The Difference Between Pre-tensioning and Post-tensioning

(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.