What is the technical specification for prestressed tensioning construction of bridges?

Recently, with the continuous promotion of highway, railway, and urban bridge construction in China, prestressed tensioning construction, as the core key process of bridge structure construction, directly affects the bearing capacity, stability, and durability of bridges. Pre stressing tensioning applies predetermined tension to prestressed steel strands and other tendons to create a reasonable stress state in bridge structures, effectively improving the bridge's crack resistance and deformation resistance. It is widely used in various bridge engineering such as beam bridges, arch bridges, and cable-stayed bridges. If the tensioning construction does not follow the standard requirements, it is easy to encounter problems such as insufficient prestress, relaxation of steel strands, and anchoring failure. In severe cases, it may cause safety accidents such as bridge structural instability and collapse. Previously, the Lianshen Line Yuegang Bridge in Xiangshui County, Yancheng City, Jiangsu Province, collapsed due to incomplete final tensioning of the prestressed steel strands and failure of the anchoring system, resulting in serious casualties and property losses.
The technical specifications for prestressed tensioning construction of bridges follow the core principles of "safety first, standardized operation, graded control, and full traceability", which run through the entire process of construction preparation, on-site tensioning, post tensioning anchoring, and acceptance. The core goal is to ensure accurate prestressing application, reliable anchoring, and long-term safe and stable operation of bridge structures. Industry experts have stated that prestressed tensioning of bridges is a highly specialized and high-risk critical process that requires strict adherence to relevant national technical specifications and industry guidelines. Based on the type of bridge structure, prestressed reinforcement specifications, and construction environment, targeted implementation of various regulatory requirements is necessary. Any violation of regulations may pose safety hazards.

The preparation work before the prestressed tensioning construction of bridges is the foundation for implementing technical specifications and preventing construction hazards. The core includes four key links: equipment control, material acceptance, scheme preparation, and personnel qualification review, each of which has clear regulatory requirements.

The control of tensioning equipment is of utmost importance in construction preparation. The specifications clearly require that intelligent tensioning systems should be used for the prestressing tensioning of post tensioned structures or components, and intelligent tensioning robots should also be used for the tensioning of prefabricated components in precast beam yards. The intelligent tensioning system needs to have the ability to fully automate the entire process of tensioning, anchoring, and topping out, integrate real-time data collection, automatic storage, remote transmission, abnormal warning, and process tracing functions, meet the requirements of full process automation and digital control, and the tensioning data needs to be transmitted in real time to the information management platform of the project construction unit. At the same time, the core components of the intelligent tensioning system, such as pressure sensors, displacement sensors, and hydraulic jacks, should be calibrated as a whole with the control system. The calibration cycle should not exceed 6 months, and the tensioning frequency should not exceed 300 times. If the equipment leaks oil, disassembles, repairs, or key components are replaced, the overall calibration must be re performed to ensure that the equipment accuracy meets the construction requirements.

Material acceptance must strictly follow the specifications and standards. For core materials such as prestressing tendons, anchorages, and fixtures, it is necessary to verify the specifications, models, quality certificates, and qualification certificates to ensure compliance with design requirements and regulatory requirements. Pre stressed steel strands need to be tested for tensile strength, elastic modulus, and other indicators. Anchors need to be tested for anchoring efficiency coefficient, ultimate tensile strength, and other properties. Unqualified materials are strictly prohibited from being put into use. The storage of materials should be standardized, and pre stressing steel strands should be treated with rust prevention. Anchors and fixtures should be stored in dry and ventilated warehouses to avoid moisture and corrosion, and to prevent affecting their performance. In addition, before tensioning, it is necessary to conduct anchor ring mouth friction loss tests on different types of anchor ring mouths, extract different types of holes for hole friction tests, and correct the tensioning control stress values based on the measured data to ensure tensioning accuracy.

The preparation of construction plans and personnel qualification review are equally indispensable. The construction unit shall prepare a special prestressed tensioning construction plan based on the characteristics of the bridge structure, the layout of prestressed reinforcement, and the construction environment, clarifying the tensioning process, technical parameters, graded loading process, safety protection measures, and emergency response plan. After approval, it shall be put into implementation. The selection criteria for initial stress should be clearly defined in the plan, and should be reasonably determined based on the length of the prestressed reinforcement. When the length does not exceed 30m, it should be taken as 10% -15%, when the length is between 30m and 60m, it should be taken as 15% -20%, when the length is between 60m and 100m, it should be taken as 25%, and when the length exceeds 100m, it should be determined through on-site testing. At the same time, tensioning operators must possess corresponding professional qualifications, and special operators must hold valid work permits before they can take up their posts. The construction unit needs to provide professional training and technical disclosure to the operators to ensure that they are proficient in the standard requirements and operating procedures.

On site tensioning construction is the core link in implementing technical specifications, and it is necessary to strictly control key points such as tensioning timing, graded tensioning, symmetrical tensioning, and process monitoring to ensure that tensioning construction meets the requirements of the specifications. The specification clearly stipulates that tensioning operations can only be carried out after the concrete has reached the strength, elastic modulus, and age required by the design and specifications. The age of prefabricated components using steam curing technology shall not be less than 72 hours, the age of prefabricated components using conventional curing technology shall not be less than 120 hours, and the age of cast-in-place components shall not be less than 168 hours. It is strictly prohibited to carry out tensioning operations when the concrete strength is insufficient to avoid causing concrete collapse and structural damage in the anchorage zone.

The tensioning process must strictly follow the principle of "graded tensioning, symmetrical loading, and double control verification". Each bundle of steel strand prestressing bars should be tensioned in stages, with each stage holding the load for no less than 2 minutes and the last stage holding the load for no less than 5 minutes. The loading rate should be uniform and controllable to avoid damage to the prestressing bars caused by sudden load changes. Symmetrical tensioning is the key to ensuring uniform stress distribution in bridge structures. It is necessary to perform symmetrical tensioning in accordance with the order required by the special plan to prevent fatigue damage to the anchoring system caused by excessive tensioning of local steel bundles, and to eliminate the hidden danger of structural instability caused by irregular tensioning sequence. The principle of flat anchor tensioning construction should be synchronous tensioning and anchoring of the entire bundle to ensure uniform tensioning force.

The specification requirements for dual control verification during the tensioning process require simultaneous control of the tensioning stress and the elongation value of the prestressed reinforcement. The tensioning stress must reach the design control stress, and the deviation between the actual elongation value and the theoretical elongation value should be controlled within the allowable range of the specification. If the deviation exceeds the range, the tensioning should be temporarily stopped, the cause should be investigated and rectified before continuing construction. At the same time, professional personnel should be arranged for on-site monitoring during the tensioning process to monitor the tensioning stress, elongation value, and structural deformation in real time. Detailed construction records should be kept to ensure traceability of the construction process. If any abnormal situation is found, the tensioning should be stopped immediately and emergency measures should be taken.

The anchoring and sealing construction after tensioning must strictly follow the regulatory requirements to ensure reliable anchoring and tight sealing. When anchoring, the anchor clip should be installed in place and firmly clamped to avoid problems such as clip slippage and anchor failure, and to prevent the relaxation of prestressed steel strands. After the tensioning is completed, the anchor head should be sealed in a timely manner. The strength grade of the sealing concrete should not be lower than the design requirements. Before sealing, the anchor head debris should be cleaned to ensure that the sealing concrete is tightly combined with the anchor head and beam body, preventing the anchor head from rusting and affecting the prestressing effect and structural durability.

The acceptance and post construction control after tensioning construction are the last line of defense to ensure construction quality, and the standards for acceptance and control requirements have been clearly defined in the regulations. During the acceptance stage, a comprehensive verification of tensioning records, material inspection reports, equipment calibration reports, and other materials is required. At the same time, effective prestress under the anchor must be randomly inspected, and the sampling frequency must comply with industry guidelines. The first 3 beams of each precast beam yard must be inspected, and 5% and no less than 6 beams of subsequent precast beams must be randomly inspected. The proportion of negative bending moment sampling for simply supported and continuous structures should not be less than 10% and no less than 2 beams, and the proportion of sampling for cast-in-place suspension and support should not be less than 10% and no less than 2 beams. After the tensioning of each prefabricated beam is completed, the arch value needs to be measured, a special ledger should be established, and compared and analyzed with the theoretical value. Any abnormalities found should be reported and dealt with in a timely manner.

In the later stage of control, regular inspections should be carried out on the prestressed structures that have been tensioned, with a focus on checking the corrosion of anchor heads and prestressed reinforcement, as well as the integrity of the sealing concrete. If corrosion, damage, and other issues are found, timely repair measures should be taken. At the same time, it is necessary to strengthen the monitoring of settlement and deformation of bridge structures to ensure the long-term stability of prestressed structures and avoid hidden dangers such as cracks and deformation caused by prestress loss.

The reporter learned that the "Technical Specifications for Construction of Highway Bridges and Culverts" (JTG/T 3650-2020) came into effect on September 1, 2020, further improving the technical requirements for bridge prestressed tensioning construction, absorbing the achievements of "four new" technologies such as intelligent tensioning, and providing clear guidance for industry construction. Industry experts remind that the prestressed tensioning construction of bridges is related to the safety of bridge structures. Construction units need to strictly implement the quality responsibility system, strengthen the standardized control of the entire process, strictly follow relevant technical specifications and industry guidelines, optimize the construction plan based on the actual project, strengthen professional training for construction personnel, and improve the level of standardized operation. In the future, with the continuous promotion of intelligent construction technology, the standardization and refinement level of bridge prestressed tensioning construction will be further improved, providing strong guarantees for the high-quality construction of bridge engineering in China and helping to create safe, durable, and efficient bridge transportation facilities.