(1) pretensioning method?
That is to say, the steel bar is tensioned first and then the concrete is poured. The main tensioning procedure is as follows: stretching the steel bar on the pedestal according to the design requirements to control the stress → temporary fixing with anchorage → pouring the concrete → cutting off and loosening the steel bar when the concrete reaches the design strength of more than 75%. The force transmission way is to prevent the elastic rebound of the steel bar by relying on the adhesive force between the steel bar and the concrete, so that the section concrete can obtain the preloading stress?
6. 1.2 Pre-stressing method
The pretensioning method is simple in construction, self-anchored by cohesive force, without consuming special anchorage, and the temporary anchorage can be reused (generally called tool anchorage or fixture), which is economical and stable in mass production. It is suitable for industrial production of small and medium-sized components.
(2) post-tensioning method?
① Bonded prestressed concrete?
Concrete is poured first, and then the steel bars (tendons) are tensioned after the concrete reaches the design strength of more than 75%. The main tensioning procedures are: burying the pipe holes → pouring concrete → drawing pipes → curing and stretching the tendons → anchoring → grouting (to prevent the steel bars from rusting). The force transmission way is to rely on the anchorage to prevent the elastic rebound of the steel bars, so that the cross-section concrete can obtain the preloading stress, as shown in Figure 6.2. This practice makes the steel bars and concrete become a whole, which is called "having".
Bonding prestressed concrete reduces the tensile stress of prestressed steel bars and the compressive stress of concrete due to the bonding force (resistance), so we should try to reduce this bonding. This method has simple equipment, no need for tensioning pedestal, flexible production and is suitable for on-site construction of large components.
② Unbonded prestressed concrete?
The main tensioning procedure is as follows: pre-stressed steel bars are coated with lubricating and anticorrosive materials such as asphalt along the whole length → wrapped with plastic paper or casing (there is no cohesive force between pre-stressed steel bars and concrete) → poured with concrete for curing → tensioned steel bars → anchored.
In construction, just like ordinary concrete, concrete can be poured directly when the steel bars are put into the design position, and there is no need to reserve holes, pierce the steel bars and grout, which simplifies the construction procedure. Because the effective preloading stress of unbonded prestressed concrete increases, the cost is reduced, and it is suitable for curved reinforced beams with long span.
There are five differences: also, whether the anchorage is reused or not, and the way of transmitting prestress: adhesive force and gripping force.