The construction of the closure section is the process of system transformation, and the bridge can complete the system transformation through the construction of the closure section. First close the side span, and finally close the middle span; The side span closure adopts hanger construction, and the side span closure section shall be constructed after the cast-in-place section is completed and the concrete strength reaches the design strength; The rigid skeleton of the span closure section can only be closed after the concrete pouring of the side span is completed and the hanging basket of the side span is dismantled.
(a) side span closure
The side span closure section is located between the cantilever end and the cast-in-place section of the support. The cast-in-place section of the support is relatively stable, while the cantilever end will expand axially, deflect vertically and deflect horizontally under the influence of temperature change, sunshine and wind. Before the prestressed reinforcement is tensioned, especially in the early stage of concrete pouring, these deformations may lead to concrete cracking in the closed section, and the construction technology should ensure that the closed section adapts to these deformations to avoid cracks.
In order to ensure that the relative position between the cantilever end and the cast-in-place section of the bracket will not change and reach the strength during the concrete pouring process of the closed section, and to resist the compressive stress caused by the longitudinal extension of the cantilever due to the temperature increase, the rigid support device of the top and bottom plates and the shear support device should be welded before closing; It is also necessary to stretch the temporary prestressed reinforcement to offset the tensile stress caused by the shortening of both ends due to the temperature drop, so that the concrete in the closed section can be protected by setting devices that bear pressure and tension.
The closure section is located on the hanging basket between the cast-in-place section of the support and the cantilever end. When the formwork is erected, it shall be tightened together with the concrete poured at both ends to ensure the smoothness of the interface.
(2) Mid-span closure
The mid-span closure section is the closure of two cantilever casting beam sections of the middle pier. Because both sides are cantilever sections, the influence of external factors such as temperature will be more significant. Before folding, the hanging basket on one side will retreat, and the hanging basket on the other side will advance in the folding position, and the hanging basket will be used for folding.
The concrete measures for mid-span closure are basically the same as those for side span closure.
(3) Construction process
The hanging basket moves forward into position? Rigid support installation of top and bottom plates? Shear brace installation welding? Wear temporary beams? Tighten the rigid bracket? Tension the temporary beam? Concrete pouring? Concrete curing reaches 80% of strength? Remove external support? Tension permanent tendons? Form removal and others.
(4) Key points of construction
1. The concrete pouring time of the closure section should be selected when the daily temperature is low and the temperature variation range is small, and the concrete of the closure section should be poured with the mouth locked.
2. The whole bridge must be balanced and symmetrical at the same time.
3. The design of rigid supports at joints and the tensioning of temporary beams must be carried out in strict accordance with the design requirements. The locking time of rigid support is determined according to the continuous observation results, which requires symmetrical, balanced and synchronous locking in the time interval with minimum relative deformation and minimum temperature change.
4. During closure construction, additional stress should not be generated in this section of construction. Therefore, it is necessary to adjust the closing construction load of the two cantilever ends in the pouring process to make their deformation equal and avoid vertical stress in the closing section. Adjust the construction load of cantilever end closure, and set up water injection adjustment water tank.
5. The concrete in the closed section should be one grade higher than that in the beam, and the early strength is required. It is best to use micro-expansive concrete, which requires special mix design.
6. The tensioning sequence of continuous prestressed tendons should comply with the design regulations. Generally, the top plate should be followed by the bottom plate and then the web plate, and the long beam should be followed by the short beam, and the tensioning should be implemented symmetrically.
7. Before closing, the elevation and axis of the top surface of the box girder should be jointly measured, and the temperature change, the relative elevation change and axis deviation of the beam body should be continuously observed, and the length change of the beam body under the influence of temperature should be observed to determine the closing time, so as to provide a basis for selecting the temporary locking method for closing.
8. During the curing period after the completion of concrete pouring in the closed section, it is necessary to do a good job of cooling down the closed section. Common cooling measures are: sprinkling water on the top of the beam, sprinkling water on the side of the beam, sprinkling water on the box girder and ventilation.
9. When the concrete strength of the closed section meets the design requirements, the temporary consolidation constraint of the bearing at the other end is lifted to complete the system transformation. After the temporary consolidation constraint is released, the settlement of the permanent bearing should be observed and recorded to check the conversion effect.
Cantilever pouring of beam concrete is generally completed by pumping, and the slump is generally controlled at 14 ~ 18 cm, which is appropriately adjusted with the change of temperature and transportation and pouring speed.
(A) the construction process
Hanging basket design and processing? Hanging basket installation and testing? Enter the next suspended irrigation section for construction (external formwork installation and correction? Steel binding, welding, prestressed pipe installation? Installation of internal mold and end mold? Concrete pouring? Tension of longitudinal prestressed tendons? Move the basket and position it? Longitudinal prestressed pipe grouting)? Tension and grouting of vertical prestressed tendons? Tension and grouting of transverse prestressed tendons.
(2) Key points of construction
1. Before pouring box girder segment concrete, the center line of the hanging basket and the elevation of the bottom die of the hanging basket must be strictly checked; Longitudinal, transverse and vertical prestressed pipes; Before pouring concrete, carefully check the positions of steel bars, anchor heads, sidewalks and other embedded parts. In order to ensure the design alignment of box girder, the alignment control scheme should be formulated and the formwork elevation at each stage should be determined.
2. If it is the whole face, it is best to finish it at one time, otherwise it should be finished in the following order.
(1) Secondary pouring: the first support from the bottom plate to the bottom of the web plate; The second time is rest.
(2) the third pouring starts from the bottom plate and is supported to the web plate; The second time, the prestressed pipeline was supported from the lower part of the web to the upper part of the web, and the third time, it was supported from the upper part of the web to the roof.
3. Concrete pouring should start from the front end of the hanging basket, so as to realize most small deformation of the hanging basket, thus avoiding cracks between new and old concrete.
4. Before concrete pouring, the hard plastic pipe should be inserted into the corrugated pipe to prevent the pipe from being crushed; The positioning bars of the pipeline shall be short steel bars, which shall be properly fixed with the steel mesh of the box girder, and the spacing between the positioning bars shall be kept at about 0.5-0.8m, so as to prevent the corrugated pipe from floating during the concrete vibrating process, resulting in the component force along the normal direction of the pipeline during prestress stretching and causing accidents.
5. After the concrete pouring of box girder is completed, check the pipeline with the through-hole device immediately to deal with the pipe blockage caused by slurry leakage.
6. During construction, rainproof awning should be set on the hanging basket to avoid the influence of sun and rain on the concrete quality.