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[This question is about constructing techniques in Brazil, so if anything is not clear or not properly worded, please let me know and/or suggest phrasing improvements]

I am building the foundations for the railings which will separate my plot of land from my neighbour. The foundations will be built with concrete blocks and reinforced with metal bars. The standard rebar size to be used is 3/8". However, I have some leftover 1/4" bars from my house which I would like to use.

So here's my thought process: up to some constant, the cross section area covered by a 3/8" bar is 9/64 (~0.14) square inches, whereas for a 1/4" bar it is 1/16=4/64 square inches. Two 1/4" bars would cover 8/64, and three bars would cover 12/64 square inches, which is more than the one 3/8" bar would.

Is my thought process correct? Similarly, could I use one 1/4 and one 5/16" bars (covering ~0.16 square inches) instead of the one 3/8" bar?

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  • For what you want, basically a fence instead of something like a building, it should not matter. Besides the diameter/sq. in difference, a 3/8 rebar is stronger than a 1/4 rebar(or three 1/4 inch rebar).
    – crip659
    Commented Feb 8 at 0:55
  • Seems that this would be determined by your local building codes. What does your local building department/building inspector have to say about it?
    – FreeMan
    Commented Feb 8 at 12:53
  • @crip659 what's your reasoning for one 3/8" being stronger than three 1/4"?
    – Huesmann
    Commented Feb 8 at 13:24

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Your reasoning is sound. Spread them out evenly, though. Don't bundle them together in a blob. The rebar's bending strength is assumed to be zero for reinforced concrete design, so the bar diameters don't matter. Steel areas are used in design (Av for shear reinforcement, As for flexural tension reinforcement, etc.). In the US, below ground rebar gets covered with a minimum of 3" of concrete between the rebar and soil.

Not in your subgrade case, but for a beam that's supported in the air, too much reinforcement can be a liability. The extra area from the 3 #2 bars could be a problem in such a case. Loading a concrete beam up to failure, you want the lower rebar to yield and stretch before the beam's upper concrete explodes. If you add a bunch of additional reinforcement to a beam designed for this stretching, the concrete may explode before the rebar stretches at all. The stretching rebar gives people an opportunity to move to safety, whereas the case without stretching is considered brittle and gets penalized in the design code.

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