Problems and Solutions in the Construction of Prestressed Concrete Bridges
China has become a world-class bridge building country. Bridge engineering construction investment is large, the construction period is long, and the quality control is difficult. In order to provide better bridge engineering projects, the application of prestressing technology is becoming more and more extensive. Because prestressing technology effectively improves the anti-cracking performance of concrete members, it has become one of the important technical forms of bridge engineering construction. With the continuous expansion of the construction scale of prestressed concrete bridges, problems in the construction process have begun to appear, and effective measures need to be taken to solve them.
Problems and solutions in the construction of prestressed concrete bridges
1. Insufficient initial stress in the tensioning stage causes the effective prestress to fail to meet the bridge design requirements
1.1 Causes of the problem
The Technical Specification for Construction of Highway Bridge and Culvert (JTG/TF50-2011) clearly stipulates the prestressed steel strand tensioning procedures and the establishment of initial stress. However, in the specific bridge construction process, prestressed steel strands of different lengths The initial stress has a very close relationship with the final control stress, and the relationship between tensile stress and deformation during construction is also different. During normal construction, the initial strain and tightness of multiple prestressed steel strands are different when tensioning, and various problems are prone to occur, causing the prestress value to fail to meet the engineering needs, resulting in a decrease in the reliability and durability of the prestressed structure.
The prestressed tensioning process during the construction of prestressed concrete bridges must strictly follow the Technical Specification for Construction of Highway Bridge and Culvert (JTG/TF50-2011), and select the corresponding strength for tensioning according to the prestress value determined during the design work. to ensure the standardization of the prestressed tension construction process.
2. The tension stress control is not accurate enough in the establishment of the effective prestress value
2.1 Causes of the problem
During the construction of prestressed concrete structures, it is necessary to strictly control the effective prestress value. If the value is inaccurate, it will have a bad effect on the quality of the project. If the effective prestress value is too large when the load is applied to the prestressed steel bar, a high-stress state will appear, causing cracks in the steel bar due to deflection or tension, and at the same time, the plasticity will be reduced, and sudden brittle failure will easily occur. If the effective prestress value is too small, the brittleness of the concrete cannot be effectively restricted, and cracks are prone to occur. The appearance of the cracks will affect the appearance of the bridge and reduce the corrosion resistance of the internal steel bars, which will threaten the safety of the bridge structure over time.
In the process of bridge construction, detailed and sufficient analysis of the properties of the project, material properties, and equipment performance is carried out, the project is specifically qualitative, and the appropriate method is selected to solve the effective prestress problem, instead of uniformly selecting the tension method to deal with prestressed concrete.
3. The concrete metal ducts are blocked
3.1 Causes of the problem
After the concrete pouring of the bridge is completed, the corrugated metal ducts are often blocked, which affects the installation and penetration of the prestressed steel strands, resulting in a difference between the actual elongation of the tensioned prestressed steel strand and the calculation result during the design process. Affect the quality of bridge construction. The main reason for this situation is that the construction unit has problems, such as the construction unit’s process control is not strict, and the relevant operation specifications are not strictly enforced, causing the metal ducts to deviate from the positioning or appear deformed or fall off; the construction unit does not strictly control the quality of the materials, The metal ducts used in the construction has quality defects and leaking and blocking the pipe; during the concrete pouring process, it needs to be vibrated, and the vibrator has technical errors, causing the metal ducts to be displaced or broken, and the concrete leaks to the metal ducts.
1) When selecting corrugated metal ducts, perform quality inspection in strict accordance with (JG225—2007) Corrugated Metal Ducts for Prestressed Concrete to ensure that its performance meets industry standards.
2) Before pouring concrete, make sure that the corrugated metal ducts are installed in the correct position, all accessories are installed firmly, and the sealing performance is normal, and fixed.
3) Education and training should be carried out for the construction personnel to strengthen the protection of the corrugated metal ducts during the construction process. If the corrugated metal ducts blockage occurs in the project, you can first determine the location of the blockage, then avoid the main reinforcement of the beam, perform drilling operations, and then clean up the block of slurry that caused the blockage to ensure the steel strand can traverse smoothly without affecting its expansion and contraction performance. After the tensioning work is completed, high-grade micro-expansion concrete is used to seal the open holes.
4. The prestressed steel strand has slipped or broken wires
4.1 Causes of the problem
The main reason for the slippage of the prestressed steel strand may be due to severe corrosion of the steel strand or the working wedge, or the surface of the steel strand or the working wedge has impurities, such as cement, oil, etc., or the size of steel strand does not meet the construction requirements or is unqualified, or the jack used in the construction process is not uniformly stressed. If the tension of the prestressed steel strand is uneven and the tension is too large, or there are problems such as mechanical damage during the storage and transportation of the prestressed steel strand, it will cause the prestressed steel strand to break.
Strict and meticulous inspection work is carried out after the prestressed strand is tensioned, and special inspections are carried out on the phenomenon of slippage and broken wires of the prestressed steel strand. If the slippage of the prestressed steel strand occurs, use a mono-strand jack to pull out the slippery prestressed steel strand, replace with a new prestressed steel strand or working wedge, and then tension it to the specified value. If the prestressed steel strand is broken, use the jack to remove all the prestressed steel strands, replace them with new prestressed steel strands, and re-tension the strands. In order to reduce the loss of prestress after the tension is completed, the construction unit generally needs to complete the grouting construction within 48 hours.
Prestressed concrete construction requires complicated crafts and technologies to ensure the smooth implementation of the project. In the process of bridge construction, designers, site managers, and construction personnel need to consider many issues. However, the problems that arise during the construction of prestressed concrete need to be solved by scientific and effective measures to ensure the smooth development of the construction of prestressed concrete bridges.