Rebar is a critical component of modern reinforced concrete. Concrete is generally very strong in compression but not in tension so steel reinforcing is required to take the tensile load. The physics of the mechanical bond that transfers load between the concrete and steel is very complex and is very dependent on the pattern of deformations. Today these deformations are very standardized, but that wasn’t always the case. If you search Google Books for ‘reinforced concrete’ you’ll find books from the early 1900’s that show all the different options that were available at the time. The below images are from: Turneaure, F. E., Maurer, E. R. (1907). Principles of Reinforced Concrete Construction. United States: J. Wiley & Sons.