This method is widely used in bridges and long-span buildings.
Problems needing attention
1, prestressed reinforcement and prestressed anchorage shall be strictly inspected and accepted in batches and properly kept. In addition to checking the appearance, accuracy and quality certificate of the anchorage, the strength (including fatigue strength) and anchoring capacity of the anchorage should also be sampled.
2. All prestressed steel strands and steel bars are not allowed to be welded, and all prestressed steel strands with joints should be removed and are not allowed to be used. Steel strand and reinforcement shall be derusted before use.
3. All prestress stretching needs extension and tension control. The value of E was determined by experiments, and the design extension was modified. The error between the measured extension and the designed extension shall meet the requirements of construction specifications. The measured elongation shall be deducted from all elongation caused by inelastic deformation.
4. Before prestress tensioning, use initial stress (0. 1 ~ 0.2σ k) to tension once, and then start measuring elongation. The tensioning procedure is: 0→ initial stress → 103% to control the tensioning stress under the design anchor (hold the load for 3min, anchor).
5. In order to ensure the quality of prestress, it is required to strictly control the process of tube drawing, steel bar positioning and tube forming. The specific requirements are as follows:
(1) When installing the pipeline, check the quality of the pipeline and the cross-sectional shapes at both ends. If slurry leakage may occur, the parts should be cut, trimmed and deburred at both ends before use.
(2) Channel positioning must be accurate and reliable, and corrugated pipes are strictly prohibited from floating. The straight line segment is 0.8m on average, and the curve segment is provided with a positioning steel bar every 0.5m.. Each positioning steel bar comprises a supporting steel bar and a U-shaped ring positioned on the supporting steel bar, and the U-shaped ring is connected with the box girder steel bar framework. After positioning, the deviation of pipeline axis is not more than 5 mm.
(3) The flat bending section of prestressed beam must be provided with anti-collapse reinforcement, and the inside of the bending arc of the anti-collapse reinforcement must be attached to the pipeline and fastened with iron wire, and the anti-collapse reinforcement itself is reliably connected with the reinforcement in the main beam.
(4) The pipeline axis at the joint of pipeline and bell mouth must be perpendicular to the anchor pad. Pipeline grouting must be dense, and the cement slurry grade shall not be lower than C40.
(5) Prestressed anchor concrete shall be constructed as soon as possible after grouting, and the wrapped steel wire mesh shall be reliably connected with the structural reinforcement, and special attention shall be paid to it during construction.
6, prestressed thick steel pipe bottom should be sealed to prevent slurry; The upper end should be closed to prevent water and sundries from entering the pipeline. Steel core can be inserted into grouting pipe, pulled out after concrete pouring to ensure smooth grouting holes, and holes must be blown.
7. When steel bars interfere with prestressed pipes or other main components in space, the position of steel bars can be adjusted appropriately to ensure the accuracy of reinforced pipes or other main components. If the ordinary steel bars at the anchorage of steel beams affect the prestress construction, they can be properly bent and restored to the original state in time after the prestress construction is completed. In case of spatial position conflict of reinforcement during construction, the layout can be adjusted appropriately, but the net protective layer thickness of reinforcement should be ensured.
8. If the spiral reinforcement under the anchor interferes with the distributed reinforcement, the distributed reinforcement or its spacing can be adjusted appropriately. The position of prestressed pipeline must be located in strict accordance with the coordinates and fixed with positioning steel bars. The positioning steel bar is connected with the stirrup of the box girder web by spot welding to prevent dislocation and pipe sagging. If the pipeline collides with the steel bar, the pipeline position should be kept unchanged and the steel bar should be moved appropriately. Check whether the bellows is sealed before pouring to prevent the pipeline from being blocked when pouring concrete.
9, box girder concrete reaches 85% of the design strength, and the concrete age is not less than 7d, before tensioning prestressed steel beam. Both ends of the positive moment steel beam in the precast beam and the negative moment steel beam in the continuous section of the pier top are tensioned at the same time, and the control stress under the anchor is the design value.
10. The steel corrugated flat pipe of the negative moment steel beam on the box girder roof should be embedded when prefabricating the box girder, and effective measures should be taken to prevent the deformation of the flat pipe from affecting the tension of the roof beam in the later period. After the box girder installation is completed, connect the corresponding flat pipes before pouring the continuous joint section.
1 1, it is forbidden to cut off the longitudinal and transverse reinforcement of the lower layer of the box girder roof at the notch of negative moment tensioning when precast box girder, and it is not appropriate to cut off the reinforcement casually when tensioning the negative moment steel beam.
12. During construction, ensure that the anchor pad is vertical to the prestressed beam, and the center of the pad is aligned with the center of the pipeline, in the dense parts of the pipeline and the anchorage zone.