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Creep coefficient over time

Creep coefficient over time

Creep coefficient over time

(OP)
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.



RE: Creep coefficient over time

I think your top left diagram is showing the effect of age at loading the concrete. Creep is less if the load is applied to old concrete, but the reduction to zero at 100 days is overly optimistic.

RE: Creep coefficient over time

(OP)

Quote (steveh49)


Which is correct ? First or second ? In the first one, the creep coefficient decrease with time, the second one increase with time.....If the coefficient decrease with time, effective modulus will be increasing with time, it seems doesnt make sense

RE: Creep coefficient over time

Creep always increases with time. The diagram showing it reducing is because it is showing final creep of concrete that is more mature when loaded. If you load concrete one day after pouring it, the creep after a year will be massive. If you load ten year old concrete, the creep after one year will be much less.

RE: Creep coefficient over time

I see now, it's a nomogram. The creep doesn't go down to zero.

RE: Creep coefficient over time

Your second graph is the ageing coefficient used in the age-adjusted effective modulus method. Neither of the graphs you posted is actually creep vs time. Creep vs time looks like this:

RE: Creep coefficient over time

(OP)

Quote (steveh49)


Do you mean the first graph ( from EUrocode) present the creep of concrete that is loaded at different age (creep of concrete will be less when the concrete is older, concrete loaded after sometime ) ? While the second graph showed that the creep of concrete over time (concrete loaded at t =0 ) ?? Am I right ?

Thus, (in second graph, as the time progress, creep will be higher? ) (In first graph, for older concrete, the creep will less noticeable than fresh concrete ? )

RE: Creep coefficient over time

Quote:

Do you mean the first graph ( from EUrocode) present the creep of concrete that is loaded at different age (creep of concrete will be less when the concrete is older, concrete loaded after sometime ) ?

That's correct for the final value of the creep, but you have to follow the five steps to use the eurocode nomogram. You can't just read directly from the graph on the left. Creep vs time looks like my Figure 2.2.


The second graph (from Bazant) is the ageing coefficient, not the creep coefficient. They're different things but the shape is similar to creep vs time. I think you should ignore the Bazant graph for the moment as it's a more advanced topic in creep. It's used to modify your equation 6.8 to improve accuracy of creep calculations:



Eurocode equation B.7 is what you are looking for.

RE: Creep coefficient over time

What is equation B.7? Can someone post it? Thanks...

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

-Dik

RE: Creep coefficient over time

Eurocode:




Australia:

RE: Creep coefficient over time

Thanks steveh... a new little SMath program...

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

-Dik

RE: Creep coefficient over time

For the first both for any comments (I'm not quite sure of what I'm doing). I don't know what alpha1 and alpha2 are for. When the bugz are out, I'll post the SMath program.



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

-Dik

RE: Creep coefficient over time

Alpha1 and Alpha2 are use to calculated the nominal creep. They are used in Eq. B.3a and B.3b.



Edit: Obviously only used in B.3b, but in that set of equations :)

RE: Creep coefficient over time

thanks Manitou... will update

Updated... can anyone provide information about what the numbers mean? I well aware of the hazards of using numbers you don't understand. I'm faintly curious about creep and I was happy to see some quantitative formulae. I've arbitrarily modified the units to 'work' and SMath treats percents as a decimal number, and shouldn't need to divide it by 100.



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

-Dik

RE: Creep coefficient over time

Correction... forgot to change the alphaprimed2



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

-Dik

RE: Creep coefficient over time

steveh: can you re-post the link; I cannot get it to download.

Thanks... there wasn't much trouble...

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

-Dik

RE: Creep coefficient over time


Thanks, got it... not to read and maybe try to understand... again, thanks. I'll post the final SMath when it's completed and checked. The reason I wrote it is that I like 'funny' formulae and I find SMath great for them.

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

-Dik

RE: Creep coefficient over time

Is there information about the values in the below formula, in particular epsiloncc and sigma0? Is there also an article that expains the methodology? I've nearly completed the formulae (as a challenge to myself) and I don't have a feeling or understanding of what I'm calculating.

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

-Dik

RE: Creep coefficient over time

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

RE: Creep coefficient over time

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).

RE: Creep coefficient over time

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

RE: Creep coefficient over time

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.

RE: Creep coefficient over time

Thanks

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

-Dik

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