Bond and splitting in bar pull-out: behavioural laws and concrete cover role

Abstract

Concrete splitting originating at the interface of longitudinal rebars is a tricky aspect of R/C structures, since the ensuing loss of bond may cause the premature activation of one of the major failure mechanisms. Consequently, the study of bond after concrete splitting, leading to the formulation of suitable behavioural laws, is still needed, and constitutes the objective of this paper, in order to quantify the loss of bond efficiency in terms of peak strength, initial stiffness, friction and bond-confinement interaction. Within this context, the authors try to give a systematic answer, starting from the results obtained in Milan in the late 1980s and early 1990s, by testing three series of specimens, reinforced with a single ribbed bar (Ø 14, 18 and 24 mm) and characterized by a preformed splitting crack, whose width was controlled during each pull-out test. In the last part of the paper, the bondconfinement law proposed by the authors is introduced into a recently developed limit analysis model, which considers the concrete embedding of the bar as an elastic-plastic fracturing material, and some plots of bond strength plotted against concrete cover are given, for different values of the transverse pressure. Although transverse pressure is always beneficial, bond strength exhibits an upper bound, since high pressures lead to bond collapse in shear, owing to the shearing off of the concrete keys between bar ribs.