by In order to avoid the premature occurrence of cracks in reinforced concrete structures and make full use of its high-strength reinforcement and high-strength concrete, the prestress of the structural members can be generated to reduce or offset the tensile stress of the concrete caused by the load before the structural members bear the load, so that the tensile stress of the structural members is not large, and even in the compression state, thus the prestressed concrete is produced.
The first part is the factors causing prestress loss.
1.
The construction equipment has poor stability due to insufficient strength and rigidity of the pedestal, which leads to deformation, overturning and sliding of the pedestal, resulting in prestress loss.
Due to the poor self-locking and self anchoring capacity of the clamp itself, the strength and hardness of the taper pin are less than the strength and hardness of the prestressed tendon, thus causing the loss of prestress.
2.
Due to the low strength of concrete, the pre stress of concrete material is greater than the compressive stress of concrete, resulting in the concrete being crushed, thus causing the loss of prestress.
Because concrete has the characteristics of shrinkage and creep, the loss of prestress will be caused when the shrinkage and creep of concrete are too large.
The loss of prestress is caused by the improper size of coarse aggregate.
The low strength of reinforcement (steel wire) causes the loss of prestress.
The poor plasticity of reinforcement (steel wire) causes the loss of prestress.
Loss of prestress caused by insufficient surface roughness of reinforcement (steel wire).
3.
Tension control stress the value of tension control stress directly affects the use effect of prestressed concrete.
If the value of tension control stress is too low, the prestressing stress produced by the prestressed reinforcement on the concrete after several losses is too small, which can not effectively improve the crack resistance and stiffness of the prestressed concrete member.
4.
Temperature difference: the prestress loss caused by the temperature difference between the tensioned prestressed reinforcement and the equipment bearing the tension when the concrete is heated and cured.
Since the temperature of the reinforcement and the pedestal during tensioning is the same as T1, the maximum temperature of the concrete during heating and curing is T2.
At this time, since the reinforcement has not been bonded with the concrete, the temperature can be freely deformed in the concrete after rising from T1 to T2.
When the temperature is stopped for curing, the concrete has been bonded with the reinforcement, and the reinforcement and the concrete will expand and contract together with the temperature change.
The stress reduced by the curing temperature rise can not be recovered, so the temperature difference stress loss is formed.
5.
The elastic deformation of concrete will lead to the loss of prestress with the passage of time due to the creep and shrinkage of concrete and the relaxation of steel tendons.
In each construction stage, the time dependence of concrete members is considered to calculate the deformation caused by creep and shrinkage.
Then the calculated deformation is used to consider the loss effect of the tensile stress of the tendon.
At each stage, the calculation results of prestress loss can be confirmed by charts.
When the tensile stress is applied to the steel bundle to maintain a certain strain, the tensile stress acting on the steel bundle gradually decreases with the passage of 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 properties of the product.
6.
Friction caused by friction curve between prestressed reinforcement and duct partition wall, and friction caused by pipe deviation.
When the curved reinforcement is tensioned, the friction resistance is caused by the normal stress between the prestressed reinforcement and the duct wall; During the construction of reserved duct, some of them are uneven and deviate from the design position.
When the reinforcement is tensioned, the normal stress between the prestressed reinforcement and the duct wall will cause friction resistance.
The second part is the method to reduce the prestress loss.
1.
Select the pedestal with high strength, high rigidity and good stability to reduce the deformation, sliding and overturning of the pedestal.
Use clamps with good self anchoring and self anchoring capacity, and make the strong hardness of the taper pin greater than that of the prestressed reinforcement.
In the pre tensioning method, the base plate shall be used as little as possible, because each additional base plate will increase the deformation of anchorage and the internal shrinkage of reinforcement by 1mm.
Increase the length of the pedestal as much as possible (if conditions permit).
2.
Select high-strength concrete because high-strength concrete can improve the bonding force between reinforcement and concrete for the members with pretensioning method; The local bearing capacity of the anchorage end can be improved for the members with post tensioning method.
Adopt high-grade cement to reduce cement consumption and water cement ratio; The well graded aggregate shall be used to strengthen the vibration of concrete, improve the compactness of concrete, and reduce the shrinkage and creep of concrete.
3.
The coarse aggregate with large particle size and rough surface shall be used within the allowable range.
The coarse aggregate with large particle size and rough surface shall be used as much as possible, so as to enhance the bonding force between concrete and reinforcement.
The prestress of concrete depends on the tension pressure of prestressed reinforcement.
When high-strength reinforcement (steel wire) is used, the reinforcement (steel wire) will break due to insufficient strength due to over tension, thus causing prestress loss.
4.
The steel bar (steel wire) with good plasticity shall be used as much as possible to enhance the tensile stress.
After 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, resulting in large pre stress.
In pretensioned members, when high-strength reinforcement (steel wire) is used, the surface shall be nicked or pressed.
When ordinary reinforcement is used, it is better to use Deformed Reinforcement to enhance the friction between concrete and reinforcement (steel wire).
5.
In order to reduce the loss of prestress caused by stress relaxation and other reasons during construction, generally, over tensioning shall be carried out, and the tensioning stress shall be controlled to be 1.05 σ~ one point one σ， Continue the load for 2 ~ 5min, then unload the load, and then apply the tensile stress to σ， At the same time, hot-rolled steel bars shall be used as much as possible, and carbon steel wires shall be used less, so as to reduce the prestress loss caused by stress relaxation..
