Creep coefficient over time 1

[dccd]

I came across 2 creep coefficient diagram. First one is from Eurocode, second one is from elsewhere. I think the second one make sense, As the time progress, the creep coefficient shall increase, hence, the effetive modulus of concrete shall decrese. Can someone explain it ? I am confused.

 

[dik]

This is the current 'bunch of numbers'... I can/will post the SMath file once I get the numbers worked out... (I started out thinking this was a complicated no-brainer).


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

Hi Dik,

sigma_0 is the sustained stress.
epsilon_cc is the creep strain due to the sustained stress.

sigma_0 / E_c is the instantaneous strain due to the applied stress. Creep strain is a multiple of the instantaneous strain. The total long-term strain due to the load is the instantaneous strain plus the creep strain. If the creep coefficient is 1.0, the total long term strain is (1+1) = double the short term; if creep = 1.5, the total long-term strain is (1+1.5) = 2.5*short term.

Where is gets interesting is when you add in shrinkage (which isn't load dependent) and the effect of restraint (eg steel doesn't creep or shrink so resists these effects in the concrete). The steel tends to take load away from the concrete, meaning the concrete stress isn't constant even if the applied load is. This effect is what the age-adjusted effective modulus method tries to address (one of the graphs in the original post was for the ageing coefficient).

 

[dik]

Thanks steveh... is there an article that describes the formulae? I can plug numbers in all day long, but I don't have a inkling of what I'm doing... There's a little more to creep than what I understood... In the 'good old days' I used to apply factors to the deflection based on material, loading, etc. and had a confidence of about 50% that they were correct... this seems a tad overkill. It's almost like masonry where use use standard deviation and then arbitrarily apply a safety factor of 5... just can't win for losin'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

The article referred to in the Australian code commentary doesn't go into details of where the equations came from. Probably just curve fitting to test data.

The factors account for:

k2 - creep vs time, with thin elements more subject to creep than thick sections (more exposed surface area).

k3 - concrete maturity. Young concrete creeps more than mature concrete (compounded because the Young's modulus is also lower, so you're multiplying by a larger instantaneous strain).

k4 - humidity in service vs the standardised test conditions.

k5 - accounts for the low water-cement ratio in high-strength concrete, which affects the response to humidity and the thickness hence k4 and alpha_2 make another appearance.

k6 - creep strain vs instantaneous strain becomes non-linear at high stress.


I'm not so familiar with Eurocode but presume some/all of the same effects are accounted for in its various factors.

 

[dik]

Thanks

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik