A simplified test method to estimate the relative rib area of rebar

Abstract

An alternative formula to the current standard is proposed for the calculation of the relative rib area of concrete reinforcing bar (rebar). The relative rib area is an important geometrical feature of rebar, as it facilitates distributed load transfer between concrete and reinforcement. The proposed formula and concomitant method are mathematically simple and easily implemented in practice. The proposed formula is based on the proposition that the projected outline of both the ribbed and un-ribbed rebar can be accurately approximated by elliptical curves, such that the net projected area of the ribs only is calculated as the difference between the areas of the two ellipses thus constructed. In contrast, the standard formula assumes a quadratic relationship between the rib height and the un-ribbed bar surface. Three available rebar specimens are evaluated against both formulae. The specimens are geometrically measured in three dimensions by a laser scanning system, the data of which are projected onto the plane perpendicular to the longitudinal axis of the rebar, and numerically integrated to calculate the actual projected area of the ribs. The standard formula accurately estimates the projected rib area in one out of three cases, while it overestimates (thus being nonconservative) for the balance. The proposed formula accurately estimates the projected area in two out of three cases, while it underestimates (thus being conservative) for the balance.