Slab reinforcement for shrinkage / temperature? 1

[Kyle_1]

Hi everyone.
For one way slab, do we need reinforcement for shrinkage / temperature purpose in secondary direction? And if so, do they need to be placed throughout the slab? and top & bottom? I am designing to AS3600.
And for primary direction, since the top reo are used at where supports are only, do I still need top reo in primary direction at mid-span for shrinkage / temperature purpose?
Thank you.

 

[hokie66]

Yes, you need shrinkage reinforcement perpendicular to the span. You can count both top and bottom reinforcement.

In the primary direction, the flexural steel in most cases will suffice also as shrinkage reinforcement. Bottom only is fine.

There are many situations where, due to restraint conditions, shrinkage reinforcement requirements will control.

 

[structuralCADspecialist]

In my local region they usually omit temperature reinforcement for top flexural reinforcing. But a local engineering association which conducts post-university courses (very good ones at that) - the instructor was indicating that top temperature reinforcing was a good design practice. That is: Anywhere flexural reinforcing occurs it should also have temperature bars traversing it. Top flexural reinforcing usually comes with at least two carry bars (for example, perimeter hooks) which would assist in satisfying any temperature reinforcing requirement. I rarely see it in the structural details however.

 

[Enable]

The answer as hokie notes is yes. The specific code you are designing to should have minimum steel requirements to help satisfy this end.

Take a look at this Ph.D Thesis out of Ryerson (it's mostly about walls unfortunately but the same logic applies). In section 7.3 various code requirements are compared (including AS3600) in terms of minimum reinforcement provisions vs crack width tolerances.

Your detailing should also reflect coatings / coverings (e.g. if an elastomeric coating is to be placed overtop of your slab crack widths are less of a concern). You'll likely want to add additional bars at rather restrained areas (corners, around columns, etc).

 

[rapt]

Section 9.5.3 answers your first question. Why is there any doubt about that bit?

The reinforcement is normally placed on the face that is in tension in the primary direction and serves as the transverse support bars for the primary reinforcement. For thicker slabs, over about 300 - 400mm, it should probably be placed in both faces.

 

[Tomfh]

What’s the basis for 0.6% strong crack control? Is there marginal benefit beyond that? I’ve seen some designs for continuous concrete pavements with greater than 1%, which has always made me wonder if 0.6% is really all that much.

 

[dik]

It's lots... 3x normal temperature reinforcing... still get the shrinkage cracking but it's more dispersed and finer...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

Tomfh, this is from the Concrete Institute of Australia's document Z7/06. There's strong and then there's stronger.

 

[Tomfh]

Thanks steveh49.

Any idea why AS3600 doesn’t have “even stronger”?

 

[rapt]

Tom,

Because they are minimums. For any situation and specific level of restraint and crack control you require, you are supposed to calculate what is required for those conditions.

To achieve .2mm crack width with full restraint, you will probably require 1%.

 

[Reo-Checka]

It's also worth noting that the qualitative language of 'minor', 'moderate' and 'strong' rather than quantitative intervals reflects the uncertainty in the underlying theory.

ReoChecka

https://reochecka.com.au/

 

[1503-44]

I was taught to distribute temperature steel as much as is practical, as top of slab may be warmer or cooler than the bottom, depending on season, so put some in both (if thickness warrants a separation).

 

[JoshPlumSE]

The reinforcement is normally placed on the face that is in tension in the primary direction and serves as the transverse support bars for the primary reinforcement. For thicker slabs, over about 300 - 400mm, it should probably be placed in both faces.

I was going to say essentially the same thing. But, I was going to point out that I prefer to continue a portion of the flexural reinforcement past where it is required for flexure. Not just for development, but for temp/Shrinkage crack control. This doesn't have to go full length for thinner slabs. Same thing with bottom reinforcement. I often extend some of the bars a good bit past where they are required to help with development and temp/shrinkage. It's a little conservative, but not all that much more expensive.

 

[1503-44]

For sure. If it cracks from shrinkage or temperature stress, basically mission failure was achieved right there and its difficult to fix.