AS3600 Section 6.1 Concrete Properties

[asixth]

Hi guys,

Does anyone know where I can find equations for the creep co-efficients that are given in section 6.1 of AS3600. I am putting together a design spreadsheet for prestressed bridge girders and I want to be able to calculate additional hogging that occurs with time due to creep.

I am using Queensland Department of Main Roads standard deck unit designs as my base. Typically, they experience a 10% gain in hogging after 28 days and 25% additional hogging after 100 days.

Typically when these units are stressed they stay in the yard for several months so they are subject to the same loading at transfer as they are at 100+ days except for some losses of prestress.

Also, I did notice that the QDMR use 26000MPa as the Young's modulus for high early strength concrete (f'cp=40MPa). This is significantly less than the calculations for Young's modulus that is given in AS3600 Section 6. Does anyone know how Young's modulus is calculated for high early strength concrete.

Any help would be appreciated.

 

[apsix]

There is some background information in the Commentary, as well as references.
Quoting from the Commentary; "the curves have been drawn from a plot of calculated point values, there being no simple governing equation...".

 

[rapt]

asixth,

The code values also do not apply to steam cured concrete!

 

[asixth]

Thanks guys,

I found a creep model in Eurocode 2 which is the same as the model in CEB-FIP 1990 which is what I think is referenced in the code. Also the rapt user manual has equations that are a best estimate of the curves that are given in AS3600. I am going to proceed with the spreadsheet using the Eurocode model as it also has allowance for different strength gain concrete. The concrete that are used in these members is 40MPa after 24 hours.

I was visiting a precast yard west of Brisbane and they throw a sheet of the members after they have been poured but I don't know whether this would classify as steam curing because they are not steaming it as such. However, my visit was on a 35 degree day, procedures may change a little now that we are in the cooler months.

 

[csd72]

I would suggest that you talk to your local concrete manufacturer as these vary from region to region, mix to mix.

 

[rapt]

asixth,

If you are getting 40MPa in 24 hours without steam curing, none of the creep, shrinkage , strength gain, Concrete Modulus gain methods in codes will give realistic results. You would need test results over time for that specific material.

 

[asixth]

Thanks everyone for the input. I was reading through the PCI bridge design manual section regarding camber and deflections and it also doesn't recommend using code projections for Youngs modulus for the concrete that they use in these precast prestressed members, so I will continue using QLD Dept. Main Roads assumption of 26,000MPa for the concrete Youngs Modulus at release.

The code says that any realistic model for creep and shrinkage can be assumed. I guess this is because of the large variability that will occur with any model that is selected. I think I will continue using the Eurocode or ACI model because it allows me to take into account the members are loaded very early, where AS3600 makes the assumption that loading occurs at 28 days.