Crack Widths - AS3600-2018

[Drapes]

In the new Australian concrete code AS3600-2018, under section 8.6.2.3- crack control by calculation of crack widths, the crack inducing strain calculation (i.e. mean strain in reo minus mean strain in concrete) appears to incorrectly include the long term shrinkage strain.

For end restraint conditions, which this calculation is presumably based on, crack widths are independent on the restrained strain, so I don't believe the shrinkage strain is required. The extent of shrinkage strain will only dictate the number of cracks, not the actual crack widths. In addition, the Eurocode calculation does not include this shrinkage strain value as part of its assessment on crack widths.

Am I missing something here, or is this an oversight?

 

[rapt]

You are missing something.

Concrete shrinks. Reinforcement does not. The concrete between the cracks shrinks away from the cracks resulting in a wider crack.

 

[Drapes]

From the Technical Report No. 67 - Movement, restraint and cracking in concrete structures, the following is stated:

"When a slab is restrained at each end, when a crack forms it opens to its design value then, when the force in the reo at the crack increases sufficiently, another crack is formed. This continues with the number of cracks being related to the total movement that has taken place. The max crack width is unaffected by the overall movement, and thus crack width calculations that relate the crack width to the restrained strain are flawed."

Do you disagree with this?

 

[rapt]

It has nothing to do with what I disagree with. AS3600 disagrees with this!

Ian Gilbert's work on this shows that shrinkage does contribute. He has presented several papers on this over the last 10+ years in CIA seminars and in papers presented at conferences and in ACI Journals.

 

[IDS]

Without getting into a debate about the correctness of the Concrete Society publication, the crack width calculation in AS 3600 is for flexural cracking, where the restraint comes from the reinforcement, not from end restraint.

I think there is no doubt that once crack spacing has reached its minimum value, shrinkage does increase crack width.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/