It has been my understanding that the requirement of placing rebar at least 3″ from the outside edges of concrete is to protect from moisture, preventing rust that will damage the concrete from within.
There is now rebar composed of non-ferrous materials like fiberglass and carbon fiber. Since those don’t have a rust problem, could that change the 3″ rule?
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Replies
Why would you want it closer?
The wonderful thing that makes steel reinforced concrete so good is that the coefficient of thermal expansion of steel and concrete is almost the same. So as temperatures fluctuate reinforced concrete is not torn apart from within.
There are quite a few requirements for rebar spacing in concrete, both between rebar parts, and from them to an external surface.
It does make sense that spacing requirements motivated by corrosion resistance would not be needed for non metallic versions, but other changes could need to be considered, based on the specific properties of the products considered.
In addition to compatible thermal expansion properties, the great thing about reinforced concrete is the complementary nature of the strength profiles. Concrete is stronger in compression than in tension, reinforcing is selected for the opposite property.
For a footing, it might be that use of composite rebar could allow a different geometry for the footing, balancing mix properties, reinforcement details, and the loads imposed between the structure and the soils. Engineering analysis would be needed for such consideration.
I'd also avoid having the rebar, even if it wont rust, 3 inches away from the edges, because then vibrations will be more likely to cause visible cracks.
What vibrations?