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Creep and relaxation

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creep is a time-dependent deformation due to  constant load. we discuss before the elastic and plastic behavior of the materials under loading and unloading. here we will discuss the effect of loading materials for a period of time. in figure no:1 the bar is loaded with a force P. at time of loading (to) the elongation equal δ0.  if the applied load on bar maintained for a long time. the strain of the material will increase gradually without any increase in applied load as shown in figure 1. the gradual increase in strain for a permanent or constant load known as creep. 
In prestressing concrete creep cause losing of prestressing force. therefore it is calculated and added to the jacking force to compensate for losses. also, creep can create undulation of bridge deck. this can be adjusted by adjusting deck level upward in an amount equal to expected creep so when creep occurred the level of deck will be in required position.
Figure-1

Relaxation of materials is another example of creep. let consider the wire shown in figure no:2. the wire is stretched between two immovable supports. at the time  of stretching  (to) stress equal σ0 . the stress on the  wire will decrease gradually with time until it becomes constant as shown in figure 2. this increase will occur without changing the support location.
in prestressing concrete. relaxation of prestressing strands will cause a loss of prestressing force and reduce the efficiency of prestressing. therefore the used strands in prestressing concrete is treated to reduce the relaxation

Figure-2




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