The differences between post tensioning System and Pretensioning System

The pre-tensioning system and the post-tensioning system an important difference is whether the steel bar is stretched

1. Prestensing system

The pre-tension method is simple in construction, and self-anchored by bonding force, no need to consume special anchors, temporary anchors can be reused.It is called tool anchor or fixture), which is economical and stable in mass production. It is suitable for chemical production of small and medium-sized components production

The first step : Stretching the steel bar , and then pour the concrete. The main stretching procedure is: stretch the steel bar on the pedestal according to the design requirements.

To control the stress → temporarily fix with anchors → pour concrete → wait until the concrete reaches more than 75% of the design strength.

The way of cutting off the loose steel bar 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 section concrete should be preloaded.

2. Post-Tension System

The post-tensioning method refers to the construction method in which the concrete is poured first, and then the prestressed steel is stretched to form a prestressed concrete member after reaching more than 75% of the design strength.

In the Post tension system, including Bonded System and Unbonded System

(1 ) Bonded Prestressed Concrete System

The procedure is: burying the control hole-+ pouring concrete-→ pipe drawing-+ curing through reinforcement tension-→ anchoring-+ grouting (to prevent the growth of steel

Rust). The way of force transmission is to rely on anchors to prevent the elastic rebound of steel bars, so that the section concrete can obtain pre-compression stress, such as shown in Figure 6.2. This approach makes the steel and concrete integrated, called bonded prestressed concrete.

Bonded prestressed concrete reduces the tensile stress of prestressed steel bar due to the effect of cohesive force (resistance), resulting in the compressive stress of the concrete is reduced, so we should try to reduce this bonding. This method is simple and does not require a tensioning table seat, flexible production, suitable for on-site construction of large components.

(2) Unbonded Prestressed Concrete System

The main tensioning procedure is to apply lubricating anti-corrosion materials such as asphalt along the entire length of the prestressed steel bar-+ plastic coating Paper or casing (the pre-stressed steel bar and concrete do not establish adhesion)-+>concrete curing -→tensioned steel bar -→anchoring

The construction is the same as ordinary concrete. The concrete can be poured directly by placing the steel bars in the design position, without preserving holes and passing through reinforcing bars, grouting, simplifying the construction procedure, because the effective pre-compression stress of unbonded pre-stressed concrete increases, the cost is reduced, It is used for beams with large span curves.