Limiting concrete crack widths due to shrinkage
Limiting concrete crack widths due to shrinkage
(OP)
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.
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
www.jaws.co.za






RE: Limiting concrete crack widths due to shrinkage
RE: Limiting concrete crack widths due to shrinkage
_________________
Jones & Wagener
www.jaws.co.za
RE: Limiting concrete crack widths due to shrinkage
Mike McCann, PE, SE (WA)
RE: Limiting concrete crack widths due to shrinkage
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.
RE: Limiting concrete crack widths due to shrinkage
Dik
RE: Limiting concrete crack widths due to shrinkage
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.
RE: Limiting concrete crack widths due to shrinkage
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.
_________________
Jones & Wagener
www.jaws.co.za