prestressed concrete

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Prestressed Concrete is a kind of concrete component, which is mainly used in construction engineering, Bridge works Medium.

Chinese name: prestressed concrete
Foreign name: Prestressed Concrete

Discipline classification: civil engineering
application area: architectural engineering , Bridge works
Role: Improve crack resistance and rigidity of components

catalog

five Material requirements

Background

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prestressed concrete

for fear of reinforced concrete structure The cracks in the structure appear prematurely. Make full use of high-strength reinforcement and high-strength concrete, and try to reduce the tensile stress generated by the component by applying external force before the concrete structure or component bears the service load Compressive stress The concrete component in the state.^[1]

working principle

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The pre compression stress is used to reduce or offset the concrete tensile stress caused by the load, so as to Structural member Of Tensile stress Control in a small range, even under pressure, to delay Concrete crack So as to improve the crack resistance and stiffness of the component.

classification

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According to the pre stressing value crack control Different classifications of degree:

one Fully prestressed concrete （ FPC ， fully prestressed concrete^[1] ）

Under the action of service load, members that do not allow tensile stress in the concrete on the section are strictly required not to appear cracks, and the section size of prestressed members and prestressed beams are strictly controlled deflection 。

two Partially prestressed concrete （PPC， partially prestressed concrete）

Cracks are allowed, but maximum crack width Not more than Allowable value The component of is allowed to crack.

3. Unbonded prestressed reinforcement

take Prestressed reinforcement Of Outer surface Apply with asphalt, grease or other lubrication Rust proof material , to decrease friction And rust proof, and use Plastic sleeve Or paper tape, Plastic tape Wrapping to prevent the coating from being damaged during construction and to isolate it from surrounding concrete, and post tensioned prestressed reinforcement that can slip longitudinally during tensioning.

Features: there is no need to reserve holes or grout, the construction is simple and fast, the cost is low, and it is easy to promote and apply.

Advantages and disadvantages

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advantage

1、 Crack resistance Good, high stiffness. Due to the application of prestress , which greatly delays the appearance of cracks. Under the action of service load, the members may not appear cracks, or the cracks may be delayed, so the stiffness of the members is improved, and the Durability 。

2. Save materials and reduce dead weight. Its structure must adopt High strength material Therefore, it can reduce the amount of reinforcement and component section size, save steel and concrete, and reduce Structure dead weight , Yes Large span And heavy load structure has obvious advantages.

3. Can reduce the vertical direction of concrete beam Shear And the main Tensile stress 。 Prestressed beam The curved reinforcement (bundle) of concrete beam can reduce the vertical shear force near the support in the beam; And because of the existence of prestress on the concrete section, the main tensile stress under the load will also be reduced. This is conducive to reducing the Web Thickness, so that the dead weight of prestressed concrete beams can be further reduced.

4. Improve the stability of compression members. When the slenderness ratio of compression member Large time After being subjected to certain pressure, it is easy to be bent, resulting in loss of stability and destruction. If yes reinforced concrete Prestress the column to make Longitudinal load-bearing reinforcement The tension is very tight, not only Prestressed reinforcement It is not easy to bend, and can help the surrounding concrete to improve its resistance to bending.

5. Improve the fatigue resistance of components. Because of the stress caused by the loading or unloading of the reinforcement with strong prestress in the use stage Change range Relatively small, so it can be improved Fatigue strength , this pair bears Live load The structure is very favorable.

6. Prestress can be used as Structural member Connecting means to promote Long-span structure New system and Construction method Development of

shortcoming

1. The process is relatively complex quality requirement High, so a skilled professional team is needed^[2] 。

2. Certain special equipment is required, such as tensioning machines and grouting equipment. Pretension method Tensioning is required Pedestal ； Posttensioning method It also needs more and reliable quality Anchorage Etc.

3、 Prestressed concrete structure The starting cost is relatively high, and the number of components is small engineering cost Higher.

4、 Prestressed inverted arch The degree is not easy to control. It follows Concrete creep The bridge deck is not smooth due to the increase of.

5. Under high temperature reinforced concrete The strength will decrease significantly, resulting in Fire endurance Therefore, there are potential safety hazards in building fire protection.

Material requirements

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yes Prestressed reinforcement Some requirements of:

(1) The strength should be high. The tensile stress of prestressed reinforcement will cause various stress losses during the whole fabrication and use of members. The sum of these losses can sometimes reach more than 200N/mm2. If the strength of the reinforcement used is not high, the stress established during tension will even be lost.

(2) And concrete Cohesion 。 Especially in Pretension method The prestressed reinforcement and concrete must have a high bond anchor strength. For some high-strength Smooth surface The steel wire will go through "nicking", "pressing wave" or "twisting" to form nicked steel wire, corrugated steel wire and twisted steel wire to increase the bonding force.

(3) It shall have sufficient plasticity and good processing performance. The higher the strength of steel, the lower its plasticity. When the plasticity of reinforcement is too low, especially under the condition of low temperature and impact load, it may occur Brittle fracture 。 Good processing performance means Welding performance Good, and adopt the heading Anchor plate When the rebar head is upset, the original mechanical property Etc.

Commonly used prestressed reinforcement in China includes Cold drawing Grade III reinforcement, cold drawn Grade IV reinforcement, cold drawn Ligature Rib reinforcement Heat treated rebar 、 High strength steel wire Etc.

Some requirements for concrete in prestressed concrete:

(1) The strength shall be high, and shall be compatible with the high-strength reinforcement, so as to ensure that the prestressed reinforcement can give full play to its role, and can effectively reduce the size of the member section and reduce the dead weight.

(2) Shrinkage creep Smaller to reduce prestress Loss.

(3) Fast hardening, early strengthening, Enable Apply prestress as early as possible to speed up Construction progress , improve Equipment utilization 。

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