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Contact person: Li Yong Tel.: 13853181576 (same wechat number) http://www.sdyzty.com/list/?1_1.html In ordinary reinforced concrete members under tension or bending, because the tensile strength and ultimate tensile strain of the concrete are very low, even if the concrete is not constrained by the external when the volume changes, there are micro cracks in the concrete.
Under the action of service load, it usually works with cracks.
Due to the cracks caused by deformation (such as temperature change, shrinkage and expansion; uneven settlement; cracks caused by external load; cracks caused by chemical action, etc.) that occur with the passage of time after the final setting of concrete, the root cause of such cracks is that the volume change of concrete is restrained and excessive tensile stress (or tensile strain) is caused inside.
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, it is possible to make the structural members produce preload before they bear the load to reduce or offset the tensile stress of concrete caused by the load, so that the tensile stress of the structural members is not large, or even in the compression state, thus producing prestressed concrete.
Factors causing prestressing loss 01 prestressing loss caused by construction equipment due to insufficient strength and rigidity of the pedestal and poor stability, leading to deformation, overturning and sliding of the pedestal and prestressing 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.
02 loss of prestress caused by concrete materials due to the low strength of concrete, the prestress 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).
03 prestress loss due to 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 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.
04 prestress loss due to temperature difference prestress loss due to temperature difference between tensioned prestressed reinforcement and equipment bearing tension during concrete heating and curing.
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.
05 the loss caused by time is due to the creep and shrinkage of concrete and the relaxation of steel tendons, and the loss of prestress will occur with the passage of time.
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.
06 the loss caused by elastic deformation of concrete exerts prestress on the concrete, and the concrete is compressed, and its length becomes small.
In this way, the length of the tendon anchored to the concrete will also be reduced, and the tensile stress of the tendon will also be reduced.
Such losses caused by elastic deformation occur in both the pretension method and the post tensioning method, but their shapes are slightly different.
When the pretensioning method is used for construction, when the tensioning force is applied to the component, the steel tendon will undergo elastic contraction, and the length will become shorter, thus resulting in prestress loss.
The post tensioning method is different from the pre tensioning method.
It does not have a special fixed support, but uses the solidified concrete as the support to tension the steel bundle.
In this way, the shrinkage phenomenon of the concrete member is the same as that of the pretensioning method, except that the measurement of the tension of the tendon is carried out after the elastic shrinkage of the concrete member, so there is no loss of tension caused by the elastic deformation of the concrete.
07 prestress loss caused by friction between prestressed reinforcement and tunnel partition wall friction caused by bend.
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 method to reduce the prestress loss is to select the pedestal with high strength, high rigidity and good stability, so as 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..
