by In order to avoid early occurrence of cracks in reinforced concrete structures, and make full use of its high-strength steel bars and high-strength concrete, it is possible to make structural members produce pre pressure to reduce or offset the concrete tensile stress caused by the load before they bear the load, so that the tensile stress of structural members is not large, or even in the compression state, thus producing prestressed concrete.
Part I Factors causing prestress loss 1.
Due to insufficient strength and stiffness of the pedestal and poor stability of the construction equipment, the pedestal will deform, overturn and slip, causing 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 those of the prestressed tendon, which causes the loss of prestress.
2.
Due to the low strength of concrete, the pre pressure 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 itself has the characteristics of shrinkage and creep, the loss of prestress will be caused when the concrete shrinkage and creep are too large.
The loss of prestress is caused by 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.
The loss of prestress caused by insufficient surface roughness of reinforcement (steel wire).
3.
Tensioning control stress The value of tensioning control stress directly affects the use effect of prestressed concrete.
If the value of tension control stress is too low, the pre compression stress produced by prestressed reinforcement on concrete after several losses is too small, which can not effectively improve the crack resistance and stiffness of prestressed concrete members.
4.
Temperature difference refers to the prestress loss caused by the temperature difference between the prestressed reinforcement under tension and the equipment under tension when the concrete is heated and cured.
Since the temperature of the reinforcement and 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 increased from t1 to t2 and then it can deform freely in the concrete.
When the temperature is stopped for curing, the concrete has been bonded with the reinforcement, and the reinforcement and concrete will simultaneously expand and contract with the temperature change.
The reduced stress due to curing temperature rise is irrecoverable, so the temperature difference stress loss is formed.
5.
Due to the creep and shrinkage of concrete and the relaxation of steel tendons, the loss of prestress will occur as time goes by.
The time dependence of concrete members shall be considered in each construction stage to calculate the deformation caused by creep and shrinkage.
Then the calculated deformation is used to consider the loss effect of tensile stress of steel tendon.
At each stage, the calculation results of prestress loss can be confirmed through charts.
When the tension stress is applied to the steel tendon to maintain a certain strain, the tension stress acting on the steel tendon gradually decreases with time, which is called relaxation.
The loss caused by relaxation varies with the size of the initial stress applied, the time experienced, and the nature of the product.
6.
Due to the creep and shrinkage of concrete and the relaxation of steel tendons, the elastic deformation of concrete will cause the loss of prestress over time.
The time dependence of concrete members shall be considered in each construction stage to calculate the deformation caused by creep and shrinkage.
Then the calculated deformation is used to consider the loss effect of tensile stress of steel tendon.
At each stage, the calculation results of prestress loss can be confirmed through charts.
When the tension stress is applied to the steel tendon to maintain a certain strain, the tension stress acting on the steel tendon gradually decreases with time, which is called relaxation.
The loss caused by relaxation varies with the size of the initial stress applied, the time experienced, and the nature of the product.
7.
Friction caused by friction bend between prestressed reinforcement and duct wall.
Friction caused by pipe deviation.
When tensioning curved reinforcement, friction resistance is caused by normal normal stress between prestressed reinforcement and duct wall; During the construction of reserved holes, some of them are uneven and deviated from the design position.
When the reinforcement is tensioned, the normal normal stress between the prestressed reinforcement and the hole wall causes friction.
Part II: Methods to reduce the prestress loss.
1.
Select the pedestal with high strength.
Select the pedestal with high strength, high stiffness and good stability, so as to reduce the deformation, sliding and overturning of the pedestal.
Clamp with good self and self anchoring capacity shall be used, and the hardness of taper pin shall be greater than that of 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 anchorage deformation and reinforcement shrinkage by 1mm.
Increase the length of pedestal as much as possible (if conditions permit).
2.
The concrete with high strength is selected because the concrete with high strength can improve the bond strength between reinforcement and concrete for the members with pretensioning method; The local bearing capacity of the anchored end can be improved for the members adopting the 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 and improve the compactness of concrete, so as to reduce the shrinkage and creep of concrete.
3.
Coarse aggregate with large particle size and rough surface shall be used within the allowable range.
Coarse aggregate with large particle size and rough surface shall be used as far as possible to enhance the bonding force between concrete and reinforcement.
The size of concrete prestress depends on the tensioning 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 tensioning, resulting in prestress loss.
4.
The steel bar (steel wire) with good plasticity shall be used as far as possible to enhance 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, resulting in large preload..
