Why Do Reinforced Concrete Structures Need Prestressing?
Reinforced concrete structures work with cracks under normal working conditions. In order to avoid the premature expansion of cracks in reinforced concrete structures, we should make full use of the tensile properties of steel bars and the compressive properties of concrete materials. We should try to reduce the tensile stress of the components by applying external force before the concrete structure or components are subjected to the service load, or even put them in a compressive stress state. The prestressed state thus generated is used to reduce or offset the tensile stress caused by the external load, that is, to make up for the lack of tensile strength using the higher compressive strength of concrete, so as to delay the cracking of concrete in the tensile zone. Structures made of prestressed concrete are also called prestressed reinforced concrete structures because they achieve prestress by tensioning steel bars.
Advantages and Disadvantages of Prestressed Concrete Structures
Advantages
(1) Good crack resistance and high rigidity. The application of prestress to the components greatly delays the appearance of cracks. Under the action of the service load, the components may not crack or the cracks may be delayed, thus improving the rigidity of the components and increasing the durability of the structure.
(2) Save materials and reduce deadweight. Since its structure must use high-strength materials, it can reduce the number of steel bars and the cross-sectional dimensions of the components, save steel and concrete, and reduce the self-weight of the structure. It has obvious advantages for large-span and heavy-load structures.
(3) It can reduce the vertical shear force and principal tensile stress of the concrete beam. The curved steel bars (bundles) of the prestressed concrete beam can reduce the vertical shear force near the support in the beam, and because of the presence of prestress on the concrete cross-section, the principal tensile stress under the load is also reduced. This is conducive to reducing the web thickness of the beam so that the self-weight of the prestressed concrete beam can be further reduced.
(4) Improve the stability of the compressive member. When the length-to-slenderness ratio of the compressive member is large, it is easy to bend after being subjected to a certain pressure, resulting in loss of stability and destruction. If prestress is applied to the reinforced concrete column, the longitudinal force-bearing steel bars are stretched very tightly; not only are the prestressed steel bars themselves not easy to bend, but also the surrounding concrete can be helped to improve its ability to resist bending.
(5) Improve the fatigue resistance of the member. Because the stress change caused by loading or unloading during the use of strong prestressed steel bars is relatively small, the fatigue strength can be improved, which is very beneficial for structures subjected to dynamic loads.
(6) Prestressing can be used as a means of connecting structural components to promote the development of new systems and construction methods for large-span structures.
Disadvantages
(1) The process is relatively complex and has high-quality requirements, so a skilled professional team is required.
(2) Certain special equipment is required, such as tensioning machinery and grouting equipment. The pre-tensioning method requires a tensioning pedestal; the post-tensioning method also consumes a large number of anchors of reliable quality.
(3) The construction cost of prestressed concrete structures is relatively high, and the cost of projects with a small number of components is relatively high.
(4) The prestressing camber is difficult to control. It increases with the increase of concrete creep, causing the bridge deck to be uneven.
(5) Under high-temperature conditions, the strength of reinforced concrete after prestressing will decrease significantly, resulting in a decrease in its fire resistance limit, so there are safety hazards in building fire protection.
What Do Pre-Tensioning and Post-Tensioning Refer to? How Are They Constructed?
A prestressed concrete structure is a structure that pre-stresses concrete before loads are applied. Prestress is generated by tensioning high-strength steel bars or wires. There are two tensioning methods:
(1) Pre-tensioning. That is, the steel bars are tensioned first, and then concrete is poured. When the concrete reaches the specified strength, the ends of the steel bars are relaxed. Before the concrete is poured, the prestressed tendons, composed of steel wire strands or steel bars, are tensioned to a certain specified stress and anchored to the piers at both ends of the pedestal with anchors. Then, the formwork, structural steel bars, and parts are installed, and the concrete is poured and cured. When the concrete reaches the specified strength, the prestressed tendons at the piers at both ends are relaxed, and the tension in the prestressed tendons is transferred to the concrete through the bonding force, generating pre-compression stress. The pre-tensioning method is more advantageous when using a long pedestal, and the longest one can be more than 100 meters, so it is sometimes also called the long-line method.
(2) Post-tensioning method. That is, pour concrete first, and when it reaches the specified strength, tension the steel bars through the reserved holes in the concrete and anchor them at both ends. The general practice is to first install the casing, structural steel bars, and parts for the post-tensioned prestressed tendons, and then install the formwork and pour concrete. The prestressed tendons can be inserted into the casing first or later. After the concrete reaches the required strength, the prestressed tendons are tensioned to the required stress with a jack and anchored at both ends of the beam. The prestressing stress is transmitted to the concrete of the component through the anchors at both ends. In order to protect the prestressed tendons from corrosion and restore the bonding force between the prestressed tendons and the concrete, the gap between the prestressed tendons and the casing must be filled with cement slurry. In addition to its anti-corrosion effect, cement slurry is also conducive to restoring the bonding force between the prestressed tendons and concrete. In order to facilitate construction, sometimes non-bonded post-tensioned prestressing can be used, in which anti-corrosion materials are applied to the surface of the prestressed tendons and wrapped with plastic casings or oil paper.
