Several key factors in quality control of prestressed concrete continuous beams.

A, prestressed steel strand installation

The key to quality control is the position of prestressed steel beam channel and whether the steel strand is twisted. The inaccurate hole position changes the stress state of the structure, and if the elevation change section of the curved hole is irregular, it will also increase the friction loss of the prestressed hole. Therefore, whether the hole location is accurate or not is directly related to whether the construction prestress is consistent with the design prestress, and has a profound impact on the structural safety and whether cracks will occur in the engineering use stage. If multiple steel strands are twisted together, the stress of each steel strand is uneven during tensioning, which increases the friction between steel strands and causes the prestress loss to increase.

In actual construction, many construction units do not attach importance to these details. The position of the tic-tac-toe frame for fixing the steel beam is inaccurate or not arranged according to the spacing specified in the specification and design, which will inevitably cause the position of the steel beam to be inconsistent with the design, and some will also produce sharp bends (the radius is too small) or the local deviation of the tunnel is too large. At present, a small number of teams still use manual beam threading, especially the long beam of multi-strand steel strands is very heavy, time-consuming and laborious, and it is easy for workers to rotate the steel strands and make them intertwine. There are 9 continuous beams in the fourth bid section of a expressway (viaduct) project in Shenyang. During construction, the tic-tac-toe spacing of fixed steel beams is 1m, and the beam height is 1.6m, so the vertical bending changes little and the spacing meets the requirements. However, due to the careless work of workers during construction, the coordinates of the sub-frame are inaccurate, so artificial beam crossing is adopted, and the beam length is 100. During the tensioning process, it is found that the elongation values of most steel strands are inconsistent with the theoretical elongation values (some of them are smaller than the theoretical values 1 1%). During the tensioning process, the internal steel strand is often twisted and pulled apart. At that time, the equipment was verified immediately, and the design unit, supervision unit and construction unit began to analyze the problem when there was no problem with the equipment. Among them, the measured elastic modulus is used to calculate the elongation of steel strand, and the values of μ and κ are recommended according to the specification. The design unit rechecked the structure and finally determined that the elongation error was within 12% under the condition of ensuring the tension, which undoubtedly reduced the safety factor of the structure.

The D ramp of expressway Sujiatun Interchange in Shenyang-Dalian is a curved continuous beam with four holes in one, with a beam length of 220 meters and a curve radius of 55 meters. Therefore, the steel beams have both horizontal and vertical bends, and the tic-tac-toe frames are arranged at intervals of 50cm, with accurate coordinates. Mechanically and manually cross the beam (the steel strand is fixed on the conical traction device, which is pulled by the winch). The above method was also adopted in Guangzhou South Expressway Project 14 Mark Bridge 100 meter continuous beam construction. Due to the special attention to control the coordinates of the tunnel and the roundness of the tunnel line, and to avoid the intertwining between the steel strands, the elongation values of the steel strands in the above two projects meet the requirements during tensioning.

Second, the prestressed steel strand tension

1. Tension control stress and elongation value: Whether the tension control stress can reach the design value directly affects the prestress effect. Therefore, the tension control stress is the focus of tension quality control, which must reach the design value, but not exceed the maximum tension control stress specified in the design. The prestress value is too large, which exceeds the design value too much. Although the crack resistance of the structure is good, because of its high crack resistance, the prestressed tendons are often in a high stress state when bearing the service load, which is close to the load when the structure cracks, and there is often no obvious warning before the damage, which will seriously endanger the safety of the structure. Therefore, in order to accurately grasp the application of prestress, the elongation value must be checked when the stress control method is used for tensioning. Therefore, it is particularly important to provide accurate theoretical elongation value, and it is necessary to have a correct understanding of the calculation of theoretical elongation value in Technical Specification for Construction of Highway Bridges and Culverts (JTJ04 1-2000):

(1) When the coordinates of the prestressed channel meet the design requirements and the curved channel is smooth, the local deviation of the channel and the friction coefficient between the prestressed reinforcement and the channel wall have little influence on the theoretical elongation value, so the median value can be taken according to the specification.

② The elastic modulus Ep of steel strand has a great influence on the theoretical elongation, and should be calculated according to the measured value.

③ Value of L: The average pulling force should be calculated according to the length of the hole, and the distance from the anchor pad to the front end of the tool holder should be added to the value of L when calculating the elongation value. In addition, when comparing the theoretical elongation value with the actual elongation value, the comparison should be based on the value from the initial stress to the control stress, because the elongation value from zero to the initial stress is calculated, with many measurements and large accumulated error.

2. Influence of formwork support: Due to prestress, concrete will inevitably undergo elastic deformation, and at the same time, it will produce axial deformation and vertical deflection. If axial shrinkage and deflection are restrained in the process of tensioning, unexpected cracks will appear in concrete, and even quality accidents will occur. Therefore, before tensioning, it is necessary to dismantle the beam side formwork which restricts the axial contraction of the beam body, and dismantle the formwork and bracket around the bearing which restrict the movement and rotation of the movable bearing along the bridge direction and the rotation of the fixed bearing. We have observed the beam length of Guangzhou Southern Expressway Project 14 Mark Bridge 100m continuous beam before and after tensioning. The results show that the beam length per meter is shortened by about 0.2mm, and if all kinds of constraints are not removed, it is likely to cause local cracking of the beam or deformation of the support. During the construction of a viaduct 120m continuous beam in Dongguan, Guangdong Province, because the steel bottom formwork around the bracket was not removed before tensioning, most of the bottom formwork was deformed after tensioning, and the fixed basin bracket rolled over.

3, tensioning points:

① Tensioning sequence: The tensioning sequence shall be as specified in the design. If there is no provision in the design, it is necessary to avoid making the section of the component in an excessively eccentric stress state, so as to avoid causing excessive tensile stress at the edge of the component. Especially for curved bridges, it should be noted that the inner and outer edges of curved beams can not bear too much tensile stress during the tensioning process, which will cause the web of beams to crack. When tensioning, the steel beams near the centroid of the section must be tensioned first. If there are multiple rows of steel beams, they must be symmetrical.

② Tension length: the steel beam of continuous beam is long, so it is recommended to tension both ends at the same time. If the equipment is insufficient, one end can be fixed first, then the other end can be tensioned, and then the fixed end can be tensioned to make up for the stress. Especially for curved prestressed tendons. When one end is tensioned, although the tensioning end reaches the control stress, due to the long tunnel length, the included angle θ of the steel beam increases and the friction increases, the prestress gradually decreases from the tensioning end to the fixed end, and the prestress near the fixed end is obviously insufficient. A expressway (viaduct) project in Shenyang 120m prestressed continuous beam has one end tensioned and the other end anchored in the beam. The elongation value can't meet the requirements during tensioning, mainly because the tunnel friction loss is too large (influenced by the tunnel angle θ and the tunnel length). On the one hand, in the event of an accident (such as broken wire, etc. When it happens, it will be difficult to deal with; On the other hand, due to the insufficient prestress exerted by steel beams on the structure, the safety of the structure is endangered.

4. Handling of broken wire and slippery wire: During construction, broken wire and slippery wire sometimes occur due to operational errors or inaccurate jack pressure, incorrect anchorage installation, poor clip quality and other reasons. When the number of broken wires or slippery wires does not exceed the standard value, the stress can be supplemented by overstretching; If it exceeds the standard value, the anchorage must be removed and the steel beam replaced. This must be handled with care, high quality and safety.

(1), stress compensation treatment: determine the stress loss value according to the number of broken wires, and compensate the stress loss caused by broken wires by increasing the stress of other steel wires. However, the stress of steel strand cannot reach 0.8Rb under any circumstances, otherwise the steel beam must be replaced.

(2), the replacement of steel beam processing method:

(1), traction to relax. Install the jack according to the tension state and wedge the steel wire into the chuck. One end is tightened, and when the steel wire is stretched, the anchor plug is slightly taken out. At this point, immediately use a steel drill (steel drill can be made of φ5mm steel wire, the end is sharpened, and the length is 20 ~ 30cm) to clamp the anchor plug thread. Then the main cylinder slowly returns oil, and the steel wire retracts. The anchor plug cannot retract with the steel wire at the same time because it is stuck. If the jack stroke is not enough, it can be repeated until the anchor plug exits. Then pull out the steel strand bundle and replace it with new steel strand bundle and anchorage.

(2), single slip line single pull. Wedge the sliding steel wire on the chuck, and press the wedge tightly after stretching.

(3) Manually smooth the wires to relax the wire bundle. Install the jack and wedge the steel wire. When the control stress at one end of the steel wire bundle still cannot pull out the anchor plug, the steel wire wedge on the jack chuck is knocked off, forcing the 1~2 steel wire to be pulled out. This means that the anchoring force between the anchor plug and the anchor ring is reduced, and it is easy to pull out the anchor plug again.

Third, the hole grouting

Prestressed pipeline grouting plays an important role in post-tensioned prestressed members: preventing prestressed steel bars from rusting; Prestressed steel and concrete are effectively bonded to realize the overall stress effect and improve the bearing capacity of the beam; Reduce the load of the anchoring system. Therefore, we must attach great importance to grouting quality. Therefore, it is required that the cement slurry pressed into the channel has reliable compactness after hardening, which can not only protect the prestressed tendons, but also have certain bond strength and shear strength, so as to effectively transfer the prestress to the surrounding concrete. In the past engineering practice, the constructors did not pay enough attention to the technology and material quality of hole grouting, which led to the premature corrosion of prestressed tendons and reduced the durability of the structure. If you want to make the grouting work successful, you must do the following:

(1), cement, water, additives and grouting equipment meet the specification requirements.

(2) The water cement ratio, bleeding rate, expansion rate and consistency of cement slurry meet the specification requirements.

(3), check whether the channel is clear before grouting.

(4) The grouting sequence is correct. According to the order of channels from low to high.

(5) Strictly control the grouting pressure and speed.

⑥ Vacuum grouting technology is adopted.

Prestressed concrete continuous beams are generally designed as fully prestressed structures, so it is extremely important to accurately determine the degree of prestress. However, in the actual construction, due to the above reasons, there are often problems such as insufficient prestress, cracks in the beam and damage to the support, so the key factors affecting the quality of prestressed construction must be strictly controlled in the construction process.

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