What are the control methods for prestress in the tensioning process of prestressed steel bars?

1) The tensile control stress of the prestressed tendon should meet the design requirements. When the prestressed tendon needs to be over-tensioned or included in the prestress loss of the anchor ring mouth during construction, it can be increased by 5% compared with the design requirements, but in any case shall not exceed the maximum tensile control stress specified in the design.

2) When the stress control method is used to tension the prestressed tendon, the elongation value should be checked, and the difference between the actual elongation value and the theoretical elongation value should be controlled within 6%, otherwise the tension should be suspended, and the tension can be continued only after the cause is identified and measures are taken to adjust.

3) The theoretical elongation value of the prestressed tendon l (mm) can be calculated according to the formula (where: the average tensile force of the PP prestressed tendon (n), the tensile force of the tension end of the straight tendon is taken, and the curved tendon of the tension at both ends is shown in Appendix G-8 for the calculation method; l The length of the tendon (mm); cross-sectional area of ap tendons (mm2); Modulus of elasticity of ep tendons.

n/mm2）。

4) When the tendon is tensioned, it should be adjusted to the initial stress δ0, which should be 10% 15% of the tensile control stress δcom, and the elongation value should be measured from the initial stress. In addition to the measured elongation value, the actual elongation value of the tendon must be added to the estimated elongation value below the initial stress. For post-tensioned members, the elastic compression value generated during the tensioning process can generally be omitted.

The actual elongation value of tensile tension l(mm), calculated according to the formula (calculated: l= l1+ l2 (where: l1 measured elongation value (mm) between the initial stress and the maximum tensile stress; The estimated elongation value (mm) below the initial stress of L2 can be used as the elongation value of the adjacent stage.

5) If necessary, the friction loss of the anchor ring mouth and the hole should be measured, and adjusted when tensioning.

6) The anchoring of the prestressed tendons should be carried out in a stable state of tensile control stress. The amount of inward shrinkage of the tendon at the tension end during the anchoring stage.

It should not be greater than the design specification or not greater than the allowable value listed in the specification.

Tension controls stress.

Tension control stress refers to the maximum stress value that is controlled and reached by the prestressed steel bar when it is tensioned. Its value is the stress value obtained by dividing the total tensile force indicated by the tensioning device (e.g., jack oil pressure gauge) by the cross-sectional area of the stressed steel bar, expressed as con.

If the value of tensile control stress is too low, the precompressive stress generated by the prestressed steel bar is too small after various losses, and the crack resistance and stiffness of the prestressed concrete component can not be effectively improved. If the tension control stress value is too high, it can cause the following problems:

1) In the construction stage, some parts of the component will be subjected to tensile force (called pretension) or even cracking, which may cause local pressure damage to the end concrete of the post-tensioning component.

2) The load value of the component when the crack occurs is very close, so that the component has no obvious warning before failure, and the ductility of the component is poor.

3) In order to reduce the loss of prestress, sometimes it is necessary to carry out overtensioning, and it is possible to make the stress of individual steel bars exceed its actual yield strength in the process of overtensioning, so that the steel bars produce large plastic deformation or brittle breakage.

The magnitude of the tension control stress value is related to the method of applying the prestress, and for the same steel grade, the value of the pre-tensioning method is higher than that of the post-tensioning method. This is due to the fact that the way in which prestressing is established is different between pretension and posttension. The pre-tensioning method is to tension the reinforcement on the pedestal before pouring concrete, so the tensile stress established in the prestressed reinforcement is the tensile control stress con.

The post-tensioning method is to tension the steel bar on the concrete member, and at the same time as the tensioning, the concrete is compressed, and the tension control stress indicated by the jack of the tensioning equipment has deducted the reinforcement stress after the elastic compression of the concrete. For this reason, the con value of the post-tensioned component should be appropriately lower than that of the pre-tensioned method.

The determination of the tensile control stress value is also related to the prestressed steel grade. Because the prestressed concrete is made of high-strength steel bars, its plasticity is poor, so the control stress can not be too high.

Based on the long-term accumulated design and construction experience, the Code for the Design of Concrete Structures stipulates that, under normal circumstances, the tensile control stress should not exceed the limits in the following table.

Tension control stress limits.

Type of reinforcing bar. First tension method.

Post-tension method. Prestressed steel wire, steel strand.

Heat-treated rebar.

Note: 1In the table, FPTK is the strength standard value of prestressed reinforcement, see Table 2-8 of Appendix 2 of the Code for Design of Concrete Structures;

2.The tension control stress value of prestressed steel wire, steel strand and heat-treated steel bar should not be less.

When the FPTK meets one of the conditions in one column, the tension control stress limits in the table can be increased.

FPTK1) requires to improve the crack resistance of the component in the construction stage, and the prestressed steel bar arranged in the compression zone in the use stage;

2) It is required to partially offset the prestress loss due to factors such as stress relaxation, friction, batch tension of the rebar and the temperature difference between the prestressed rebar and the tension pedestal.

The main control method is the double inspection system.

That is: control of fuel gauge readings and elongation.

After the design tension is reached, whether the steel strand reaches the design elongation and whether the elongation value meets the requirements of the specification.

Hello, now I am here to answer the above questions for you. What are the prestressed tensile specifications, prestressed tensile specifications, I believe that many friends still don't know, now let's take a look at the pre-response. <>

The tensile control stress of the tendon must comply with the design regulations.

When the stress control method is used to tension the prestressed tendon, the elongation value should be checked.

3. The difference between the actual elongation value and the theoretical elongation value should meet the design requirements; When the design is not specified, the difference between the actual elongation value and the theoretical elongation value should be controlled within 6.

4. Otherwise, the tensioning should be suspended, and the tensioning can only be continued after the reason is found out and measures are taken.

When the prestress tension is made, it should be adjusted to the initial stress The initial stress should be the tension control stress 10 l5, and the elongation value should be measured from the initial stress.

The anchorage of the prestressed tendon should be carried out in a stable state of tension control stress, and the inward shrinkage of the tensile end of the tensile tendon in the anchoring stage shall not be greater than the design or specification.

Tension control stress refers to the maximum stress value that is controlled by the pre-lift stress reinforcement during tensioning. Frank.

The value of the tension control stress is the total tension force indicated by the tensioning device (e.g., a jack pressure gauge) divided by the cross-sectional area of the prestressed reinforcement, expressed in con.

Within the limits of elasticity, the deformation of an object under the action of an external force is proportional to the external force exerted. The deformation varies depending on the direction in which the force acts, and the force that causes the object to extend is called "pull" or "tension". (Push, pull, lift, pressure, and buoyancy are collectively referred to as: pull).

Rebar refers to steel for reinforced concrete and prestressed reinforced concrete, which has a circular cross-section and sometimes a square with rounded corners.

The prestress tension control should be mainly based on stress control, and the elongation value should be used as the control method for review. In other words, tension control is the main control.

1. Determine the reading of the oil gauge according to the calibrated jack and the oil gauge.

2. According to the drawings or design requirements, determine the initial stress, which is generally 10%-20% of the control stress.

3. Convert stress into force value according to the number of steel strands and cross-sectional area.

4. Bring the initial control stress and the force value of the control stress conversion into the linear equation of the top report, so that you can get the corresponding oil gauge reading, which is the main control data when you want to tension, and the tension must reach this value.

5. According to the different section lengths given in the drawings, the theoretical elongation value of the steel strand should be calculated.

6. Compare the elongation value generated by the tension value of the oil gauge reading with the theoretical elongation value to see if the error is within the range of the specification requirements (generally plus or minus 6%).

Beam prestressed tendons are generally divided into three types: longitudinal prestressed tendons, vertical prestressed tendons and transverse prestressed tendons. Longitudinal tendons:

It is mainly to apply longitudinal pressure to the concrete of the lower part of the beam body to ensure that the concrete of the lower part of the beam body does not appear tensile stress under the vertical load of the beam body. Vertical prestressed tendons: mainly apply vertical pressure to the beam to ensure that the beam does not have vertical shear tensile stress.

Transverse prestressed tendons: It is mainly to apply transverse pressure to the beam to ensure that the concrete does not have transverse shear tensile stress under the appointment of torsional force of the beam.

1. Different definitions:

Prestressed tension is to add tensile force in advance in the component, so that the applied prestressed tension member bears tensile stress, and then makes it produce a certain deformation, to cope with the load of the steel structure itself, including the load of the weight of the roof itself, wind load, snow load, load and so on.

In order to save steel, the method of cold drawing or cold drawing is often used to improve the strength of hot-rolled steel bars, cold drawing is a method of stretching steel bars to a certain stress that exceeds their yield strength, and then unloading to zero to improve the strength of steel bars. The yield strength of the steel bar is increased after cold drawing, but the plasticity is reduced accordingly.

Second, the production materials are different:

Prestressed steel bars should be carbon steel wires, scored steel wires, steel strands and heat-treated steel bars, as well as cold-drawn grade, grade and grade steel bars. For the prestressed steel bars of small and medium-sized components, Class A cold-drawn low-carbon steel wire and cold-drawn low-alloy steel wire can be used.

Cold drawn steel can only be used as structural reinforcement, can not be used as a stress reinforcement, can be used as a lifting ring, the material of prestressed steel bar and cold drawn steel bar is very different, can not be discussed.

3. Different strength levels:

The cross-section of the cold-drawn reinforcement will generally become smaller and cannot be restored to the original cross-section (i.e., there is plastic deformation), and it is possible to restore the cross-section after the tension of the pre-tensioned reinforcement (i.e., elastic deformation or elastic tension).

The concrete strength grade of prestressed concrete components should not be lower than C30; When carbon steel wire, steel strand, and heat-treated steel bar are used as prestressed steel bars, it should not be lower than C40.

Summary. Dear, I am glad to answer for you: what are the ways of prestressed tensioning?

What are the conditions under which they apply? Answer: Pre-tensioning and post-tensioning in pro-pro-prestress tension are two different methods of applying prestress to concrete components; The pre-tensioning method is used for prefabricated components with small shapes, and has a tensioning pedestal, which is processed in the prefabrication yard, such as prestressed hollow core slabs, bridge slabs with small spans, etc.; The post-tensioning method is suitable for large bridge plates, T beams, etc. with a span of more than 25m, and can be processed on site without tensioning the pedestal.

Before the concrete is poured, the prestressed steel bar is stretched to the design tension, and then the concrete is poured, and the concrete is poured until the concrete reaches a certain strength (generally more than 80% of the design strength), the prestressed steel bar is relaxed, and the prestressed steel bar is self-anchored to the prestressed steel bar through the concrete, so as to complete the prestressed member. The post-tensioning method is to pour the concrete component first, and bury the prestressed pipe (generally the bellows) before pouring or the embedded hose is pulled out to form the pipeline at a certain time after pouring. Then, after the concrete reaches a certain strength (generally more than 80% of the design strength), the prestressed steel bar is inserted into the pipeline, tensioned, and then locked and anchored with anchors.

After all tensioning is completed, the grouting in the pipeline is completed, so as to complete the prestressed member.

What are the ways of prestressed tensioning? What are the conditions under which they apply?

Dear, I am glad to answer for you: what are the ways of prestressed tensioning? What are the conditions under which they apply?

Answer: Pre-tensioning and post-tensioning in pro-pro-prestress tension are two different methods of applying prestress to concrete components; The pre-tensioning method is used for prefabricated components with small shapes, and has a tensioning pedestal, which is processed in the prefabrication yard, such as prestressed hollow core slabs, bridge slabs with small spans, etc.; The post-tensioning method is suitable for large bridge plates, T beams, etc. with a span of more than 25m, and can be processed on site without tensioning the pedestal. Before the concrete is poured, the prestressed steel bar is stretched to the design tension, and then the concrete is poured, and the concrete is poured until the concrete reaches a certain strength (generally more than 80% of the design strength), the prestressed steel bar is relaxed, and the prestressed steel bar is self-anchored to the prestressed steel bar through the concrete, so as to complete the prestressed member.

The post-tensioning method is to pour the concrete component first, and bury the prestressed pipe (generally the bellows) before pouring or the embedded hose is pulled out to form the pipeline at a certain time after pouring. Then, after the concrete reaches a certain strength (generally more than 80% of the design strength), the prestressed steel bar is inserted into the pipeline, tensioned, and then locked and anchored with anchors. After all tensioning is completed, the grouting in the pipeline is completed, so as to complete the prestressed member.

What are the ways of concrete segment joint splicing? What are the characteristics of each?

There are three main types of joints in cement concrete pavement: 1. Shrinkage joints. In the concrete hardening process, the transverse and longitudinal joints are cut at certain intervals (4 6m) to prevent irregular cracks from forming due to concrete cooling shrinkage and dry shrinkage; 2. Expansion seams.

Set up expansion joints in places where there are obstacles to restrict the thermal expansion and contraction of concrete pavement; 3. Construction joints. The pavement concrete pouring construction is not continuous, and the joints set at the pause part should coincide with the shrinkage joints or expansion joints as much as possible.