decompression 2

[marblaze]

Eurocode 2 speaks of decompression with regards to prestressed concrete design.

Can anyone explain to me what decompression means. many thanks

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[parrot77]

Decompression refers to the loss of compression stress in a prestressed concrete member. This may by from relaxation of strand, elastic shortening of concrete, creep, shrinkage etc

 

[marblaze]

I am particularly refering to pargraph 3 of section 7.2 in Eurocode 2:

. . . Under severe exposure conditions when DECOMPRESSION is required, the design proces tends to be dominated by the serviceability requirements of cracking, rather than by ultimate strength as in reinforce concrete.

hope you can explain further in conjucntion with the above.

thanks

 

[cooperDBM]

With prestressing you are compressing the concrete around the tendon. You can think of the concrete going from zero stress to an effective (after losses) compressive prestress. Decompression is simply the loss of that compression (or prestress) due to an increase in the load on a member (on the flexural tension side of the section) back to a concrete stress of zero. If the load continues to increase after full decompression then the concrete goes into tension, potentially resulting in cracking.

The decompression stress in the tendon is the increase in tension from the point of effective prestress to the point where the stress in the concrete is zero under increasing load. If you need to determine the decompressed tensile stress in a tendon as part of a cracking check, or for a strain compatibility analysis, it's equal to the final stress minus the effective stress and minus the decompression stress. In other words the tensile stress added to the tendon after the concrete around the tendon has returned to zero. This decompressed tendon stress would be proportional to the concrete strain and similar to the stress in any surrounding rebar.

Your partial quote is a bit out of context so I'm not sure of it's complete meaning other than that serviceability requirements often govern over strength for prestressed concrete. What is section 7.2 about?

 

[rapt]

barblaze,

In EC2, Decompression is the point at which the stress in the concrete is 0. In other words, the tendon cannot be in the tension zone under the loading nominated in the code for the particular cracking condition you are investigating.