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Correctly tensioning the prestressed concrete strands can improve the bending resistance and rigidity of the components. This article will specifically introduce the precautions for tensioning prestressed concrete strands.
Before engineering structural components bear external loads, tensioning the prestressed concrete strands in the tension module can improve the bending resistance and rigidity of the components, delay the occurrence of cracks, and increase the durability of the components. The following are the matters needing attention when tensioning prestressed concrete strands.
1. The influence of formwork support
Because of the application of prestressing, the concrete will inevitably produce elastic deformation, and at the same time produce axial deformation and vertical deflection. If it restricts its axial shrinkage and deflection during tension, it will cause unexpected cracks in the concrete or even quality accidents.
Therefore, it is necessary to remove the beam side template that restricts the axial contraction of the beam body before tensioning and remove the template around the support that restricts the movement and rotation of the movable support in the direction along the bridge, and the rotation of the fixed support.
2. Tension points
(1) Tension sequence
The tensioning sequence should be carried out in accordance with the design regulations. If the design does not specify, excessive eccentric stress on the cross-section of the component should be avoided, and excessive tensile stress should not be generated at the edge of the component.
Especially for curved bridges, it should be noted that excessive tensile stress should not be generated on the inner and outer edges of the curved beam during tension, and cracks will occur in the web of the beam. When stretching, the steel strands close to the cross-section centroid must be stretched first. If there are multiple rows of steel strands, they must be symmetrical.
(2) Tension length
The length of the continuous beam steel bundle is large, and it is recommended to stretch both ends at the same time. If the equipment is insufficient, you can fix one end first, stretch the other end, and then stretch the fixed end to make up for the stress. This is especially true for curved prestressed tendons.
When one end is tensioned, although the tension end has reached the controlled stress, the long hole length causes the steel bundle rotation angle θ to increase and the friction force increases, so that the prestress gradually decreases from the tension end to the fixed end, near the fixed end The prestress is obviously insufficient.
3. Treatment of broken wire and slippery wire
During the construction process, due to operating errors or inaccurate jack pressure or anchor installation errors, poor quality of the clip, etc., wire breakage and slippage sometimes occur. When the number of broken wires or sliding wires does not exceed the specified value, over-tensioning can be used to compensate for the stress. If it exceeds the specified value, the anchor must be unloaded and the steel bundle must be replaced. You must be cautious when dealing with this to ensure quality and safety.
4. Complementary stress treatment
Determine the stress loss value according to the number of broken wires, and make up for the stress loss caused by broken wires by increasing the stress of other steel wires, but under no circumstances should the stress of the steel strand reach 0.8Rb, otherwise the steel bundle must be replaced.
5. The treatment method of replacing the steel bundle
(1) Tow relaxation
Install the jack in a tensioned state, and wedge the steel wire in the chuck tightly. One end is stretched. When the wire is stretched under stress, the anchor plug is slightly taken out. At this time, the anchor plug thread is immediately clamped with a steel drill (the steel drill can be made of φ5mm steel wire, the end is sharpened, and the length is 20-30cm).
Then the main cylinder returns oil slowly, the steel wire retracts, and the anchor plug cannot be retracted simultaneously with the steel wire due to being stuck. If the jack stroke is not enough, it can be repeated until the anchor plug withdraws. Then pull out the wire harness to replace the new wire harness and anchor.
(2) Single slip wire single wire supplementary
Wedge the sliding steel wire on the chuck, and press the wedge tightly after the tension reaches the stress.
(3) Artificial sliding wire to relax the wire bundle
Install the jack and wedge each wire tightly. When one end of the wire bundle is stretched until the control stress of the wire is still unable to pull out the anchor plug, knock off the wedge of the wire on the jack chuck, forcing 1 or 2 wires to spin.
At this time, the anchoring force between the anchor plug and the anchor ring is reduced, and the anchor plug is easily pulled out by pulling the anchor plug again.
6. Hole grouting
The grouting work of prestressed pipes plays a pivotal role in post-tensioned prestressed components: preventing corrosion of prestressed steel; effectively bonding prestressed steel to concrete, realizing the overall stress effect, and enhancing the load-bearing capacity of the beam; reducing the anchorage system Load.
Therefore, we must attach great importance to the quality of grouting, and require that the cement slurry pressed into the pores should have reliable compactness after being hardened, which can play a protective role of the prestressed tendons, and also have a certain bond strength and shear strength.
In order to effectively transmit the prestress to the surrounding concrete. In the past engineering practice, because the construction personnel did not pay enough attention to the process and material quality of grouting for pores, the pre-stressed tendons rusted prematurely and reduced the durability of the structure. To make the grouting work successful, the following must be done:
(1) Cement, water, admixtures, and grouting equipment meet the requirements of the specification.
(2) The water-cement ratio, bleeding rate, expansion rate, and consistency of cement slurry meet the requirements of the specification.
(3) Check whether the tunnel is unblocked before grouting.
(4) The grouting sequence is correct. Proceed in the order of the channel from low to high.
(5) Strictly control the grouting pressure and speed.
(6) Using vacuum grouting technology. In actual construction, problems such as insufficient prestressed, cracks in the beam, and damage to the support are often caused by the above reasons. Therefore, the key factors that affect the quality of prestressed construction must be strictly controlled during the construction process. If you want to know more about prestressed concrete strands after reading the above, we can provide you with a comprehensive solution.
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