## 2 Background  
Concrete, as a structural material, is known to be strong in compression, but weak in tension. When bending occurs in the beam owing to an external force, a compressive force is generated on the upper surface of the beam, and a tensile force is generated on the lower surface of the beam. Therefore, concrete with a weakness in tension is not suitable for use as a flexural member because it cracks and breaks at the lower surface, where the tensile force is generated. To compensate for the disadvantages of concrete, reinforcing bars are placed inside the concrete to strengthen the structure against tension [13].  
However, a reinforced concrete structure also requires a larger cross-section as the span increases, and accordingly, its own weight gradually increases, making it an inefficient structure for long spans. Moreover, it is very difficult to avoid deflection of the structure caused by the working load of the vehicle and the occurrence of cracks owing to the tensile force [14; 15].  
To compensate for the limitations and disadvantages of the reinforced concrete structure, a steel strand was placed in the concrete such that no tensile force was generated in the concrete, and a compressive force was applied to the concrete by pulling and fixing it strongly from both ends. Pre-stressed concrete (PSC) is a concrete structure that is restrained. Reinforced concrete structure is a concept that compensates for these shortcomings by reinforcing concrete that is weak in tension with reinforcing bars. Introducing the pre-stress introduces the compressive force of the beam caused by the pre-stress to be greater than the tensile force on the lower surface of the beam generated by the external force; thus, the tensile force is not generated in the concrete when an external force is applied [7; 16]. An overview of this procedure is shown in Figure 3[16].  
In bridge construction, pre-stressed concrete girders are constructed according to the method of introducing pre-stressing. They are divided into tension and pretension methods. The post-tension method introduces a tension force into the concrete structure as a reaction force by inserting a tension material, which is a compression material, into the buried sheath pipe (steel duct, etc.) after the concrete is cured. After placing and stretching the tension member, the concrete was poured to form the structure. After the concrete was cured, the stretched tension member was cut to introduce the elastic restoring force of the tension member into the structure.  
Figure 2: Pre-tensioning mechanism using internally placed concrete piles as a reaction arm and forces applied through strands to anchor blocks for (**a**) Arrangement of anchor block-hydraulic cylinder-internal reaction arm (**b**) After concrete pouring and curing.  
As the post-tension method introduces tension by inserting a tension material into a concrete structure in which the sheath pipe is embedded, it is necessary to insert the tension material, and an elongated amount of the tension material is maintained. The pre-tension method requires an external reaction force on the tension member. The reaction arm can be manufactured in a factory and can be produced repeatedly.  
In the post-tension method, tension loss can occur owing to the introduction of tension, and can be divided into short-term and long-term losses. Short-term loss includes the friction loss of the tension member and sheath pipe, loss of the anchorage device, and loss due to concrete elastic shrinkage. etc. However, friction loss is not accounted for in the pre-tension method. Overall, it is reduced by 5-20% compared to the post-tension method, so the number and amount of tension members can be reduced, the length of the structure can be extended, or the height of the structure can be lowered.