Specifying Concrete Shrinkage
Specifying Concrete Shrinkage
(OP)
I work in Australia and one practice that confuses me, is the specification of 56 day shrinkage microstrain on concrete on the drawings (specification notes). I am interested to know how these shrinkage values are derived, what are appropriate shrinkage strains for different applications etc. I have tried to look up the AS3600 commentary but there is no commentary available for the latest version (2009) of the code, so can anyone provide advice or point me to a reference which discusses these points?
Thanks.
Thanks.






RE: Specifying Concrete Shrinkage
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Specifying Concrete Shrinkage
I'm just looking over some of the specifications from the previous consultants that I have worked at and found a 450 microstrain specified for columns and walls in multi-storey buildings.
RE: Specifying Concrete Shrinkage
The important point in all of this, explained in the Commentary mentioned above, is that the 56 day shrinkage value from the tests is not equivalent to the codes "final drying basic shrinkage strain" with default values of 800 to 1000 micro strain. The code values are 30 year values. The 56 day test value needs to be converted to a 30 year value as shown in an example the new Commentary. 650 will probably really be closer to 800-850 and 750 closer to 950 as a starting value for design.
So, if you are using RAPT for design, you do not input the 650 value in the materials properties, you need to convert it to a 30 year value. We are adding a converter function to do this for you.
RE: Specifying Concrete Shrinkage
I was also interested in why the shrinkage values would be less for columns than for beams or slabs.
Do we approach creep values in the same way as shrinkage i.e. get testing done to identify creep which verifies the creep values determined in the design? I personally haven't seen this approach in specifications, but it does seem to follow the same logic as shrinkage.
Both creep and shrinkage do seem important as they effect the long term behaviour for deflection checks and do inform reinforced concrete detailing particularly at restraints and changes in geometry of concrete members.
RE: Specifying Concrete Shrinkage
I think the materials properties written into AS3600 for shrinkage, creep, E-modulus etc. were all based on materials relevant to the Australian suppliers and what get produced in the industry.
RE: Specifying Concrete Shrinkage
Creep tests can be performed. If you want to. Boral has done this with its new Envisia concrete in Sydney. But most additive suppliers simply test shrinkage and do not worry about creep. Not sure why, other than that consultants probably only ask for control over shrinkage properties.
RE: Specifying Concrete Shrinkage
1) Shrinkage tests are easier/cheaper
2) No-one else does it
3) Creep is seen as being less variable than shrinkage.
I don't know that 3) is true. Certainly Bazant doesn't seem to think so.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Specifying Concrete Shrinkage
RE: Specifying Concrete Shrinkage
RE: Specifying Concrete Shrinkage
Since 2009, the code in clause 3.1.7.1(b) has specifically said that the design shrinkage strain can be determined from tests after 8 weeks of drying "modified for long term value".
RE: Specifying Concrete Shrinkage
I have appreciated this discussion and it does give me some direction in considering shrinkage and creep in my design and especially reinforces the point that if you request a certain shrinkage limit, you should understand why and know how it effects the structure you have designed.
RE: Specifying Concrete Shrinkage
When designing using L/D ratios and simplified deflection acceptance criteria, there is no way to account for shrinkage directly. But consultants will often nominate a shrinkage value that is lower than the code default value so that they have a little up their sleeve.
The misconception in the industry has been that the drying basic shrinkage strain is the 56 day test value. That has been addressed in the current version of the code and the commentary. The problem is getting industry in general to understand this. The 2001 code did not specify the difference, just left it to engineers to figure out that the basic shrinkage should be a 30 year value, not the 56 day test value. That is why the wording in the code was changed.
Many designers using RAPT do define the shrinkage in RAPT to correspond to what they are specifying on the drawings. Many PT companies specify lower shrinkage concrete and allow for it in design as do many of the larger consultancies with RC concrete. The problem is to get them to define the correct value, the 30 year estimate from the 56 day test value! That is why I am changing the RAPT input to do the conversion to 30 year value from a nominated 56 day test value for them.
RE: Specifying Concrete Shrinkage
Or even to know that the commentary has been published!
I forget where I found out that it had been published, but it certainly wasn't from SAI Global. From my brief examination so far it seems to be an excellent document, and certainly not one that should be ignored.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Specifying Concrete Shrinkage
RE: Specifying Concrete Shrinkage
http://www.concreteinstitute.com.au/Australian-Sta...
This gives you on-line access to all Australian Standards for $220/year (for Institute individual members). You can download pdfs of any standard, but these expire after a short time (2 hours I think). There is no limit to how many times you connect or download though.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Specifying Concrete Shrinkage
RE: Specifying Concrete Shrinkage
https://newtonexcelbach.wordpress.com/2015/03/21/g...
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Specifying Concrete Shrinkage