, and the bending angle and length of Z-type penetrating rebar. Based on the results, the influence of different parameters of the improved PBL on the bearing capacity is discussed.

To investigate the shear performance of the improved PBL in accordance with Eurocode 4 [

According to the code GB50010-2010 [

Portland cement | Fly ash | Mineral powder | Medium sand | Broken gravel | Water reducer | Water | |
---|---|---|---|---|---|---|---|

Material per cubic meter of concrete (kg/m^{3}) |
400 | 40 | 40 | 716 | 1074 | 5.8 | 150 |

Theoretical mix ratio | 1 | 0.10 | 0.10 | 1.79 | 2.69 | 0.015 | 0.38 |

Types of steel (MPa) | Yield strength (MPa) | Tensile strength (MPa) | Young’s modulus (GPa) |
---|---|---|---|

Q345b | 374 | 521 | 202 |

HRB335 | 479 | 597 | 200 |

During the casting process, six groups of 150 mm × 150 mm × 150 mm cubic concrete samples with a standard strength of 50 MPa (C50) were prepared, and the performance of the concrete materials was evaluated. The test results indicated that the concrete exhibited a cubic compressive strength fcu of 54.6 MPa, an axial compressive strength fc of 44.5 MPa, a cubic splitting strength ft of 4.8 MPa, and an elastic modulus Ec of 42.6 GPa.

Materials | Compressive strength (MPa) | Flexural strength (MPa) | Tensile strength (MPa) | Young’s modulus (GPa) | Poisson’s ratio |
---|---|---|---|---|---|

C50 | 54.6 | 6.8 | 3.1 | 38.5 | 0.2 |

After transferring the specimen to the test bench and before the start of the experiment, the vertical line method and the spherical angle steel plate were applied to align the center, then the specimen was preloaded to 100 kN, and its position was fine-adjusted. When the reading error of the relative slip by the four dial indicators was within ±0.1 mm, the specimen was considered to be under axial compression. Therefore, the loading process of PBL can be regarded as an uneccentric axial compression process.

In the preloading steps, the load was 100 kN and the loading speed was 2 kN/s, respectively. In the formal loading, each stage had a displacement of 0.4 mm with a duration of 2 min under the load. After the displacement reached 4 mm, the displacement of each stage was 0.2 mm while the duration under the load was 2 min, until the specimen was destroyed by the loading machine.

As shown in

To better analyse the failure pattern of the specimens, The concrete blocks were cut off in two directions after the failure. One direction was parallel to perfobond steel plates, as shown in

The ultimate bearing capacity of the specimen in this study stems from the peak load in the load-slip curve.

Specimen number | Ultimate bearing capacity/kN | Bearing capacity of a single PBL shear connector/kN | Mean ultimate bearing capacity/kN | Finite element calculation value/kN |
---|---|---|---|---|

Straight 1 | 412.23 | 206.11 | 211.07 | 214.14 |

Straight 2 | 432.05 | 216.03 | ||

Z-type 1 | 481.72 | 240.86 | 233.07 | 231.20 |

Z-type 2 | 483.76 | 241.88 | ||

Z-type 3 | 437.55 | 218.77 | ||

Z-type 4 | 467.56 | 233.78 | ||

Z-type 5 | 435.63 | 217.81 | ||

Z-type 6 | 490.63 | 245.31 |

The elastic phase of the specimen under load is characterized by a linear phase with high stiffness, which mainly relies on the bond between the steel plate and the concrete to resist the applied force. Initially, the relative slip is negligible. However, with the increase of load, the bond at the steel-concrete interface gradually weakens. Once this binding force is eliminated, the load is gradually borne by the penetrating steel bar. Therefore, the relative slip gradually increases until the peak load is reached. Subsequently, with the continuous increase of load, the concrete cracks and the bearing capacity of the specimen decrease rapidly. It is noteworthy that cracks were observed on the concrete blocks, and this is mainly caused by insufficient bonding capacity of the transverse straight penetrating rebar. By comparing the peak load of the two types of specimens, the ultimate bearing capacity of the specimen with Z-type penetrating rebar is enhanced by 10.42% compared with the straight penetrating rebar. The high strength shown in the specimen using Z-type penetrating rebar may be due to the increased adhesion of the Z-type rebar.

After the concrete cracked, the specimen was in the post-yield hardening stage with rapidly increasing relative slip, while the bearing capacity of the specimen with straight penetrating rebar slowly decreased, and the bearing capacity of the specimen with Z-type penetrating rebar decreased first and then increased gradually. The specimen with a Z-type penetrating rebar achieved an average peak load of 380.8 kN in the third stage, while the specimen with a straight penetrating rebar attained an average peak load of 328.5 kN during the same stage. The utilization of Z-type penetrating rebar resulted in a significant increase of 15.9% in the average peak load of the third stage.

Therefore, the specimen with Z-type penetrating rebar did not fail immediately after reaching concrete cracking. It still had some shear resistance, which was beneficial for improving the ductility of PBL shear connectors. Finally, the perforated plate and the penetrating rebar were pushed out from the concrete block, leading to the failure of the specimen.

The final slip of the eight specimens in this test was greater than 6.00 mm. According to European specification 4, the ductility of the shear connectors can meet the shear requirements when the slip is greater than 6.00 mm [

Researchers have defined the shear stiffness of perforated connectors by using the cutting slope of important inflection points on the load-slip curve. The Japanese specification [

Specimen type | The secant stiffness at 1/3 of the ultimate load (kN/mm) | The secant stiffness at 1/2 of the ultimate load (kN/mm) | The secant stiffness at 0.2 mm (kN/mm) |
---|---|---|---|

Straight penetrating rebar | 521.39 | 297.73 | 426.80 |

Z-type penetrating rebar | 539.92 | 314.38 | 472.33 |

The shear stiffness of the Z-type perforated connector is consistently superior to that of the straight perforated connector in all three shear stiffness calculation methods. According to the results obtained by the three methods, the shear stiffness of Z-type perforated connectors is 3.55%, 5.59%, and 10.67% higher than that of straight perforated connectors, respectively. Among the three calculation methods, the maximum increase is 10.67%.

In this study, the grating strain sensor is used to measure the average strain of the steel rebar with a range of 60 mm under the sensor. The test captures strains of the penetrating steel rebars throughout the entire loading process.

The whole loading process can be divided into four stages: elastic stage, plastic stage, bending of penetrating rebar stage, and failure stage. At the initial stage of loading, the shear force transmitted by the steel-perfobond connector surface is small, mainly due to the bond resistance between the I shape steel and concrete. The strain values of the two types of penetrating steel rebars are very small, and the rebars are in the elastic stage.

After the bonding resistance is eliminated, the test enters the plastic stage. The penetrating steel rebar begins to be stressed under the action of load. The shear force is resisted by the penetrating steel rebar and the circular concrete tenon. The upper surface of the rebars is compressed, and the strain monitored by the sensor gradually increases.

After the test entered the bending stage of steel rebars, the strain of straight steel rebars and Z-type penetrating rebars continued to increase.

At the failure stage of the test, the strain values of both types of penetrating steel rebars did not change significantly. This is attributed to the concrete having seriously cracked by this time, losing its constraining effect on the penetrating steel rebars, with the specimens now only supported by the residual strength of the concrete. It is noteworthy that under the same load, Z-type penetrating rebars exhibit greater strain, which means that Z-type penetrating rebars bear more shear during loading.

3-D nonlinear finite element (FE) models of the push-out tests are established by ABAQUS to investigate the shear behavior of the PBL with Z-type penetrating rebar. As shown in

The bottom of the concrete block is fixed in the model, and the downward displacement loading is employed on the I shape steel, which is the same as the tests’ boundary condition and the loading condition. The perforated plate and the I shape steel share the same node. The concrete and the rebar are connected by the Embed technique. The surface-to-surface contact is adopted between the concrete and the perforated plate, as well as between the concrete and the I shape steel. The contact surface of I shape steel and the perforated plate are set as the main surface, and the concrete contact surface is set as the slave surface. A friction coefficient of 0.5 [

In this paper, the ideal elastic-plastic bifold constitutive model is selected for the I-shape steel and the rebar [

The uniaxial stress damage constitutive model proposed by Ding et al. [

The relevant parameters of concrete are substituted into ^{3}, the elastic modulus of 3.45 × 10^{4} MPa and Poisson’s ratio of 0.2.

The mean ultimate bearing capacity of two PBL specimens with straight penetrating rebar in test is 211.07 kN with a relative slip of 0.96 mm, while the FE analysis yields a mean ultimate bearing capacity of 214.14 kN and a relative slip of 0.97 mm. The actual mean ultimate bearing capacity of six PBL specimens with Z-type penetrating rebar in test is 233.07 kN with a relative slip of 1.07 mm, while the FE analysis yields a mean ultimate bearing capacity of 231.20 kN and a relative slip of 1.02 mm. These results demonstrate that the developed numerical modelling method is reasonable and effective in analysing the mechanical behavior of PBL shear connectors.

To investigate the influence of different factors on the load-slip behavior of the PBL with Z-type penetrating rebar, 15 PBL models were parameterized by the validated FE. The relevant parameters include concrete strength grade, perforated plate aperture size, penetrating bar diameter, bending angle and length of Z-type penetrating rebar.

Parameters | Range |
---|---|

Concrete cubic strength _{c} (Mpa) |
40, 50, 60 |

Diameter of perforated plate’s hole _{h} (mm) |
30, 40, 50 |

Diameter of penetrating rebar _{r} (mm) |
10, 14, 18 |

Bending angle of penetrating rebar | 40°, 45°, 50° |

Bending length of penetrating rebar _{r} (mm) |
40, 50, 60 |

Three FE models with concrete grades of C40, C50, and C60 were used to explore the impact of concrete strength on the shear behavior of PBL connectors, and other factors remained unchanged like a 14-mm-diameter rebar, a 40-mm-diameter perforated plate aperture, and a 45-degree bend in the Z-type rebar.

Three FE models were developed to examine the impact of perforated plate aperture sizes (30, 40, and 50 mm) on the shear behavior of PBL connectors, and other parameters kept constant. The models featured 14-mm-diameter rebar, C50 concrete, and a 45-degree bend in the Z-type rebar. As shown in

Three FE models were created to investigate the impact of penetrating rebar diameter on the shear behavior of PBL connectors. These models featured Z-type rebars with diameters of 10, 14, and 18 mm. All other parameters were maintained constant, i.e., the 40-mm-diameter perforated plate aperture, the C50 concrete, and the Z-type rebar’s bending angle at 45 degrees.

To investigate the effects of the bending angle and length of the Z-type penetrating rebar on the shear behavior of PBL connectors, three FE models were created for each variable. In the first set, the models had rebars with bending angles of 40°, 45°, and 50°, and in the second set, the rebars had bending lengths of 40, 50, and 60 mm. All other parameters were consistent among models: a concrete grade of C50, a rebar diameter of 14 mm, and an aperture size of 40 mm in the perforated plate. The load-slip curves, depicted in

Based on the results of the parametric study of PBL with Z-type penetrating rebar, the conclusions are as follows:

The main factor affecting the bearing capacity and load-slip behavior of the PBL with Z-type penetrating rebar is the concrete grade. The higher the grade of concrete, the higher shear capacity and better ductility.

Increasing the perforated plate’s aperture has a very limited effect on the improvement of the shear bearing capacity of PBL. Increasing the penetrating rebar’s diameter has little impact on the improvement of the shear bearing capacity of PBL.

The bending angle of the rebar and the length of the bending part should match the diameter of the perforated plate aperture. In this study, the perforated plate’s aperture is 40 mm, with the Z-type penetrating rebar’s bending angle of 45° and the penetrating rebar’s bending length of 50 mm, the shear bearing capacity of the PBL with Z-type penetrating rebar is the largest.

In this study, the push-out tests of the PBL specimens with Z-type penetrating rebar and the PBL specimens with straight rebar were carried out to compare their load-slip curve, strain, ultimate bearing capacity and failure mode. Through FE analysis, the effects of concrete grade, perforated plate’s aperture, Z-type penetrating rebar’s diameter, Z-type penetrating rebar’s bending angle and length on shear behavior were discussed. The main conclusions are as follows:

Under the same material conditions, the PBL specimens with Z-type penetrating rebar is better than that of the PBL specimens with straight penetrating rebar. The ultimate bearing capacity of Z-type perforated connector is 10.42% higher than that of the straight rebar, and the maximum shear stiffness increased by 10.67%.

According to the load-slip curve of the specimen, the bearing capacity of the specimen with straight penetrating rebar slowly decreased, and the bearing capacity of the specimen with Z-type penetrating rebar decreased first and then increased gradually. Thus, the specimen containing Z-type penetrating rebar did not fail immediately after concrete cracking and it still retained shear resistance, enhancing the ductility of the specimen. By comparing the peak load of the two types of connectors, the ultimate bearing capacity with Z-type penetrating rebar is enhanced by 10.42% compared with straight penetrating rebar. This enhanced performance can be mainly attributed to the increased adhesion of the transverse penetrating rebar. The use of Z-type rebar can significantly improve the bonding ability to the concrete.

The stress of the PBL shear connector with Z-type perforated connector is more uniform than the PBL shear connector with a straight perforated connector, and the overall deformation is more uniform. Under the same load, Z-type penetrating rebars exhibit greater strain, which means that Z-type penetrating rebars bear more shear force during loading. It makes full use of the shear and tensile properties of the penetrating rebars, which promotes the shear bearing capacity and ductility of the integrated PBL shear connector.

Based on the results of the parametric study of PBL connectors with Z-type penetrating rebar, it can be observed that the higher the grade of concrete, the higher shear capacity and the better ductility. Increasing the perforated plate’s aperture and penetrating rebar’s diameter have a very limited effect on the improvement of the shear bearing capacity of PBL. The bending angle of the rebar and the length of the bending part should match the diameter of the perforated plate aperture.

None.

The authors received no specific funding for this study.

Study conception and design: Caiping Huang; data collection: Zihan Huang; analysis and interpretation of results: Wenfeng You; draft manuscript preparation: Caiping Huang, Zihan Huang, Wenfeng You. All authors reviewed the results and approved the final version of the manuscript.

All data generated or analysed during this study are included in this published article.

The authors declare that they have no conflicts of interest to report regarding the present study.