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The Reasons for Excessive Elongation of Prestressed Concrete Steel Strands

The Reasons for Excessive Elongation of Prestressed Concrete Steel Strands

Issue Time:2021-08-04
the specific reasons for the excessive elongation of the prestressed concrete steel strands

The prestressed steel strand is the lifeline of prestressed concrete engineering, which is equivalent to the bones of the human body. The quality of its construction directly affects the safety of the structure. Therefore, tension control stress is the focus of pre-stress construction. 

The problem often encountered in post-tensioned pre-stress construction is that the elongation value of the steel strand exceeds the standard, which will directly affect the establishment of the effective pre-stress value in the structure. The following are the specific reasons for the excessive elongation of the prestressed concrete steel strands.

1. The calculation error of the elongation value of the steel strand

The design drawings of many projects now clearly give the theoretical elongation value of the steel strand under the corresponding tension control stress. As a result, some construction units control the elongation according to the data given on the design drawings. 

In fact, this approach is not advisable. Because most of the design drawings use standard parameters for calculating the elongation of the steel strand. For example, EP is 1.95×105MPa, L is calculated based on the effective prestress length, and the duct friction coefficient is also based on The value of experience value.

However, in actual situations, the elastic modulus of the stranded wire is not always equal to 1.95×105 MPa. The construction unit should randomly select samples for the elastic modulus test to obtain the actual elastic modulus value and the anchor pad opening during the tensioning process. The steel strand in the middle to the tip of the tool clip is also stretched, so when calculating the elongation value, the elongation of this part should also be calculated.

Especially for some important large-scale projects, the channel friction coefficient must be calculated based on the actual test data on-site. As long as the elongation value is carefully checked and rechecked during the on-site construction, the calculation error of the elongation value of the steel strand can be completely avoided.

2. Calculation errors of tensile stress and oil gauge readings

The calculation error of the oil meter reading will directly lead to the error of the tensile stress of the steel strand, and the calculation of the oil meter reading is directly based on the linear regression equation established by the jack and the corresponding oil meter, so you should first ensure that the calibration report is true and valid.

On the basis of confirming that the oil meter reading is correct, check whether the matching relationship between the on-site jack and the oil meter is consistent with the calibration on the top calibration report. If they are inconsistent, it means that the force output by the jack cannot be truly reflected on the oil gauge, and it is also impossible to determine how much force the steel strand is subjected to. Of course, it is impossible to talk about right or wrong elongation.

3. The position of the bellows is incorrect

In actual construction, many construction units do not pay much attention to these details such as the positioning of the steel strands. Inaccurate positioning of the positioning ribs of the fixed steel beam, or not laying in accordance with the spacing required by the construction specifications and design, or not firmly fixed, will cause the position of the steel beam to be inconsistent with the design, and sometimes even sharp bends (the bending radius is too small).

Although the stress on the steel strand is correct, it changes the stress state of the entire structure, which is very dangerous. At the same time, because the position of the stranded wire is different from the design, it is inevitable that its elongation value differs greatly from the theoretical elongation value.

4. The steel strands are twisted to each other

At present, there is still a small part of the team in the construction that uses manual threading, especially for the long bundles of multiple steel strands. Manual threading is time-consuming and laborious. It is easy to cause workers to rotate the steel strands to penetrate and make the steel strands entangle with each other. Together. We often hear the sound from the structure during the tensioning process, which is due to the sounding phenomenon when the strands are entangled and pulled apart.

If multiple steel strands are twisted together, the force of each steel strand will not be uniform during tension, and the force of the steel strands with more windings will be small, which will increase the strength of each steel strand. The friction between the strands and the stress loss of the strands also increase.

At the same time, due to the mutual entanglement of the steel strands, the originally assumed homogenous system of the steel strands is changed into an inhomogeneous body, and its elastic modulus EP also changes accordingly, so its elongation value often fails to meet the requirements. If it is changed to use a hoist or other equipment or a whole bundle of steel strands, the probability of mutual entanglement will be greatly reduced.

5. The corrugated pipe leaks in the slurry or the corrugated pipe is flattened during the process of pouring concrete

In the construction process, as long as a little attention is paid to that link, it may cause leakage in the corrugated pipe or the corrugated pipe being flattened by the vibrating pump. In the light of the case, the length of the tension is too short, and in the case of the heavy, the steel strand cannot be passed through. Through the tunnel, the structure needs to be opened. Even if there is no abnormal phenomenon during the tensioning process, the stress loss of the steel strands in the leakage section will be too large, so that the structure will completely become reinforced concrete without prestressing or the prestress degree is very high in some sections. Small.

6. Slippery wire appears on one end

When one end is stretched and the other end is slightly slippery, the elongation of the steel strand will also increase a lot.

7. Improper operation

The unskilled or unsatisfactory work attitude of the tensioning operator can also lead to misjudgment and misunderstanding of the elongation. The specific manifestation is that the oil gauge readings of each stage of the tensioning oil pump are not well controlled, and the measuring staff reads the ruler incorrectly or the elongation value does not reach a stable value.

If the installation angle of the anchor pad is not right and there are too many objects behind the anchor pad, it will directly affect the elongation of the prestressed concrete strand. If you want to learn more about prestressed concrete strands after you have reviewed the above, you can contact us for a comprehensive solution.

As a professional manufacturer of prestressed concrete products, we have won unanimous praise from many customers with our excellent technology and considerate service. We have an experienced manufacturing team and staff, and strictly follow the production standards, we can provide customers with safe and high-quality products. At the same time, we always put the needs of customers first, which can improve the thoughtful one-stop service and effective solution technology. If you are interested in our prestressed concrete strands, please contact us immediately!
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