Post-tensioning prestressing involves first fabricating components, reserving ducts for prestressed tendons within them, and threading the tendons. After the component strength reaches the specified requirement, tensioning is carried out, followed by grouting and anchorage sealing in the ducts. This technology has been increasingly applied in concrete bridges in recent years. It enables large-scale manufacturing of components to save labor and reduce labor intensity, allows component fabrication to be unaffected by seasons, and supports simultaneous construction of upper and lower parts to shorten the construction period. Therefore, serious research on the construction technology of post-tensioning prestressed concrete bridges is of far-reaching significance for ensuring bridge construction quality and realizing its economic and social benefits.

1. Support Installation and Formwork Erection  

Before the erection of the support, the local foundation bearing capacity shall be treated in combination with the actual situation to ensure that the bearing capacity of the support meets the requirements. After the support is erected, formwork installation can be carried out. The formwork shall be installed in the order of bottom formwork, side formwork, and then top formwork. Camber shall be reserved as required. Finally, the flatness and verticality shall be controlled to meet the specification requirements, and the joints shall be tight, straight, and neat. Settlement observation points shall be set under the support to facilitate regular observation during construction.  

2. Steel Bar Binding  

In the steel bars of bridge engineering, there are both stress-bearing bars and erection bars. When binding steel bars, the plane or three-dimensional skeleton shall be first fabricated and welded firmly. After the steel bars are bound, the coordinates of each prestressed steel strand shall be marked on the skeleton with lines, and then the positioning bars shall be welded and the fixing bars shall be bound. After the ordinary steel skeleton is installed, the positioning bars shall be spot-welded, and the guide bars for the steel strands shall be bound on the positioning bars. Alternatively, the steel bars can be bound in two times: the first time to bind the bottom plate and web steel bars, and the second time to bind the top plate and wing plate steel bars after the concrete of the bottom plate and web is poured. The bottom of the steel skeleton shall be padded with high-grade cement mortar blocks to ensure the concrete protective layer.  

3. Duct Installation  

During the binding process of steel bars, the prestressed duct can be fixed by the track fixing method to ensure it is smooth and round. If plastic corrugated pipes are used, it shall be ensured that they have sufficient strength and rigidity. Generally, the length of the casing shall be determined according to the length of the pipeline. If there are joints, the joint parts shall be wrapped with sealing paper for multiple layers on site to prevent grout leakage. For the installed corrugated pipe, it shall be ensured that the deviation in any direction does not exceed 4mm within the range of 4m from the mid-span and does not exceed 6mm in other parts, and it shall be effectively fixed to avoid phenomena such as upward floating and pressing bending during concrete pouring. In the bending part, the positioning shall be accurate and the shape shall be smooth.  

4. Steel Strand Installation  

Before the installation of prestressed steel strands, it shall be checked whether there are damages, rust, and oil stains. During the installation process, if welding of steel bars is encountered, the method of blocking with wet cardboard shall be used to prevent the welding sparks from piercing the corrugated pipe. Once a break occurs, it shall be promptly repaired. When threading through, all steel strands in one duct shall be threaded at one time, and the positions of each steel strand shall be marked. After threading, the position of each steel strand shall be checked to avoid cross-winding in the duct. Then, each steel strand shall be corresponding to the hole of the disc-type anchorage when threading in. After the anchorage is installed, the clamp-type anchorage shall be placed in each anchorage hole.  

5. Concrete Pouring and Curing

Before concrete pouring, the corrugated pipe, embedded parts, etc. shall be accepted again, and concrete pouring can only be carried out after passing the acceptance. A suitable pouring time for concrete pouring construction should be selected to ensure proper formwork entry temperature control. In the event that the temperature is too high or too low in summer, measures shall be taken to ensure that its formwork entry temperature meets the requirements. Concrete pouring is generally carried out in two times: the first time to pour the concrete of the bottom plate and web, and the top plate and wing plate concrete can be poured after the steel bars of the top plate and wing plate are installed. If the concrete thickness is large, layered pouring shall be adopted, but the longitudinal direction shall be poured at one time, and the upper and lower layers can also be poured at the same time. Generally, the distance between the front end of the upper layer and the front section of the lower layer is controlled at about 2m, and it shall not be less than 1.5m. For the poured concrete, due to the dense internal steel bars, inserted vibrators and inserting pins shall be used for vibration. During the vibration process, the corrugated pipe and formwork shall be avoided from being damaged. The vibration process shall be carried out in accordance with the principle of fast insertion and slow extraction, until the concrete surface is pulpy and there is no bubble emission, so as to avoid the phenomena of missing vibration and over-vibration. The poured concrete shall be cured in time, and the temperature and humidity shall be controlled during the curing process. In general, wet curing or steam curing methods can be adopted. During the curing process, it is imperative to ensure that there is no excessive temperature difference between the concrete surface, interior, and the environmental temperature.

6. Prestress Tensioning

The prestress tensioning process can only be carried out after the strength of the constructed concrete reaches 90% of the design strength and the age is not less than 7 days. Before tensioning, the duct shall be cleaned with compressed air, and at the same time, the anchorage and the end of the steel strand shall be thoroughly cleaned. Tensioning shall be executed in symmetrical fashion across both segments and in accordance with the design numbering. The first batch is tensioned to 50%, the second batch to 80%, and the third batch to 100%. During the tensioning process, stress control shall be taken as the standard and the elongation shall be taken as the check. When tensioning, the steel strand shall be slightly tensioned first to eliminate its relaxation state, and at the same time, it shall be checked whether the duct axis, anchorage, and jack are on the same straight line. When the tensioning stress reaches 10% of the design value, a mark shall be made on the steel wire as a reference value for the elongation on both sides, and it shall be checked whether there is any sliding of the steel strand at this moment. During the tensioning process, checking shall be carried out once every 5MPa to control the oil pressure difference of tensioning at both ends within 5MPa. In the whole tensioning process, close attention shall be paid to the change of the steel strand, and whether there are phenomena of broken wires and slipped wires, so as to avoid the decrease of the jack pressure or the obvious difference in the elongation of a certain steel strand from other steel strands. If there are broken wires and exceed the allowable range, measures shall be taken in time to increase the prestress value of other strands. After the tensioning force reaches 105% of the design value, it shall be held for 2 minutes, and then the tensioning force shall be restored to the design value and the elongation of the steel strand shall be measured. If it meets the requirements, the anchorage can be closed and the jack can be removed. If the difference between the measured elongation and the design value exceeds 6%, the tensioning shall be suspended, and the construction can be continued only after taking measures to correct it.

7. Duct Grouting

Grout Preparation: The used cement shall be portland cement or ordinary cement with an age of not more than 1 month and a strength of not less than 42.5, and the mix ratio of the cement grout shall be determined through tests. When mixing, water shall be added first, and then cement shall be added. After the two are fully mixed, admixtures can be added. The mixing time shall not be less than 2 minutes, and the mixing result shall be uniform and viscous. The mixed cement grout shall be used up within 45 minutes, and the cement grout shall be continuously stirred in the middle to prevent initial setting.

Duct Cleaning: In the construction of corrugated pipes, especially metal corrugated pipes, a large number of rust residues, dust, oil stains, and other impurities will exist in the pipeline. Before grouting, it shall be cleaned, which can be rinsed through the drain hole. If there are oil stains, neutral detergent shall be added for cleaning. The cleaning result shall be that the water quality at the water outlet is the same as that at the water inlet. After cleaning, the standing water in the duct shall be washed out with oil-free compressed air.

Grouting: After the prestress tensioning is completed, grouting shall be carried out in time. If the interval time exceeds 3 days, anti-rust measures shall be taken for the prestressed strands. Grouting is to press the mixed cement grout into the duct with a grouting pump. When grouting, it shall be ensured that the duct is pressed in from the grouting hole at the lowest point until it is discharged from the air vent and the bleeding hole at the highest point. The lower duct shall be grouted first. The grouting process shall be slow and uniform, and no interruption is allowed. The air vent at the highest point shall be opened and closed in turn to ensure the smooth exhaust of the duct. For concentrated or adjacent ducts, continuous grouting shall be carried out first. When continuous grouting cannot be carried out, it shall be washed with pressure water before grouting. When grouting to the other end is full and thick grout seeps out, the hole can be blocked with a wooden plug, and then the valve of the grouting pipe is closed after stabilizing the pressure for 10 seconds, and the grouting pipe is disassembled for the next duct grouting. After an interval of 30-45 minutes, the wooden plugs of the air vents at both ends can be pulled out, the grouting pipe is installed at the other end, and all the cut-off valves of the grouting pipe are opened, and the second grouting is carried out. After the air vent flows out thick grout, the hole is blocked with a wooden plug and the valve of the grouting pipe is closed. In order to ensure that all the steel strands are grouted, the grouting port shall be closed, and all the wooden plugs or air valves shall be kept from moving or opening before the cement grout solidifies.

8. Anchor Sealing

Upon completion of grouting, the cement grout at the beam end shall be rinsed clean immediately, and the support pad, anchorage, and end concrete shall be removed, and the end concrete shall be chiseled. After 3 days of grouting, the concrete anchor sealing can be carried out. Before anchor sealing, the concrete surface at the beam end shall be rinsed clean, and then the steel mesh can be bound and the anchor end concrete can be poured. The anchor sealing length is subject to the sum of the anchor sealing lengths at both ends and the beam length, but the total length shall be controlled not to exceed the design beam length.

Conclusion

The post-tensioning prestress tensioning method is widely used in bridge construction and is a crucial component of the entire process. Its construction quality will affect the force and safety of the whole structure. Therefore, in the construction, it shall be carried out in strict accordance with the process requirements, so as to promote the widespread application and continuous development of the post-tensioning method, ultimately leading to improved economic and social benefits.