What are post-tension and pre-tension? What does it do?


What are post-tension and pre-tension? What does it do?

Tension control stress refers to the maximum stress value controlled by prestressed reinforcement during tension. The value is the stress value obtained by dividing the total tensile force indicated by the tensioning equipment (such as a jack gauge) by the section area of the stressed bar, expressed as σcon. The value of the tension control stress directly affects the use effect of prestressed concrete. If the value of the tension control stress is too low, the prestressed stress generated by the prestressed steel bar after various losses is too small, which can not effectively improve the crack resistance and stiffness of the prestressed concrete members. 

If the value of tension control stress is too high, the following problems may be caused: 

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

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

(3) In order to reduce the loss of prestress, sometimes need to be overstretched, it is possible to make the stress of individual steel bars in the process of overstretching exceed its actual yield strength, resulting in large plastic deformation or brittle fracture of steel bars. The value of tension control stress is related to the method of prestressing. For the same steel, the value of the first tension method is higher than that of the second tension method. This is due to the difference in the way prestressing is established between the first and the second tensioning methods. The tensioning method is to stretch the bar on the bench before pouring the concrete, so the tensile stress established in the prestressed bar is the tension-controlled stress σcon. The post-tensioning method is to stretch the steel bar on the concrete member. At the same time, the concrete is compressed. The tension control stress indicated by the jack of the tensioning equipment has been deducted the stress of the steel bar after the elastic compression of the concrete. Therefore, the σcon value of the post-tensioning component should be lower than that of the pre-tensioning component. The determination of tension control stress value is also related to the steel type of prestressing. Because prestressed concrete adopts high strength reinforcement, its plasticity is poor, so the control stress can not be too high. 

According to long-term accumulated experience in design and construction, Code for Design of Concrete Structures stipulates that, under normal circumstances, the tensile control stress should not exceed the limit value in the table below.