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Shrinkage Cracking - Minimum Reinforcement - Concrete Thickness

Shrinkage Cracking - Minimum Reinforcement - Concrete Thickness

Shrinkage Cracking - Minimum Reinforcement - Concrete Thickness

(OP)
To control cracking due to shrinkage, I understand there should be sufficient steel in the concrete such that when a crack occurs the reinforcement will not yield. On this basis, the minimum area of reinforcement increases as the concrete thickness increases, which is reflected in most codes of practice.

However, concrete is less prone to drying shrinkage cracking as the concrete thickness increases, because thick concrete members dry out more slowly than thin ones over the same drying period. This effect is represented in most codes by its theoretical thickness or hypothetical thickness. On this basis, then, if the concrete is less prone to drying shrinkage cracking as the concrete thickness increases, shouldn’t the minimum area of reinforcement be lower?

To me, it appears the requirement of providing minimum requirement is not based on how susceptible the concrete is to shrinkage cracking, but is rather based on simply ensuring the steel will not yield when and if a crack does occur. What do others make of this?

RE: Shrinkage Cracking - Minimum Reinforcement - Concrete Thickness

ACI has a separate document on pouring of "mass concrete". I looked into it at one point, but I don't remember it too much. My belief is it had totally different provisions for preventing cracking.

RE: Shrinkage Cracking - Minimum Reinforcement - Concrete Thickness

A few points

Yes, shrinkage reduces with thickness increase, but the stress caused by significant restraint to shrinkage will still be sufficient to crack the concrete so you need the minimum reinforcement.

The reinforcement is for "shrinkage and temperature". Temperature shortening does not reduce with depth of the whole depth reduces in temperature.

Some codes only require that the edge 200-250mm of concrete be used to calculate the reinforcement quantity. So the reinforcement is the same for a 500mm deep slab as a 1000mm deep slab.

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