Creep Shrinkage in Draft AS3600 1

[degenn]

The creep shrinkage properties in the new draft of AS3600 have everything happening much faster. For example Figure 6.1.8.2(A) creep 50% complete in 200 days for a 200 thickness where Figure 3.1.7.2 in the draft has 50% complete in 60 days. Does anyone know of data to support this.

It makes a big difference to the effectiveness of pour strips.

 

[asixth]

I haven't had the chance to look deeply into the differences between AS3600:2001 and DR05252, but I have been told that the new AS3600 will be out shortly and the Concrete Institue is organising full-day seminars on the changes between the old and new codes. I am also hesitant to start programming my spreadsheets to the draft code in-case there are any amendments before the final issue.

To be more specific to your question, I believe (not 100% sure) that the creep model in AS3600:2001 was based on some papers that were put out by Prof Gilbert from UNSW. The factor which was a function of time that gave the creep curve it's S-shape was:

k=t^0.7/(t^0.7+0.15*th)...AS3600:2001

where t is time (in days) and th is the hypothetical thickness of the cross-section.

The draft model follows:

k=t^0.8/(t^0.8+0.15*th)...DR05252

I just quickly put those equations in excel and found that old model took 113 days to achieve 50% of the long-term creep (30 years) while the draft model took 67 days to achieve 50% of the long-term creep. So things are happening faster with the draft model, I don't know what this draft model is based on but I can ask around (may not happen until these Concrete Institute Seminars kick off).

Also, I would wait for the new code to issued before using this model and I believe the BCA does give a 2 year clearance before AS3600:2009 is "Deemed-to-have-satisfied".

Good pick up, I will be mindful of this when the new code comes out.

 

[rapt]

degenn,

It was developed by Gilbert. I am seeing him later this week so I will ask (he is not answering his phone as he is probably out trying to get the CIA congreess organised).

But it is not creep that controls the pour strips timing, it is shrinkage.

The reason all of this had to change was because the old code could not be applied to higher strength concretes, especially for shrinkage. I have not checked the new equations against the old for lower strength concretes.

 

[rowingengineer]

has the hall marks of Gilbert all over it. Try searching for R.I. Gilbert and Creep.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[degenn]

The original post should have said creep and shrinkage.

The k1 equation (shrinkage) and k2 equation (creep) both use the same time scale.

The ACI document ACI 209.2R-08 give info on a number of creep and shrinkage models (GL2000 and CEB). They agree better with the existing code compared with the draft.

 

[hokie66]

Couldn't be that contractors have 'gotten to' the code writers in order to shorten construction time?

 

[rapt]

Hokie66,

They definitely did not "get at" me!
After the fights between the main committee and certain industry groups over many parts of the new code, I do not think anyone could think that any of those doing the main developemnt work on the code are open to influence, or we would have had a new code 3 years ago!!

Ian does not have time to compare them at the moment as he is one of the main organisers of the CIA congress starting Thursday. But does not think there is a big difference.

The formula is very similar to the Eurocode one except for 1 factor which is not a function of time. Compared to what I use in RAPT for AS3600, for the old version we used loadedperiod to a power of .7. In the new code it is to a power of .8, so there is a difference, but I do not think it is much as you suggest.

Looking at the table in the current code, for 200 thick with an interior environment, 30 year factor is about .75 and the 100 day factor is about .35, so the 100 day figure is pretty close to 50%.

 

[hokie66]

rapt,

Will await those results. I haven't done any numbers myself, but asixth is normally on the money, and degenn sounds like he knows what he is talking about.

Based purely on observation, I have thought that the existing estimates of shrinkage rates were too fast, so if they are being further accelerated, I for one won't be using them.

Someone on this site recently posted a plot of ultimate shrinkage vs age for various slab thicknesses. I can't remember the source, but will try to find it and reference it here. This plot showed about 12 months for 50% shrinkage of a 200 slab.

 

[asixth]

I wouldn't say I'm normally on the money, I still get them wrong more often than I get them right.

 

[hokie66]

Miecz found the graph I was remembering.

thread507-254286

 

[asixth]

Does anyone know the status of the new code. I heard that there are some factors that are still being debated and has pushed the release date back to January?

Not that I am keen to get a new code release, I think the '01 code is a relevant document. Despite the debate over development lengths, high-strength concrete and Clause 8.2.12.4.c.

 

[rapt]

asixth,

As far as I know nothing is being debated in the code committee. It is finalised as far as they are concerned.

I am not sure if there is any holdup with the Building Control Board. I will see if I can find out.

No matter when it is released, it willl not be referenced in BCA until at least 2011!

RE the existing code, any section that you now know to be unconservative (eg development lengths for small bars), you should no longer use (legally). You should use the new code rules in these areas. Plus the new code now defines rules for things not covered in the old code, like ties in columns with concrete strength greater than 50MPa which night be useful if anyone is actually designing a tallish building in the current economic climate.

 

[rowingengineer]

I have to get a new name or reduce my ego, when you write RE I always think you are using my initials.

It could be like the amendment to the wind code, the committee ticked this off more than three years ago i think, but for some reason it still hasn't been published.

yes there was a very good article in some magazine a while back about a designer from Germany that got done for not using the most up to date information for design from his knowledge bank. Given he was a part of the committee, but still worth the consideration.

I have been using the draft code for designing truss-ties models since it was published, since it was a lot better than the AS3600-2001

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rowingengineer]

CIA latest email "Concrete Institute of Australia has been advised by Colin Blair, Deputy Chief Executive, Standards Australia, that AS3600-2009 will be published by the end of October 2009"

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rapt]

AS3600 - 2009 is delayed apparently by ACBC who have to give a final approval. I have not been able to find out the reasons for the delay but will in the next week or so.

My contact has suggested it may not be out until the end of the year, so it still may end up being AS3600 - 2010!!

 

[rapt]

Dyslexia strikes again.

That should have been ABCB, Australian Building Codes Board!

 

[rowingengineer]

Rapt,
Since you have contacts in the right places, can you Please find out what they did with the wind code revision at the same time?





Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[asixth]

Are they revising the wind code?

 

[rowingengineer]

I just went and checked the SAI website and there is a draft for comment, thus I assume they decided to do away with t amendment and go to a complete revision. Maybe Woolcock will be able to publish his book revision to the portal frame book next year.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rapt]

RE and asixth,

Yes, there is a new draft of the wind code out for comment. They had extra changes they wanted to introduce eg wind pressures inside buildings such as on partitions etc, plus some others that they decided were too much for an amendment so it is going to be a new code release instead. So now you will have to pay for a new copy!

The draft is available fro download for those who want to see what has changed and possibly comment.