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Contact: Li Yong Tel.: 13853181576 (WeChat account) http://www.sdyzty.com/list/?1_1.html In ordinary reinforced concrete tension or bending members, because the tensile strength and ultimate tensile strain of concrete are very low, even if the concrete is not subject to external constraints when the volume changes, there have been microcracks in the concrete.
Under the action of service load, it usually works with cracks.
After the final setting of concrete, cracks caused by deformation (such as temperature change, shrinkage, expansion, uneven settlement, cracks caused by external load, cracks caused by chemical action, etc.) occur with the passage of time.
The root cause of such cracks is that the volume change of concrete is constrained and excessive tensile stress (or tensile strain) is caused inside the concrete.
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.
Factors causing prestress loss 01 The prestress loss caused by construction equipment is due to insufficient strength and stiffness of the pedestal and poor stability, which leads to deformation of the pedestal, overturning and sliding, 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.
02 Loss of prestress caused by concrete materials.
Because the strength of concrete is not high, the prestress is greater than the compressive stress of concrete, resulting in the concrete being crushed, resulting in 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).
03 Prestress loss caused by 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.
04 The prestress loss caused by temperature difference is 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.
05 The loss caused by time will lead to prestress loss as time goes by due to the creep and shrinkage of concrete and the relaxation of steel strands.
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.
06 The loss caused by the elastic deformation of the concrete exerts prestress on the concrete, and the concrete is compressed, and its length becomes smaller.
In this way, the length of the steel tendon anchored to the concrete will become smaller, and the tensile stress of the steel tendon will also become smaller.
Such losses caused by elastic deformation occur in both pre tensioning and post tensioning methods, but their shapes are slightly different.
When the pretensioning method is used for construction, the elastic shrinkage of the steel tendon will occur at the moment when the tensioning force is applied to the component, and the length will become shorter, thus causing prestress loss.
The post tensioning method is different from the pre tensioning method.
It has no special fixed support, but uses the solidified concrete as the support to tension the steel tendon.
In this way, the shrinkage of concrete members is the same as that of the pretensioning method, except that the tension measurement of steel tendons is conducted after the elastic shrinkage of concrete members, so there will be no tension loss caused by the elastic deformation of concrete.
07 Prestress loss caused by friction between prestressed reinforcement and duct wall Friction caused by bend.
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 the reserved duct, 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 duct wall causes friction.
The method to reduce the prestress loss is to select a 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..
