×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Significant change in new AS3600 shrinkage strain for high strength concrete - why ?

Significant change in new AS3600 shrinkage strain for high strength concrete - why ?

Significant change in new AS3600 shrinkage strain for high strength concrete - why ?

(OP)
Hi all,

I've noticed that in the 2018 code the 30 year shrinkage strain values for 80 and 100 MPa concrete mixes are significantly larger than those in the 2009 code. Referring to table 3.7.1.2, for th = 400 and Tropical environment, the 2018 code recommends 500 microstrains - nearly 50% greater than the 2009 code recommended 340 microstrains (the difference is actually higher since the base drying shrinkage used to make the table is smaller in the 2018 code than in 2009). This increase is exacerbated for smaller values of th.

I've done a back of the envelope comparison with CEB-FIP 2010 and while the 2009 code was in the same ballpark as FIP, the new one is quite off (image attached)


Can anyone advise on why there is such a large increase in shrinkage for the new standard ? I've looked through Gilbert's research online (I assume he was the driving force behind this ?) but haven't found anything to explain the change.

RE: Significant change in new AS3600 shrinkage strain for high strength concrete - why ?

There were serious problems with the old formulae. Boral research people and myself found the problem in 2015 when we were doing some work on some new reduced shrinkage and creep concrete products they were developing. The problem was with the combination of the drying and autogenous shrinkage formulae and the concrete strength term in the drying shrinkage formula.

Using the 2009 formulae, If you do the conversion from 56 day to 30 year for 100MPa concrete, it shows a 30 year basic shrinkage strain as a starting value of about 3300 for the test shrinkage being achieved, instead of the 800 defined in the code. Working the other way, based on the code defined Basic Shrinkage of 800, the formula gave 56 day shrinkage values for a shrinkage test of 290 for 100MPa concrete and Boral testing was giving about 700 for the test samples!

The 2009 formulae had never been checked for concrete strengths above 50Mpa as that was the maximum concrete strength when the code was first written. We found in 2015 that the formulae were wrong for anything above 50MPa concrete and the error increases as concrete strength increases.

We pointed this out to Ian Gilbert and the AS3600 committee in January 2016 and they modified the formulae to give more logic results. The new results match the results being achieved with Australian concretes based on a survey done in 2016 in conjunction with the development of the changes.

Unfortunately the change has not been made in AS5100 yet as I understand it.

I have not compared to other codes.

RE: Significant change in new AS3600 shrinkage strain for high strength concrete - why ?

(OP)
Thanks for the reply rapt. Quite concerning since I imagine it would affect most tall buildings built in the last 10 years. Then again, restraint from reinforcement may bring shortening down to more agreeable levels.

Quote (rapt)

The new results match the results being achieved with Australian concretes based on a survey done in 2016 in conjunction with the development of the changes.
You wouldn't happen to have this survey on hand would you?

RE: Significant change in new AS3600 shrinkage strain for high strength concrete - why ?

It was done by CCAAA for Standards at CIA and Ian Gilberts request, so it is a standards document. I do not think it was ever published. I think Ian Gilbert presented something on this at one of the CIA conferences, probably Adelaide in 2017. He might have given information on it then. It is also the basis for changing the Basic Shrinkage for all states in Australia to 800 in the 2018 code from the previous variable value depending on location as the latest figures showed little variation compared to 30 or so years ago when the last survey was done.

Creep was not affected and I would think it would be the real worry for tall buildings as it causes differential shortening between different supports (relatively lightly loaded concrete cores compared to highly loaded exterior columns under gravity loading.). I would be more worried about bridges where higher strength concrete is nearly always used and shrinkage is important and AS5100 is still wrong. Remember the error starts at about 50MPa concrete.

PS RAPT has had this close to right since Jan 2016!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close