The quality of the connection between old and new concrete interfaces is a key issue in structural strengthening projects. The bond slip of the interface, the analysis of the force performance, the study of interface treatment and interface agents are all the directions that the engineering community at home and abroad has been exploring.
Research tests and engineering practice have shown that it is difficult to meet the demand of concrete durability and actual bearing capacity by relying only on the interface treatment and effective bonding of old and new concrete, therefore, the use of mechanical connectors to strengthen the interface strength of old and new concrete is proposed.
1 A preliminary investigation of mechanical connection methods for old and new concrete
In structural reinforcement projects, the most basic reinforcement method is the expanded section method, where new concrete and reinforcement are added to the original old concrete surface, and the force at the joint surface of the old and new concrete is always a weak link. By adding mechanical connectors to the old and new concrete joints, not only can the interface bearing capacity be improved, but also the amount of new concrete and reinforcement can be reduced, lowering the project reinforcement cost.
In recent years, the most commonly used mechanical connectors are reinforcement, anchors, bolts and injection screws. The form, number, distribution and hole depth of the connectors have a great impact on the connection effect, however, the reinforcement design and parameter selection methods of such connectors basically remain at the stage of empirical design and data reference, and as the main stress-bearing members subjected to vertical loads, the theoretical study of the reinforcement design of compressive members is even less. Concrete columns are more likely to have insufficient bearing capacity due to various reasons, and the shear force at the joint surface of the old and new concrete cannot be transferred effectively.
For the reinforcement of concrete column, this paper proposes a composite reinforcement method combining self-locking anchor mechanical joint and steel pipe self-consolidating concrete, in which the self-locking anchor is arranged at a certain spacing around the original column, so that there is a certain gap between the surface of the outer clad compression steel plate and the original compressed member, and the self-locking anchor is welded to the steel plate and fixed, and then the self-consolidating concrete is used to fill the gap.
The self-locking anchor is a new type of anchor developed by Wuhan Wuda Jucheng Reinforcement Industry Co., Ltd. In the construction, the inorganic material is filled after the hole is reamed first, and there is self-locking force at the end due to the axial pressure. This anchor combines mechanical self-locking with inorganic bonding material, forming a new anchoring technology with heat resistance, water resistance and high efficiency, which has obvious advantages compared with the common reinforcement planting technology.
In the combination of old and new concrete, the self-compacting concrete acts as the “new” concrete material, and its self-weighted flowing compactness can fill the gap between the steel pipe and the original concrete column, and its micro-expansion makes the “new” concrete and the steel pipe as well as the “new” concrete and the “new” concrete. The micro-expansion makes the bond between the “new” concrete and the steel pipe and the “old” and “new” concrete tighter.
In addition, the old and new concrete, as the core part of the steel pipe concrete column, is in a three-way pressure state, this reinforcement technology uses the interaction of two different materials, steel pipe and concrete in the process of force, and finally the strength of concrete can be increased, plasticity and toughness is improved. Therefore, the composite reinforcement method combining self-locking anchor mechanical connectors and steel pipe self-compacting concrete has better safety, economy and reliability through the connection of self-locking anchor and steel pipe.
2 Analysis of force performance after reinforcement of old and new concrete
A bridge pier of Yunlong Xijiang Special Bridge in Wuzhou City, Guangxi adopted a composite reinforcement method combining self-locking anchor mechanical connectors and steel pipe self-compacting concrete. Based on the on-site inspection of the repaired structure, this paper uses ANSYS large finite element software to conduct numerical simulation of the bridge pier column, and the simulation results are compared with the data of the on-site inspection report to further study the reinforced force performance of the circular bridge pier column and the accuracy of the simulation model.
2.1 Establishment of finite element model
The simulation model is established by solid modeling, hexahedral meshing, integral model for reinforcement and concrete, and separate model for anchor and steel plate.
2.1.1 Unit type
Solid65 3D solid unit with reinforcement is used for the old and new concrete, beam188 unit is used for the self-locking anchors, and solid45 solid unit is used for the steel plate set.
2.1.2 Ontogenetic relationship
The concrete principal structure model adopts the elasto-plastic model with multilinear random strengthening, which reflects the joint working performance of concrete and reinforcement after unit cracking by considering the tensile strengthening effect when calculating the unit stiffness and stress. The damage criterion uses the ANSYS default tensile stress criterion and the W-W criterion. The input stress-strain curves under uniaxial compression use the formulas in China’s Code for Structural Design of Concrete, and do not consider the falling section.
2.1.3 Material parameters
The original column concrete grade is C25, its weight is taken as 2600kg/m3, modulus of elasticity is 2.8E+4MPa, Poisson’s ratio is 0.18, compressive strength is taken as 39MPa from the data of the core testing report of the bridge pier column; the filled micro-expansion concrete grade is C30, its weight is taken as 2600kg/m3, modulus of elasticity is 3.0E4MPa, Poisson’s ratio is 0.2, compressive strength is taken as 48MPa. The compressive strength is taken as 48MPa; the longitudinal reinforcement of the bridge pier and column are all Grade II reinforcement and the hoop reinforcement are all Grade I reinforcement, after calculation, the longitudinal reinforcement rate of the pier and column is 0.49% and the circumferential reinforcement rate is 0.078%. 2.1.4 Establishment of model
The modeling of the entire reinforced pier B column is shown in Figure 2, and the model includes the original column concrete unit, new concrete unit, reinforcement unit, self-locking anchor unit and steel plate sleeve unit.
2.2 Result Analysis
In order to verify the correctness of the model, the theoretical results output from ANSYS are compared with the measured data from the inspection report. As shown in Table 1 and Table 2.
From Table 1 and Table 2, it can be seen that the finite element theoretical simulation results of the bridge pier and column are basically consistent with the actual engineering test results, which indicates the correctness of the simulation model in this paper, and the actual engineering can be simulated and analyzed more accurately by using the finite element software ANSYS.
In different working conditions, the bridge pier column with the water and the horizontal direction of the bridge displacement changes in the range of 3mm-5mm, the measured data does not show a certain regular change, the reason is the measurement error caused by the tilt is approximately 0, indicating that the bridge deck through the steam-20 vehicle load did not increase the horizontal displacement and tilt of the pier column, therefore, the reinforced bridge pier stiffness Good. And the maximum vertical compressive stress under various conditions is also small, no tensile stress, indicating that the pier column safety, the old and new concrete interface shear stress transfer is reliable, the old and new concrete and steel sleeve bond between good, can be jointly stressed.
This paper proposes a composite reinforcement method using self-locking anchor mechanical connectors and steel pipe combined with self-compacting concrete. The overall force performance of the old and new concrete mechanical connection is analyzed by the test data of the engineering example, and the reinforced bridge pier column members are simulated by the large-scale finite element software ANSYS, and the calculation model of the self-locking anchor connection to enhance the joint working performance of the old and new concrete is established.
By comparing and analyzing the data with the field test report, the data of the theoretical study and the test are similar, which shows the correctness of the reinforced cylindrical model studied in this paper. And the stress-strain state and deformation of the bridge pier column members and more detailed information can be obtained quickly through numerical simulation, which can greatly reduce the workload of the staff for on-site inspection, improve the efficiency and save the reinforcement cost.
Therefore, the reinforcement method of combining self-locking anchor mechanical connectors and steel pipe self-compacting concrete can effectively enhance the load-bearing capacity of the old and new concrete joint, thus improving the overall stiffness to achieve a good reinforcement effect.