In the water resources allocation project in Pearl River Delta, in order to optimize the structural design, the deep buried tunnel adopts the composite lining structure. However, the weakest link in a complex structure is the connection between two different interfaces. This paper reports the findings of an experimental study that was undertaken to investigate the interface mechanical performance of steel self-compacting concrete composite structure subjected to cyclic loads. In this study, different shear connectors are considered, and six different specimens were designed and tested, respectively. The test is used to research the effect of the different shear connectors on the bearing capacity and interface mechanical properties of composite structure in an experimental study. According to these test results, a detailed analysis was carried out on the relationships, such as the stress-strain and load-displacement relationships for the specimen. These tests show that the shear connectors will significantly enhance the bearing capacity and interface mechanical properties of the composite structure. Among them, the comprehensive performance of the specimens using the stud-longitudinal ribs shear connectors is the best. Additionally, a finite element analysis (FEA) model was developed. The comparison of the simulation results with the experimental results shows that this FEA is applicable for this type of experiment.
This experiment takes the water resources allocation project in Pearl River Delta as the background, in which the deep-buried tunnel accounts for 75% of the total length. The deep-buried tunnel has the characteristics of large burial depth, long line length, and high internal and external water pressure, and the tunnel construction is in an extreme environment. Compared with ordinary tunnels, the high internal and external water pressures often cause the tunnels to be under extreme load conditions [
Diagram of composite lining.
In steel-concrete composite structures, the advantages of steel and concrete can be fully utilized, and shear connectors are directly related to the bearing capacity of the whole composite structure as the key part of the stress [
Stud connectors are widely used in bridges with smaller spans, and scholars have conducted many studies on them [
In the design of composite lining structure, it is believed that the opening of the interface will lead to the failure of the structure. At present, similar projects lack reasonable design methods and can only adopt safe and conservative methods, which are not scientific and economical. Therefore, in order to improve the performance of the interface, it is necessary to comprehensively compare different interface processing methods and observe the mechanical properties and interface slippage properties of composite beams under different interfaces. Compared with the traditional single connection, the method of composite shear connection will be more economical and safe for the project, and there are few related researches at present. In order to compare the influence of different composite shear connectors on the bearing capacity of steel-self-compacting concrete composite beams, the interfacial mechanical properties of steel-self-compacting concrete composite beams under reciprocating loads were studied in detail. At the same time, high performance concrete is gradually replacing ordinary concrete [
This paper mainly studies the mechanical properties of the steel-self-compacting concrete interface in the composite lining structure. Since the composite lining bears various loads together, the shear connectors need to transmit tensile force, shear force, and bending moment. At the same time, the tunnel involves repeated water filling and discharging; the change of water pressure is equivalent to repeated loading and unloading, so the test adopts the loading and unloading mode of reciprocating cycle. In order to fully study the mechanical properties of the steel-concrete interface, this study intends to start from the microscopic force mechanism of the interface and conduct the test of the bending and shear properties of the steel self-compacting concrete composite structure and study the behavioral characteristics of the steel-concrete interface under the cyclic loads. Through the above research, the connection mode of steel-concrete interface is optimized to improve the overall bearing capacity of the structure.
All steel types used in the test are Q345C. According to the code GB/T 2975-2018 [
Mechanical properties of steel.
Model | No. | ||
---|---|---|---|
Q345C | 1 | 336.12 | 203 |
Q345C | 2 | 338.20 | 200 |
— | Ave. | 337.16 | 201.5 |
The strength grade of self-compacting concrete is C30, the coarse aggregate is ordinary gravel, and the daily drinking water is mixed. All indexes of water quality meet the requirements of concrete mixing water. According to JGJ/T 98-2010 [
Concrete mix design.
Strength grade | Water to cement ratio (mass ratio) | Water-reducing agent (kg/m3) | Cement (kg/m3) | Water (kg/m3) | Gravel (kg/m3) | Sand (kg/m3) | Rheological properties (mm) |
---|---|---|---|---|---|---|---|
C30 | 0.55 | 3.61 | 368.90 | 201.11 | 786.00 | 801.72 | 650 |
Rheological properties of concrete.
Concrete strength.
Time | Maximum bearing capacity (kN) | Specimen strength (MPa) |
---|---|---|
7 days | 640.7 | 28.46 |
14 days | 798.49 | 35.49 |
28 days | 825.39 | 36.68 |
In order to better study the force and interface failure characteristics of composite members, in this experiment, a total of 6 groups of steel self-compacting concrete composite structural specimens were designed for load test. The length of the specimen is 3000 mm and the width of the specimen is 800 mm. The height of the specimen is 314 mm, of which the thickness of the concrete is 300 mm and the thickness of the steel plate is 14 mm. The model size is shown in Figure
Specimen size.
As summarized in Table
Details of the specimens.
Specimen no. | Types of shear connectors | |||||
---|---|---|---|---|---|---|
SP1 | 314 | 800 | 3000 | Studs, longitudinal ribs | 36.68 | 337.16 |
SP2 | 314 | 800 | 3000 | Sparse studs, longitudinal ribs | 36.68 | 337.16 |
SP3 | 314 | 800 | 3000 | Reinforcing cage, longitudinal ribs | 36.68 | 337.16 |
SP4 | 314 | 800 | 3000 | Longitudinal ribs, transverse ribs | 36.68 | 337.16 |
SP5 | 314 | 800 | 3000 | Longitudinal ribs | 36.68 | 337.16 |
SP6 | 314 | 800 | 3000 | Two longitudinal ribs | 36.68 | 337.16 |
1
Photographs of the specimen: (a) SP1, (b) SP2, (c) SP3, (d) SP4, (e) SP5, and (f) SP6.
The test setup and instrumentation for the specimens are presented schematically in Figure
Test setup and instrumentation of specimens: (a) schematic diagram of test set-up and (b) general view.
This experiment adopts the method of hierarchical loading. Before the loading test, preloading is carried out to determine whether the beam is loaded eccentrically. At the same time, check the readings of each device, confirm that the displacement gauge and strain gauge data are in good condition, and unload to zero. After the completion of preloading, the graded reciprocating loading is started. Before concrete cracking, the load control level is 20 kN; after concrete cracking, the load control level is 50 kN. The loading method adopts the reciprocating loading methods of 5–50 kN, 5–100 kN, 5–150 kN, 5–200 kN, 5–250 kN, and 5–300 kN. After cyclic loading for 5 times in each stage, the load increases to the cycle of the next stage until the specimen is destroyed.
Displacement meter and strain gauge were used for data collection. Four strain gauges were arranged from top to bottom at both ends of the specimen and at the side of the specimen mid-span. Three strain gauges were arranged in each column at the bottom, middle, and both ends of the specimen. Displacement measurement using displacement meter is shown in Figure
Point position map: (a) displacement meter layout, (b) arrangement of strain gauges, and (c) arrangement of strain gauges.
This section presents the main experimental results of the tested steel self-compacting concrete beams, including the failure modes, mid-span load deflection, load-strain relationship, slip curve and analysis of cycle times, and mid-span deflection.
The brittle failure of the six specimens is shown in Figure
Brittle failure of specimens.
The main crack distribution of the specimen: (a) SP1, (b) SP2, (c) SP3, (d) SP4, (e) SP5, and (f) SP6.
The synergistic behavior of steel-self-compacting concrete beams under pure bending test and the deformation mechanism of composite members can be obtained by mid-span load-deflection curve.
The load-span deflection curves of the six specimens in the test are shown in Figure
Specimen load-deflection curve.
Test results of specimens.
Specimen no. | ||||
---|---|---|---|---|
SP1 | 310.7 | 5.233 | 253.3 | 2.532 |
SP2 | 305.6 | 6.632 | 241.7 | 2.322 |
SP3 | 292.5 | 5.685 | 149.9 | 2.217 |
SP4 | 308.5 | 6.743 | 205.9 | 2.469 |
SP5 | 256.2 | 5.406 | 170.5 | 1.932 |
SP6 | 299.1 | 4.914 | 250.0 | 3.274 |
1
It can be seen from the test results that the ultimate bearing capacity of the SP1 and the SP4 is remarkably improved. According to the results, the ultimate bearing capacity of SP5 was only 256.2 kN; however, the ultimate bearing capacity of SP1 and SP4 was about 310 kN. Compared with the SP5, the ultimate bearing capacity of the specimen is increased by about 24%.
The deformation properties of the composite members SP1 and SP4 under cyclic load are also significantly improved. The SP5 has passed from the elastic stage to the elastic-plastic stage at 200 kN, and the deflection becomes larger and the specimen is destroyed. During the loading process, SP1, SP4, and SP6 were in the elastic phase before the load reached 290 kN. The deflection increased linearly with the load and then entered the elastoplastic phase, and the load continued to increase. The specimen entered the plastic phase until the specimen was destroyed. The ultimate bearing capacity of SP2 and SP3 is high, but the elastic-plastic stage enters the elastoplastic stage at about 200 kN, and its deflection changes greatly. In short, the ultimate load-bearing performance of SP1 and SP4 is the best.
By observing the slip curve, the restraining effect of different shear connectors on the steel-concrete interface can be obtained. The slip curve of the six specimens is shown in Figure
Slip curves: (a) SP1, (b) SP2, (c) SP3, (d) SP4, (e) SP5, and (f) SP6.
For SP1, it can be seen from Figure
The slip curve of the SP2 is shown in Figure
The slip curve of SP3 is shown in Figure
The slip curve of SP4 is shown in Figure
The slip curves of SP5 are shown in Figure
The slip increment of the SP6 before complete damage is about 0.01 mm, which indicates that the concrete and the steel plate have good combined force, which can effectively limit the lateral displacement of the concrete. When the load is 230 kN, the slip gradually increases, and the specimen starts partially. Cracks occurred until the load became 300 kN, and the slip suddenly became large, and the specimen broke, as shown in Figure
According to the slip curve, SP3 and SP6 had the worst performance and the worst constraint effect on the lateral displacement of concrete. SP1, SP4, and SP5 showed outstanding performance, and the lateral displacement always changed in a small range, among which SP1 and SP5 had the smallest fluctuation in the process of change.
In order to further study the influence of the pressurization and unloading process on the specimen during the cycle, the development of the deflection of the six specimens with the increase of the number of cycles is obtained, as shown in Figure
Cycle times and mid-span deflection curve.
The development law of the mid-strain strain with load of the six specimens is shown in Figure
Strain-load curves for each measuring point on the side of the specimens: (a) SP1, (b) SP2, (c) SP3, (d) SP4, (e) SP5, and (f) SP6.
It can be seen from the test results that the strains of the SP1, SP2, and SP5 are in accordance with the development law, and the concrete and the steel plate have better combined force performance. Among them, SP1 and SP2 did not appear to suddenly increase the strain of the specimen but showed the property of first elastic and then elastoplastic change. As the load of SP3, SP5, and SP6 increases, the strain at a certain measuring point suddenly increases, and the crack in the specimen causes the joint to be broken, and the force on the steel plate suddenly increases.
The load-strain curves of the steel plates at the bottom of SP1, SP4, and SP5 are shown in Figure
Strain-load curves for each measuring point at the bottom of the specimens: (a) SP1, (b) SP4, and (c) SP5.
Figure
Section height-strain curve.
The bearing capacity of the studs and ribs shear connectors were affected by the material properties of concrete, the material properties of the studs and ribs shear connectors, and the bond properties of the interface. Among them, the interaction between shear connectors and concrete in spatial scale determines the mechanism of load distribution and the distribution characteristics of concrete damage and then affects the bearing capacity of the two materials, respectively. Therefore, based on force mechanism analysis, the shear connectors and concrete under the three-dimensional scale of relative motion are decomposed; the interaction of shear connectors and concrete separately for the equivalent of two kinds of materials to the interface method of mutual extrusion, toroidal at the interface between the mutual friction and reinforcing rib protrusions and concrete local extrusion, the load transfer mechanism of the corresponding description and quantitative calculation method was proposed to characterize the damage effect of concrete influence on bearing performance, thus establishing the stud three-dimensional constitutive relationship of concrete interface model, as shown in Figure
Three-dimensional relative motion decomposition and load transfer of interface.
Through the above methods, the relative motion of concrete and stud shear connectors in the spatial coordinate system is decomposed, and the corresponding load transfer mechanism is determined in the three directions of interface normal, interface ring, and stud axis, respectively. When the composite member is under load pressure or tension, the normal stress between the concrete and the contact surface of the stud is equivalent to the mutual extrusion between the stud and the concrete. The annular displacement of the interface is equivalent to the friction between the stud and the concrete. The axial failure of the stud is regarded as the local extrusion between the stud and the concrete. Assuming that the circumferential friction along the interface of the model meets the molar coulomb criterion, the circumferential stress and the normal stress can be calculated according to (
Quantitative expression.
Additionally,
In this paper, the nonlinear analysis software Link3D developed by Maekawa [
FEM simulation model.
In addition, considering the interaction between concrete and steel plate, a bond element is set at the surface between concrete and steel plate. The bond element is shown in Figure
Bond element.
Material properties of the bond element.
Shear stiffness in closure mode (kgf/cm3) | Normal stiffness in closure mode (kgf/cm3) | Frictional coefficient | Shear stiffness in open mode (kgf/cm3) | Normal stiffness in open mode (kgf/cm3) |
---|---|---|---|---|
10000 | 1000000 | 0.5 | 100 | 10 |
Material properties of the model: (a) material properties of the concrete element and (b) material properties of the steel plate element.
To verify the validity of the FEM model, load-deflection curve by the above-mentioned FEM model was compared with the tested results in this paper, as shown in Figure
Comparison of load-deflection curve between simplified model and test results.
It can be seen from the simulation results that the numerical results obtained by the simulation method in this paper are slightly larger than the experimental results. However, the deviation is small, which can reflect the failure of steel-self compacting concrete composite members more accurately. The applicability of the model is verified, which has an important reference value for the study of similar tests and the prediction of mechanical properties of steel self-compacting concrete composite members.
The behavior of steel self-compacting concrete beams has been studied under cyclic loads. And the finite element model is developed. Further research is also still needed on steel self-compacting concrete beams that are used in the field. From the range of the test parameters studied in this paper, the following conclusions can be drawn based on the above results: Under the early low load, the steel and concrete meet the plane section assumption, and the interaction between them is not obvious. After the load is gradually increased to a certain degree, the concrete will be under pressure and generate microcracks at the interface. The load will be transferred to the steel plate through the shear connector. At this time, the constraint effect of the shear connector on the concrete becomes obvious. The setting of shear connectors can greatly enhance the ultimate bearing capacity of steel-concrete composite structures, in which the ultimate bearing capacity of composite members composed of studs-ribbed shear connectors is increased by about 24% compared with that of composite members composed of only longitudinal ribbed shear connectors. In such projects, the interface processing method of SP1 is the most reasonable. From the slip curve, the steel self-compacting concrete beam with the stud or the transverse rib as the connecting piece can be seen; during the whole loading process, there is no obvious relative slip phenomenon. The concrete and steel plate always work together, and the overall bearing capacity is good. The restraining effect is the best, and the slip does not change. The stud-longitudinal rib shear connectors and transverse rib-longitudinal rib shear connectors perform best under the cyclic load. When the SP1 is broken, the crack is concentrated at the compression axis, and when the SP4 is broken, the crack is concentrated at the transverse rib. When the transverse ribs are used as shear connector, the cracks produced by the specimen under stress tend to expand diagonally, which is more prone to shear failure. When considering slippage, SP1 and SP4 interface treatment method is the most reasonable, but the crack of SP4 is easy to occur at the shear connector, and the SP1 interface treatment method is the best choice. As a new material, the compressive strength of self-compacting concrete can meet the compressive strength requirements of ordinary concrete. It has higher applicability for large-scale construction site and complex underground construction, and the test has a higher reference significance for future engineering application. Based on the finite element modeling theory, the steel-concrete coupling frame with interface load transfer mechanism is analyzed, and the steel-self compacting model was established. The finite element software Link3D is used to simulate the tests of composite components. By comparing the test results with the simulation results, the deviation of the results is small. The model has a certain reference value for similar tests in the future and has an important significance in predicting the mechanical properties of steel-self compacting concrete.
The test data are included within the article and can be made freely available.
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.
This research was funded by the Research Program of the Pearl Delta (nos. WW2018231 and WW2018225), the Chinese National Natural Science Foundation (Grant nos. 51979017 and 51979021), and Major Projects of Chongqing Education Committee (Grant no. KJZD-M201900702).