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How to Control the Prestress Loss of Concrete Members ?

Thomas Huang posted this article in June 30, 2020

In order to avoid the premature occurrence of cracks in reinforced concrete structures and make full use of its high-strength steel and high-strength concrete, the structural members can be preloaded before the load is applied to reduce or offset the tensile stress of the concrete caused by the load, thereby making the structure The tensile stress of the component is not large, even in a compressed state, so prestressed concrete is produced.
Factors causing loss of prestress
1. Construction equipment
Due to insufficient strength and rigidity of the pedestal and poor stability, the pedestal is deformed, and prestress losses are caused by overturning and slipping. Due to the poor self-locking and self-anchoring ability of the fixture itself, the strength and hardness of the tapered pin are less than the strength and hardness of the prestressed ribs, thereby causing the loss of prestress.
2. Concrete materials
Because the strength of the concrete is not high, the prestress is greater than the compressive stress of the concrete, which causes the concrete to be crushed, thereby causing the loss of the prestress. Because the concrete itself has the characteristics of shrinkage and creep, when the concrete shrinkage and creep is too large, it will cause the loss of prestress. Due to the improper size of coarse aggregate particle size, the loss of prestress is caused. The low strength of the steel bar (steel wire) causes the loss of prestress. The poor plasticity of the steel bar (steel wire) causes the loss of prestress. The insufficient surface roughness of the steel bar (steel wire) causes the loss of prestress.
3. Tension control stress
Tension control stress value directly affects the use effect of prestressed concrete. If the value of the tensile control stress is too low, the pre-stressed steel will cause too little pre-stress to the concrete after several losses, which cannot effectively improve the crack resistance and rigidity of the pre-stressed concrete member.
 4. Temperature difference
As the concrete is heated and cured, the temperature difference between the prestressed steel bar under tension and the equipment under tension causes prestress loss. Since the temperature of the steel bar and the pedestal are both t1 during tension, the maximum temperature during heating and curing of the concrete is t2. At this time, since the steel bar has not yet been bonded to the concrete, the temperature can be freely deformed in the concrete after rising from t1 to t2. When the temperature is stopped and maintained, the concrete has been bonded to the steel bar, and the steel bar and the concrete will simultaneously expand and contract together with the temperature change. The stress reduced due to the maintenance temperature rise can no longer be recovered, so the temperature difference stress loss is formed.
5. Time
Due to the creep and shrinkage of the concrete and the relaxation of the steel beam, the loss of prestress will occur over time. Calculate the deformation caused by creep and shrinkage in each construction stage considering the time dependence of concrete members. Then the calculated deformation is used to consider the loss effect of the tensile stress of the steel beam. At each stage, the calculation result of the prestress loss can be confirmed by a graph. When a tensile stress is applied to the steel beam to maintain a certain strain, the tensile stress acting on the steel beam gradually decreases with time. This phenomenon is called relaxation. The loss caused by relaxation varies with the magnitude of the initial stress applied, the time elapsed, and the nature of the product.
6. Elastic deformation of concrete
Due to the creep and shrinkage of the concrete and the relaxation of the steel beam, the loss of prestress will occur over time. Calculate the deformation caused by creep and shrinkage in each construction stage considering the time dependence of concrete members. Then the calculated deformation is used to consider the loss effect of the tensile stress of the steel beam. At each stage, the calculation result of the prestress loss can be confirmed by a graph. When a tensile stress is applied to the steel beam to maintain a certain strain, the tensile stress acting on the steel beam gradually decreases with time. This phenomenon is called relaxation. The loss caused by relaxation varies with the magnitude of the initial stress applied, the time elapsed, and the nature of the product.
7. Friction between the prestressed steel bar and the partition wall
Friction caused by curves. Friction caused by pipeline deviations. When tensioning curved steel bars, friction resistance is caused by the normal normal stress between the prestressed steel bars and the walls of the holes; some unevenness occurs in the construction of the reserved holes, which deviates from the design position. When tensioning the steel bars, the prestressed steel bars and the walls of the holes The normal normal stress between them causes frictional resistance.
Method to reduce prestress loss
1. Choose a high-strength pedestal
   Choose a platform with high strength, high rigidity and good stability, so as to reduce the deformation, slippage and overturning of the platform. Use fixtures with good self-anchor and self-anchoring capabilities, and at the same time make the cone pin stronger than the prestressed tendons. In the pretensioning method, as few backing plates as possible are used, because for each additional backing plate, the value of anchor deformation and steel bar shrinkage increases by 1 mm. Increase (as far as possible) the length of the pedestal as much as possible.
2. Use high-strength concrete
Because the high-strength concrete can improve the bonding force between the steel bar and the concrete to the member using the pretensioning method; it can increase the local bearing capacity of the anchoring end to the member using the posttensioning method. Use high-grade cement to reduce the amount of cement and reduce the water-cement ratio; use better graded aggregate to strengthen the vibration of concrete and improve the compactness of concrete, thereby reducing the shrinkage and creep of concrete.
3. Use coarse aggregate with large particle size and rough surface
Within the allowable range, use coarse aggregate with a larger particle size and a rough surface as much as possible to enhance the adhesion between concrete and steel. The size of the concrete prestress depends on the tension of the prestressed steel bar. When high strength steel bar (steel wire) is used, the steel bar (steel wire) will break due to insufficient strength due to over-tension, which will cause loss of prestress.
4. Adopt steel bars with good plasticity (steel wire)

Try to use steel bars with good plasticity (steel wire) to strengthen the tensile stress. When the tensile stress is removed, the shrinkage of the steel bar (steel wire) is related to the plasticity of the steel bar (steel wire). The plasticity is good and the shrinkage is large, which results in a large preload. In the pretensioning method, when high-strength steel bar (steel wire) is used, the surface should be scored or compressed. When ordinary steel bar is used, it is better to deform the steel bar to enhance the friction between the concrete and the steel bar (steel wire).
5. Construction
In order to reduce the prestress loss caused by stress relaxation and other reasons during construction, it is generally necessary to super-tension, first control the tensile stress to 1.05σ~1.1σ, continue the load for 2~5min and then remove the load, and then apply the tensile stress to σ At the same time, use hot rolled steel bars as much as possible, and use less carbon steel wire, thereby reducing the value of prestress loss due to stress relaxation. For longer components that can be stretched at both ends, the length of the hole can be calculated according to half the length of the component. Tension at both ends can reduce the loss caused by friction.
6. Adopt larger diameter ring members
In components with bends, the surface of the prestressed steel wire should be as smooth as possible, and grease can be applied when necessary, thereby reducing frictional resistance caused by the bend. When reserving holes, strictly follow the specifications to minimize the unevenness of the surface of the holes as much as possible, thereby reducing the loss of prestress. Due to the compression of the prestressed ribs on the concrete, the diameter of the ring member is reduced, and the tensile stress in the prestressed ribs will be reduced, resulting in loss of prestress. Therefore, ring members with larger diameters should be used as much as possible to reduce the prestress loss.

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