Well, for one creep is what concrete does and continues over a long period of time as long as the stress exists. Relaxation, OTOH, is what steel does, occurs fairly quickly and there is a limiting strain value (the strain does not continue to increase indefinitely).
@hetgen, I guess it's possible that the additional 1% strain in the steel in the period between 10 hours and 3 years could be considered creep of the steel, but what is used for prestressed design (and calculated as steel relaxation for design) is the 1% that occurs in the first few hours.
I haven't seen the research data that went into developing the equations for deflections due to creep, so I don't know if they include the effect of the steel 'creep' shown in the graph or not. It's likely that they do.
Steel creep is a known issue at high temperatures, comes up in boilers and stuff like that. I've not heard of that being an issue at room temperatures.
Just scanning through the first few paragraphs of this document, they mention the distinction in the two:
Creep in steel strands I think is due to the mechanical slipping of the twisted wires in an individual strand over time - not a creep in the actual steel material itself.
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It is not just slipping of the twisted wires (it is actually worse for prestres bar due to the different manufacturing process!). It is also much worse in normal reinforcing steel, which is why the early attempts at prestressing in the early 1900's using normal reinforcing bars failed and didn ot work until prestress steel was developed. Even then, the strand was Normal Relaxation and had about 3 times the relaxation that modern (since mid 1970's) low relaxation strand has.
And it does happen at normal temperatures but increases significantly at higher temperatures. Australian code has a temperature factor for relaxation of T/20 where T is in degrees C. The basic test is at 20C.
There is no relaxation at stress below about 40% UTS and it increases fairly linearly up to about 70-80% UTS depending on PT steel type and then increase much more rapidly (3 or more times the rate as stress increases).
rapt - is there some measure or knowledge of what percent of "creep" is in the material of a strand (material creep) vs. strand slipping?
Your post reminded me of an investigation I once did on a hanging walkway - a heavy steel truss and precast panel walkway that was suspended from a steel structure above at midspan with two steel 7 wire strand cables. The walkway had apparently sagged downward right at the hanger strands. No evidence of connection slippage at either end and no evidence of structural deflection from above. A local precast prof was called on and he indicated there was no way that the strand could have crept over time (it was a low lax strand). We never totally concluded the cause- simply added additional support under the walkway as a safety response.
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I have never seen any estimate of the wire slipping effect in the strand. It would be interesting to see a comparison of a wire relaxation test versus a strand test to see if there is a difference from a wire slipping effect. As I said above, bar has worse relaxation and it has no possibility of a wire/strand slipping effect.
Design code rules to allow for relaxation include a reduction due to creep and shrinkage which would not have happened in your case. You would have had the full intrinsic relaxation which is not 0 for LR.
Typical pure relaxation for NR and LR strand are shown on page 84 in
Even with LR (group 2),relaxation is 10% at a stress of 80% UTS. It would all depend on the strand stress. Both in terms of strand stretching under stress and then relaxation increasing this. If the stresses were high in the strand, strand extension could have gone non-linear plus the relaxation effect really increases at high stress levels.
