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.

Bond-slip response of deformed bars in rubberised concrete

“…As pointed out in studies dealing with normal concrete, for low and medium levels of confinement (σl,cc/fc<0.25 [20] or σl,cc/fc<0.30 [39]), the relationship between τb,max/σl,cc and σl,cc/fc follows a linear trend [29]. On the other hand, for high confinement levels, above the previously mentioned limits, the confinement has minimal impact on τ b,max .…”

“…R00) have limited resistance to fracture with limited fracture energy. Consequently, as splitting cracks occur, they are typically uncontrolled [29]. For intermediate level of confinement (CL), R00 and R20 were governed by splitting; R40 had mixed response, whilst in the R60 samples, the rebar pulled out.…”

“…In such tests, premature splitting failures can be arrested through confining action by stirrups [27,28] or external devices [29][30][31]. Tests showed nonetheless that the bond strength exhibits an upper bound, since there is a limit of confining pressure which forces bond failure due to shearing of the concrete keys between the rebar ribs [26,29].…”

Influence of the relative rib area on bond behaviour

“…Due to the bursting forces on the surrounding concrete as a result of the wedge action of the ribs Jones, 1995a, 1995b), it has been observed that splitting cracks may develop longitudinally along the bar (Gambarova and Rosati, 1997;Giuriani et al, 1991;Tepfers, 1973). Splitting cracks impair bond mechanical behaviour and make the bond very sensitive to confinement Tepfers, 1973).…”

Influencia del diámetro de la barra y del recubrimiento en las características adherentes del hormigón armado

“…Este tipo de fallo es frágil y puede ser originado por ejemplo por un cambio de volumen de una barra debido a corrosión. Sin embargo, conociendo la relación tensión de adherencia local deslizamiento y las condiciones de confinamiento es posible comprobar aunque el recubrimiento se encuentre fisurado si las condiciones del anclaje son o no fiables desde el punto de vista de la seguridad (8).…”