AS3600 concrete elastic modulus

[flin8812]

Hi all,
I'd like to know how elastic modulus for 32MPa of various curing day concretes can be determined
e.g. 7days/ 14 days/ 28 days/ 30yrs

Is it okay to assume that because creep and shrinkage effects are so miniscule in 7 , 14 and 28 day concretes,
modulus of elasticity are taken as:
(rho^1.5) x (0.043x SQRT(fcmi)) for fcmi <40MPa
(rho^1.5) x (0.024x SQRT(fcmi)+0.12) for fcmi > 40MPa

Whilst for 30yr concretes, since creep and shrinkage must be accounted for, one must use coefficient of shrinkage to find
Ec.

Plus,could anyone englighten me on matters associated with the behaviour of 7day,14day and 30yr concrete cylinder before/after they're cracked and how the the choice of concrete may influence the calculations of Ix or Ec and etc etc. Anything relevant would be good.

cheers

 

[cooperDBM]

AS3600 is the Australian concrete code, not a Eurocode.

The modulus of elasticity is the material stiffness of the concrete. It varies with concrete strength growth but not by shrinkage, creep, or cracking. The 2nd moment of inertia, I, can be calculated for either uncracked or cracked conditions. The inertia would also account for different composite materials (different E) by transforming them to a base E, and would be time dependent as the E of the concrete varies with strength growth. Deflections can then be calculated from E and I. Long term growth of those deflections can then be calculated to account for creep. Differential shrinkage and creep in composite beams should also be accounted for, particularly for deflections.

 

[rapt]

flin8812
I do not understand how shrinkage is going to affect Ec. There is no relationship between them. For deflection calculations, it is possible to allow for creep by modifying the Ec for a long term value but not shrinkage.

Also creep and shrinkage are not miniscule at 28 days. About 50% of shrinkage and creep would occur over the first 28 days.

 

[asixth]

I know of a few Australian design engineers who are using a model proposed by Prof. Mendis from one of the Melbourne universities for E-modulus. From what I am led to believe it has approximatley a 10% increase from the values in AS3600 and correlates more closely to the ACI model.

Not sure I would be prepared to use it for a flexural member like a slab or beam but I would be using it for shortening calculations.

 

[rapt]

I do not know why they would be using something outside the code unless they do their own testing. The AS3600 committee looked at the available models and decided that the one in As3600-2009 most closely matches local Australian concretes. That is why they used it!