Discontinuous yielding in bare and embedded reinforcing bars: Implications on the determination of steel and bond shear stresses from strain measurements

Abstract

Distinct jumps from the yield strain to the strain at the onset of hardening occur in bare reinforcing bar exhibiting a yield plateau once the yield limit is reached. These strain discountinuities known as Lüders bands nucleate at single locations and travel along the bar axis at a constant load. In contrast, no such strain jumps were observed in recent experiments with such reinforcing bars embedded in concrete and instrumented using distributed fibre optical strain sensing (DFOS). Instead, the strain increased gradually at locations where the yield limit was exceeded. This observation, made possible by the high sensing resolution of DFOS, cannot be explained entirely by the known effect of tension stiffening. While bond stresses cause a variation of steel stresses along the bar axis and, hence, indeed prevent Lüders bands from propagating along embedded bars, distinct strain jumps would still be expected at the locations where the yield stress is reached. The different steel stress–strain behaviour is rather attributed to the three-dimensional stress state induced by the locally concentrated bond forces at the bar ribs. These forces presumably (i) cause the direction of maximum shear stress to vary over the bar cross-section, leading to a loss of the macroscopic yield phenomena, and (ii) reduce the nominal yield stress in axial tension. As a consequence of the altered yield behaviour, a simple conversion from strains to stresses using a steel stress-strain relationship derived from uniaxial tensile tests leads to incorrect results for the strain range slightly below the yield limit up to the onset of hardening. These findings are most relevant for experimental studies investigating bond with DFOS, since researchers rely on the validity of the steel material law to derive and validate bond stress-slip relationships.This paper presents the results of the mentioned experiments, discusses the observed differences between bare and embedded reinforcing bars along with potential explanations, and outlines the implications on the determination of steel and bond shear stresses based on strain measurements in reinforcing bars embedded in concrete.