What is the practicality of prestressed concrete?

1. The application of prestress can delay the cracking of concrete in the tension zone, thereby improving the stiffness of the component; 2. The prestress effect can produce deformation opposite to the external load on the component, and the above two points can reduce the total deformation of the component, thereby reducing the cross-section of the component, reducing the self-weight of the structure and improving the applicable span.

It should be noted that prestressing does not improve the ultimate bearing capacity of the component, its main function is to reduce the total deformation of the structure to increase the span and reduce the self-weight of the structure.

The standard value of tensile strength of prestressed tendons is generally 1000 2000MPa, much higher than that of ordinary steel bars (less than 500MPa, and now there is a new type of cold-rolled ribbed steel bar strength standard value of up to 600MPa, but it is still in the test stage), and the amount of steel used in the structure can be reduced under the same bearing capacity conditions, thereby reducing the cost and saving resources.

In addition, prestressing can make the concrete not easy or not cracking, thus playing a role in protecting the steel bars from rust and improving the service life of the structure.

The prestressed structure has good seismic performance, although it is still slightly worse than the ordinary concrete structure, but it can fully guarantee safety through reasonable design.

Finally, compared with ordinary concrete components, the fire resistance of prestressed concrete components is relatively poor, the main reason is that the cracks of prestressed concrete structures are smaller, and under the action of high temperature, the water vapor in the concrete is not easy to emit out, which causes bursting, so that the steel bars are exposed to direct fire, and when the temperature of the steel bars exceeds 400 degrees, the strength will drop sharply, resulting in premature damage to the components.

First, the floor height can be reduced;

Second, the large space and large span can improve the use function;

the third is to improve the overall stiffness of the structure;

Fourth, reduce the amount of materials.

This technique has been successfully applied to the 2008 Olympic Games National Stadium, shooting hall and judo taekwondo competition hall, etc.

Prestressed concrete structures will be strong and strong, and have excellent seismic resistance.

Meet the structural use and durability.

Prestressed concrete is to make up for the phenomenon of premature cracks in concrete, before the use (loading) of the stool piece, give a pre-pressure to the concrete in advance, that is, in the tensile area of the concrete, the steel bar is stretched by the method of artificial afterburner, and the retraction force of the steel bar is used to make the concrete tensile area under pressure in advance.

Advantages: 1. Good crack resistance and high stiffness. Because the prestress is applied to the component, the appearance of the crack is greatly delayed, and under the action of the use load, the crack can not appear, or the crack is delayed, so the stiffness of the component is improved, and the durability of the structure is increased.

2. Save materials and reduce dead weight. Because the structure must be made of high-strength materials, it can reduce the amount of steel bars and the cross-sectional size of the components, save steel and concrete, reduce the weight of the structure, and have obvious advantages over large-span and heavy-load structures.

3. The vertical shear force of the concrete beam can be reduced.

and principal tensile stress. The curvilinear reinforcement (bundle) of the concrete beam of the prestressed beam can reduce the vertical shear force near the support in the beam; Due to the existence of prestress on the concrete section, the main 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 compressive components. When the compression member is relatively slender, it is easy to be bent after being subjected to a certain pressure, resulting in loss of stability and destruction. If a prestress is applied to a reinforced concrete column, the longitudinal stress reinforcement is applied.

Zhang Lasen is very tight, not only prestressed steel bars.

It is not easy to bend, and it can help the surrounding concrete to improve its ability to resist bending.

5. Improve the fatigue resistance of components. Because the reinforcement with strong prestress, the stress change caused by loading or unloading in the use stage is relatively small, so this mold can improve the fatigue strength, which is very beneficial for the structure bearing 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 more complex and has high quality requirements, so it needs to be equipped with a skilled professional team.

2. There is a need for certain special equipment, such as tensioning machines, grouting equipment, etc. First tension method.

It is necessary to have a tension pedestal; The post-tensioning method also requires a large number of anchors of reliable quality.

Wait. 3. Prestressed concrete structure.

The start-up cost is larger, and the project cost of the small number of components is higher.

4. The prestressed reverse camber is not easy to control. It increases with the increase of concrete creep, resulting in an uneven bridge deck.

1.Good crack resistance: the crack resistance of prestressed concrete components is much higher than that of ordinary reinforced concrete components, which can delay the emergence of cracks, reduce the deformation of components, improve the durability of the structure, and expand the scope of application of concrete structures.

2.Save materials and reduce the weight of the component: the use of prestressed concrete components, due to the improvement of the stiffness of the components, can reduce the cross-sectional size of the components, save the amount of steel and concrete, reduce the weight of the structure, especially for long-span structures.

3.It can make full use of high-strength materials: because the prestressed steel bar has a certain tensile stress before being subjected to external load, and the concrete is subjected to high pre-compressive stress, the material strength of high-strength steel bar and concrete can be fully utilized.

4.It can improve the shear resistance of the component: the test shows that the longitudinal prestressed steel bar plays the role of anchor bolt, which can prevent the appearance and development of oblique cracks, so as to improve the shear resistance of the component.

5.Improve the fatigue resistance of the component: in the prestressed structure, due to the prior tension of the reinforcement, under the action of reciprocating moving load, the stress of the reinforcement does not change much (less than 10%), and will not cause fatigue of the reinforcement.

The disadvantage of prestressed concrete components is that the process is more complex, and the quality requirements are high, so it is necessary to be equipped with a more skilled professional team, and there is a need for certain special equipment, such as tensile machinery, grouting equipment, etc., and the starting cost of prestressed concrete structure is larger, and the engineering cost of less component quantity is higher.

Prestressed concrete elements have the following main advantages:

Good crack resistance and high stiffness.

Save materials and reduce the weight of the structure.

Improve the shear resistance of the member.

Improve the fatigue resistance of components.

Improve the stability of compressive members.

The application range of reinforced concrete structures has been expanded.

But everything is relative, and prestressed concrete structures have their own limitations:

The production process is complex and has high requirements for construction quality.

It is necessary to have certain special equipment, such as tensioning machines, grouting equipment, etc.

The start-up cost of prestressed concrete structure is larger, and the cost of engineering with a small number of components is higher.

The concrete used in prestressed concrete structural components needs to meet the following requirements: (1) high strength. This is because the prestressed concrete is subjected to higher precompressive stresses from the prestressed tendons.

In pre-tensioning components, high-strength concrete can improve the bond strength between steel bars and concrete. For post-tensioned members, high-strength concrete can improve the local compressive bearing capacity of the anchor end.

2) Shrinkage and creep are smaller. It can reduce the prestress loss caused by concrete shrinkage and creep, and establish a high precompressive stress.

3) Fast hardness, early strengthening. Prestressing can be applied as early as possible to improve the turnover rate of tensioning equipment and speed up construction. The Code for the Design of Concrete Structures (2015 Edition) (GB50010-2010) stipulates that the concrete strength grade of prestressed concrete structures should not be lower than C40 and should not be lower than C30.