Using higher concrete mix than specified as per design. 1

[civilmate09]

What is the consequence of using higher concrete mix design than specified as per design.

Say 50MPa of concrete mix was used instead of 40MPa on bridge overpass crosshead. Engineer's intention is to removed the forms at an earlier days as specified.

I am working in Melbourne, Australia where the specs that we are using are based on VicRoads Specs. Please refer to attached file

Can you please provide comments and assessments whether the scenario is acceptable considering catching up on forecast program schedules. Thanks.

 

[hokie66]

Although not strictly in compliance with the 7 day clause for removal, I believe a good case has been made in this instance. If this were an element where early age deflection under self weight was a problem, my answer would be different, but bridge crossheads are typically very stiff, so I would lean toward allowing the early removal.

 

[civilmate09]

Thanks hokie66 for the comments.

One of our engineer's concern is that the increased cement content in the 50MPa mix will increase the risk of early thermal cracking in the massive pier due to higher heat of hydration during concrete setting and also due to higher shrinkage which will occur in the long term.

Cheers.

 

[hokie66]

We use a lot of 50Mpa concrete in buildings without much thought to those issues. In bridges, I don't know. The extra cement is a minor component of what causes shrinkage cracking.

 

[TXStructural]

That amount of excess is probably not much of an issue. If heat is an issue (for DEF formation), then that's a whole other issue that would need to be addressed by monitoring temps. If the least dimension is greater than 1.3 meters, mass concrete concerns should be examined.
Shrinkage will be an issue is the mix design does not consider it, but this is not purely a function is 28 day strength.