1. Tension control stress and elongation value
Whether the tension control stress can reach the design specified value directly affects the prestressing effect, so tension control stress is the focus of quality control in tension. The tension control stress must reach the design specified value, but cannot exceed the maximum tension control stress specified in the design. If the prestressing value is too high and exceeds the design value too much, although the structure has good crack resistance, due to the high crack resistance, the prestressing reinforcement is often in a high stress state when subjected to the use load, which is close to the load when the structure cracks. Often, there is no obvious warning before the failure, which will seriously endanger the safety of the structure’s use. Therefore, in order to accurately grasp the application of prestress, it is necessary to verify the elongation value when using stress control methods for tensioning. Therefore, it is particularly important to provide accurate theoretical elongation values. It is necessary to have a correct understanding of the calculation of theoretical elongation value in the “Technical Specification for Construction of Highway Bridges and Culverts” (JT041-2000): ① In the case where the coordinates of the prestressed duct meet the design requirements and the curved duct is smooth, the local deviation of the duct and the friction coefficient between the prestressed reinforcement and the duct wall have little effect on the theoretical elongation value, and the median value can be taken according to the specification. ② The value of elastic modulus Ep of steel strand has a significant impact on the theoretical elongation value, and should be calculated based on the measured value. The value of L: When calculating the average tension force, it should be calculated according to the length of the hole. When calculating the elongation value, the value of L should be added to the distance from the anchor plate to the front end of the tool clamp. In addition, when comparing the theoretical elongation value with the actual elongation value, the value from the initial stress to the control stress should be used as the reference for comparison. Because the elongation value from zero to initial stress is calculated and measured multiple times, it generates a large cumulative error.
2. The impact of template support
Due to the application of prestress, concrete will inevitably undergo elastic deformation, as well as axial deformation and vertical deflection. If the axial shrinkage and deflection are constrained during tensioning, unexpected cracks may occur in the concrete, and in severe cases, quality accidents may occur. Therefore, before tensioning, it is necessary to remove the beam side formwork that constrains the axial shrinkage of the beam, and remove the formwork and brackets around the support that constrain the movement and rotation of the movable support in the direction of the bridge, as well as the rotation of the fixed support. Given practice, if various constraints are not removed, it is likely to cause local cracks in the beam or deformation of the supports.
3. Key points of tensioning
① Tensioning sequence: The tensioning sequence should be carried out according to the design specifications. If there is no specification in the design, it should be avoided to make the cross-section of the component in an excessively eccentric stress state, and to prevent excessive tensile stress from being generated at the edges of the component. Special attention should be paid to curved bridges, and excessive tensile stress should not be generated on the inner and outer edges of the curved beam during tensioning, which may cause cracks in the beam belly. When tensioning, the steel bundle near the centroid of the section must be tensioned first. If there are multiple rows of steel bundles, they must be symmetrically tensioned.
② Tensioning length: The continuous beam steel bundle has a relatively long length, and it is recommended to tension both ends simultaneously. If the equipment is insufficient, you can first fix one end and tension the other end, and then tension the fixed end to make up for the stress. This is especially true for curved prestressed tendons. When one end is tensioned, although the tensioning end reaches the control stress, due to the large length of the channel, the angle θ of the steel bundle increases, the friction force increases, and the prestress gradually decreases from the tensioning end to the fixed end. The prestress near the fixed end is significantly insufficient. The elongation value during tensioning cannot meet the requirements, mainly due to the large frictional loss of the channel (affected by the large angle θ value of the channel and the length of the channel). The method of stretching the steel strand at one end is a failure. On the one hand, once an accident occurs (such as a broken wire), it will be difficult to handle; On the other hand, due to insufficient prestress applied to the structure by steel tendons, it poses a threat to the safety of structural use.
4. Handling of broken and slippery wires
During the construction process, wire breakage and slippage may occur due to operational errors, inaccurate jack pressure, installation errors of anchorages, poor quality of clamps, and other reasons. When the number of broken or slipped wires does not exceed the specified value, the over tensioning method can be used to supplement the stress. If it exceeds the specified value, the anchor must be unloaded and the steel bundle replaced. We must handle this with caution and ensure quality and safety.
(1) Stress compensation treatment: Determine the stress loss value based on the number of broken wires, and supplement the stress loss caused by broken wires by increasing the stress of other steel wires. However, under no circumstances should the stress of the steel strand reach 0.8Rb, otherwise the steel bundle must be replaced.
(2) The method for replacing the steel bundle:
① Relax the fiber bundle. Install the dry top in a tensioned state and wedge the steel wire tightly inside the chuck. One end is tensioned, and when the steel wire is stretched under force, the anchor plug is slightly pulled out. At this time, the anchor plug thread is immediately clamped with a steel drill, and then the main cylinder slowly returns oil. The steel wire shrinks inward, and the anchor plug cannot shrink inward at the same time as the steel wire due to being stuck. If the stroke of the jack is not enough, repeat this process until the anchor plug exits. Then pull out the steel wire harness and replace it with a new one and anchor.
② Single sliding wire, single supplementary pull. Wedge the sliding steel wire onto the chuck, tension it to a stress level, and then press the wedge tightly.
③ Manually loosen the steel wire bundle by sliding the wire. Install the jack and wedge each steel wire tightly. When one end of the steel wire bundle is tensioned to the control stress of the steel wire but still cannot pull out the anchor plug, remove the wedge of the steel wire on a jack chuck, forcing 1-2 steel wires to produce wire pulling. At this point, the anchoring force between the anchor plug and the anchor ring decreases, making it easier to pull out the anchor plug when pulled again.
