Reinforcement bars are circular to ensure even stress distribution and avoid weak points. This shape improves bonding with concrete and prevents slippage. Circular bars are easier to manufacture and bend without cracking. They also avoid stress concentration at corners. Overall, the circular design enhances strength, durability, and construction efficiency.

Steel bars inside concrete are round (not square) to ensure uniform stress distribution, better bonding with concrete, easier handling, and improved strength due to a higher shape factor.

This is due to several important engineering and material science considerations.

First, one of the primary reasons is the superior bond strength that round bars offer with the surrounding concrete. The circular shape allows for a more uniform and effective mechanical interlock, especially when combined with surface deformations like ribs or ridges. This promotes better load transfer between the steel and concrete, minimizing the risk of slippage or localized failures. In contrast, square bars with sharp corners would create uneven stress distribution, leading to potential weak points where cracks can initiate and propagate.

Second, round cross-sections are more efficient at handling stress concentrations. The absence of sharp edges ensures that tensile and compressive forces are distributed evenly across the bar’s surface, reducing the likelihood of premature failure.

Third, round bars are more economical and practical to manufacture, easier to bend into desired shapes without weakening the structure, and less prone to corrosion since they lack corners where moisture might accumulate.

For additional information, square bars have been used historically up to about 1970 approximately. both plain square bars and square twisted bars. Square twisted were more common. In the UK the contractor was free to use whatever was economical provide the area of steel was greater than that specified by the designer. Have only seen plain square bars on one site, with poor compaction around the square bars in the bottom of the beam leading to large areas of spalling when the steel started to corrode.