Limiting concrete crack widths due to shrinkage

[gharli]

Dear All,

We are busy designing a large reinforced concrete structure that sits within a return water dam as part of an Ash Handling Facility. The retaining walls are thick (varies from 500 to 1400mm thick). The thickness is governed by a number of criteria of which I will not go into.

Part of the design requirement is to limit crack widths to 0.2mm. We are using the crack models in BS8007, yes I realize that this has been replaced with EN 1992-3 (also won't go into that).

The question is...

When calculating reinforcing areas for controlling cracks widths developed from temperature effects only (T1 and T2), can one place the required reinforcing in multiple layers within the "surface zone"? i.e, say we have a 700mm thick wall section, the surface zone on each side is 250mm thick, can we place 2 layers in the outer 250mm face/surface zone and an additional 2 layers in the inner 250mm face/surface zone to meet the reinforcing requirements?

Some guidance would be highly appreciated.

Thanks.

_________________
Jones & Wagener

http://www.jaws.co.za

 

[oldestguy]

It would seem that you also need to specify details about the concrete, such as water/cement ratio, curing, etc.

 

[gharli]

@oldestguy thanks for the reply. The question is unrelated to the concrete properties as it is of a general nature regarding placement of the reinforcing.

_________________
Jones & Wagener

http://www.jaws.co.za

 

[msquared48]

I think you could vary the distribution tiu describe if there was a constant temperature variation across the thickness of the sections at any point in time. Sections appear to be 20 to 54 inches in thickness,

Mike McCann, PE, SE (WA)

 

[KootK]

When calculating reinforcing areas for controlling cracks widths developed from temperature effects only (T1 and T2), can one place the required reinforcing in multiple layers within the "surface zone"? i.e, say we have a 700mm thick wall section, the surface zone on each side is 250mm thick, can we place 2 layers in the outer 250mm face/surface zone and an additional 2 layers in the inner 250mm face/surface zone to meet the reinforcing requirements?

I vote yes, especially if it's going to alleviate congestion issues.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

KootK's correct as well as OG (in particular). The concrete has a real impact on the shrinkage involved... including aggregate, cement type, fly ash, slump, and admixtures... Whatever reinforcing crosses the potential crack helps reduce the width. Can use Vandex additive or post applied to help.

Dik

 

[rapt]

The closer the reinforcement is to the exposed surface, the better crack control it will provide. Any reinforcement more than 70-100mm from the surface will have very little effect on the width of the cracks at the surface.

The 250 limit on calculating minimum reinforcement for shrinkage/temperature reinforcement for a surface in my opinion does not suggest that you put half of the reinforcement 200mm into the member. It just gives the area that you need to provide at the exposed surface.

Sop I would vote to provide it in one layer at minimum cover to the exposed surface.

If you look at the Eurocode crack width formula and the old BS8110 one (not sure if it is the same as the one in 8007), depth to the reinforcement is critical in the crack width calculations.

 

[gharli]

Thanks for the views guys. Just as a point, I'm not referring to cracking as a result of applied flexure/tension.

The formula given in BS8110 for addressing crack widths is only related to this phenomenon.

The forumals given in BS8007 are for checking cracking due to thermal and shrinkage strains as well as flexure. For thicker sections, the first case usually governs.






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Jones & Wagener

http://www.jaws.co.za