The Difference Between Pre-tensioning and Post-tensioning
An important difference between pre-tensioning and post-tensioning is the order of component production. Pre-tensioning is the first tensioning and then making the component, and post-tensioning is the first making the component and then tensioning the prestressed tendons.
Pre-tensioning means that the steel bars are firstly tensioned and then the concrete is poured. The main tensioning procedures are as follows: stress the steel bars to control stress according to the design requirements on the pedestal → temporarily fix them with anchorages → pour concrete → cut off the steel bars when the concrete reaches more than 75% of the design strength. The way of force transmission is to rely on the bonding force of the steel bar and concrete to prevent the elastic rebound of the steel bar so that the cross-section concrete can obtain prestress.
The pre-tensioning method is simple in construction, self-anchoring by adhesive force, no special anchorages are required, and temporary anchorages can be reused (generally mono strand barrels or wedges are used). It is economical and stable in mass production, suitable for small and medium components factory production.
(1) Bonded prestressed concrete
Pour the concrete first, and when the concrete reaches more than 75% of the design strength, stress the steel bars (rebar bundles). The main tensioning procedures are: burying duct to make hole → pouring concrete → drawing pipe → curing → pushing and tensioning the strands→ anchoring → grouting (to prevent steel rust). The way of force transmission is to rely on anchorages to prevent the elastic rebound of steel bars so that the cross-section concrete can be prestressed. In this way, the steel bars and concrete are integrated as a whole, which is called bonded prestressed concrete.
Bonded prestressed concrete reduces the tensile stress of the prestressed steel bar due to the effect of the cohesive force (resistance), resulting in a decrease in the compressive stress of the concrete, so we should try to reduce this bonding. This method has simple equipment, no need for the tensile pedestal, flexible production, and is suitable for on-site construction of large components.
(2) Unbonded prestressed concrete
The main tensioning procedures of unbonded prestressed concrete are: prestressed steel bars are painted with lubricating anti-corrosion materials such as asphalt along the entire length of the surface → covered with plastic paper or casing (the prestressed steel bars and concrete do not establish cohesive force) → pouring concrete and curing → tensioning steel bars → anchored.
The construction is the same as ordinary concrete, the concrete can be poured directly by placing the steel bars in the design position. There is no need to reserve holes, push strands, and grout, which simplifies the construction procedure. Because the effective prestress of unbonded prestressed concrete is increased and the cost is reduced, it is suitable for beams with large spans and curved reinforcement.